Heat Treatment for Flake Graphite Cast Iron and Spheroidal Graphite Cast Iron

摘要

Characteristics of heat treatment for flake graphite cat iron and spheroidal graphite cast iron not containing Ni, Cr, Mo and other elements which affect the hardenability of cast iron were studied.The characteristics of heat treatment of these cast irons can be summarized as follows, 1) The heat treatment characteristics of flake graphite cast iron and spheroidal graphite cast iron are similar. 2) In the case of short cooling time (cooling time from Ac_1 or Ac_3 to 773 K is less than about 6s), the microstructures after heat treatment consist of high carbon martensite and retained austenite. The hardness of these microstructures at room temperature ranges from 470 to 690 HV10, and the higher the Si content of the cast iron or the higher the austenitizing temperature, the higher is the hardness. Subzero treatment after heat treatment is considered to increase the hardness of cast irons, because retained austenite transforms to martensite. 3) In the case of cooling time from Ac_1 or Ac_3 to 773 K between about 6 and 30 s, bainite, ferrite and pearlite coexist with martensite and retained austenite. The hardness decreases from about 690 HV10 to about 230 HV10 with increasing cooling time. 4) In the case of long cooling time over 60 s, the microstructures are ferrite and pearlite. The longer the cooling time, the more ferrite is produced. Hardness after heat treatment increases with an increase in the Si content of the cast iron or austenitizing temperature.The hardness of cast irons having large sized primary solidified proeutectoid austenite is higher than that with small sized proeutectoid austenite. The control of the solidification process is relatively important in the manufacture cast irons for heat treatments.