Heat Treatment of Martensitic Stainless Steel
13 July 2018There’s nothing simple about heat treatment technology. Even the various microcrystalline forms of steel make the process hard to regulate. Among these forms, martensitic stainless steel ranks as a super-hard alloy. It features a dense tetragonal crystalline structure, but where does this dense configuration occur in the heat treatment process? These and other questions will be addressed in the following post. Here, the stainless steel workpiece is approaching a furnace right now.
Directing Austenitic Tranformations
Heated in a furnace until the stainless steel piece radiates 670°C of raw thermal energy, the stainless steel is quickly quenched. As the heat is rapidly absorbed by the quenching medium, the atomic structure reorganizes inside the metal. Instead of a cubic, face-centred crystalline structure, the carbon atoms in the alloy move inwards. The hard yet ductile tetragonal arrangement relies on the fast, diffusionless trapping of the carbon, plus a high quantity of added chromium. Of course, having produced martensitic stainless steel, what comes next in the heat treatment operation?
Martensitic Stainless Steel: Heat Treatment Provisos
Quenching rates and carbon content are the two process-influencing variables to address this time. Additionally, certain alloying metals are known to hamper the hardening phase. By adding more carbon to the steel, there’s more material to relocate into the tetragonal matrices, which means the stainless steel becomes harder. However, alloying metals can attenuate the process. The result is transformation retardation and excessive grain growth. It’s the same for quenching, with certain quench rates causing undesirable carbide precipitates. In plain English, bereft of engineering complexities, this hardened stainless steel type is heat treatment sensitive, so all process variables must be rigorously managed.
Regulating the Process Properly
Employ the normal heat treatment methods, but beware of the control variables used during the various operations. Air cooling works well as a hardening mechanism, but oil quenching is also a recognized hardening solution, one that avoids carbide precipitates. In the meantime, over in the furnace, the next batch of martensitic stainless steel components are being heat treated in the furnace. They’re thermally subjected to between 925°C and 1060°C of furnace heat, at which point they gain corrosion resistant vigour and strength. That’s a product strength, the notion that different quantities of applied thermal energy make it easy to produce completely different strength ratios.
If there’s one phase of the process that receives more attention than the furnace work, it has to be the tempering of the alloy. Granted, martensitic stainless steel is clearly a super-hard alloy, but that hardness quotient also equals an undesirable quantity of process brittleness. Carefully managed tempering work is required to balance that renowned quality against an equally substantial amount of material ductility.
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