QUANTITATIVE MICROSTRUCTURAL ANALYSIS OF M-2 GRADE HIGH-SPEED STEEL DURING HIGH-TEMPERATURE TREATMENT

The microstructural changes of M2 grade high speed steel Were examined quantitatively to identify the mechanisms responsible for changes dur ing high temperature treatment. Reheating at high temperature leads to major microstructural changes involving phase transformation, spheroi dization and coarsening of carbides formed in the as-cast M2 grade hig h speed steel. The M2C carbide decomposes rapidly to form MC and M6C i n less than 1 h after attaining the reheating temperatures of 1150 or 1200 degrees C. Further reheating brings about the eutectic M6C carbid e spheroidization and general coarsening of both M6C and MC. Carbide p articles situated at the original austenite grain boundaries grow pref erentially until impingement of carbides along these boundaries takes place. It is the diffusion of tungsten in gamma-Fe which controls sphe roidization rate of M6C carbides, while diffusion of vanadium and tung sten in gamma-Fe is the rate controlling mechanism for coarsening of M C and M6C carbides, respectively. The process of spheroidization of al loy carbides such as M6C, is somewhat different from that of cementite where impingement of the already spheroidized carbide particles make the analysis more difficult. (C) 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.