Title: Welding Metallurgy 2
1Welding Metallurgy 2
2Welding Metallurgy 2
- Objectives
- The region of the weld where liquid does not
form - Mechanisms of structure and property changes
associated with these regions
3Heat Affected Zone Welding Concerns
4Heat Affected Zone Welding Concerns
- Changes in Structure Resulting
- in Changes in Properties
- Cold Cracking Due to Hydrogen
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6Look At Two Types of Alloy Systems
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8Cold Worked Alloy Without Allotropic
Transformation
Introductory Welding Metallurgy, AWS, 1979
9- Welding
- Precipitation
- Hardened Alloys
- Without Allotropic
- Phase Changes
- Welded In
- Full Hard Condition
- Solution Annealed Condition
10Annealed upon Cooling
11Precipitation Hardened Alloy Welded in Full Hard
Condition
Introductory Welding Metallurgy, AWS, 1979
12Precipitation Hardened Alloys Welded in
Solutioned Condition
Introductory Welding Metallurgy, AWS, 1979
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14Steel Alloys With Allotropic Transformation
Introductory Welding Metallurgy, AWS, 1979
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16Introductory Welding Metallurgy, AWS, 1979
17Hydrogen Cracking
- Hydrogen cracking, also called cold cracking,
requires all three of these factors - Hydrogen
- Stress
- Susceptible microstructure (high hardness)
- Occurs below 300C
- 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
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22Why 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
23Using 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
T base
Cooling rate µ (T - Tbase)
Cooling rate µ (T - Tbase)
T base
24Interaction of Preheat and Composition
CE C Mn/6 (CrMoV)/5 (SiNiCu)/15
- Carbon equivalent (CE) measures ability to form
martensite, which is necessary for hydrogen
cracking - CE lt 0.35 no preheat or postweld heat treatment
- 0.35 lt CE lt 0.55 preheat
- 0.55 lt CE preheat and postweld heat treatment
- Preheat temp. depends on CE and plate thickness
25Why 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
26Postweld Heat Treatment and Hydrogen Cracking
- Postweld heat treatment ( 1200F) 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
27Base Metal Welding Concerns
28Lamellar 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
29Multipass 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
30Multipass Welds
- Exhibit a range of microstructures
- Variation of mechanical properties across joint
- Postweld heat treatment tempers the structure
- Reduces property variations across the joint