Calorimetric Analysis of Aging Reaction in Ferritic and Martensitic Stainless Steel

The aging reactions of ferritic and martensitic iron alloys containing 11–30%Cr and 0–10%Co, and 13%Cr maraging steel containing Co and Mo have been investigated by means of calorimetric analysis and hardness testing. The specific heat versus temperature curves of the high Cr ferritic and martensitic alloys are characterized by the appearance of an evolution peak due to the formation of Cr rich zones in the temperature range between 330 and 540°C, and also an absorption peak due to the reversion of them in the higher temperature range between 500 and 590°C. The amount of evolution increases with increasing content of not only Cr but also Co. This result shows that the addition of Co promotes the formation of Cr rich zones resulting in the so-called 475°C embrittlement.In 13%Cr maraging steel, another evolution peak due to the precipitation of the intermetalic compound containing Mo appears in the higher temperature range between 520 and 660°C. On the low temperature aging in which the intermetalic compound never precipitates, however, the hardness increases more greatly than in the Mo free martensitic alloys. Thus, it is suggested that Mo as well as Cr forms the rich zones in the same temperature range between 330 and 540°C on the specific heat versus temperature curve. On the 500°C isothermal aging, for example, the Mo rich zones seem to be replaced by the intermetalic compound in the later stage of hardening. Lastly, the two evolution peaks and one absorption peak are separated on the specific heat versus temperature curve of as quenched 13%Cr maraging steel.