Welding Metallurgy 2

Welding Metallurgy 2

Objectives

• The region of the weld where liquid does not form

• Mechanisms of structure and property changes associated with these regions

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

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

Introductory Welding Metallurgy,AWS, 1979

Steel Alloys With Allotropic Transformation

Introductory Welding Metallurgy,AWS, 1979

Hydrogen cracking, also Hydrogen cracking, also called cold cracking, requires called cold cracking, requires all three of these factorsall three of these factors HydrogenHydrogen StressStress Susceptible microstructure (high Susceptible microstructure (high

hardness)hardness) Occurs below 300°COccurs below 300°C Prevention by Prevention by

Preheat slows down the cooling Preheat slows down the cooling rate; this can help avoid rate; this can help avoid martensite formation and martensite formation and supplies heat to diffuse hydrogen supplies heat to diffuse hydrogen out of the materialout of the material

Low-hydrogen welding procedureLow-hydrogen welding procedure

Hydrogen CrackingHydrogen Cracking

PreheatPreheat reduces the temperature reduces the temperature differential between the weld region and differential between the weld region and the base metal:the base metal: Reduces the cooling rate, which reduces the Reduces the cooling rate, which reduces the

chance of forming martensite in steelschance of forming martensite in steels Reduces distortion and shrinkage stressReduces distortion and shrinkage stress Reduces the danger of weld crackingReduces the danger of weld cracking Allows hydrogen to escapeAllows hydrogen to escape

Why Preheat?Why Preheat?

Using Preheat to Avoid Using Preheat to Avoid Hydrogen CrackingHydrogen Cracking

If the base material is preheated, heat flows If the base material is preheated, heat flows more slowly out of the weld regionmore slowly out of the weld region Slower cooling rates avoid martensite formationSlower cooling rates avoid martensite formation

Preheat allows hydrogen to diffuse from the Preheat allows hydrogen to diffuse from the metalmetal

Cooling rate T - Tbase)

Cooling rate T - Tbase)

T base

T base

Carbon equivalent (CE) measures ability to Carbon equivalent (CE) measures ability to form martensite, which is necessary for form martensite, which is necessary for hydrogen crackinghydrogen cracking CE < 0.35CE < 0.35 no preheat or postweld heat treatmentno preheat or postweld heat treatment 0.35 < CE < 0.550.35 < CE < 0.55 preheatpreheat 0.55 < CE0.55 < CE preheat and postweld heat treatmentpreheat and postweld heat treatment

Preheat temp. Preheat temp. depends on CE and plate depends on CE and plate thickness thickness

Interaction of Preheat Interaction of Preheat and Compositionand CompositionCE = %C + %Mn/6 + %(Cr+Mo+V)/5 + %(Si+Ni+Cu)/15

The fast cooling rates associated with The fast cooling rates associated with welding often produce martensitewelding often produce martensite

During postweld heat treatment, During postweld heat treatment, martensite is tempered (transforms to martensite is tempered (transforms to ferrite and carbides)ferrite and carbides) Reduces hardnessReduces hardness Reduces strengthReduces strength Increases ductilityIncreases ductility Increases toughnessIncreases toughness

Residual stress is also reduced by the Residual stress is also reduced by the postweld heat treatmentpostweld heat treatment

Why Post-Weld Heat Why Post-Weld Heat Treat?Treat?

Postweld heat treatment (~ 1200°F) Postweld heat treatment (~ 1200°F) tempers any martensite that may have tempers any martensite that may have formedformed Increase in ductility and toughnessIncrease in ductility and toughness Reduction in strength and hardnessReduction in strength and hardness

Residual stress is decreased by postweld Residual stress is decreased by postweld heat treatmentheat treatment

Rule of thumb: hold at temperature for 1 Rule of thumb: hold at temperature for 1 hour per inch of plate thickness; minimum hour per inch of plate thickness; minimum hold of 30 minuteshold of 30 minutes

Postweld Heat Postweld Heat Treatment and Treatment and Hydrogen CrackingHydrogen Cracking

Base Metal Welding Concerns

Occurs in thick plate subjected to high Occurs in thick plate subjected to high transverse welding stresstransverse welding stress

Related to elongated non-metallic Related to elongated non-metallic inclusions, sulfides and silicates, lying inclusions, sulfides and silicates, lying parallel to plate surface and producing parallel to plate surface and producing regions of reduced ductilityregions of reduced ductility

Prevention byPrevention by Low sulfur steelLow sulfur steel Specify minimum ductility levels in transverse Specify minimum ductility levels in transverse

directiondirection Avoid designs with heavy through-thickness Avoid designs with heavy through-thickness

direction stressdirection stress

Lamellar TearingLamellar Tearing

Heat from subsequent passes affects the Heat from subsequent passes affects the structure and properties of previous structure and properties of previous passespasses TemperingTempering Reheating to form austeniteReheating to form austenite Transformation from austenite upon coolingTransformation from austenite upon cooling

Complex MicrostructureComplex Microstructure

Multipass WeldsMultipass Welds

Multipass WeldsMultipass Welds Exhibit a range of Exhibit a range of

microstructuresmicrostructures Variation of Variation of

mechanical mechanical properties across properties across jointjoint

Postweld heat Postweld heat treatment tempers treatment tempers the structurethe structure Reduces property Reduces property

variations across the variations across the jointjoint