Welding Metallurgy 2 - Welding Metallurgy, AWS, 1979 Cold Worked Alloy WithoutAllotropic...

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Transcript of Welding Metallurgy 2 - Welding Metallurgy, AWS, 1979 Cold Worked Alloy WithoutAllotropic...

  • Welding Metallurgy 2

  • Welding Metallurgy 2

    Lesson Objectives

    When you finish this lesson you will


    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


    5. Do homework


    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


  • Heat Affected Zone Welding


    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


    Hardened Alloys

    Without Allotropic

    Phase Changes

    Welded In:

    Full Hard


    Solution Annealed


    Introductory Welding Metallurgy,

    AWS, 1979

  • Annealed upon


  • 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


  • Hydrogen Cracking Hydrogen cracking, also called cold

    cracking, requires all three of these factors



    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

    Cracking in Welds

  • 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

  • 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


    Cooling rate ( ( ( (T - Tbase)3

    T base

    T base

  • Interaction of Preheat and


    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


  • 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

  • Postweld 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


  • 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

  • Improve Cleanliness

    Improve through thickness properties


  • Multipass Welds

    Heat from subsequent passes affects the

    structure and properties of previous passes


    Reheating to form austenite

    Transformation from austenite upon cooling

    Complex Microstructure

    Carbon and Low-Alloy Steels

  • 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


  • 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

  • Knife-Line Attack in the HAZ

    Cr23C6 precipitate in HAZ

    Band where peak temperature is 800-1600F

    Can occur even in stabilized grades

    Peak temperature dissolves titanium carbides

    Cooling rate doesnt allow them to form again



    Knife-line attack

    Stainless Steel