Effect of Destabilizing Heat Treatment on Solid-State Phase Transformation in High-Chromium Cast Irons
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INTRODUCTION
HIGH-Cr cast irons are well known to be widely used for castings that undergo intensive wear (abrasive, erosion, corrosive-abrasion, etc.) under operation.[1–4] Excellent wear resistance of these materials is associated with a great amount of hard carbide phases distributed in the metallic matrix. In the as-cast condition, the microstructure of high-Cr cast irons consists of eutectic carbides M7C3 and dendrites of primary austenite or pearlite. To obtain the highest wear performance, ascast high-Cr irons are subjected to heat treatment, which leads to appropriate microstructure change as a result of solid-state phase transformation. The influence of heat treatment on the microstructure and properties of high-Cr cast irons was comprehensively described in numerous investigations.[5–9] As follows from the published works, two kinds of heat treatment are usually applied for these materials: (1) destabilization heat treatments (heating at 1073 K to 1373 K (800 C to 1100 C) with 1 to 6 hours soaking[5–7,10–12]); and (b) subcritical heat treatment, which implies soaking at temperatures 573 K to 973 K (300 C to 700 C), that is, below the A1 critical point.[9,13,14] Destabilization is aimed at destabilizing primary stable austenite to martensite transformation by means of secondary carbide precipitation.[6,10,12] The subcritical VASILY EFREMENKO, Professor, and YULIIA CHABAK, Postgraduate Student, are with Priazovskyi State Technical University, Mariupol 87500, Ukraine. Contact e-mail: [email protected] KAZUMICHI SHIMIZU, Professor, is with the Muroran Institute of Technology, Muroran, Hokkaido 050-8585, Japan. Manuscript submitted December 28, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
heat treatment is performed with the purpose of decreasing the austenite (primary or retained) amount in microstructure or tempering quenched martensite in cast irons.[8] Soaking at a subcritical range of temperatures leads to austenite transformation by eutectoid reaction at 773 K to 973 K (500 C to 700 C)[13] or by bainite reaction at 573 K to 723 K (300 C to 450 C).[14] The transformation can be preceded by dispersed secondary carbide precipitation effecting an increase of bulk hardness; thereby, subcritical soaking is widely used to improve the hardness and wear resistance of Ni–hard cast irons.[7] Destabilizing heating is carried out at high temperatures in the range above the A1 critical point to stimulate Cr-enriched carbide phase (M7C3, M23C6) precipitation.[15] According to Bedolla-Jacuinde et al.,[10] Kmetic et al.,[14] and Maratray,[16] the highest precipitation rate is attributed to a certain temperature—1223 K to 1273 K (950 C to 1000 C)—and precipitation kinetics can be presented in time-temperature-transformation (TTT) diagrams by C-shaped curves. The features of the secondary carbide precipitation process and its influence on high-Cr cast iron properties are quite fully described in many works.[12,15–17] The common conclusion is that the secondary carbide occurrence is followed by primary auste
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