Welding Metallurgy 2

Welding Metallurgy 2

Lesson ObjectivesWhen 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 Activities1. View Slides; 2. Read Notes, 3. Listen to lecture4. Do on-line

workbook5. 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 Without Allotropic Transformation

WeldingPrecipitationHardened AlloysWithout AllotropicPhase 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 CleanlinessImprove 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 stress

concentration

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