i | P a g e FABRICATION OF METALS FAUZIAH BINTI HAMDAN SOFEA LING BINTI ABDULLAH SURAYA BINTI MUSTAFFA POLITEKNIK KOTA BHARU
ii| P a g e Jabatan Kejuruteraan Mekanikal, Politeknik Kota Bharu, KM 24, Kok Lanas, 16450 Ketereh, Kelantan. FABRICATION OF METALS First Edition 2023 © 2023 Fauziah Binti Hamdan/ Sofea Ling Binti Abdullah/ Suraya Binti Mustaffa All right reserved. No part of this publication may be reproduced, stored in retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise without prior permission of the publisher. FABRICATION OF METALS/ © 2023 Fauziah Binti Hamdan/ Sofea Ling Binti Abdullah/ Suraya Binti Mustaffa
iii| P a g e APPRECIATION Thanks to Allah for the strength that has been given to us in preparing this book. I would also like to take the opportunity to express my gratitude to the Head of Mechanical of Engineering Department, En Wan Abdul Halim Amir Bin Wan Muhamad for the trust given in carrying out this task. A word of thanks also to Pn. Ruzila Binti Mat Ghani, JKM’s E-Learning Coordinator who jointly made revisions and edits comrades as well- in -arms who contributed their thoughts and time directly in strengthening the content of this book. Not forgetting the family who have given a lot of support to make this ebook a success. FABRICATION OF METAL/ Fauziah Binti Hamdan/ Sofea Ling Binti Abdullah/ Suraya Binti Mustaffa Jabatan Kejuruteraan Mekanikal, Politeknik Kota Bharu, KM 24, Kok Lanas, 16450 Ketereh, Kelantan
iv| P a g e SYNOPSIS This book was written for topic 6, Fabrication of Metals for the subject Material science and engineering. Subtopics discussed include introduction to Fabrication of Metals, hot work, cold work, forging, rolling, extrusion, casting process, powder metallurgy, welding and heat treatment. The titles found in this book refer to the course curriculum DJJ 30113 Material Science and Engineering, Polytechnic Malaysia. The content summary has been formulated by the lecturer who teaches the subject DJJ 30113 and translated as scientific writing. Therefore, this book is very suitable for students who are new to the field of Material Science.
v | P a g e AUTHOR’S BIOGRAPHY FAUZIAH BINTI HAMDAN was born on Sept 1968 in Batu Pahat, Johor. She received early education at Sekolah Kebangsaan Pekan Baru, Sekolah Menengah Pekan Baru and Sekolah Menengah Teknik Bukit Piatu. She holds a Diploma in Mechanical Engineering and Degree in Mechanical Engineering from Universiti Teknologi Malaysia. She currently works as a lecturer of Mechanical Engineering Department at Politeknik Kota Bharu, Kelantan. She had experienced in teaching Material Science since 2002 and her interest includes Material Science & Engineering, AutoCAD Design and Entrepreneurship. SOFEA LING BINTI ABDULLAH was born on May 1983 in small town called Pendang, Kedah. She received early education at Sekolah Kebangsaan Sungai Tiang, Pendang, Kedah. She was accepted as a student of Sekolah Menengah Kebangsaan Pendang, Kedah to continue her studies at secondary level. She holds a Degree in Mechanical Engineering from Universiti Teknologi Tun Hussein Onn Malaysia. She currently works as a lecturer of Mechanical Engineering Department at Politeknik Kota Bharu, Kelantan. She had experienced in teaching Material Science since 2014 and her interest includes Material Science & Engineering and AutoCAD Design. SURAYA BINTI MUSTAFFA holds a Degree in Mechanical Engineering from University Teknologi Mara, UiTM Shah Alam and also Diploma in Mechanical Engineering (Manufacturing) in Politeknik Port Dickson, Negeri Sembilan. She is a lecturer in Department of Mechanical Engineering at Politeknik Kota Bharu since 2007. Her teaching interest include Mechanics Of Machine and Strength of Material.
vi | P a g e TABLE OF CONTENT 1.O INTRODUCTION 1 1.1 Fabrication of Metals 1 2.0 HOT WORK 1 2.1 Advantages of Hot Work 3 2.2 Disadvantage of Hot Work 3 3.0 COLD WORK 3 3.1 Advantages of Cold Work 4 3.2 Disadvantages of Hot Work 5 4.0 FORGING 5 4.1 Open Die Forging 6 4.2 Closed Die Forging 7 4.2.1 Advantages of Hot Forging 8 4.2.2 Disadvantages of Hot Forging 8 5.0 ROLLING 8 6.0 EXTRUSION 10 6.1 Advantages of Extrusion 11 6.2 Disadvantages of Extrusion 11 7.0 WIRE AND TUBE DRAWING 11 7.1 Tube Drawing with Fixed Mandrel 13 8.0 CASTING PROCESS 14 8.1 Sand Casting 14 8.2 Pressure Die Casting 18 8.3 Lost Wax/ Investment Casting 20
vi | P a g e TABLE OF CONTENT 9.0 POWDER METALLURGY 21 10.0 WELDING 25 11.0 HEAT TREATMENT OF STEEL 28 11.1 Annealing 29 11.1.1 Full annealing 30 11.1.2 Stress relief 31 11.1.3 Spherodizing 32 11.2 Normalizing 34 11.3 Quenching/Hardening Process 38
1 | P a g e FABRICATION OF METALS 1.0 INTRODUCTION In building construction, furniture manufacturing and other engineering fields, metal items play an important role. Metal is one of the most widely used materials because it is easy to obtain, has high strength, is durable and easy to work with. Therefore, metal needs to undergo certain metal work processes before it can be produced to meet the needs of use. Metal fabrication involves two main categories which is hot work and cold work. 2.0 HOT WORK Hot work is the formation of a metal that is carried out slightly above the recrystallization temperature. Re-crystallization temperature is the temperature at which atomic mobility can be repaired when any defect was present in the metal caused by the working process. In this process, the metal is heated to the plastic state, and then the pressure is applied to get various size and shapes. When the pressure is applied, the metal grain size will be varied, and the metal’s mechanical properties are improved. If the pressure is applied by hand hammer, then it is called as hand or smith forging. If hand hammering is replaced by power hammers, then it is called hammer forging. Such type hot working of metals is called as hot forging. Hot-working can be used for rolling, forging, extrusion and drawing, etc.
2 | P a g e FABRICATION OF METALS Figure 1: Hot rolling processes When metals are worked above the re-crystallization temperature, then it becomes plastic and causes the growth of grains. During the hot working, the grains become loosened in their structure, and they realign in a proper manner. Only small pressure is required to shape the metal. If the heating temperature is too high, the re-crystallization will take place in an even shorter period, then this the material is more easier to be worked on without the risk of cracking. If the temperature is low, re-crystallization will occur slowly and not only more force (compression force) is needed to distorted the metal, even the risk of cracking will increase. If the metal material is over heated, say burned, this means that oxidation has occurred on the boundaries of the grain and this weakens the material. Figure 2: Changes in microstructure due to hot rolling processes
3 | P a g e FABRICATION OF METALS Advantages of Hot Work: i. The hot working process will be able to break the rough crystal structure contained in the ingot. ii. Metal grain structure will be refined. iii. Hot-working leads to homogeneous structure of metal without defects and blowholes. iv. Mechanical, physical, and chemical properties of metals can be improved; strength, ductility and toughness is better. v. Metalworking is done under high temperature; therefore, larger deformation is possible. Disadvantages of Hot Work: i. The exact final dimensions for the workpiece are quite difficult to obtain. This is because the hot metal when cooling will occur shrinkage of metal. ii. Hot working leads to poor surface finish due to oxidation, because the material will be worked under high temperature. iii. Sometimes it leads to lower strength due to loss of carbon due to oxidation. iv. On account of the loss of carbon from the surface of the steel piece being worked, the surface layer loses its strength, which is a disadvantage when the part is put to service. v. It is a costly process. 3.0 COLD WORK Cold working is the formation of a metal that is carried out at a temperature below the re-crystallization temperature. This process needs comparatively higher pressure than hot working.
4 | P a g e FABRICATION OF METALS Soft, ductile, and malleable metals can be easily worked with cold working. But this process leads to hardness and distorted grain structure. The cold working process also affects the following mechanical properties of metals significantly; hardness, yield strength, ductility, tensile strength. When a piece of metal or ingot is worked out, the structure of the crystal is changed, becoming distorted and elongated in the direction of work. The metal will become hard and the strength as the internal stress will increase causing the ductility criteria to decrease. As a result of this the properties of metals in terms of strength and electrical resistance increase. Therefore, the metal will become harder and brittle after being worked on. If we continuously carry out a cool working process against this metal, then it will break and crack. Advantages of Cold Work: i. No heating of metals. ii. Better surface finish can be achieved since there is no oxidation during the cold working of metals. iii. Dimensional accuracy can be maintained. iv. Strength and hardness of the metal are increased. v. Due to cold working, metal gains strength and hardness. vi. Better strength and wear properties of the material can be achieved. Figure 3: Changes in microstructure due to cold rolling processes
5 | P a g e FABRICATION OF METALS Disadvantages of Cold Work: i. It leads to brittleness if the metals are worked under cold working. ii. Metals hardness is increased. iii. Cold worked components require a heat-treatment process. iv. Only ductile and malleable materials are suitable for cold working. v. Greater force is essential for deformation. Therefore, powerful machines are required. Cool works such as rolling and drawing are usually carried out on metals that have been worked on. It is also a finishing process in production and it function as follows: Membolehkan dimensi yang tepat didapati pada hasil kerja. 1. Gets clean and good surface finishing. 2. Gets different degrees of hardness by imposing various cold work. 3. Improve machinability and yield point. 4.0 FORGING Hot Forging is a process of metal work in a hot temperature or above recystallization temperature. Where the materials is shaped by compressive forces which can be exerted by manually or with power hammer or by special forging machine. Figure 4: Hot forging
6 | P a g e FABRICATION OF METALS Metal is heated to a temperature above the recrystallization temperature. A desirable compressive force is applied to manipulate metal to a desired shape. During this period, the metal is subjected to a number of strains hardening effects. The temperature will depend on the metal to be forged. For example: copper alloys require between 710 to 800°C, steel alloys require between 1100 to 1150°C, aluminium alloys require between 350 to 520°C. The required temperature must be maintained above the recrystallization point or the point at which the metal begins to cool. Recrystallization can form microscopic crystals that warp when the metal is reheated causing it to "strain harden", which makes it unworkable. Environmental factors can influence the hot forging process where contact with the atmosphere can cause oxidation. To avoid this, forging may be completed in an environmentally controlled chamber or isothermal forging, which is similar to a vacuum. 4.1 Open Die Forging Also referred to as the free/Smith forging. In this metal forging, metal is subjected to a compressive force without being confined within dies. Most forged parts require machining. Open die forging is carried out between two flat dies or simple shapes. This process is used mostly for large objects and smaller quantities. It is also used to perform the work piece for closed die forging.
7 | P a g e FABRICATION OF METALS Figure 5: Open die forging 4.2 Closed Die Forging A heated workpiece is placed in a stationary mold half. The upper mold half is pushed against the workpiece with an applied pressure force and deformed and shaped by the two mold halves. It often requires several strokes before the mold cavity is fully filled. To ensure that such a material is sufficient to fill the mold the workpiece volume is slightly larger than the mold. The design allows for excess material to flow outside the required shape and is trimmed away in a subsequent operation. In some cases, the part is repositioned in the mold to apply further strokes to achieve final size and volume. The method allows for Precision forging where ingot size is carefully controlled. Figure 6: Closed die hot forging
8 | P a g e FABRICATION OF METALS Advantages of Hot Forging: 1. Hot forged components possess increased ductility which makes them desirable for many configurations. 2. As a technique hot forging is more flexible. 3. The excellent surface quality allows a wide range of finishing work such as polishing, coating or painting, tailored to customers’ specific needs. 4. Hot forging materials are available all over the world, which has a positive impact on their final price. 5. Produces uniform grain size and flow. 6. Less porosity. 7. Less Machining. Disadvantages of Hot Forging: 1. Less precise dimensional tolerance is another possible disadvantage of hot forged components compared to the cold forged ones. 2. The cooling process should be also performed under special conditions; otherwise, there is a risk of warping or cracking. 3. The grain structure of forged metals may vary and there is always a possibility of reactions between the atmosphere and the workpiece. 4. Highly cost of maintenance 5. Rapid Oxidation might occur on the metal surface at high temperature. 5.0 ROLLING Rolling is a metal forming process in which metal stock is passed through one or more pairs of rolls to reduce the thickness, to make the thickness uniform, and/or to impart a desired mechanical property. The concept is similar to the rolling of dough. Used in the production of sheets and strips.
9 | P a g e FABRICATION OF METALS The main purpose of this process is to thin out hot metal ingot or thick cross-section to get the desired thickness. Sometimes it's a level where the cross section is too thin and does not get to retain heating heat of re-crystallization. In this operation the ingot will go through two sets of rollers. Then go through several other rollers if it is necessary to get the desired final thickness. The thickness of the material that is rolled will become thinner every time after being rolled. Special shaped such as railway crossings, construction frames, round bars can be obtained by using special shaped rollers. Discontinuities in the ingot of the castings been closed or welded under the action of great pressure and provide one homogeneous structure. Figure 8: Differences in microstructure of cold rolling and hot rolling 6.0 EXTRUSION Figure 7: Hot rolling process
10 | P a g e FABRICATION OF METALS Hot extrusion process is performed at above the material recrystallization temperature to keep the material from work hardening or to keep the material ductile. This process used to create objects of a fixed cross-sectional profile. A material is pushed through a die of the desired cross-section. This process has an ability to create very complex cross-sections, and to work materials that are brittle, because the material only encounters compressive and shear stresses. It also forms parts with an excellent surface finishing. Figure 9: Extrusion This process involves pushing a hot billet through a mould with hydraulic power. Through this process the ingot is pressed until it flows under high preassure, through an ’orifice channel’ or ’die’ like a toothpaste cream is squeezed out of its tube. Figure 10: Extrusion Advantages of extrusion: 1. Complex solid or hollow shapes can be produced. 2. Small quantities can be economically produced.
11 | P a g e FABRICATION OF METALS 3. Delivery times are often shorter than alternatives processes. Disadvantages of extrusion: 1. High equipment set up and maintenance cost. 2. High temperature needed for metals extrusion process. 3. Die must preheat to increase its life, so there are chances of oxidation of hot billet. 4. Process Wastage is higher as compared to rolling. 5. Non-homogeneous. 7.0 WIRE AND TUBE DRAWING The process runs at below recrystallization temperature or at room temperature. The metal to draw must be ductile enough to be drawn through the hard tungsten carbide conical shape die. The metal wire will pass through a number of dies whose size is decreasing. This mean, the cross-sectional area reducing each time the wire pulls through die. In the process cleaning and lubricating are needed as the wire flows through the dies. The cleaning is essential to remove any scale or rust present on the surface of the wire which may severely effect the dies and also decreasing elevated temperature due to friction to avoid recrystallization occurring during the process. Figure 11: Wire withdrawal process in cold work
12 | P a g e FABRICATION OF METALS Tube drawing is similar to wire drawing except that a mandrel of appropriate diameter is required to form the internal hole. Tube drawing also a cold metal working, means the operation run at room temperature or under recrystallization temperature. The tube drawing process involves reducing the cross section and wall thickness of cylinder/tube/pipe through a draw die. The cross section can be circular, square hexagonal, triangle or in any shape. Figure 12: Tube withdrawal process in cold work 7.1 Tube Drawing with Fixed Mandrel: The process is shown in Figure (c). The tube is drawn through a die and a mandrel. The position of mandrel may be adjusted by the bar attached to its rear end in order to change the thickness of tube and the internal diameter. The external diameter is determined by the die diameter. The surface quality of both the surfaces, internal as well as external gets improved. The pull required is certainly more than that in tube sinking because of the additional
13 | P a g e FABRICATION OF METALS deformation in the thickness of tube and also due to frictional force between the tube and the mandrel. Figure 13: Tube withdrawal with fixed mandrel Wire Drawing Tube Drawing Picture above shows Wire Drawing die to produce wire with variety cross sections Picture above shows Tube Drawing die to produce tube/pipe with variety cross sections and wall thickness Die without mandrel Some of the Die use mandrel to form internal hole eg: electrical wire, spring, paper clips, stringed musical instrument etc eg: seamless steel pipe, copper tubing pipe, steel oval tube, titanium flat oval pipe Table 1: Differences Between Wire And Tube Drawing 8.0 CASTING PROCESS The casting process is a production process in which materials are required, been heated so that it melts and then poured into the mold and let it to solidify through its own cooling before being removed for cleaning or re-machined. The circumstances that exist during metal melting and during metal pouring are important in determining the quality of product to be produced.
14 | P a g e FABRICATION OF METALS This casting process can be categorized according to the way the melting metal is filled into the mold and the type of mold material used. The usual casting processes are: a) Sand casting. b) Pressure die casting. c) Lost wax/investment casting. 8.1 Sand Casting Sand casting is a manufacturing process in which liquid metal is poured into a sand mold, which contains a hollow cavity of the desired shape and then allowed to solidify. Figure 14: Sand Casting Mold Typical sand molds have the following parts: 1. The mold is made of two parts, the top half is called the cope and bottom part is the drag. 2. The liquid flows into the gap between the two parts, called the mould cavity. The geometry of the cavity is created by the use of a wooden shape, called the pattern. The shape of the patterns is almost identical to the shape of the part we need to make. 3. A funnel shaped cavity; the top of the funnel is the pouring cup; the pipeshaped neck of the funnel is the sprue – the liquid metal is poured into the pouring cup and flows down the sprue.
15 | P a g e FABRICATION OF METALS 4. The runners are the horizontal hollow channels that connect the bottom of the sprue to the mould cavity. The region where any runner joins with the cavity is called the gate. 5. Some extra cavities are made connecting to the top surface of the mould. Excess metal poured into the mould flows into these cavities, called risers. They act as reservoirs; as the metal solidifies inside the cavity, it shrinks, and the extra metal from the risers flows back down to avoid holes in the cast part. 6. Vents are narrow holes connecting the cavity to the atmosphere to allow gasses and the air in the cavity to escape. 7. Cores are made by baking sand with some binder so that they can retain their shape when handled. The mould is assembled by placing the core into the cavity of the drag, and then placing the cope on top, and locking the mould. After the casting is done, the sand is shaken off, and the core is pulled away and usually broken off. Figure 15: Steps in sand casting process
16 | P a g e FABRICATION OF METALS Procedure to produce sand casting products: 1. The drag is placed on a flat and clean board. Make sure the floor on which the drag is placed is flat. The pattern is placed into the drag. Separator powder is sprinkled on the pattern and board. 2. A fine sand mixture is laid around the pattern. The sand mixture around the pattern is pressed with the finger to ensure its position. Add more sand mixture into the drag and use sand rammer to compact the sand and repeat the step until the compacted sand mixture exceeds the same height of the drag. A flat bar is used to level the sand surface. 3. The drag is reversed so that the pattern is at the top, then the cope is placed in the correct position on the drag and carefully locked to ensure it is always aligned. The pattern is placed in the correct position and then the sprue is placed in the appropriate position. Separator powder is sprinkled onto the upper surface of the drag and repeat the step of compacting the fine sand mixture until the compacted sand mixture exceeds the same height of the cope.
17 | P a g e FABRICATION OF METALS Figure 16: Process of Sand Casting Mold 4. The sprue is moved so that it is loose and can be removed out. Pouring blades are used to flatten the surface around the sprue cavity. Use the hollow strings to create ventilation around the sprue cavity. Then separate the cope from the drag to remove out the pattern by using a pattern puller and place the core at the right position. Then make a gate for the molten metal flow into pattern cavity. 5. Place back the cope at the right position on the drag and carefully locked to ensure it is always aligned. 6. The mold that been prepared earlier was brought near the furnace to be poured molten metal. The molten metal is poured out and it flows to fill the cavity of sprue and pattern. After the metal has solidified, the casting product can be removed by breaking the mold.
18 | P a g e FABRICATION OF METALS 8.2 Pressure Die Casting This process is used for materials that have a low melting temperature such as aluminum and alloy zinc. This process is carried out by sequencing a metal liquid into a metal mold under a certain pressure. Because the mold is made of metal, a higher cooling rate can be obtained compared to sand mold. This allows metals such as alloys based on aluminum and zinc able to produce a uniform crystal structure with a fine grains. Figure 17: Products of Pressure Die Casting This type of casting mold is made of special steel, known as die. These castings are used to produce products by injecting the molten metal at high pressure into the die. This process is suitable for mass-production as the die can be used repeatedly. The high pressure generated in this process can produce complex shaped products with a good surface finishing. Examples of products produced with pressure die casting are automotive components, toy items, jewelry items and home appliances.
19 | P a g e FABRICATION OF METALS Figure 18: Pressure Die Casting Procedure to produce pressure die casting products: 1. Molten metal is poured into the chamber. 2. The plunger starts traveling forward and builds up a pressure forcing the molten metal into the die cavity. 3. The core is removed and the die is moved backwards. 4. The ejector pin will reject the casting product out of the die. Figure 19: Process of Pressure Die Casting
20 | P a g e FABRICATION OF METALS 8.3 Lost wax/ Invesment Casting Lost-wax process of metal casting in which a molten metal is poured into a mold that has been created by means of a wax model. Once the mold is made, the wax model is melted and drained away. A hollow core can be effected by the introduction of a heatproof core that prevents the molten metal from totally filling the mold. Procedure to produce lost wax products. 1. Wax casting patterns are produces using wax (paraffin, beeswax, acrawax). The wax pattern is dipped into a heat-resistant coating material concentrate to produce a smooth surface on the inside of the mold wall. Figure 21: Wax casting pattern 2. A wax pattern coated with a heat-resistant material is placed into a metal mold container. The melting material is poured into the mold container. Then it is left to Figure 20: Example of product resulting from Lost Wax sting
21 | P a g e FABRICATION OF METALS harden in the entire mold container to form a mold. The melting material used consists of hardener and silica sand. Figure 22: Position of pattern in the mold container 3. The wax mould is heated in a furnace at a range of temperature of 1000C to 2000C. The wax pattern will melt and flow out to create a mold cavity. Figure 23 : The wax melts and flows out 4.The mold will be removed from the furnace and been reversed. Molten metal will be poured into the mold cavity. When the molten metal has soldified, the casting product can be removed. 9.0 POWDER METALLURGY (PM) Is a term covering a wide range of ways in which materials or components are made from metallic powders.
22 | P a g e FABRICATION OF METALS Figure 24: The process of Powder Metallurgy Blending and mixing of powder composition. It is important to make a uniform homogenous mixture of powder composition (to get better result in the subsequent operations). Pressing powder. Different types of pressing depend on mixture nature – powder injection molding, powder rolling, powder extrusion, powder forging, isostatic pressing.
23 | P a g e FABRICATION OF METALS Figure 25: The process of powder pressing Sintering of parts. It is a heat treatment process in which ranges between 70 to 90% of the material melting temperature. This step is done to achieve the maximum possible hardness and strength needed in the final product. Figure 26: The process of Sintering Sintering is all about compacting and forming a solid mass of material by heat or pressure without melting to liquefaction.
24 | P a g e FABRICATION OF METALS Table 2: A production of a product in Powder Metallurgy The sintering process start with the metal in powdered form. The powdered metal is poured into the feed ram. The feed ram moves across the die opening and deposits the metallic powder into the die. The compression ram compressed the metallic powder in the die with substancial force. The metallic powder fuses together. The compressed part is ejected out of the die. The feed ram then moves the compressed part away and the new cycle begins again.
25 | P a g e FABRICATION OF METALS Example of product: Tungsten carbide (WC). WC is used to cut and form other metals and is made from WC particles bonded with cobalt. It is very widely used in industry for tools. Sintered filters. Porous oil-impregnated bearings. Electrical contacts. Diamond tools. Advantages of powder metallurgy 1.Eliminate machining and scrap. 2.Reduce waste through improve material utilization. 3.Complex features and good dimensional accuracy can be achieved. 10.0 WELDING Is a fabrication process that joins materials usually metals or thermoplastics, by using high heat to melt the parts together and allowing them to cool, causing fusion. Fusion welding is a generic term for welding processes that rely on melting to join materials of similar compositions and melting points. There are many types of welding processes, but the most common welding processes are: 1. MIG Welding (GMAW; Gas Metal Arc Welding) 2. TIG Welding (GTAW; Gas Tungsten Arc Welding) 3. Stick Welding (SMAW; Shielded Metal Arc Welding) 4. Flux Cored Arc Welding (FCAW; Flux Cored Arc Welding)
26 | P a g e FABRICATION OF METALS Figure 27: Welding process They are all arc welding processes, meaning the workpiece, the grounding clamp, and the electrode complete an electrical circuit when the electrode makes contact. Lifting creates an arc. This arc of electricity reaches thousands of degrees, melting the metal and causing separate pieces to flow together. The electrode uses a filler metal that melts and fills the gap, becoming part of the single piece of metal. Figure 28: Arc Welding Gas welding also known as oxyacetylene welding. It is most versatile welding processes. It is still widely used for welding pipes and tubes, as well as repair work. Oxy-fuel cutting are processes that use fuel gases (or liquid fuels such as gasoline) and oxygen to weld or cut metals.
27 | P a g e FABRICATION OF METALS Figure 29: Gas Welding Advantages of welding: 1. There are processes that can be performed manually, semi-automatically, or completely automatically. 2. Some processes can be made portable for implementation in the field for erection of large structures on site or for maintenance and repair of such structures and equipment. 3. Continuous welds provide fluid tightness (so welding is the process of choice for fabricating pressure vessels. 4. Welding (better than most other joining processes) can be performed remotely in hazardous environments (e.g., underwater, in areas of radiation, in outer space) using robots. 5. For most applications, costs can be reasonable. The exceptions to the last statement are where welds are highly critical, with stringent quality requirements or involving specialized applications (e.g., very thick section welding). Disadvantages of welding: The single greatest disadvantage of welding is that it precludes disassembly. While often chosen just because it produces permanent joints, consideration of ultimate
28 | P a g e FABRICATION OF METALS disposal of a product (or structure) at the end of its useful life is causing modern designers to rethink how they will accomplish joining. 11.0 HEAT TREATMENT OF STEEL The term heat treatment applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally, heating and cooling often occur incidentally during other manufacturing processes such as hot forming or welding. Main purpose of heat treatment is to change the mechanical properties of steel such as changing the size, shape of crystals and distribution of steel composition. Figure 30: Phase Diagram of Heat treatment for steel There are three stages of heat treatment: 1. Heat the metal slowly to ensure that the metal maintains a uniform temperature (depends on carbon content). 2. Soak, or hold, the metal at a specific temperature for an allotted period of time (depends on size of component).
29 | P a g e FABRICATION OF METALS 3. Cool the metal to room temperature. Figure 31: Typical heat treatment cycle Types of Heat Treatment: 1. Annealing. 2. Normalizing. 3. Quenching/Hardening. 11.1 Annealing Annealing is a heat treatment process that changes the physical and sometimes also the chemical properties of a material to increase ductility and reduce the hardness to make it more workable. The annealing process requires the material above its recrystallization temperature for a set amount of time before cooling. The cooling rate depends upon the types of metals being annealed. Advantages: The main advantages of annealing are in how the process improves the workability of a material, increasing toughness, reducing hardness and increasing the ductility and machinability of a metal.
30 | P a g e FABRICATION OF METALS The heating and cooling process also reduces the brittleness of metals while enhancing their magnetic properties and electrical conductivity. Disadvantages: The main drawback with annealing is that it can be a time consuming procedure, depending on which materials are being annealed. Materials with high temperature requirements can take a long time to cool sufficiently, especially if they are being left to cool naturally inside an annealing furnace. 11.1.1 Full Annealing The main purpose of full annealing is to originate a uniform and stable microstructure, to eeliminate strain from forging and casting process and improving steel machinability Heating temperature is above the austenitic temperature: For steel with carbon content less than 0.83% C, it is heated to temperature 25–50°C above the above upper critical temperature (A3), and steel with carbon content more than 0.83% C, it is heated 50° C above the lower critical temperature (A1) and then cooled very slowly in the furnace. Figure 32: The temperature range for full annealing
31 | P a g e FABRICATION OF METALS Figure 33: Microstructure of 0.4% C x 500 after full annealing 11.1.2 Stress Relief Annealing The main purpose of stress relief annealing is to return the ductility in steel which suffers from severe stress due to cold work. If a steel is heated or cooled unevenly, internal stresses can arise, also known as residual stresses. Such residual stresses are often induced during welding, for example, because the workpiece is heated not evenly but only locally at a certain point and then cooled down. However, residual stresses can also occur in the workpiece during milling or turning, as high temperatures can occur in the machining area of the workpiece. In stress relief annealing, the workpiece is annealed below the PSK-line in the range between 550 °C and 650 °C. The effect of the stress relief is based on the fact that the strength of the heated material decreases at a higher temperature.
32 | P a g e FABRICATION OF METALS Figure 34: The temperature range for stress relief annealing Figure 35: Microstructure of 0.15% carbon stress relief annealing 400°C 11.1.3 Spherodizing Spheroidizing refers to a heat treatment material modification process that is used to convert granular structures of the material into a spheroidal form. Spheroidizing is performed by annealing steels with more than 0.8% carbon. The metal is heated to a temperature of about 650°C and maintained at this temperature for a predetermined amount of time to convert its microstructure. This allows for a cementite steel structure to change from a lamella formation to an alpha ferrite matrix. The alpha ferrite matrix is made up of particles of spheroidal cementite formations.
33 | P a g e FABRICATION OF METALS Figure 36: The temperature range for spherodizing Figure 37: On the left is the raw material of flake graphite, and on the right is the product after spheroidization of flake graphite.
34 | P a g e FABRICATION OF METALS Figure 38: Spheroidize annealed microstructure of type WI carbon tool steel (Fe-1.05% C-0.25% Mn-0.2% Si) which colored both the cementite particles (brownish red) and the ferrite matrix 11.2 Normalizing Normalizing heat treatment helps to remove impurities and improve ductility and toughness. During the normalizing process, material is heated to between 750-980 0C (1320-1796 0F). The exact heat applied for treatment will vary and is determined based on the amount of carbon content in the metal. Figure 39: The temperature range for Normalizing After heating, the material is cooled to room temperature. The rate of cooling significantly influences both the amount of pearlite and the size and spacing of the pearlite lamellae. At higher cooling rates, more pearlite forms, and the lamellae are
35 | P a g e FABRICATION OF METALS finer and more closely spaced. Both the increased amount of pearlite and the greater fineness of the pearlite result in higher strength and higher hardness. Normalizing will typically produce a uniform pearlite structure in combination with either ferrite grains or grain-boundary carbide present depending on the base material’s carbon content. Normalizing is very similar to annealing as both involve heating a metal to or above its recrystallization temperature and allowing it to cool slowly in order to create a microstructure that is relatively ductile. The main difference between annealing and normalizing is that annealing allows the material to cool at a controlled rate in a furnace. Normalizing allows the material to cool by placing it in a room temperature environment and exposing it to the air in that environment. This difference means normalizing has a faster cooler rate than annealing. The faster cooler rate can cause a material to have slightly less ductility and slightly higher hardness value than if the material had been annealed. Normalizing is also generally less expensive than annealing because it does not require additional furnace time during the cool down process. Figure 40: Microstructure of 0.45% carbon after Normalizing
36 | P a g e FABRICATION OF METALS 11.3 Quenching/Hardening Process Quenching is when you rapidly cool metal in air, oil, water, brine, or another medium. Usually quenching is associated with hardening because most metals that are hardened are cooled rapidly with quenching, but it is not always true that quenching or otherwise rapid cooling results in hardening. Water quenching, for example, is used to anneal copper, and other metals are hardened with slow cooling. In metallurgy, quenching is most commonly used to harden steel by inducing a martensite transformation. The process of quenching is a progression, beginning with heating the sample. Most materials are heated to between 815 and 900 °C (1,500 to 1,650 °F), with careful attention paid to keeping temperatures throughout the workpiece uniform. Figure 41: The temperature range for Quenching The second step in the quenching process is soaking. Workpieces can be soaked in air (air furnace), a liquid bath, or a vacuum. The recommended time allocation in salt or lead baths is up to 6 minutes. As in the heating step, it is important that the temperature throughout the sample remains as uniform as possible during soaking.
37 | P a g e FABRICATION OF METALS Once the workpiece has finished soaking, it moves on to the cooling step. During this step, the part is submerged into some kind of quenching fluid; different quenching fluids can have a significant effect on the final characteristics of a quenched part. Water is one of the most efficient quenching media where maximum hardness is desired, but there is a small chance that it may cause distortion and tiny cracking. When hardness can be sacrificed, mineral oils are often used. These oil-based fluids often oxidize and form a sludge during quenching, which consequently lowers the efficiency of the process. The cooling rate of oil is much less than water. Quick cooling rate will get a new phase known as “martensite”. Martensite is very hard and very stressed. Martensite is basically ferrite that has too much carbon trapped inside. All this carbon distorts the crystal, so we call the new phase martensite. This distorted crystal is very hard, but quite brittle. Swords, knives, and other tools are made of martensitic steel to obtain high strength. Figure 42: The distorted crystal of Martensite To get martensite, the steel will need to change phases when while it’s quenched. Below the austenitizing temperature, nothing will happen when it’s quenched. Austenite, the Face-Centered Cubic (FCC) form of iron which is usually represented by γ, can dissolve more carbon atoms than ferrite (the Body-Centered Cubic (BCC)
38 | P a g e FABRICATION OF METALS form of iron, represented by α). Austenite can dissolve 2% carbon, while ferrite can dissolve 0.025% carbon. This is because of the size and number of interstitial sites in FCC vs BCC structures. Figure 43: Structural changes during Quenching When the steel is quenched, it cools rapidly and wants to transform from BCC austenite to FCC ferrite. However, the cooling rate is too fast for the carbon atoms to move out of the way, so they essentially get trapped in the FCC phase. FCC iron with carbon trapped in it is no longer called ferrite; now this is martensite.
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