Question:
I have a customer who wants me to process some 410 stainless steel. I know that the carbon content of the material has to be on the higher side to ensure that it will fully harden with gas quenching, but I don't know what the minimum value is. Is there a minimum carbon content used by industry where they can or will declare their material "air hardenable"? I told my customer they can buy material specifically graded this way, but in case they don't, I want to be able to determine this from the mill report.

Since 410 stainless steel has a specification that simply states a "maximum 0.15%C," my concern is that the carbon content could be .01% and still qualify (although I doubt I would get a good hardness response). We have encountered this in the past where we have heat treated ½-inch-diameter pieces and ¾-inch-diameter pieces together only to find that after hardening and tempering, the ¾-inch parts are 40 HRC and the ½-inch parts are 32 HRC. The cause was due to the purchase of two different bar sizes, and one was not graded "air hardenable."

I know of two suppliers, although I am not sure if one is still around today, that have two grades of 410SS where one has a suffix indicating that the material is air hardenable. The job I am looking at is ¼-inch fasteners hardened and tempered to 35-40 HRC. Perhaps this is not too difficult a task, and I have done it before. With today's shift to offshore suppliers, however, I want to make sure the material has good chemistry before I go through the heat-treating procedure to find it did not. Therefore, I was curious if there was an industry standard or "rule of thumb" carbon content that would draw the line between air hardenable, or "bright quench" as one supplier calls it, and a material that would require oil quench.

Answer:
You are indeed correct that the specification for 410 stainless steel is typically given as 0.15%C maximum. And depending where the material is purchased (domestic or offshore) the composition (and also grain size) may vary considerably. Typical composition is in the 0.11-0.14%C range, which is what I would consider the "air (gas) hardening" range for the product that you describe that would produce parts with equivalent properties to that of an oil quench. Hardening response in this carbon range is typically in the 38-45 HRC range.