COMPETENCY BASED LEARNING MATERIAL
Sector:
METALS AND ENGINEERING
Qualification:
Flux-Cored Arc Welding (FCAW) NC 4
Unit of Competency:
Weld Carbon Steel Plates
Module Title:
Performing Groove Welding on Carbon Steel Pipe
TECHNICAL EDUCATION AND SKILLS DEVELOPMENT AUTHORITY
East Service Road, South Superhighway, Taguig City, Metro Manila
METALS AND ENGINEERING
INDUSTRY SECTOR
NATIONAL CERTIFICATE LEVEL 4
QUALIFICATION LEVEL
COMPETENCY-BASED LEARNING MATERIALS
No. Unit of Competency Module Title Code
1. Weld carbon
1. Performing groove
steel Pipes welding on carbon
using FCAW steel pipes
HOW TO USE THIS COMPETENCY BASED LEARNING MATERIAL
Welcome to the Competency Based Learning Material or Module on Performing Groove
Welding on Carbon Steel Pipes. This module contains training materials and activities for you
to complete.
The unit of competency "Weld Carbon Steel Pipes Using FCAW" contains the knowledge,
skills and attitudes required of a Welder. It is one of the core modules at National Certificate
level I (NC I).
You are required to go through a series of learning activities in order to complete each
learning outcomes of the module. In each learning outcome there are learning resources to
help you better understand the required activities. Follow these activities on your own and
answer the self-check at the end of each learning outcome/activity. Use the blank answer
sheet at the end of each module to reflect your answers for each self-check. If you need
clarification on the technical terms used in this module, refer to the “Definition of Terms”. If you
have questions, please don’t hesitate to ask your trainer for assistance.
Recognition of Prior Learning (RPL)
You may already have some or most of the knowledge and skills covered in this module
because you have:
been working for some time
already completed training in this area.
If you can demonstrate to your trainer that you are competent in a particular skill, you don’t
have do the same training again.
If you feel you have some of the skills, talk to your trainer about having them formally
recognized. If you have a qualification or certificates from previous training, show them to your
trainer. If the skills you acquired are still relevant to the module, they may become part of the
evidence you can present for RPL.
At the end of this module is a Learner’s Diary. Use this diary to record important dates, jobs
undertaken and other workplace events that will assist you in providing further details to your
trainer or an assessor. A Record of Achievement is also provided for your trainer to
accomplish once you have completed the module.
DEFINITION OF TERMS:
Alternating Current (AC) - an electrical current that reverses its direction at regular intervals, such
as 60 cycles alternating current (AC) or 60 Hz.
Amperage - the measurement of electricity flowing past a given point in a conductor per second.
Current is another name for amperage.
Arc - the physical gap between the end of the electrode and the base metal is the physical gap that
causes heat due to resistance of current flow and arc rays.
Arc Length -distance or air space between the tip of the unmelted electrode wire and the work for
GMAW - arc voltage determines arc length.
Arc Voltage – voltage measured across the welding arc between the electrode tip and the surface
of the weld puddle.
Automatic welding (AU) - uses equipment' which welds without the constant adjusting of controls
by the welder or operator. Equipment controls joint alignment by using an automatic sensing device.
Circuit -The complete path or route traveled by the electric current circuit for GMAW can include
the welding machine, weld cables, wire feeder, gun assembly, arc, base metal and work clamp with
cable.
Complete Joint Penetration - occurs when the weld metal completely fills the groove, and good
fusion to the base metal is present.
Concentric - Having a common center or a common axis.
Constant current (CV) Welding Machine - These welding machines have limited maximum short
circuit current. They have a negative volt-amp curve and are often referred to as "drooper". The
voltage will change with different arc length while only slightly varying the amperage, thus the name
constant current or variable voltage.
Constant Voltage (CV), Constant Potential (CP) Welding Machine - "Potential" and "Voltage" are
basically the same in meaning. This type of welding machine output maintains a relatively stable,
consistent voltage regardless of the amperage output. It results in a relatively flat volt-amp curve as
opposed to the drooping volt-amp curve of typical SMAW (stick) machine.
Current - another name for amperage. The amount of electricity flowing past a given point in a
conductor every second.
Defect - one or more discontinuities that cause a testing failure in a weld.
Direct Current Electrode Positive (DCEP) - the specific direction of current flow through a welding
circuit when the electrode lead is connected to the positive terminal and the work lead is connected
to the negative terminal of a DC welding machine also called direct current reverse polarity (DCRP).
Discontinuity - Any change in a metal's properties, discontinuities are either allowable after testing,
or they are failures and are defects.
Distortion - the warpage of a metal due to the internal residual stresses remaining after welding
from metal expansion (during heating), and contraction (during cooling).
Duty Cycle – this is the number of minutes out of a 1O minute time period an arc welding machine
can be operated at maximum rated output. An example would be 60% duty cycle at 300 amps. This
would mean that 300 amps .the welding machine can be used for 6 minutes and then must be
allowed to cool with the fan motor running for 4 minutes. (Some foreign welding machines are
based on a 5 minutes/cycle).
Edge Joint - a joint that occur when the surfaces of the two pieces of metals to be joined are
parallel or nearly parallel, and the weld is made along their edges.
Electrode extension - while welding, the length of unmelted electrode extending beyond the tip of
the contact tube.-,Also referred as electrical stick.out.
Electron - this is the atomic particle which carries a negative electrical charge. Electrons can move
from one place to another in atomic structures. It is electrons that move when electrical current
flows in an electrical conductor.
Excessive melt-thru - a weld defect occurring in a weld joint when weld metal no longer fuses the
base metal being joined. Rather, the weld metal falls through a weld joint or "burns through; also
referred as EXCESS PENETRATION.
Face - the surface of the weld as seen from the side of the joint on which the weld was made.
Ferrous - refers to a metal that contains primarily iron, such as steel, stainless steel and cast iron.
Fillet weld - a weld that is used to join base metal surfaces that are approximately 90 degrees to
each other, as in a T-joint, corner joint or lap joint ; the cross-sectional shape of a fillet weld is
approximately triangular.
Fit-up – this refers to the manner in which 'two members are brought together to be welded, such
as the actual, space or any clearance or alignment between two members to be welded. Proper fit-
up is important if a good weld is to be made. Tacking, clamping or fixturing is often done to ensure
proper fit-up. Where it applies, base metal must be beveled correctly and consistently. Also, any
root openings or joint angles must be consistent for the entire length of a joint. An example of poor
fit-up can be too large of a root space in a V-groove butt weld.
Flat position - When welding is done from the top side of a joint, it is in the flat position if the face of
the weld is approximately horizontal; sometimes referred as DOWN HAND welding. The axis angle
can be from 0-15 degrees in either direction from a horizontal surface.
Flux- Cored Arc Welding (FCAW) – this is the arc welding process which melts and joins metals
by heating them with an arc between a continuous consumable electrode wire and the work;
shielding is obtained from a flux contained within the electrode core; depending upon the type of flux
cored wire, added shielding may or may not be provided from externally supplied gas or gas
mixture.
Gas Metal Arc Welding (GMAW) – this is an arc-welding process which joins metals by heating
them with an arc. The arc is between a continuously fed filler metal (consumable) electrode and the
workpiece. Externally supplied gas or gas mixtures provide shielding for GMAW.
Gas-Shielded electrode- A flux - cored wire electrode used in the presence of a shielding gas to
provide double protection and improved mechanical properties for the weld zone.
Gas Tungsten Arc Welding (GTAW) - Sometimes called TIG welding (Tungsten Inert Gas), it is a
welding process which joins metals by heating them with a tungsten electrode which should not
become part of the completed weld. Filler metal is sometimes used and argon inert gas or inert gas
mixtures are used for shielding.
Glass - term often used in GMAW to refer to small islands of residue left on top of a weld bead,
usually made of silicate; must be removed for very critical welds; sometimes referred to as DROSS,
glossy islands, or inclusions.
Globular Transfer – non-axial directed transfer between a short circuit and a spray arc transfer;
CO2 shielding gas parameters above the short circuit range result in a globular transfer.
GMA Spot Welding – this is an arc welding process where the gun is not moved. Two pieces of
material can be joined by completely penetrating the top piece and burning into the bottom piece.
Standard GMAW equipment is used along with a special timer control and a special gun nozzle.
Groove Angle - When the groove is made between two materials to be joined together, the groove
angle represents the total size of the angle between the two beveled edges. This is also referred to
as INCLUDED GROOVE ANGLE.
Gun Technique -/Refers to the position of the gun as it progresses along the weld joint. A
perpendicular technique would have the wire being fed 90 degrees into the weld.
Drag Technique - has the gun pointed back at the weld as the gun is "dragged away" from the
deposited weld metal; the drag technique is sometimes referred to as PULL technique.
Push Technique - has the gun pointed away from the weld as the gun is "pushed" away from the
weld.
Heat Affected Zone (HAZ) - the portion of a weldment that has not melted, but has changed due to
the heat of welding. The HAZ is between the weld deposit and the unaffected base metal. The
physical makeup or mechanical properties of this zone are different after welding.
Helix – this refers to the distance in height between one end of a looped wire and the other end.
The wire should be laid flat and not restrained. The wire should be cut off to form one loop, or a
maximum of 10 ft. For steel, 4" spool, 0.045" diameter and less, helix should be 1/2" maximum. For
steel spools larger than 4" and for wires 0.030" in diameter and smaller, helix should be 1"
maximum. For steel spools larger than 4" and for wires 0.035" in diameter and larger, helix should
be 1" maximum. For stainless steel 12" spools, helix should be 1" maximum.
Horizontal Position - occurs when the axis of the weld is from 0-15 degrees from the horizontal,
and the face rotation is from either 80-150 degrees or 210-280 degrees for groove welds, or from
either 125-150 degrees 0r 21 0-235 degrees or fillet weld.
Incomplete Fusion - molten metal rolling over a weld edge but failing to fuse to the base metal;
also referred to as COLD.
Inductance - inductance (an inductor) will slow down the changes in current, as if the electrons
were sluggish.
Inductor (Stabilizer) - for GMAW and related processes, an inductor changes the welding
machine’s rate of response and number of short circuit; per second. An inductor limits the amount of
spatter and generally improves wetting out of the puddle.
Inverter - power sources which increases the frequency of the incoming primary power, thus
providing smaller size machines and improved electrical characteristics for welding, such as faster
response time and more control for pulse welding.
Joint Design - a cross sectional design and the given measurements for a particular weld;
generally includes included angle, root opening, root face, etc.
Lap Joint - joint that is produced when two or more members of a weldment overlap one
Load Voltage - Measured at the output terminal of a welding machine while under load; it includes
the arc voltage (measured while welding), and the voltage drop through connections and weld
cables.
Nonferrous - refers to a metal that contains no iron, such as aluminum, copper, bronze, brass, tin,
lead, gold, silver, etc.
Open Circuit Voltage (OCV) - as the name implies, no current is flowing in the circuit because the
circuit is open. The voltage is impressed upon the circuit, however, so that when the circuit is
completed, the current will flow immediately. For example, a welding machine that is turned on but
not being used for welding at the moment will have an open circuit voltage applied to the cables
attached to the output terminals of the welding machine
Overhead Position - when the axis angle is between 0-80 degrees and the face rotation is from 0-
80 degrees or 280-360 degrees; for groove welds or from 0-125 degrees or 235-360 degrees for
fillet welds, the weld position is considered to be in the overhead position
Plasma - the electrically charged, heated ionized gas which conducts welding current in a welding
arc
Plug welding - a weld made by filling (or partially filling) a hole in one member of a joint, fusing that
member to another member
Positioner - a device which moves the weldment when a stationary arc is used. Positioners
include^ turning rolls, head and tail stocks and turntables
Pounds per Square inch - a measurement equal to a mass or weight applied to one square inch of
surface area
Primary Power - often referred to as the input line voltage and amperage available to the welding
machine from the shop's main power line; often expressed in watts or kilowatts (KW), primary input
power is AC and may be single-phase or three-phase; welding machine with the capability of
accepting more than one primary input voltage and amperage must be properly connected for the
incoming primary power being used
Puddle - more often referred to as Molten Weld Pool, the weld puddle is the liquid state of a weld
prior to solidification
Pulsed Spray Welding - also called PULSED ARC WELDING) his method of welding uses two
separate currents, and alternates between them to produce less heat than a constant spray
transfer. One current is in the spray transfer current range and the other current is lower
Purging - cleaning, purifying or removing something from a container
Quenching - the dipping of a heated metal into water, oil, or other liquid to obtain necessary
hardness
Reactor - For welding purposes, a reactor is a device within a welding machine which allows a
welder some degree of slope control. For example, a 14 turn slope reactor would have 14 wraps of
wire around the reactor iron core
Rectifier - An electrical device that allows the flow of electricity in basically only in one direction; its
purpose is to change alternating current (AC) to direct current (DC)
Residual Stress – this is the stress remaining in a metal resulting from thermal or mechanical
treatment or both. When welding, stress result when the melted material expands and then cools
and contracts. Residual stresses can cause distortion as well as premature weld failures.
Resistance - -The- opposition to the flow of electrical current in a conductor. This opposition to
current flow changes electric energy into heat energy. Resistance is measured in ohms with an
ohmmeter
Response Time - the time it takes for a welding machine to go from OCV to a short circuit, and
then to a welding voltage and amperage
Root - tne deepest point of fusion in a weld bead; for instance, the root of a joint would be where
the joint members are in the closest point of contact in the weld
Root Opening - the separation of the members to be welded together at the root of the joint
Secondary Power - refers to the actual power output of a welding machine; this includes the load
voltage while welding, measured at the output terminals and the current (amperage) flowing in the
circuit outside the welding machine; secondary amperage can be measured at any point along the
secondary circuits
Semiautomatic - an FCAW process where welding is controlled as an operator squeezes or
releases a gun trigger
Sensitization - the changing of a stainless steel's physical properties when being exposed to a
temperature range of 800 to 1600 F for a critical period of time; see also Carbide Precipitation
Shielded Metal Arc Welding (SMAW) - an arc-welding process which melts and joins metals by
heating them with an arc, between a covered metal electrode and the work; shielding gas is
obtained from the outer coating, often called as flux; filler metal is primarily obtained from the
electrode core
Short Circuiting Transfer - a method of transfer in which metal is deposited only when the wire
actually touches the work. No metal is transferred across the open arc
Single Phase - when an electrical circuit produces only one alternating cycle within a 360 deg.
Time span, it is a single-phase circuit
Slope - slope refers to the shape of the volt-amp curve. By varying the number of turns of slope in
the welding circuit, a welder can change the amount of short circuit current and in some cases the
welding machine's rate of response
Solenoid - an electrical device which either stops or permits the flow of water used to cool a
welding gun or the flow of gas used to shield the weld puddle and arc
Spatter - the metal particles blown away from the welding arc; these particles do not become part of
the completed weld
Spray Transfer - movement of a stream of tiny molten droplets across the arc from the electrode to
the weld puddle
Submerged Arc Welding (SAW) – this is a process by which metals are joined by an arc or arcs
between a bare metal electrode or electrodes and the work. Shielding is supplied by a granular,
fusible material usually brought to the work from a flux hopper. Filler metal comes from the
electrode and sometimes from a second filler rod
T-Joint – a joint produced when two members are located approximately 90 deg to each other in
the shape of a Tee
Three-Phase - refers to an electrical circuit that delivers three cycles within a 360 degree time span;
and the cycles are 120 electrical degrees apart, it is a three-phase circuit
Transitional Parameters - a transition point occurs when metal transfer changes, such as when
short-circuit transfer becomes a globular transfer or globular transfer becomes spray transfer. This
can happen when the voltage and amperage setting-are too high for the short-circuit transfer, or too
low for a spray transfer
Transverse - a measurement made across an object, or basically at or near a right angle to a
longitudinal measurement
Travel Angle - the angle at which the gun is positioned from the perpendicular as the weld
progresses; travel angles are usually 5 to 15 degrees
Undercut – this refers to a groove melted into the base metal usually along the toes of the weld.
Undercut can also occur on either side of the first pass of a full-penetration weld, such an open-
groove butt weld. Undercutting produces a weak spot in the weld, and since it is considered a
defect, it must be corrected in many cases
Vertical Position - when the axis of the weld is between 15-80 degrees and the face rotation is
between 80-280 degrees for groove welds or 125-235 degrees for fillet welds, the weld position is
considered to be in the vertical position. When the axis angle is increased between 80-90 degrees
the face rotation can be any angle from 0-360 deg for both groove and fillet welds.
Voltage – this is the pressure or force that pushes the electrons through a conductor. Voltage does
not flow, but causes amperage or current to flow. Voltage is sometimes termed Electro Motive Force
(EMF) or difference in potential
Wagon Tracks - defect within a weld resulting in an inclusion or a void when a pass is made over
an undercut area at the toes of the weld; also occurs if a weld is made over unremoved “glass”
Weave - the side-to-side motion or oscillation of the electrode wire while traveling along a joint; the
purpose of a weave is to increase the volume of the weld bead and ensure proper joint fusion
Weld Metal - the electrode and base metal that was melted while welding was taking place; this
forms the welding bead
Weld Transfer – this is the method by which metal is transferred from the wire to the molten
puddle. There are several methods used in GMAW; they include: short-circuit transfer, spray-arc
transfer, globular-transfer, buried-arc transfer and pulsed-arc transfer
Welder - a person who performs manual or semi-automatic welding; sometimes incorrectly used to
describe a welding machine
Welding Operator - a person who operates a machine or automatic welding equipment
Wire Feed Speed - expressed in millimeter per second (mm/s) or inch per minute (in/min) and
refers to the speed and amount of filler metal fed into a weld; generally speaking the higher the wire
feed speed, the higher the amperage
Work Angle - the angle to either side of the gun between the workpiece and the gun on a flat plate
butt weld, a normal work angle would be 90 deg to either side of the gun
QUALIFICATION : FCAW NC IV
UNIT OF COMPETENCY : Weld Carbon Steel Pipes
MODULE : Performing Groove Welding on Carbon Steel Pipes
INTRODUCTION:
This module contains information and practices in welding groove welding in all positions. It
includes information on how to apply your acquired knowledge skills in depositing beads on a
joint.
You need to complete this module before you can perform other welding processes. Practices in this
module will help you develop hand control to attain correct electrode angle, speed, manipulation and
other factors essentials to Flux-Cored Arc Welding.
Upon completion of this module, you have to submit yourself for assessment.
Summary of Learning Outcomes:
At the end of this module you will able to:
LO1. Weld Carbon-Steel Pipes in 1G Position
LO2. Weld Carbon-Steel Pipes in 2G Position
LO3. Weld Carbon-Steel Pipes in 5G Position
LO4. Weld Carbon-Steel Pipes in 6G Position
Assessment Criteria:
1. Weld bead of workpiece is free of defects in accordance with specifications
2. Weld size is not greater than 2mm for side lap and 3mm for crown height
3. Weld profile is in accordance with WPS industry standards
4. Penetration minimum of flush to base metal and convexity of not greater than 3 mm in height in
accordance with WPS
5 Work angle (MIG gun/torch/rod) is in accordance with ISO standards
QUALIFICATION : FCAW NC IV
UNIT OF COMPETENCY : Weld Carbon Steel Pipes
MODULE : Performing Groove Welding on Carbon Steel Pipes
LEARNING OUTCOME #1 : Weld Carbon Steel Pipe in Flat Position (1G)
ASSESSMENT CRITERIA:
1. Weld bead of workpiece is free of defects in accordance with specifications
2. Weld size is not greater than 2mm for side lap and 3mm for crown height
3. Weld profile is in accordance with WPS/Industry standards.
4. Penetration is minimum of flush to base metal and convexity of not greater than 3 mm in height in
accordance with WPS
5. Work angle (MIG gun/torch/rod) is in accordance with ISO standards.
RESOURCES:
1. Gas Metal Arc Welding and Flux Cored Arc Welding (2nd edition), The Mid-America Vocational
Curriculum Consortium, Inc.
2. Welding Fundamentals by Mike Gellerman
3. Basic welding Techniques by I.H.Griffin, E.M. Roden and C.W. Briggs
4. The Procedure Handbook by Lincoln Electric Co.
5. NMYC Learning Elements on Welding
6. Pipe Welding Techniques by Griffin,Roden,Jeffus,Briggs
7. Basic TIG and MIG Welding 3 rd Edition by Griffin,Roden,Briggs
8. Flux-Cored/MIG/SMAW Pipe and Tube Welding by Ed Craig's WeltfReality.Com
9. Modern Welding Technology 3rd Edition by Howard B. Gary
10. Internet: www.mo2.biz/welding-cutting gases.htm
11. Internet: www.the-land-rover.com/WeldShop/MasterWelder/Ch10.htm
12. Internet: www.weldreality.com/flux%20cored20%2Q2.htm
13 Internet: www.Magnatech-lp.com/images/articles/million.htm
LEARNING EXPERIENCES / ACTIVITIES
LO#1 – Weld carbon steel pipes in flat position (1G)
LEARNING ACTIVITIES SPECIAL INSTRUCTIONS
1. In the learning resource center you will Review WGF-FCAW- IS-009
read and interpret WF-FCAW-OS-007 in Review WGF-FCAW- IS-012
this learning material. You will also review Review WGF-FCAW-1S-001
how to setup the FCAW equipment and Review WGF-FCAW- IS-002
its accessories Review WGF-FCAW-OS-001
Review WGF-FCAW-OS-002
Review WGF-FCAW-OS-003
Review WGF-FCAW-OS-004
2. If you are ready, you can experiment by Review will be done in the learning
welding in flat position observing specified resource center
shielding gas setting, gun angles, current Get your materials from the tool room
and voltage setting, ESO setting and Review WGF-FCAW-IS-007
travel speed in the training workshop. Review WGF-FCAW-IS-009
Review WGF-FCAW-IS-010
Review WGF-FCAW-IS-011
Review WGF-FCAW-IS-012
Review WGF-FCAW-OS-002
Review WGF-FCAW-OS-003
Review WGF-FCAWOS-004
Perform WGF-FCAW-OS-010
3. Check your work by comparing to the
performance standard in Observation You can ask assistance from your
Checklist No. 1 instructor
Refer to welding Codes and Standards,
industry standards for acceptance criteria
4. If you find difficulty and did not meet the
standard, please go over/again Ask your trainer to demonstrate the
process again
5. If you are ready, you can take the
assessment If you pass, you can proceed to the next
activity
INFORMATION SHEET No. 1
Flux-cored arc welding (FCAW) is different from gas-metal arc welding. Flux-cored arc
welding is a semiautomatic welding process in which a tubular wire is fed continuously from a
spool into the weld pool. The wire contains a flux inside that helps to shield the weld pool from
contaminants on the base metal and in the atmosphere. The flux also helps stabilize the arc.
Because the flux is inside the wire, it is protected and is unable to soak up moisture. The flux-
cored arc welding process allows higher amperage than shielded-metal arc welding. As a result,
wire is deposited into the weld more quickly and with deeper penetration.
Welding machines that are set up for gas-metal arc welding can be adapted for flu-cored arc
welding, although lower amperage machines limit the size of wire that can be used. Flux-cored
arc welding is most effective at higher amperages, allowing greater rates of deposition. Power
sources designed for high-amperage welding feature water-cooled and gas-cooled guns with
their own exhaust systems. At high amperages, guns give off heat and produce a larger volume
of fumes, requiring ventilation.
Flux-cored arc welding is really shielded
metal arc welding on a spool. Because these
two processes are so much alike, more
training is necessary to develop competency in
this area than gas-metal arc welding
processes. As in shielded-metal arc welding,
the gun angle is important in flux-cored arc
welding to counter the effects of gravity. Care
must also be taken with this process when
removing slag to avoid slag inclusions (slag
trapped within the weld).
Flux-cored arc welding has many
applications, including the welding of storage
vessels, railroad cars, earth-moving
equipment, and steel structures. Although flux-
cored arc welding is a self-shielding welding
process, a shielding gas can be added.
Carbon dioxide is a popular choice. The
composition of the wire determines whether
the wire is self-shielding or requires a shielding
gas.
Because the wire used in flux-cored arc Fig. 1 Flux-Cored Arc Welding Equipment
welding contains a flux, slag is created that
must be removed. If all welding parameters
are set correctly, the slag should chip off
easily.
Although flux-cored arc welding is appropriate for all welding positions, it is especially useful
for welding plate (metal that is thicker than 3/16 inch) in the flat » welding position. Flux-cored
arc welding is also a popular process for surfacing or building up equipment that are subject to
wear.
1. POWER SOURCE
a. The FCAW process requires a constant-voltage DC power source with a minimum
amperage range of 200 to 400 amps.
b. The process requires an electrode input cable, a work cable, and ground.
c. FCAW requires a reel of continuous electrode wire and an electrode feed unit.
d. FCAW requires a gun and cable assembly.
Fig. 2 FCAW equipment with gun and cable assembly
2. FLUX - CORED WIRE FEEDER
Basic Types of Wire Feeders
a. Integrated - Drive-roll and control unit are attached to the wire reel housing and the entire
unit is mounted on a mobile dolly so the equipment can be moved close to the work area.
Fig. 3 Wire feeder
b. Remote - Drive-roll and control unit are detached from the wire reel housing so longer
work-to-electrode distances can be used and the control unit can be moved closer to
confined work area.
Fig. 4 Drive roll and control unit
Guidelines for using drive rolls and guide tubes
a. Drive rolls and guide tubes for wire feeders have the wire sizes for which they are
designed stenciled on them; use only the wires that are specified.
b. Clean drive rolls and guide tubes after each roll of wire electrode is used.
c. Remove accumulated dirt and shavings with a wire brush, but do not use a solvent to
clean drive rolls.
d. Some worn drive rolls can be reversed, but when drive rolls are badly worn, replace
them.
e. When drive rolls slip while being fed into the cable, check to see if the end of the wire
has been improperly prepared or check for shavings that may build up in the cable.
3. WELDING GUNS (Torches)
Carbon dioxide gas is cold before it enters the arc, and so it will help to cool an air-
cooled gun. Even so, the main cooling for an air-cooled gun comes directly from the
surrounding air.
Fig. 5 An air-cooled welding gun used for welding with current up to 300 amperes.
The gun assembly must deliver the electrode wire, shielding gas, and welding current.
Hollow tubes of copper alloy, threaded on one end, are screwed into the hollow gun nozzle.
They are commonly known as welding tips. These tips usually have the electrode size they
will take (their inside dimension, bore size) stamped on the tip. Of course, only the correct
diameter tip should be installed for the electrode wire diameter being used. Tip cleaners are
available for free, even wire feeding.
Straight and curved-nozzle guns are available, making it easy to weld in hard-to-reach
places. The tips on these nozzles may be 4" to 12" [102 to 305 mm] long and are available
for different models.
Types of FCAW guns and their uses
Straight-nozzle guns are used for the self-shielded process, mostly on fillet welds and on
applications where the work flows under the gun.
Fig. 6 Straight-nozzle FCAW gun
Curved-nozzle guns are used in the self-shielding process, mostly for welding heavy
metals because curved-nozzle guns are flexible and easy to manipulate.
Fig. 7 Curved-nozzle FCAW gun
c. Air-cooled and water-cooled guns are used for the gas-shielded process with air-cooled
guns generally used for operations up to 600 amps, and water-cooled guns for
operations over 600 amps.
Air-cooled gun Water-cooled gun
Fig. 8 Air-cooled and water-cooled guns
Air-cooled guns are used for automatic processes where side shielding is required, and
water-cooled guns are used for automatic processes where concentric shielding is required.
Fig. 9 Uses of air-cooled and water-cooled guns
WELDING CABLE ASSEMBLIES
The cable assembly carries the electrode wire, welding current, shielding gas, and cooling
water to the welding gun. The parts of the cable assembly are protected in either a molded
jacket or a cable sheath.
Flexible liners, made of metal or plastic, may run the full length of the cable from the wire
feeder to the gun. The flexible liner must be used for electrodes less than 1/16" [1.6 mm] in
diameter to keep the wire from buckling or kinking while it is pushed down the cable.
Because the flux-cored cable assembly is complicated, it is very expensive. The welder has
to be careful not to step on, kink, cut, or weld too close to the cable assembly. Any of those
could easily cause costly damage.
As the welding current is increased, the size of the welding cable must also be increased,
especially if the cable is very long before it gets to the wire feeder and then on to the welder's
gun. The table below gives the recommended cable sizes in relation to the distance between
the power source and the wire feeder.
Recommended Sizes for the Welding Cable from the Welding Machine to the Wire Feeder.
Welding Distance from the Welding Machine to the Wire
Current Feeder (Feet)
(Amperes) 50 100 150 200
100 1 1 0 0
150 1 1 00 0000*
200 1 0 0000 0000
300 0000 0000
400 0000 0000
500 0000(2R) 0000(2R)
*This may also be written as 4/0. (1 ft. = 304.8mm)
SELF-CHECK 1
Select the best answer from the choices given. Write your answer on the space provided before
each number:
_____ 1. What do we call an arc welding process in which a tubular wire is fed continuously
from the spool into the weld pool?
A. semiautomatic B. automatic C. GMAW D. FCAW
_____ 2. You're setting up for a flux-cored operation that has long, linear weld specified. You
should:
A. select an integrated wire feeder on a dolly so it can be moved along the work
piece.
B. select a remote wire feeder so you can keep the gun as close to the work piece as
possible
_____ 3. You're setting up for a flux-cored operation that requires out-of-position welding in a
confined area. You should:
A. select an integrated wire feeder and position it as close to the confined area as
possible
B. select a remote wire feeder so you can move the control unit close to the confined
area
Modified True or False: Write T if the statement is correct. If the statement is wrong supply the
correct word or group of words on the space provided.
_____ 4. Air-cooled guns are used for the self-shielded process.
_____ 5. Water-cooled guns are used for operations having a current of more than 600
Amperes.
_____ 6. In the use of the welding cable, as the welding current increases, the size of the
welding cable will decrease.
_____ 7. The FCAW process requires a constant voltage AC power source with a minimum
amperage range of 200-400 amps.
FEEDBACK 1
1) d
2) a
3) b
4) gas-shielded
5) T
6) increase
7) DC
In order to produce high-quality welds, the flux-cored arc welding process, like any other
welding process, must be correctly applied and carefully controlled. Although joints designed for
other types of welding may be welded with this process, it is a good idea to modify those joints,
if possible, for best results.
Shielding Gas Cylinders
The first step in setting up the equipment is to connect the shielding gas cylinders to the wire
feed unit. From the wire feed unit, the shielding gas will be distributed down the welding cable.
Here, common safety rules apply when handling the gas cylinders.
In brief, the cylinders must always be kept upright and moved around carefully. After the
protective valve caps have been unscrewed, the valves must be cracked to clean out the seat
before the regulators are attached. No oil should be used on the regulators and flowmeters.
Because two or more carbon dioxide cylinders must be used for enough shielding gas flow,
care must be taken to carefully connect and purge the manifold assembly before connecting the
manifold to the wire feeder.
Generally speaking, safety rules concerning the setting up of oxygen cylinders also apply to
carbon dioxide cylinders.
Adjusting the Welding Machine
When the shielding gas cylinders have been correctly set up, the welding machines itself is
adjusted. The welding machine is usually separate from the wire feed machine. The machine
should be adjusted for the recommended current and polarity. Usually, flux-core arc welding
requires reverse-polarity DC.
Both bead height and width are affected by the amperage used.
Adjusting the Wire-feed Machine 1100
The wire-feed machine actually does many 1000 015
jobs in the welding process, and each may 900
required a separate adjustment.
800
It many or may not provide an adjustment
for the arc voltage. 700
It must pull the electrode wire form the 600 1/16
welding wire spool and push it down the
cable assembly. This adjustment is called WIRE FEED AND SPEED 500
the wire feed speed.
400 3/32
It must control the flow of shielding gas 1/8
through the welding cable. While the
flowmeter controls how much shielding gas 300
flows, the wire-feed machine controls
whether or not the gas flows into the 200
welding gun.
100
It must control the flow of cooling water
through the cable assembly to the welding 0
gun and back to the drain or cooling tank. 0 100 200 300 400 500 600 700 800 900
It must pass the welding current from the WELDING CURRENT (AMPERES)
welding machine to the gun.
Fig. 10 The welding current and electrode wire
diameter help decide what wire-feed speed to use.
These steps should be followed:
1. Open the shielding gas valve. Depress the gun trigger and adjust the gas flow to the
flowmeter. For most electrode wires the flow should be 30 to 35 cfh [14.2 to 16.5L/min].
2. Adjust the wire feed speed.
Many times, specification tables will not give the wire feed in inches per minute (ipm) or
millimeters per second [mm/s]. Often wire feed must be adjusted after a few trial-and-error
welds. The wire feed is too fast if the electrode is being pushed down into the work, welding
itself to the workpiece. On the other hand, the wire feed is too slow if the electrode
disappears up into the shielding gas nozzle, breaking the arc. The wire feed will depend a
great deal on the welder's own technique, the welding job being done, the amperage being
used, and the electrode wire diameter.
An approximate wire feed speed may be determined by comparing the welding current
and electrode wire diameter with graphs that manufacturers have calculated for determining
the speed. A graph for finding the approximate wire feed speed in inches per minute for the
currents shown. Since thicker wires will be harder to melt, they will need slower speeds and
higher amperages than thinner wires.
As an example, a graph might call for 400 amperes with a 1/16" [1.6mm] electrode curve.
Then the line has been continued left from the point where the 400-ampere line crossed the
1/16" [1.6mm] welding wire curve. At the left of the graph is the wire feed speed. The line
crosses the wire feed speed line at about 435 ipm [184 mm/s]. This would be an average,
approximate setting.
Fig. 11 As the amperage is changed, the bead height and bead width change.
Fig. 12 As the arc voltage increases, the bead becomes flatter and wider.
3. Adjust the rheostat on the front of the wire-feed machine for the right voltage.
By changing the arc voltage, you will be able to change the weld bead height or width.
Generally speaking, the weld bead becomes flatter and wider as the arc voltage increases.
Although the voltage setting depends on welding conditions, it is usually from 23 to 32
volts. A quick practice weld can help show that the wire feed speed and voltage adjustments
are giving the right current for a smooth arc. If you find that the arc is uneven, the wire-feed
speed should be adjusted to a new current setting. Also, readjusting the voltage control may
help smooth out the arc.
Adjusting the Travel Speed
The travel speed refers to how fast the welder moves the welding gun along the weld
path. The travel speed, then, is one adjustment that cannot be made until you are actually
welding.
The travel speed is usually given on most specification charts. The travel speed is listed
in inches per minute, or ipm. Usually, it is hard to time yourself and measure the inches
[mm] while welding, so you should try a few "dry runs" by moving along the weld path
without the arc and see how long it takes at certain speeds.
Travel speed will also affect the weld bead and height. Generally speaking, the bead
gets flatter and narrower as the travel speed is increased. Overall, the bead gets smaller as
the speed is increased above that recommended.
Fig. 13 As the travel speed increases, the bead size gets smaller.
The nozzle angle cal also be used to change the bead height and bead width. A trailing
nozzle angle tends to produce a high, narrow bead. As the training angle is reduces, the
bead height decreases and the width increases. This will keep happening as the welder
moves the gun from the trailing angle range into the leading angle range. When the leading
angle is increased too much, the bead will become narrower again.
Stickout
Before striking an arc, trigger the welding gun to push the electrode wire out of the
contact tube about 1" [25.4mm]. If the wire accidentally pushes out more than 1" [25.4mm],
wire cutters can be used to cut it off at the desired length.
The amount of stickout and the wire feed speed both affect the weld penetration because
they change the welding current slightly. Increasing the stickout can reduce the current by
almost 100 amperes. This, of course, will reduce penetration.
With semiautomatic flux-core arc welding, stickout can be adjusted during welding. This
adjustment makes it easy for the welder to allow for any variation in the joint without having
to stop the arc. Also because of this, stickout can be used as a control for changing weld
penetration during welding.
Stickout changes the current by changing how much the electrode wire us preheated. As
the stickout is increased, the wire is preheated more before it actually melts. Because the
wire is preheated more, the welding machine doesn't need to put out as much current to
melt the wire at the feed rate being used. Due to the self-regulating nature of the constant
voltage welding machine, the overall result is a decrease in current. Since the current
decreases, the weld penetration also decreases.
Fig. 14 Weld deposit
The amount of electrode
On the other hand, as the stickout decreases, the wire preheating also decreases. When
this happens, the welding machine is forced to put out more current to melt the wire at the
feed rate being used. The increase in current then sauces an increase in penetration.
Decreased stickout, therefore, will also increase the weld deposit rate.
A 3/32" [2.4mm] diameter electrode wire with about 500 amperes current will penetrate
well into the insulating effect of the molten weld pool on thick pieces. Thicker weld beads
can deposited, or narrower included angles used, if a root gap is used in the weld joint.
Flux-cored arc welds will usually bridge the
gap in a joint making it possible to weld many
joints without using a backing strip. Standard flux-
cored arc welds will bridge gaps with slightly less
than full penetration. However, when compete
penetration is needed, it is hard to control the
difference between full penetration is needed
without back gouging backup bars are
recommended.
The applied techniques and end results in the
MIG welding process are controlled by these
variables and must be understood by the student.
The variables are adjustments that are to be made
to the equipment and also manipulations by the
operator.
Fig. 15 As the stickout is increased, more
weld metal is deposited because the
electrode wire has received more preheat.
The variables can be divided into three areas.
Preselected variables
Primary adjustable variables
Secondary adjustable variables
PRESELECTED VARIABLES
Preselected variables depend on the type of material being welded, the thickness of the
material, the welding position, the deposition rate and the mechanical properties. These
variables are:
Type of electrode wire
Size of electrode wire
Type of inert gas
Inert-gas flow rate
Charts are references for the new MIG welding student. Manufacturers'
recommendations also serve as a guide to be followed in these areas.
PRIMARY ADJUSTABLE VARIABLES
These control the process after preselected variables have been found. They control the
penetration, bead width, bead height, arc stability, deposition rate and weld soundness.
They are:
Arc voltage © Welding current © Travel speed
SECONDARY ADJUSTABLE VARIABLES
These variables cause changes in the primary adjustable variables which in turn cause
the desired change in the bead formation. They are:
Stickout
Nozzle angle
Wire-feed speed
OPERATING VARIABLES WITH FLUX-CORED ELECTRODE WIRES
a. Arc voltage - when all other variables are held constant,arc voltage variations will have these
effects:
(1) High arc voltage will produce a wider and flatter bead.
(2) Excessive arc voltage will tend to produce porosity.
(3) Low voltage tends to produce a convex, ropey bead.
(4) Extremely low voltage will cause the wire to stub on the plate or cause the wire to pierce
the plate, strike the joint bottom, and push the gun up.
b. Current - when all other variables are held constant, current variations will have these
effects:
(1) Increasing current will increase melt-off and deposition rate.
(2) Excessive current will produce a convex bead that looks bad and wastes metal.
(3) Low current will result in reduced penetration.
c. Travel speed - when all other variables are held constant, travel speed variations will have
these effects:
(1) Excessive travel speed makes the bead concave with uneven edges.
(2) Too slow a travel speed produces slag inclusions and a rough, uneven bead.
d. Electrode extension - when all other variables are held constant, electrode extension
variations will have these effects:
(1) Increased extension decreases current and decreased extension increases current.
(2) Increased extension lowers voltage across the arc, the lower arc voltage makes the bead
more convex, and the tendency for porosity is reduced.
(3) Short extension gives greater penetration than long extension.
Electrode Extension and Visible Electrode Extension
a. Electrode extension is the length of the electrode from the contact tip within the nozzle to the
workpiece.
b. Visible electrode extension is the length of the electrode than can actually be seen between
the end of the nozzle and the workpiece.
Note: Electrode extension is sometimes referred to as electrode stickout, and visible
electrode extension is sometimes called visible stickout. When the contact tip and the end of
the nozzle terminate at the same point, electrode extension and visible electrode extension
are the same.
Fig. 16 Electrode extension
With gas shielding, typically short electrode extensions (1/2" to 1-1/4") are used for most
wire diameters. The use of larger electrode diameters (greater than 3/32") and the use of
100% CO2 shielding gas can result in welds having deep penetration. This is desirable for
some welding applications to reduce required weld size or weld joint volume. Gas-shielded
FCAW electrodes are most popular for automatic, semi-automatic and robotic welding of
mild and low alloy steels. Representative applications include bridges, mining machinery,
offshore drilling rigs, ships, structural and general fabrication.
Setting Up to Weld
Follow the operator's manual when setting up the welding equipment for gas metal arc
welding or flux cored arc welding. The successful completion of this unit partly depends on
knowing what a quality weld looks like. If you can recognize a quality weld, you should be able
to set up any welding machine to make quality welds even if the adjustments are not labeled.
1. When setting up for welding, remember that the voltage and wire feed adjustments work
together. If the wire feed adjustment is too high for the voltage, the wire will not melt properly
into the weld pool, and an excessive buildup of filler metal ill result.
2. If the wire feed adjustment is too low for the voltage, the wire will burn back into the contact
tube. To save the contact tube from becoming fused to the wire during welding, begin with a
high wire feed and low voltage adjustments.
3. Some welding machines are equipped with fine adjustments for slope and inductance.
Slope can be used in combination with the voltage adjustments. Increasing slope causes a
decrease in current (amperage), which can cut back on spatter. Increasing inductance
causes an increase in arc time (wetness of the weld pool) as the current rises. Inductance
can also cut back on spatter by controlling the rate of current rise. Begin by setting these
parameters in the middle range.
4. There is a relationship between travel speed across the joint and penetration. Penetration
increases as travel speed decreases and vice versa. However, penetration should not be
determined by travel speed alone. The adjustment f voltage in relation to wire feed speed is
also involved.
5. The width of the weld bead is a primary consideration for setting the voltage, but travel
speed and wire feed (amperage) are also important.
6. The height of the weld bead is a primary consideration for setting the voltage, and travel
speed and wire feed (amperage) are also important.
7. The electrode extension and the gun angle are to more secondary adjustments that can
affect welding. Use a sidecutter and cut the wire to keep the stickout close when beginning
the arc.
8. Check the nozzle to see that it is not plugged and that the opening of the contact tube is
round. An anti-spatter spray or gel can be used to help prevent the spatter from sticking to
the nozzle.
9. The sound of the arc during welding is an indication that the proper welding parameters are
in place. Does the arc sound like eggs frying? It should.
10. Relax and find a comfortable position to weld. Good welding technique depends on being
comfortable.
SELF-CHECK 2
Complete statements about operating variables with flux-cored electrode wires. Encircle the
material that best completes each of the following statements.
Arc voltage
When all other variables are held constant, arc voltage variations will have these effects:
1. (High) (Low) arc voltage will produce a wider and flatter bead.
2. (Excessive) (Low) arc voltage will tend to produce porosity.
3. (High) (Low) voltage tends to produce a convex, ropey bead.
4. Extremely (low) (high) voltage will cause the wire to stub on the plate or cause the wire to
pierce the plate, strike the joint bottom, and push the gun up.
Current
When all other variables are held constant, current variations will have these effects:
5. (Increasing) (Decreasing) current will increase melt-off and deposition rate.
6. (Excessive) (Too little) current will produce a convex bead that looks bad and wastes metal.
7. (Low) (High) current will result in reduced penetration.
Travel speed
When all other variables are held constant, travel speed variations will have these effects:
8. (Excessive) (Slow) travel speed makes the bead concave with uneven edges.
9. (Too slow) (Too fast) a travel speed produces slag inclusions and a rough, uneven bead.
Electrode extension
When all other variables are held constant, electrode extension variations will have these
effects:
10 (Increased) (Decreased) extension decreases current and (increased) (decreased)
extension increases current.
11. (Increased) (Decreased) extension lowers voltage across the arc, the lower arc voltage
makes the bead more convex, and the tendency for porosity is reduced.
12. (Short) (Long) extension gives greater penetration than (short)(long) extension.
FEEDBACK 2
1) High
2) Excessive
3) Low
4) Low
5) Increasing
6) Excessive
7) Low
8) Excessive
9) Too slow
10) Increased, decreased
11) Increased
12) Short, long
1. Flux-cored arc welding, using a shielding gas, gives off about the same amount of smoke
and fumes as does manual shielded arc welding. The smoke and fumes may be irritating,
although they are not necessarily harmful.
When welding on zinc or cadmium-plated steels, however, the elder needs to guard
against the poisonous or harmful fumes that these metals give off. It would be a good idea to
wear a respirator while welding metals with these coatings. A respirator is a breathing unit
which filters out the poisonous gas and fumes. If a respirator is not available, the welding
area must be well ventilated with exhaust fans.
As mentioned earlier, the poison gas carbon monoxide if formed when the carbon
dioxide shielding gas breaks down at the extreme arc heat. Fig. 17 shows the typical carbon
monoxide concentration at certain distances from the shielded arc.
You will not usually be affected by the carbon monoxide if you keep your face at least 4"
[100 mm] from the smoke cone. For long periods of constant welding, remain at least 7"
[200 mm] from the smoke cone. When using this and other welding processes, local exhaust
fans or other ventilating systems should be used. When welding in areas with poor
ventilation, always use a self-contained respirator.
As with any welding process, the welder should be prepared to face bright light, heat,
and hot sparks. For the safest possible welding, do not wear trousers with cuffs, boots with a
welt bead around the top of the toe.
Fig. 17 The carbon monoxide concentration at different distances from the carbon dioxide shielded arc.
(Hob Brothers Company)
2. Wear safety glasses with side shields. Safety glasses protect your eyes fro slag and from
grinding particles. They also deflect ultraviolet rays which are given off by the welding arc
and help prevent arc flash. Safety glasses are so important that no one should be in a
welding shop without having them on. However, safety glasses are no protection against
careless shop practices.
3. Wear earplugs in the shop to safeguard your hearing. Besides offering protection from
nerve-damaging noise, earplugs help prevent hot slag and sparks from reaching the
eardrum.
4. Use leather welding gloves to protect your hands and lower arms from burns and shock.
Only gloves made for welding do an adequate job of handling heat without losing their
shape. If you sue tongs to handle hot metal, your welding gloves will last longer and not
become stiff.
5. Wear cotton clothing, not synthetic fibers that will burn or melt to the skin, to shield exposed
skin from arc rays which are more intense than sunlight. A leather jacket, leather sleeves,
and chaps, although expensive, are preferred because they will save your clothing from the
sparks and spatter of welding.
6. Be sure the shop is clear of any flammable or volatile materials. Gasoline and solvents have
no place in a welding shop.
7. Keep cigarette lighters under pressure out of the welding shop.
8. Always wear a welding helmet with a filter lens of the correct shade to protect against arc
flash. Arc flash is a painful, but usually a temporary, eye condition caused by the light of the
welding arc. If you suspect arc flash is more serious, get proper medical attention. Replace
cracked lenses or damaged helmet. Check the shade number on the lens. Gas metal arc
welding and flux-cored arc welding require a shade number of 10 to 14.
9. Welding shop should be equipped with fire extinguishers. Know their location and
classification.
10. Use the ventilation system to remove smoke and fumes.
11. Protect yourself against electric shock. Use gloves, and be sure your clothing and workplace
are dry. If you stay dry while welding, you act as an insulator and not a conductor of
electricity. Electricity always seeks the path of least resistance, so stay dry when welding to
stay safe.
12. Steel-toe leather boots offer some protection against foot injury. Leave your athletic shoes at
home.
13. Wear dark clothing, which absorbs light rays, to reduce the dangers of arc flash.
14. Before turning on the welding machine, be sure to stretch out the gun's power lead and the
workpiece lead. Never wrap either lead around your body. Always pace the workpiece
connection on the work or as close to the work as possible for good electrical contact. The
workpiece connection can be welded, clamped, or screwed into position.
15. Never weld on pressurized cylinders. A stray arc strike on a gas cylinder can cause an
explosion.
16. As a common courtesy, always warn others in the area before striking an arc, and always
begin an arc strike at the point of the welding.
17. Be alert to strange smells. Visibility is limited under a welding helmet, and your sense of
smell becomes important. Unusual odors could indicate a fire or a toxic substance, alerting
you to possible danger.
18. Pay attention to the work at hand. Never give in to daydreaming or distractions.
19. Keep track of wire cut from the spool. Cut wire is a shop hazard. It is sharp enough to
puncture the skin.
20. Turn off the welding machine if you must leave it unattended.
21. Replace parts of the gun when they wear out.
22. Secure the gas cylinder when setting up for welding, and take the necessary precautions to
avoid an arc strike on a gas cylinder.
23. Follow procedures for safety operating machines and electrical components at all times.
24. When working with flux-cored wires that produce excessive smoke and fumes use a welding
gun with a smoke removal attachment.
25. Sparks generated by the FCAW process can be excessive and erratic, so screen the
welding area from nearby workers and people passing by.
SELF-CHECK 3
Complete the following statements about safety requirements for FCAW. Circle the material
that best completes each of the following statements.
1.) Use personal safety (preferences) (clothing and equipment) when working with FCAW
equipment.
2.) Follow procedures for safely operating machines and (gun) (electrical) components at all
times.
3.) When working with flux-cored wires that produce excessive smoke and fumes, use a
(welding gun with a smoke removal attachment) (a portable exhaust cape)
4.) Sparks generated by the FCAW process can be excessive and erratic, so screen the
welding area from nearby (equipment) (workers) and people passing by.
5.) The poison gas (carbon monoxide) (ozone) is formed when the CO2 shielding gas breaks
downs
FEEDBACK 3
1.) clothing and equipment
2.) electrical
3.) welding gun with a smoke removal attachment
4.) workers
5.) carbon monoxide
POSITIONS FOR PIPE WELDING
Pipe welds are made under many different requirements and in different welding situations.
The welding position is dictated by the job. In general, the position is fixed, but in sane cases
can be rolled for flat-position work. Positions and procedures for welding pipe are outlined
below.
a. Horizontal Pipe Rolled Weld
(1) Align the joint and tack weld or hold in position with steel bridge clamps with the pipe
mounted on suitable rollers. Start welding at point C, figure 18, progressing upward to
point B. When point B is reached, rotate the pipe clockwise until the stopping point of the
weld is at point C and again weld upward to point B. When the pipe is being rotated, the
torch should be held between points B and C and the pipe rotated past it.
Fig. 18 Diagram of tack-welded pipe on rollers
(2) The position of the torch at point A (fig. 18) is similar to that for a vertical weld. As point B
is approached, the weld assumes a nearly flat position and the angles of application of
the torch and rod are altered slightly to compensate for this change.
(3) The weld should be stopped just before the root of the starting point so that a small
opening remains. The starting point is then reheated, so that the area surrounding the
junction point is at a uniform temperature. This will insure a complete fusion of the
advancing weld with the starting point.
(4) If the side wall of the pipe is more than 1/4 in. (6.4mm) in thickness, multipass weld
should be made.
b. Horizontal Pipe Fixed Position Weld.
(1) After tack welding, the pipe is set up so that the tack welds are oriented approximately as
shown in figure 19. After welding has been started, the pipe must not be moved in any
direction.
Fig. 19 Diagram of horizontal pipe weld with uphand method.
(2) When welding in the horizontal fixed position, the pipe is welded in four steps as
described below.
Step 1. Starting at the bottom or 6 o'clock position, weld upward to the 3 o'clock
position.
Step 2. Starting back at the bottom, weld upward to the 9 o'clock position.
Step 3. Starting back at the 3 o'clock position, weld to the top.
Step 4. Starting back at the 9 o'clock position, weld upward to the top overlapping
the bead.
(3) When welding downward, the weld is made in two stages. Start at the top and work down
one side to the bottom, then return to the top and work down the other side to join with
the previous weld at the bottom. The welding downward method is particularly effective
with arc welding, since the higher temperature of the electric arc makes possible the use
of greater welding speeds. With arc welding, the speed is approximately three times that
of the upward welding method.
Figure 20. Diagram of horizontal pipe weld with downhand method.
(4) Welding by the backhand method is used for joint in low carbon or low alloy steel piping
that can be rolled or are in horizontal position. One pass is used for wall thickness not
exceeding 3/8 in. (9.5 mm), two passes for wall thickness 3/8 to 5/8 in. (9.5 to 15.9 mm),
three passes for wall thickness 5/8 to 7/8 in. (15.9 to 22.2 mm), and four passes for wall
thickness 7/8 to 1-1/8 in. (22.2 to 28.7 mm).
c. Vertical Pipe Fixed Position Weld. Pipe in this position, wherein the joint is horizontal, is
most frequently welded by the backhand method (fig. 21). The weld is started at the tack
and carried continuously around the pipe.
Figure 21.
SELF-CHECK 4
1.) What thickness of pipe side wall needs a multi-pass weld?
_____________________________________________________________________
2.) Backhand or forehand welding method is necessary in welding. What method is used for
joint in low-carbon steel pipe that can be rolled or are in horizontal position?
_____________________________________________________________________
FEEDBACK 4
1.) More than ¼" (6.4 mm)
2.) Backhand method
WELDING PROBLEMS AND SOLUTIONS
STRESSES AND CRACKING
a. n this section, welding stresses and their effect on weld cracking are explained. Factors
related to weldment failure include weld stresses, cracking, weld distortion, lamellar tearing,
brittle fracture, fatigue cracking, weld design, and weld defects.
b. When weld metal is added to the metal being welded, it is essentially cast metal. Upon
cooling, the weld metal shrinks to a greater extent than the base metal in contact with the
weld, ad because it is firmly fused, exerts a drawing action. This drawing action produces
stresses in and about the weld which may cause warping, buckling, residual stresses, or
other defects.
c. Stress relieving is a process for lowering residual stresses or decreasing their intensity.
Where parts being welded are fixed too firmly to permit movement, or are not heated
uniformly during the welding operation, stresses develop by the shrinking of the weld metal
at the joint. Parts that cannot move to allow expansion and contraction must be heated
uniformly during the welding operation. Stress must be relieved after the weld is completed.
These precautions are important in welding aluminum, cast iron, high carbon steel, and
other brittle metals, or metals, with low strength at temperatures immediately below the
melting point. Ductile materials such as bronze, brass, copper, and mild steel yield or stretch
while in the plastic or soft conditions, and are less liable to crack. However, they may have
undesirable stresses which tend to weaken the finished weld.
d. When stresses applied to a joint exceed the yield strength, the joint will yield in a plastic
fashion so that stresses will be reduced to the yield point. This is normal in simple structures
with stresses occurring in one direction on parts made of ductile materials. Shrinkage
stresses due to normal heating and cooling do occur in all three dimensions. In a thin, flat
plate, there will be tension stresses at right angles. As the plate becomes thicker, or in
extremely thick materials, the stresses occur in three directions.
e. When simple stresses are imposed on thin, brittle materials, the material will fail in tension in
a brittle manner and the fracture will exhibit little or no pliability. In such cases, there is no
yield point for the material, since the yield strength and the ultimate strength are nearly the
same. The failures that occur without plastic deformation are known as brittle failures. When
two or more stresses occur in a ductile material, and particularly when stresses occur on
three directions in a thick material, brittle fracture may occur.
f. Residual stresses also occur in castings, forgings, and hot rolled shapes. In forgings and
castings, residual stresses occur as a result of the differential cooling that occurs. The outer
portion of the part cools first, and the thicker and inner portion cools considerably faster. As
the parts cool, they contract and pick up strength so that the portions that cool earlier go into
a compressive load, and the portions that cool later go into a tensile tress mode. In
complicated parts, the stresses may cause warpage.
g. Residual stresses are not always detrimental. They may have no effect or may have a
beneficial effect on the service life of parts. Normally, the outer fibers of a part are subject to
tensile loading and thus, with residual compression loading, there is a tendency to neutralize
stress in the outer fibers of the part. An example of the use of residual stress is in the shrink
fit of parts. A typical example is the cooling of sleeve bearings to insert them into machined
holes, and allow them to expand to their normal dimension to retain then in the proper
location. Sleeve bearings are used for heavy, slow machinery, and are subject to
compressive residual loading, keeping them within the hole. Large roller bearings are
usually assembled to shafts by heating to expand them slightly so they will fit on the shaft,
then allowing to cool, to produce a tight assembly.
h. Residual stresses occur in all arc welds. The most common method of measuring stress is
to produce weld specimens and then machine away specific amounts of metal, which are
resisting the tensile stress in and adjacent to the weld. The movement that occurs is then
measured. Another method is the use of grid marks or data points on the surface of
weldments that can be measured in multiple directions. Cuts are made to reduce or release
residual stresses from certain parts of the weld joint, and the measurements are taken
again. The amount of the movement relates to the magnitude of the stresses. A third method
utilizes extremely small strain gauges. The weldment is gradually and mechanically cut from
adjoining portions to determine the change in internal stresses. With these methods, it is
possible to establish patterns and actually determine amounts of stress within parts that
were caused by the thermal effects of welds.
i. Figure 22 shows residual stresses in an edge weld. The metal close to the weld tends to
expand in all directions when heated by the welding arc. This metal is restrained by adjacent
cold metal and is slightly upset, or its thickness slightly increased, during this heating period.
When the weld metal starts to cool, the upset area attempts to contract, but is again
restrained by cooler metal. This results in the heated zone becoming stressed in tension.
When the weld has cooled to room temperature, the weld metal and the adjacent base metal
are under tensile stresses close to the yield strength. Therefore, there is a portion that is
compressive, and beyond this, another tensile stress area. The two edges are in tensile
residual stress with the center in compressive residual stress, as illustrated.
Figure 22. Edge welded joint - residual stress pattern.
j. The residual stresses in a butt weld joint made of relatively thin plate are more difficult to
analyze. This is because the stresses occur in the longitudinal direction of the weld and
perpendicular to the axis of the weld. The residual stresses within the weld are tensile in the
longitudinal direction of the weld and the magnitude is at the yield strength of the metal. The
base metal adjacent to the weld is also at yield stress, parallel to the weld and along most of
the length of the weld. When moving away from the weld into the base metal, the residual
stresses quickly fall to zero, and in order to maintain balance, change to compression. This
is shown in figure 23. The residual stresses in the weld at right angles to the axis of the weld
are tensile at the center of the plate and compressive at the ends. For thicker materials
when the welds are made with multipasses, the relationship is different because of the many
passes of the heat source. Except for single-pass, simple joint designs, the compressive and
tensile residual stresses can only be estimated.
Figure 23. Butt welded joint - residual stress pattern.
k. As each weld is made, it will contract as it solidifies and gain strength as the metal cools. As
it contracts, it tends to pull, and this creates tensile stresses at and adjacent to the weld.
Further from the weld or bead, the metal must remain in equilibrium, and therefore
compressive stresses occur. In heavier weldments when restraint is involved, movement is
not possible, and residual stresses are of a higher magnitude. In a multipass single-groove
weld, the first weld or root pass originally crates a tensile stress. The second, third, and
fourth passes contract and cause a compressive load in the root pass. As passes are made
until the weld is finished, the top passes will be in tensile load, the center of the plate in
compression, and the root pass will have tensile residual stress.
l. Residual stresses can be decreased in several ways, as described below:
(1) If the weld is stressed by a load beyond its yield, strength plastic deformation will occur
and the stresses will be more uniform, but are still located at the yield point of the metal.
This will not eliminate residua! stresses, but will create a more uniform stress pattern.
Another way to reduce high or peak residual stresses is by mean of loading or stretching
the weld by heating adjacent areas, causing them to expand. The heat reduces the yield
strength of the weld metal and the expansion will tend to reduce peak residual stresses
within the weld. This method also makes the stress pattern at the weld area more
uniform.
(2) High residual stresses can be reduced by stress relief heat treatment. With heat
treatment, the weldment is uniformly heated to an elevated temperature, at which the
yield strength of the metal is greatly reduced. The weldment is then allowed to cool
slowly and uniformly so that the temperature differential between parts is minor. The
cooling will be uniform and a uniform low stress pattern will develop within the weldment.
(3) High-temperature preheating can also reduce residual stress, since the entire weldment
is at a relatively high temperature, and will cool more or less uniformly from that
temperature and so reduce peak residual stresses.
m. Residual stresses also contribute to weld cracking. Weld cracking sometimes occurs during
the manufacture of the weldment or shortly after the weldment is completed. Cracking
occurs due to many reasons and may occur years after the weldment is completed. Cracks
are the most serious defects that occur in welds or weld joints in weldments. Cracks are not
permitted inmost weldments, particularly those subject to low-temperature when the failure
of the weldment will endanger life.
n. Weld cracking that occurs during or shortly after the fabrication of the weldments can be
classified as hot cracking or cold cracking. In addition, weld may crack in the weld metal or
in the base metal adjacent to welds metal, usually in the heat-affected zone. Welds cracks
for many reasons, including the following:
(1) Insufficient weld metal cross section to sustain the loads involved.
(2) Insufficient ductility of weld metal to yield under stresses involved.
(3) Under-bead cracking due to hydrogen pickup in a hardenable type of base material.
o. Restraint and residual stresses are the main causes of weld cracking during the fabrication
of a weldment. Weld restraint can come from several factors, including the stiffness or
rigidity of the weldment itself. Weld metal shrinks as it cools, and if the parts being welded
cannot move with respect to one another and the weld metal has insufficient ductility, a
crack will result. Movement of welds may impose high loads on other welds and cause them
to crack during fabrication. A more ductile filler material should be used, or the weld should
be made with sufficient cross-sectional area so that as it cools, it will have enough strength
to withstand cracking tendencies. Typical weld cracks occur in the root pass when the parts
are unable to move.
p. Rapid cooling of the weld deposit is also responsible for weld cracking. If the base metal
being joined is cold and the weld is small, it will cool quickly. Shrinkage will also occur
quickly, and cracking can occur. If the parts being joined are preheated even slightly, the
cooling rate will be lower and cracking can be eliminated.
q. Alloy or carbon content of base material can also affect cracking. When a weld is made with
higher-carbon or higher-alloy base material, a certain amount of the base material is melted
and mixed with the electrode to produce the weld metal. The resulting weld metal has higher
carbon and alloy content. It may have higher strength, but it has less ductility. As it shrinks, it
may not have enough ductility to cause plastic deformation, and cracking may occur.
r. Hydrogen pickup in the weld metal and in the heat-affected zone can also cause cracking.
When using cellulose-covered electrodes or when hydrogen is present because of damp
gas, damp flux, or hydrocarbon surface materials, the hydrogen in the arc atmosphere will
be absorbed in the molten weld metal and in adjoining high-temperature base metal. As the
metal cools, it will reject the hydrogen, and if there is enough restraint, cracking can occur.
This type of cracking can be reduced by increasing preheat, reducing restraint, and
eliminating hydrogen from the arc atmosphere.
s. When cracking is in the heat-affected zone or if cracking is delayed, the cause is usually
hydrogen pickup in the weld metal and the heat-affected zone of the base metal. The
presence of higher-carbon materials or high alloy in the base metal can also be a cause.
When welding high-alloy or high-carbon steels, the buttering technique can be used to
prevent cracking. This involves surfacing the weld face of the joint with a weld metal that is
much lower in carbon or alloy content than the base metal. The weld is then made between
the deposited surfacing material and avoids the carbon and alloy pickup in the weld metal,
so a more ductile weld deposit is made. Total joint strength must still be great enough to
meet design requirements. Underbead cracking can be reduced by the use of low-hydrogen
processes and filler metals. The use of preheat reduces the rate of cooling, which tends to
decrease the possibility of cracking.
t. Stress Relieving Methods:
(1) Stress relieving in steel welds may be accomplished by preheating between 800
and 1450°F (427 and 788°C), depending on the material, and then slowly cooling.
Cooling under some conditions may take 10 to 12 hours. Small pieces, such as butt
welded high speed tool tips, may be annealed by putting them in a box of fire
resistant material and cooling for 24 hours. In stress relieving mild steel, heating the
completed weld for 1 hour per 1.00 in. (2.54 cm) of thickness sis common practice.
On this basis, steel % in. (0.64 cm) thick should be preheated for 15 minutes at the
stress relieving temperature.
(2) Peening is another method of relieving stress on a finished weld, usually with
compressed air and a roughing or peening tool. However, excessive peening may
cause brittleness or hardening of the finished weld and may actually cause cracking.
(3) Preheating facilities welding in many cases. It prevents cracking in the heat affected
zone, particularly on the first passes of the weld metal. If proper preheating times and
temperature are used, the cooling rate is slowed sufficiently to prevent the formation
of hard martensite, which causes cracking. Table 1 lists preheating temperatures of
specific materials.
Table 1. Preheating Temperatures*
METAL Temperature
F C
Low carbon steels (up to 0.30 percent carbon) 200 to 300 93 to 140
Medium carbon steels (0.30 to 0.55 percent carbon) 300 to 500 140 to 260
High carbon steels (0.55 to 0,83 percent carbon) 500 to 800 260 to 427
Carbon molybdenum steel (0.10 to 0.30 percent carbon) 300 to 600 149 to 316
Carbon molybdenum steels (0.30 to 0.35 percent carbon) 500 to 800 260 to 427
High strength constructional alloy 100 to 400 38 to 204
Manganese steels (up to 1.75 percent carbon) 300 to 900 140 to 482
Manganese steels (up to 1 5.0 percent manganese) Usually not required
Nickel steels (up to 3.50 percent nickel) 200 to 700 93 to 371
Chromium steels 300 to 500 149 to 260
Nickel and chromium steels 200 to 11 00 93 to 593
Stainless steels Usually not required
Cast iron 700 to 900 371 to482
Aluminum 500 to 700 260 to 371
Copper 500 to 800 260 to 427
Nickel 200 to 300 93 to 149
Monel 200 to 300 93 to 149
Brass and bronze 300 to 500 149 to 260
•The preheating temperatures for alloy steels are governed by the carbon as well as the alloy content of
the steel.
(4) The need for preheating steels and other metals is increased under the following
conditions:
(a) When the temperature of the part or surrounding atmosphere is at or below
freezing.
(b) When the diameter of the welding rod is small in comparison to thickness of the
metal being joined.
(c) When welding speed is high.
(d) When the shape and design of the parts being welded are complicated.
(e) When there is a great difference in mass of the parts being welded.
(f) When welding steels with a high carbon, low manganese, or other alloy content.
(g) When steel being welded tends to harden when cooled in air from the welding
temperature.
u. The following general procedures can be used to relieve stress and to reduce cracking:
(1) Use ductile weld metal.
(2) Avoid extremely high restraint or residual stresses.
(3) Revise welding procedures to reduce restraint.
(4) Utilize low-alloy and low-carbon materials.
(5) Reduce the cooling rate by use of preheat.
(6) Utilize low-hydrogen welding processes and filler metals.
(7) When welds are too small for the service intended, they will probably crack. The
welder should ensure that the size of the welds are not smaller than the minimum
weld size designated for different thickness of steel sections.
IN-SERVICE CRACKING
Weldments must be designed and built to perform adequately in service. The risk of failure of a
weldment is relatively small, but failure can occur in structures such as bridges, pressure
vessels, storage tanks, ships, and penstocks. Welding has sometimes been blamed for the
failure of large engineering structures, but it should be noted that failures have occurred in
riveted and bolted structures and in castings, forgings, hot rolled plate and shapes, as well as
other types of construction. Failures of these types of structures occurred before welding was
widely used and still occur in unwelded structures today. However, it is still important to make
weldments and welded structures as safe against premature failure of any type as possible.
There are four specific types of failures, including brittle fracture, fatigue fracture, lamellar
tearing, and stress corrosion cracking.
a. Brittle Fracture. Fracture can be classified into two general categories, ductile and brittle.
(1) Ductile fracture occurs by deformation of the crystals and slip relative to each other.
There is definite stretching or yielding and a reduction of cross-sectional area at the
fracture (fig. 24).
Figure 24. Ductile fracture surface.
(2) Brittle fracture occurs by cleavage across individual crystals The fracture exposes the
granular structure, and there is little or no stretching or yielding. There is no reduction
of area at the fracture (fig. 25).
Figure 25. Brittle fracture surface.
(3) It is possible that a broken surface will display both ductile and brittle fracture over
different areas of the surface. This means that the fracture which propagated across
the section changed its mode of fracture.
(4) There are four factors that should be reviewed when analyzing a fractured surface.
They are growth marking, fracture mode, fracture surface texture and appearance,
and amount of yielding or plastic deformation at the fracture surface.
(5) Growth markings are one way to identify the type of failure. Fatigue failures are
characterized by a fine texture surface with distinct markings produced by erratic
growth of the crack as it progresses. The chevron or herringbone pattern occurs with
brittle or impact failures. The apex of the chevron appearing on the fractured surface
always points toward the origin of the fracture and is an indicator of the direction of
crack propagation.
(6) Fracture mode is the second factor. Ductile fractures have a shear mode of
crystalline failure. The surface texture is silky or fibrous in appearance. Ductile
fractures often appear to have failed in shear as evidenced by all parts of the fracture
surface assuming an angle of approximately 45 degrees with respect to the axis of
the load stress.
(7) The third factor is fracture surface and texture. Brittle or cleavage fractures have
either a granular or a crystalline appearance. Brittle fractures usually have a point of
origin. The chevron pattern will help locate this point.
(8) An indication of the amount of plastic deformation is the necking down of the surface.
There is little or nor deformation for a brittle fracture, and usually a considerable
necked down area in the case of a ductile fracture.
(9) One characteristics of brittle fracture is that the steel breaks quickly and without
warning. The fractures increase at very high speeds, and the steels fracture at
stresses below the normal yield strength for steel. Mild steels, which show a normal
degree of ductility when tested in tension as a normal test bar, may fail in a brittle
manner. In fact, mild steel may exhibit good toughness characteristics at roan
temperature. Brittle fracture is therefore more similar to the fracture of glass than
fracture of normal ductile materials. A combination of conditions must be present at
the same time for brittle fracture to occur. Some of these factors can be eliminated
and thus reduce the possibility of brittle fracture. The following conditions must be
present fro brittle fracture to occur: low temperature, a notch or defect, a relatively
high rate of loading, and triaxial stresses normally due to thickness of residual
stresses. The microstructure of the metal also has an effect.
(10) Temperature is an important factor which must be considered in conjunction with
microstructure of the material and the presence of a notch. Impact testing of steels
using a standard notched bar specimen at different temperatures shows a transition
from a ductile type failure to a brittle type failure based on a lowered temperature,
which is known as the transition temperature.
(11) The notch that can result from faulty workmanship or from improper design
produces an extremely high stress concentration which prohibits yielding. A
crack will not carry stress across it, and the load is transmitted to the end of the crack.
It is concentrated at this point and little or no yielding will occur. Metal adjacent to the
end of the crack which does not carry load will not undergo a reduction of area since
it is not stressed. It is, in effect, a restraint which helps set up triaxial stresses at the
base of the notch or the end of the crack. Stress levels much higher than normal
occur at this point and contribute to starting the fracture.
(12) The rate of loading is the time versus strain rate. The high rate of strain, which is a
result of impact or shock loading, does not allow sufficient time for the normal slip
process to occur. The material under load behaves elastically, allowing a stress level
beyond the normal yield point. When the rate of loading from impact or shock
stresses, occurs near a notch in heavy thick material, the material at the base of the
notch is subjected very suddenly to very high stresses. The effect of this is often
complete and rapid failure of a structure and is what makes brittle fracture so
dangerous.
(13) Triaxial stresses are more likely to occur in thicker material than in thin material. The
direction acts as a restraint at the base of the notch, and for thicker material, the
degree of restraint in the through direction is higher. This is why brittle fracture is
more likely to occur in thick plates or complex sections than in thinner materials.
Thicker plates also usually have less mechanical working in their manufacture than
thinner plates and are more susceptible to lower ductility in the z axis. The
microstructure and chemistry of the material in the center of thicker plates have
poorer properties than the thinner material which receives more mechanical working.
(14) The microstructure of the material is of major importance to the fracture behavior
and transition temperature range. Microstructure of a steel depends on the chemical
composition and production processes used in manufacturing it. A steel in the as-
rolled condition will have a higher transition temperature or liner toughness than the
same steel in a normalized condition. Normalizing, or heating to the proper
temperature and cooling slowly, produces a grain refinement which provides for
higher toughness. Unfortunately, fabrication operations on steel, such as hot and cold
forming, punching, and flame cutting, affect the original microstructure. This raises
the transition temperature of the steel.
(15) Welding tends to accentuate some of the undesirable characteristics that contribute
to brittle fracture. The thermal treatment resulting from welding tends to reduce the
toughness of the steel or to raise its transition temperature in the heat - affected zone.
The monolithic structure of a weldment means that more energy is locked up and
there is the possibility of residual stresses which may be at yield point levels. The
monolithic structure also causes stresses and strains to be transmitted throughout the
entire weldment, and defects in weld joints can be the nucleus for the notch or crack
that will initiate fracture.
(16) Brittle fractures can be reduced in weldments by selecting steels that have sufficient
toughness at the service temperatures. The transition temperature should be below
the service temperature to which the weldment will be subjected. Heat treatment,
normalizing, or any method of reducing locked-up stresses will reduce the triaxial
yield strength stresses within the weldment. Design notches must be eliminated
and notches resulting from poor workmanship must nor occur Internal cracks within
the welds and unfused roots areas must be eliminated.
b. Fatigue Failure. Structure sometimes fail at nominal stresses considerably below the
tensile strength of the materials involved. The materials involved were ductile in the
normal tensile tests, but the failures,.generally exhibited little or no ductility. Most of these
failures developmewfafter the structure had been subjected to a large number of cycles
of loading. This type of failure is called a fatigue failure.
(1) Fatigue failure is the formation and development of a crack by repeated or fluctuating
loading. When sudden failure occurs, it is because the crack has increased enough to
reduce the load-carrying capacity of the part. Fatigue cracks may exist in some
weldments, but they will not fail until the load-carrying area is sufficiently reduced.
Repeatedly loading causes progressive enlargement of the fatigue cracks through the
material. The rate at which the fatigue crack increases depends upon the type and
intensity of stress as well as other factors involving the design, the rate of loading,
and type of material.
(2) The fracture surface of a fatigue failure is generally smooth and frequently shins
concentric rings or areas spreading from the point where the crack initiated. These
rings show the propagation of the crack, which might be related to periods of high
stress followed by periods of inactivity. The fracture surfaces also tends to become
rougher as the rte of propagation of the crack increases. Figure 6-57 shows the
characteristic fatigue failure surface.
Figure 26. Fatigue fracture surface.
(3) Many structures are designed to a permissible static stress based on the yield point
of the material in use and the safety factor that has been selected. This is based on
statically loaded structures, the stress of which remains relatively constant. Many
structures, however, are subject to other than static loads in service. These changes
may range from simple cyclic fluctuations to completely random variations, in this
type of loading, the structure must be designed for dynamic loading and considered
with respect to fatigue stresses.
(4) The varying loads involved with fatigue stresses can be categorized in different
manners. These can be alternating cycles from tension to compression, or pulsating
loads with pulses from zero load to a maximum ensile load, or from a zero load to a
compressive load, or loads can be high and rise higher, either tensile or compressive.
In addition to the loadings, it is important to consider the number of times the
weldment is subjected to the cyclic loading. For practical purposes, loading is
considers in millions of cycles. Fatigue is a cumulative process and its effect is in no
way healed during periods of inactivity. Testing machines are available for loading
metal specimens to millions of cycles. The results are plotted on stress vs. cycle
curves, which show the relationship between the stress range and the number of
cycles for the particular stress used. Fatigue test specimens are machined and
polished, and the results obtained on such a specimen may not correlate with actual
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