構造用合金鋼の恒温オーステナイト化曲線

抄録

Time-temperature-austenitization diagrams (T-T-A Diagrams) were plotted to provide a guide for rapid heating and quenching of structural alloy steels like Ni-Cr (C 0.31%, Ni 1.45%, Cr 0.63%), Cr(C 0.39%, Cr 1.06%) and Cr-Mo (C 0.38%, Cr 0.96%, Mo 0.18%) steels. Specimens, 3 mmφ×10 mm, were preheated to impart them different structures: lamellar structure and fine spheroidized structure (quenched and tempered structure). For this purpose, they were heated in lead bath for different durations and then rapid-quenched in water. The T-T-A diagrams were plotted from the changes in hardness and microstructure. (1) From these diagrams, the starting and finishing times of austenitization were determined with the process of austenitization made clear. (2) Austenitization started nearly at the same time in all steels, but finished much sooner in ones of fine spheroidized structure than in ones of lamellar structure. After finishing of austenitization, with increase of the holding time, the coarsening of martensite acicula obtained by quenching occurred slower in the former than in the latter. The former structue would be more effective for the purpose of quenching. (3) In all steels, austenitization began nearly at the same time, but finished in Ni-Cr as well as 0.40%C steel at the shortest time, followed by Cr and Cr-Mo steel, the last one taking the longest time. For appropriate quenching, the holding time should be chosen for each steel.