Avoiding distortion during welding stainless steels

Introduction

Although classed as readily weldable, the austenitic stainless steels can be prone to distortion during and after welding. This is due mainly to their specific combination of physical properties, mainly their relatively high thermal expansion rate and low thermal conduv
As they also readily work harden on cold working, localised partial stress relieving of machined or formed parts can also be a cause of distortion. Movement both during welding and after cooling is sometimes a problem especially where precision components are being made.

Removing cold work after forming or machining operations

Compared to other steels, including the ferritic and martensitic stainless steels, the austenitics strength levels are increased significantly during any form of cold working. This includes forming or machining of any sheet / plate, bar, tube or other product forms.
Where complex cold worked shapes need to be subsequently welded, solution annealing can be considered as a method of completely removing residual stresses.
Adequate protection of the surfaces will be needed to avoid unacceptable surface oxide film formation and any post heat treatment can reintroduce cold work stresses. Full solution annealing is usually accompanied by rapid cooling, but to avoid distortion slow (air) cooling may be preferable.
Use of the low carbon grades will avoid subsequent intercrystalline corrosion risks. See Stress relieving austenitic stainless steels.

Section sizes to be joined

Distortion is more likely when one member is considerably thicker than the other. Machining tapers onto the thicker section may help reduce distortion.

Fixturing and joint fit up

The use of fixtures should help reduce distortion risks as well as possibly reducing welding times and giving better finished fabrication tolerances. Fixtures should be stainless steel, to reduce the risk of introducing iron contamination.Iron contamination and rust staining 
Either as an alternative to fixturing, or as an additional measure, tack (spot) welds should be done evenly. It is important that tack welds are placed closer than when welding carbon steels. As a guide half the spacing compared to carbon steels should be used. It is also important that the tack welds are done in a sequence that evenly distributes their effect. Starting at one side and moving across a joint can result in the joint closing up at the opposite end. The TIG welding method is well suited to tack welding and wire brushed or ground before the final weld bead is laid on top.

Weld heat input

The heat input / /welding speed should be kept within the parameters for the electrode type and size. Attempts to weld at faster rates can be a cause of distortion, bearing in mind the combined affects of lower thermal conductivity and higher thermal expansion rates of the austenitic stainless steels compared to other stainless steels and carbon steels.

 

Source: http://www.bssa.org.uk/