Hi all,
We have thick forgings for shafts (7.5" diameter, finished machining). We currently have 'too much' stock on the shaft for rough machining that is used during heat treat / quench. We currently have an extra 1.25" diametrical. The vendor has suggested 1/4" diametrically.
My questions:
From my understanding, martensite is formed during rapid cooling. I reason that the center of the shaft may not have any martensite or very little (rapid cooling not taking place). How deep from the surface will martensite form? How much martensite is needed for a 400 series stainless (410 in this case) for the stainless to be considered 'martensitic stainless steel'?
Thanks and cheers,
The stainless steel is a martensitic stainless steel regardless of the state of heat treatment. The term martensitic stainless steel is used to describe the family of stainless steel. Any of the martensitic stainless steels will respond to a quench because they contain sufficient carbon. This is in contrast with the ferritic stainless steels that may contain similar percentages of chrome, but insufficient carbon to respond to quenching.
The maximum hardness is a function of the maximum austenitizing temperature and the quench rate. The quench rate is dependent on the quenchant; air, oil, water, or brine. There are synthetic quenchants available, but they are often compared to the "traditional" quenchants I listed.
Martensitic stainless steels can be air quenched and attain a high hardness. As you noted, there is a relationship between hardness and strength. The maximum strength is reached when the maximum hardness is reached. However, the maximum quench rate can also result in quench cracks, so there must be a balance that is maintained.
As you noted, the highest hardness readings indicative of a martensitic microstructure will be on the outside surfaces of the shaft. Less martensite will be formed toward the center of the shaft. One of the advantages of the martensitic stainless steels is the depth of the hardening. But still, with a large diameter shaft, it is unlikely the full thickness will be fully hardened.
Rough machining would be usually accomplished while the type 410 martensitic stainless steel is in the annealed condition. Once rough machined, it could be heat treated to harden it and then the shaft ground to the final dimensions. Usually, the areas that would receive finish grinding would be those areas that receive bearings. Then again, I obviously don’t know what the shaft will be used for, the reason the shaft will be hardened, or the tolerances involved.
Al