WELDING METALLURGY

DPP C2 (b)

Institut Kemahiran MARA
Kuching, Sarawak

INFORMATION SHEET

COURSE : STK SEMESTER : 3
SESSION : TMW 3032 METALLURGY SHEET NO : 6.2
CODE/SUBJECT : WEEK : 18
LECTURER :

TOPIC : HEAT TREATMENT OF STEEL
SUB-TOPIC : 1. PREHEATING AND POST WELD HEAT TREATMENT (PWHT)

LEARNING At the end of the lesson, students will be able to :
OUTCOME :
1. Identify purpose of Preheating and Post Weld Heat Treatment
(PWHT)
In Welding

TAJUK: PREHEATING AND POST WELD HEAT

PENERANGAN:

Involve heating the base metal, either in entirety or just the region surrounding the joint,
to a specific temperature, prior to welding. Are specific temperature is call preheat temperature.

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When And Why To Preheat
To determine need or not preheat, the following should be consider;
- Code requirements
- Section thickness
- Base metal chemistry
- Restraint
- Ambient temperature
- Filler metal hydrogen content
- Previous cracking problem

If a welding code must be followed, then the code generally will specify the minimum
preheat temperature for a given base metal, welding process and section thickness. Minimum
value may be can increased if necessary.

f no codes governing the welding, one must determine whether preheat required, and if
so, what preheat temperature appropriate. In generally no preheat required for low carbon steel
less than 1 in (25mm) thick.

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preheat temperatures suggested for several ranges of the CE

Carbon Equivalent Suggested Preheat
Temperature
Up to 0.45% optional
0.45% to 0.60%
200 to 400°F
Above 0.60%
400 to 700°F

5.3.2 Why Preheat?
There are four primary reasons
- It slows the cooling rate in the weld metal and base metal, producing a more ductile

structure.
- The slower cooling rate provides an opportunity for hydrogen may be present to diffuse

out harmlessly.
- It reduce the shrinkage stresses in the weld and adjacent base metal, which is especially

important in high strained joint.
- It raise some steels above the temperature at which brittle fracture would occur in

fabrication.

5.2.3 When Should Preheat?
- Post heat is most often used on the highly hardenable steels, CE more than 0.60%.
- Sometimes used on less hardenable steels (CE above 0.45%) if adequate preheat is

difficult because of the size of the sections being welded.
- Post heat is rarely used alone; it is almost always used in conjunction with preheat.

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QUESTION.
1. State the types of heat treatment as u known ?
2. Give 5 reason what the purpose of heat treatment ?
3. Discuss about the types of normalizing heat treatment process ?
4. Why should be pre heat ?
5. When should be pre heat ?

5.5 REFERENCE.
1. Ellis D J : Mechanized narrow gap welding ‘joining and material 1988 Vol 1.
2. Handbook of structure welding, Abington publishing 1992.
3. Yeo R B G :’ joining and Material 1989 vol 2

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