Hot cracking is SS welds - Boron? 2

[TEV]

We are experiencing some hot cracking problems in welding thin gauge 304L stainless sheet into tubing. Analysis of the weld area showed the presence of Boron, among other elements. According to the literature, Boron is the number one bad actor for causing hot cracking in welding. But when we approached the material supplier, he claims that B is routinely added to stainless sheet to prevent hot shortness during hot rolling.

My questions are - Is this generally true for sheet products intended for welding fabrication, such as tube making? Is there a Boron maximum or range acceptable for sheet products intended for welding to tube? Are there ways to mitigate the effects of the Boron during welding? Current problem lots of material are in the range of 2 to 4 ferrite number; would imposing a spec of, say, 6 to 10 ferrite number help?

 

[GRoberts]

Since you didn't metion it, I am assuming that you are not adding filler metal, in which case I think increasing the FN might help. I know in the case of P and S it would help because Ferrite has a higher solubility for P & S than Austenite does, so it helps reduce the amount of P and S available later to be pushed to the edge of the austenite grain boundaries and cause cracking. Whether it has the same effect on B, I do not know. Also, you should be somewhere near the edge of primary austinite solidification at 2-4 FN depending on actual composition and cooling rate, and a higher FN would definatley promote primary ferrite solidification.

Other factors that might affect hot cracking depend on the process and joint geometry. Make sure you are not getting too high of a depth to width ratio, or producing concave welds. Are you adding filler metal, or is it an option?

If you get a chance to read it, "Welding Metallurgy" by Kou has a lot of info on this.

 

[mcguire]

TEV
I know first hand that some major producers put unreported additives including boron in stainless steel to help hot-workability. This is especially true for material intended for tubing which you, more than anyone perhaps, know has elevated sulfur. The final boron level is generally under 50ppm.
I am not familiar with the idea that at these levels boron would promote weld cracking. I think it's action is to promote nucleation of sulfides which keeps them from forming grain-boundary films which cause the intergranular decohesion you're trying to avoid. That's why the steel producers use it.
I have found that keeping ferrite between 5 and 10% is useful for both hot cracking resistance and preventing hot rolling defects. Low nitrogen helps, also.