Heat Affected Zone Welding Concerns

1. Welding Metallurgy 2

2. Welding Metallurgy 2
Lesson Objectives
When you finish this lesson you will
understand:
• The various region of the weld where liquid
does not form
• Mechanisms of structure and property
changes associated with these regions
Learning Activities
1. View Slides;
2. Read Notes,
3. Listen to lecture
4. Do on-line
workbook
5. Do homework
Keywords:
Heat affected zone, Base metal, Solutionizing treatment, Aging,
welding procedure, heat input, Hydrogen cracking, Carbon
equivalent, Lamellar Tearing, Reheat Cracking, Knife-line attack,

3. Heat Affected Zone Welding
Concerns

4. Heat Affected Zone Welding
Concerns
• Changes in Structure Resulting
in Changes in Properties
• Cold Cracking Due to Hydrogen

6. Look At Two Types of Alloy Systems

8. Introductory Welding Metallurgy,
AWS, 1979
Cold Worked Alloy Without Allotropic Transformation

9. Welding
Precipitation
Hardened Alloys
Without Allotropic
Phase Changes
Welded In:
• Full Hard
Condition
• Solution
Annealed
Condition
Introductory Welding Metallurgy,
AWS, 1979

10. Annealed upon
Cooling

11. Introductory Welding Metallurgy,
AWS, 1979
Precipitation Hardened Alloy Welded in Full Hard Condition

12. Introductory Welding Metallurgy,
AWS, 1979
Precipitation Hardened Alloys Welded in Solutioned Condition

13. Turn to the person sitting next to you and discuss (1 min.):
• Precipitation hardened austenitic stainless steel is used for
high strength applications like rocket components etc.
Reviewing the various procedures for welding precipitation
hardened steels, what procedure would you recommend?
Does it make any difference that this is austenitic stainless
steel and not just plain carbon steel?

15. Introductory Welding Metallurgy,
AWS, 1979
Steel Alloys With Allotropic Transformation

17. Introductory Welding Metallurgy,
AWS, 1979

18. Turn to the person sitting next to you and discuss (1 min.):
• As we saw, the cooling rate can depend upon the preheat
and the heat input. Many codes actually specify the range of
heat inputs that can be used to weld certain materials. We
had an equation to determine the heat input before. What is
it? What processes have the highest Heat Inputs? The
lowest?

19. Hydrogen Cracking
• Hydrogen cracking, also called cold
cracking, requires all three of these
factors
– Hydrogen
– Stress
– Susceptible microstructure (high
hardness)
• Occurs below 300°C
• Prevention by
– Preheat slows down the cooling rate;
this can help avoid martensite
formation and supplies heat to diffuse
hydrogen out of the material
– Low-hydrogen welding procedure
Cracking in Welds
0.1.1.5.2.T12.95.12

24. Dickinson

25. Why Preheat?
• Preheat reduces the temperature differential
between the weld region and the base metal
– Reduces the cooling rate, which reduces the
chance of forming martensite in steels
– Reduces distortion and shrinkage stress
– Reduces the danger of weld cracking
– Allows hydrogen to escape
Carbon and Low-Alloy Steels
0.1.1.5.1.T9.95.12

26. Using Preheat to Avoid Hydrogen
Cracking
• If the base material is preheated, heat flows more
slowly out of the weld region
– Slower cooling rates avoid martensite formation
• Preheat allows hydrogen to diffuse from the metal
Cooling rate T - Tbase)2
Steel
Cooling rate T - Tbase)3
T base
T base

27. Interaction of Preheat and
Composition
• Carbon equivalent (CE) measures ability to form
martensite, which is necessary for hydrogen
cracking
– CE < 0.35 no preheat or postweld heat
treatment
– 0.35 < CE < 0.55 preheat
– 0.55 < CE preheat and postweld heat
treatment
• Preheat temp. as CE and plate thickness
CE = %C + %Mn/6 + %(Cr+Mo+V)/5 + %(Si+Ni+Cu)/15
Steel

28. Why Post-Weld Heat Treat?
• The fast cooling rates associated with welding
often produce martensite
• During postweld heat treatment, martensite is
tempered (transforms to ferrite and carbides)
– Reduces hardness
– Reduces strength
– Increases ductility
– Increases toughness
• Residual stress is also reduced by the postweld
heat treatment
Carbon and Low-Alloy Steels
0.1.1.5.1.T10.95.12

29. Postweld Heat Treatment and
Hydrogen Cracking
• Postweld heat treatment (~ 1200°F) tempers any
martensite that may have formed
– Increase in ductility and toughness
– Reduction in strength and hardness
• Residual stress is decreased by postweld heat
treatment
• Rule of thumb: hold at temperature for 1 hour per
inch of plate thickness; minimum hold of 30
minutes
Steel

31. Base Metal Welding Concerns

32. Lamellar Tearing
• Occurs in thick plate subjected to high transverse
welding stress
• Related to elongated non-metallic inclusions,
sulfides and silicates, lying parallel to plate
surface and producing regions of reduced ductility
• Prevention by
– Low sulfur steel
– Specify minimum ductility levels in transverse direction
– Avoid designs with heavy through-thickness direction
stress
Cracking in Welds
0.1.1.5.2.T14.95.12

33. Improve Cleanliness
Improve through thickness properties
Buttering

34. Multipass Welds
• Heat from subsequent passes affects the
structure and properties of previous passes
– Tempering
– Reheating to form austenite
– Transformation from austenite upon cooling
• Complex Microstructure
Carbon and Low-Alloy Steels
0.1.1.5.1.T11.95.12

35. Multipass Welds
• Exhibit a range of
microstructures
• Variation of
mechanical properties
across joint
• Postweld heat
treatment tempers the
structure
– Reduces property
variations across the
joint
Steel

36. Reheat Cracking
• Mo-V and Mo-B steels susceptible
• Due to high temperature embrittlement of the
heat-affected zone and the presence of residual
stress
• Coarse-grained region near fusion line most
susceptible
• Prevention by
– Low heat input welding
– Intermediate stress relief of partially completed welds
– Design to avoid high restraint
– Restrict vanadium additions to 0.1% in steels
Cracking in Welds
0.1.1.5.2.T15.95.12

37. Knife-Line Attack in the HAZ
• Cr23C6 precipitate in
HAZ
– Band where peak
temperature is 800-
1600°F
• Can occur even in
stabilized grades
– Peak temperature
dissolves titanium
carbides
– Cooling rate doesn’t
allow them to form
again
Weld
HAZ
Knife-line attack
Stainless Steel