Avoiding Distortion During Welding Stainless Steel
Although classed as readily weldable, the austenitic stainless steel 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 conductivity. 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 steel, including the ferritic and martensitic stainless steel, the austenitic strength levels are increased significantly during any form of cold working. This includes forming or machining of any stainless steel 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 stress.
Adequate protection of the surfaces will be needed to avoid unacceptable surface oxide film formation and any post heat treatment can reintroduce cold work stress. 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. Refer to Stress relieving austenitic stainless steel.
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 tolerance. Fixture 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 steel. 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 steel compared to other stainless steel and carbon steel.
Related References:
1. Welding Process and Letter Designations
2. Welding Stainless Steel to other Steel
3. Welding and Post Fabrication Cleaning for Construction and Architectural Application
4. Welded Stainless Steel Pipe
5. Post weld cleaning and finishing of stainless steel
6. Avoid PWHT Post Weld Heat Treatment
7. Fume Associated With Welding Stainless Steel
8. Avoiding Distortion During Welding Stainless Steel
9. Design Strength of Welded Connections
10. Processes for Welding Stainless Steel
11. Brazing Stainless Steel
12. Soldering Stainless Steel
13. Welding Stainless Steel
14. Selection of Welding Consumables Filler
15. Selection of Welding Consumable for Welding Stainless Steel
16. Filler Metals For Welding Stainless Steel
17. Schaeffler and Delong Diagrams for Predicting Ferrite Levels
18. Welding Properties of Stainless Steel
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