Gas–Solid Interactions During Nonisothermal Heat Treatment of a High-Strength CrMnCN Austenitic Steel Powder: Influence
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RECENTLY developed high-strength CrMnCN austenitic steels feature excellent mechanical properties and good corrosion resistance.[1–5] In an analogy to Hadfield steels,[6] these materials achieve their strength from their high interstitial content. Being alloyed with carbon and nitrogen, CrMnCN austenites reach yield strengths and true fracture strengths of up to 600 MPa and 2500 MPa, respectively.[4] Furthermore, as a consequence of the combined presence of C + N in the solid solution, the concentration of free electrons is considerably increased, leading to improved short-range atomic ordering, higher solubility of the interstitials in the melt, and an enhanced ductile metallic character of the NIKOLAJ KRASOKHA, Scientific Assistant, STEPHAN HUTH, Junior Professor, and WERNER THEISEN, Chairholder, are with the Chair of Materials Technology, Ruhr-University Bochum, Universita¨tsstr. 150, 44801 Bochum, Germany. Contact e-mail: [email protected] SEBASTIAN WEBER, Group Leader, is with the Chair of Materials Technology, Ruhr-University Bochum, and with Helmholtz-Zentrum Berlin fu¨r Materialien und Energie GmbH, 14109 Berlin, Germany. KATHRIN ZUMSANDE, Scientific Assistant, is with the Helmholtz-Zentrum Berlin fu¨r Materialien und Energie GmbH. Manuscript submitted August 30, 2011. Article published online May 30, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
interatomic bonding.[3,4,7,8] In their experimental investigations of C + N austenites, Berns et al.[4] measured elongation values at fracture of up to 74 pct and a specific fracture energy of approximately 700 J/cm3. With respect to the constitution, CrMnCN austenites are typically alloyed with 18 – 20 mass-pct Cr and Mn each and with up to 1 mass-pct C + N.[1,2,5] In these steels, chromium is necessary not only for corrosion resistance but also in combination with a high amount of manganese to enhance nitrogen solubility in the melt.[4,5,9] Because of the additional effects of carbon, i.e., stabilization of the face-centered cubic (fcc) phase at higher temperatures and reduction of the d-ferrite content during solidification, CrMnCN austenites can be produced at atmospheric pressure and do not require cost-intensive pressurized electroslag remelting.[4,5,9] As the beneficial effects of Cr, N, and C (corrosion resistance and mechanical properties) can be exploited only when these elements are dissolved in the metallic matrix, CrMnCN steels have to be free of precipitates.[5,10,11] However, the potential to form carbides or nitrides during cooling from the solution-annealing temperature increases with increasing C + N content.[4,9] With respect to this limitation, the influence of the amounts of carbon and nitrogen on heat treatability was intensively investigated at the Chair of Materials VOLUME 43A, NOVEMBER 2012—4237
Technology of the Ruhr-Universita¨t Bochum (Germany).[12,13] It was found that there is an optimum ratio (C/N)op at which the phase field of homogeneous austenite is extended to the lowest temperature Top (Figure 1). Hence, in addition to the overall inte
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