316L SS WELD CRACKING

[WVAENGINEER]

We take a small triangular bar and weld either round rod or flat bar material to it. When using the round rod the welds look good but when using the flat bar we are getting cracking. This appears to be a problem with the flat bar. We have had test done on the 316L material and everything checks ok. All metals being used are 316L ss material and fusion welded using no filler material. Does anyone have any suggestions where to start looking as a possible cause. I have more information on the problem if someone has any ideas.

 

[Salvatio]

Will it because of the Heat-Affected-Zone?
The flat bar may cause localized cooling to the weld, as the round rod may be less prone to it.
The cooling of the weld should be controlled if possible in order to avoid intergranular corrosion, since 316L contains quite high amount of Cr.

 

[unclesyd]

There are several things that could contribute to your problem. The first is since you are not using a filler is weld constraint. You don't have enough weld metal to hold the joint.

Come back with a little more description of the joint, sizes and configuration.
Where is the weld cracking?
Can you add filler metal?

 

[metengr]

Given the limited information, it sounds to me that you may have a potential solidification cracking problem with the autogenous welding of the flat bar, and not with the round bar. I would check the chemistry of the flat bar very closely, and compare with it the round bar base material. It may well be that you are not generating any delta ferrite from dilution affects. Typically, delta ferrite is necessary to avoid solidification cracks in austentitic weld deposits.

 

[WVAENGINEER]

Thank you for the response. We are taking triangular shaped formed metal .125 wide and .225 tall and welding the point perpendicular to the small side of .375 X .750 flat bar. This is done under extreme pressure to ensure contact of all of the triangular material to the flat bar. This has worked fine in the past with different heat numbers of the same size material. We have even welded two different heat numbered flat bars to the triangular material and one works the other cracks in a U shape around the point of the triangle. We have a spec to have low phosphorus in the metal for years this fixed the problem once before. We have had all different heat numbers analyzed and nothing stands out as being different. We are basically only getting the required 316L SS specs analyzed though. It looks like something is wrong with the metal and we cannot find it. Recently we ordered some new metal and it also cracked, just not as severe. Thanks again for your help.

 

[andyenergy]

both unclesyd and metengr seem to have identified the most probable causes - autogenous welding of stainless can result in soldification cracking due to poor delta ferrite levels in the base materials. Look closely at suplhur levels in the base materials - this is the element that generally contibutes most to solidification cracking. Try looking at the Ni and Cr equivalents for the materials and plotting them on a schaeffler diagram to see where the chemistry puts them for delta ferrite levels.

 

[TEV]

P, S, and Si are the elements to which weld cracking is generally attributed in austenitic stainlesses. These elements are always present in SS, but control of these elements within a narrow range is preferable. But, as andyenergy has stated, the detrimental effects of these elements is greatly alleviated by specifying a composition that will have some delta ferrite present in the weld. If you look at the Schaeffler diagram, you will see that 316 can have a considerable variation in delta ferrite content over the permissable composition ranges for the alloy, from 0 to fairly high levels. I would suggest you include in your specifications a required ferrite number range of, say 6 to 10.