Welding Metallurgy 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,
Heat Affected Zone Welding
Concerns
Heat Affected Zone Welding
Concerns
• Changes in Structure Resulting
in Changes in Properties
• Cold Cracking Due to Hydrogen
Look At Two Types of Alloy Systems
Cold Worked Alloy Without Allotropic Transformation
Introductory Welding Metallurgy,
AWS, 1979
Welding
Precipitation
Hardened Alloys
Without Allotropic
Phase Changes
Welded In:
• Full Hard
Condition
• Solution
Annealed
Condition
Introductory Welding Metallurgy,
AWS, 1979
Annealed upon
Cooling
Precipitation Hardened Alloy Welded in Full Hard Condition
Introductory Welding Metallurgy,
AWS, 1979
Precipitation Hardened Alloys Welded in Solutioned Condition
Introductory Welding Metallurgy,
AWS, 1979
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?
Steel Alloys With Allotropic Transformation
Introductory Welding Metallurgy,
AWS, 1979
Introductory Welding Metallurgy,
AWS, 1979
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?
Cracking in Welds
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
0.1.1.5.2.T12.95.12
Dickinson
Carbon and Low-Alloy Steels
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
0.1.1.5.1.T9.95.12
Steel
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
T base
Cooling rate T - Tbase)3
Cooling rate T - Tbase)2
T base
Interaction of Preheat and
Composition
Steel
CE = %C + %Mn/6 + %(Cr+Mo+V)/5 + %(Si+Ni+Cu)/15
• Carbon equivalent (CE) measures ability to form
martensite, which is necessary for hydrogen
cracking
– CE < 0.35
treatment
– 0.35 < CE < 0.55
– 0.55 < CE
treatment
no preheat or postweld heat
preheat
preheat and postweld heat
• Preheat temp. as CE and plate thickness
Carbon and Low-Alloy Steels
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
0.1.1.5.1.T10.95.12
Postweld Heat Treatment and
Hydrogen Cracking
Steel
• 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
Base Metal Welding Concerns
Cracking in Welds
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
0.1.1.5.2.T14.95.12
Improve Cleanliness
Improve through thickness properties
Buttering
Carbon and Low-Alloy Steels
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
0.1.1.5.1.T11.95.12
Steel
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
Cracking in Welds
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
0.1.1.5.2.T15.95.12
Stainless Steel
Knife-Line Attack in the HAZ
HAZ
Weld
Knife-line attack
• Cr23C6 precipitate in
HAZ
– Band where peak
temperature is 8001600°F
• Can occur even in
stabilized grades
– Peak temperature
dissolves titanium
carbides
– Cooling rate doesn’t
allow them to form
again