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
Introductory Welding Metallurgy,
AWS, 1979
Cold Worked Alloy WithoutAllotropic Transformation
Welding
Precipitation
Hardened Alloys
Without Allotropic
Phase Changes
Welded In:
• Full Hard
Condition
• Solution Annealed
Condition
Introductory Welding Metallurgy,
AWS, 1979
Annealed upon
Cooling
Introductory Welding Metallurgy,
AWS, 1979
Precipitation Hardened Alloy Welded in Full Hard Condition
Introductory Welding Metallurgy,
AWS, 1979
Precipitation Hardened Alloys Welded in Solutioned Condition
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?
Introductory Welding Metallurgy,
AWS, 1979
Steel Alloys With Allotropic Transformation
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?
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
Dickinson
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
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
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
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
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
Base Metal Welding Concerns
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
Improve Cleanliness
Improve through thickness properties
Buttering
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
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
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
– Dress the weld toe region to remove possible areas of
Cracking in Welds
0.1.1.5.2.T15.95.12
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