Page 61 - CITS - Welder - Trade Theory

WELDER - CITS

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

Method of control
Prevention or minimization of distortion
Several ways can be used to minimize distortion caused by shrinkage:
1 Do not over weld

2 Control fitup.
3 Use intermittent Welding
4 Use as few weld passes as possible
5 Place welds near the neutral axis
6 Balance welds around the neutral axis
7 Use back step welding

8 Anticipate the shrinkage forces
9 Plan the welding sequence
10 Remove shrinkage forces after welding
11 Minimize welding time

12 Use the smallest leg size permissible when fillet welding.
Prevention or minimization of distortion
For groove welds, use joints that will minimize the volume of weld metal. Consider double-sided joints instead of
single-sided joints.

48

CITS : C G & M - Welder - Lesson 12&13