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Section IX of the ASME BPVC

ASME P-Numbers

Structural Welding








Welding involves highly localized heating of the metal being joined together.  The temperature distribution in the weldment is therefore nonuniform.  Normally, the weld metal and the heat affected zone (HAZ) are at temperatures substantially above that of the unaffected base metal.  Upon cooling, the weld pool solidifies and shrinks, exerting stresses on the surrounding weld metal and HAZ. 

If the stresses produced from thermal expansion and contraction exceed the yield strength of the parent metal, localized plastic deformation of the metal occurs. Plastic deformation results in lasting change in the component dimensions and distorts the structure.  This causes distortion of weldments.

Several types of distortion are listed below:

  •   Longitudinal shrinkage
  •   Transverse shrinkage
  •   Angular distortion
  •   Bowing
  •   Buckling
  •   Twisting

distortion of butt welddistortion of fillet weld

Factors affecting distortion

If a component were uniformly heated and cooled distortion would be minimized.  However, welding locally heats a component and the adjacent cold metal restrains the heated material.  This generates stresses greater than yield stress causing permanent distortion of the component.  Some of the factors affecting the distortion are listed below:

  •   Amount of restraint
  •   Welding procedure
  •   Parent metal properties
  •   Weld joint design
  •   Part fit up

Restraint can be used to minimize distortion.  Components welded without any external restraint are free to move or distort in response to stresses from welding.  It is not unusual for many shops to clamp or restrain components to be welded in some manner to prevent movement and distortion.  This restraint does result in higher residual stresses in the components.

Welding procedure impacts the amount of distortion primarily due to the amount of the heat input produced.  The welder has little control on the heat input specified in a welding procedure.  This does not prevent the welder from trying to minimize distortion.  While the welder needs to provide adequate weld metal, the welder should not needlessly increase the total weld metal volume added to a weldment.   

Parent metal properties, which have an effect on distortion, are coefficient of thermal expansion and specific heat of the material.  The coefficient of thermal expansion of the metal affects the degree of thermal expansion and contraction and the associated stresses that result from the welding process.  This in turn determines the amount of distortion in a component. 

Weld joint design will effect the amount of distortion in a weldment.  Both butt and fillet joints may experience  distortion.  However, distortion is easier to minimize in butt joints.   

Part fit up should be consistent to fabricate foreseeable and uniform shrinkage.    Weld joints should be adequately and consistently tacked to minimize movement between the parts being joined by welding.

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Copyright 1999 Robert McCabe                    
Last modified: February 17, 2015