Effect of Vanadium on subcritical heat treatment behavior of hypoeutectic 16 wt% Cr cast iron containing 2 wt% Mo

บทคัดย่อ

Subcritical heat treatment of austenite in alloyed cast iron by tempering at the temperature below pearlite transformation (A1), has been used to improve matrix hardness without hardening. Hypoeutectic high chromium cast irons containing 16 wt% Cr - 2 wt% Mo without and with V were prepared in order to clarify the effect of holding temperature and holding time on behavior of hardness and decomposition fraction of austenite in matrix (f) in subcritical heat treatment. As-cast specimens were held at temperature from 723 K to 923 K for 14.4 - 43.2 ks, and then cooled by fan air. In the as-cast state, macro-hardness and micro-hardness increased gradually with an increase in V content. The f increased with the raising V content. In subcritical heat treatment state, the hardness curves showed a secondary hardening due to precipitation of secondary carbides in matrix and transformation of destabilized austenite into martensite during cooling. The degree of secondary hardening was greater in the V-free specimen than that in the V-containing specimens. The f increased greatly when the holding temperature increased over 823 K. The maximum hardness in subcritical heat treatment (H_STmax) stage was obtained by treatment condition of 823 - 873 K for 14.4 - 43.2 ks where the f was about 60-80%. The macro-hardness curves showed similar behavior to the micro-hardness curves. The H_STmax increased gradually with an increase in V content. The largest H_STmax value of 848 HV30 was obtained in the 3 wt% V specimen. The highest abrasive wear resistance was obtained in the specimen with H_STmax