Modeling Hardenable Stainless Steels Using Calculated Martensite Start Temperatures in Thermodynamic Equilibrium Calcula
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IN contrast to austenitic, ferritic, and duplex stainless steels, martensitic stainless steels usually require a certain amount of carbon, nitrogen, or both to enable their hardenability. Commonly, the interstitials content is about 0.4 wt pct.[1] In addition to a high hardness, corrosion resistance is the second key property of stainless martensites. Corrosion resistance is provided by a chromium amount above 10.5 wt pct.[2] Both the interstitials and the chromium have to be dissolved in the matrix to be effective. If the goal is to raise hardness and corrosion resistance at the same time, the high affinity of chromium to carbon leads to the formation of carbides of the M7C3 or M23C6 type.[1,3] These withdraw chromium and carbon from the matrix and thus decrease hardenability and corrosion resistance of the matrix. This contradiction can be partly resolved by alloying with nitrogen because precipitation of chromium-rich carbides is restrained.[4–6] Additionally, less chromium is dissolved in chromium-rich nitrides of the MN or M2N type.[7] However, a certain amount of carbides—or hard phases in general—is often requested to increase a third key property: the wear resistance, e.g., in case of bearings or knives for industrial use.[8,9] Here, the amount of carbides can be adjusted by means of the austenitization temperature (TAus), i.e., the higher TAus, the lower the amount of carbides.[1] This results in a
MERLIN SEIFERT, Research Assistant, and WERNER THEISEN, Chair Holder, are with the Chair of Materials Technology, Ruhr-Universita¨t Bochum, 44780 Bochum, North Rhine-Westphalia, Germany. Contact e-mail: [email protected] Manuscript submitted July 21, 2016. Article published online October 13, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
variation of the matrix composition depending on TAus. Calculation of the martensite start temperature (MS) by empirical formulae[10–12] should be performed on the basis of the matrix and not on the overall composition. MS of steels containing carbides changes for different TAus because the dissolution of carbides leads to an enrichment of alloying elements in the matrix. The amount of carbides and the resulting matrix composition can easily be calculated by thermodynamic equilibrium calculations for different TAus. Consequently, MS at a specific TAus can also be calculated because software packages such as Thermo-CalcÒ allow equations to be introduced.[13] The same holds true for the pitting resistance equivalent number (PREN), which is used to estimate the resistance against pitting corrosion.[14,15] These characteristic values allow the estimation of hardenability and hardness (MS) and corrosion resistance (PREN), respectively, in comparison to a known alloy.[9] For this purpose, adequate conditions concerning TAus or composition, e.g., comparable amounts of C, N, or C + N in solution at TAus, have to be taken into account. This results in a less uncertain development of hardenable (and corrosion resistant) steels, because the risk of failure is minimized. In this stud
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