Question: We are heat treating the Sandvik 7C27Mo2 at 1040°C, but our loading basket is getting bent. Our existing loading material is SS306. We need to find the right material for loading. Can you give us a solution for this issue?
Answer: Yes, the Sandvik 7C27Mo2 is a martensitic stainless steel that can nicely handle the 1040°C (1900°F) heat-treating temperature, but the austenitic stainless steels (such as 304, 316, etc.) will soften (anneal) at those temperatures. They can be easily deformed by twisting, dropping, bending, etc., since it will not retain much strength after such high-temperature thermal treatments. If the loading baskets are made from these austenitic stainless grades and the bar stock used to create the basket are thick bars, the problem can be even worse.
People make the incorrect assumption that the heavier the bar stock they use to make heat-treat baskets, the less will be the distortion due to the inherent strength of the heavy bar stock. That is totally incorrect. Distortion due to heat treating is a strong function of the temperature differential between the outside surface of the part and the middle of the same part.
When thick bar stock is used, there will be a much greater temperature differential between the inside and outside of the bar stock, which can cause significant distortion over time. To minimize this distortion, the racks/baskets should be made from higher thermal-conductivity material and thinner material!
One ideal solution is the use of carbon-fiber-reinforced carbon (C/C) fixturing, which is very lightweight compared to steel, has good thermal conductivity to minimize any temperature differentials and can remain distortion-free over a much longer period of time than steel fixturing. Obviously, you could also use a very high-temperature-capable superalloy for fixturing (e.g., Inconels, Incoloys, Hastelloys, Rene products) as long as it is able to retain strength properties up to and beyond the temperatures you are using for heat treating.
You also mention “bending” in your question, which I assumed to mean “distortion.” However, if bending is an actual issue due to careless handling of the parts, then that may be a worker-education problem that your company needs to address with its production people. This would be especially true if the C/C type of fixturing materials were to be used instead of the austenitic stainless materials.
A more extensive article about the use of C/C fixturing can be found on my website.
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