Study of Temper Embrittlement in HAZ of Cr-Mo Steels (Report 1). Four Types of Temper Embrittlement observed in HAZ of 21/4Cr-1Mo Steel.

The temper embrittlement occurring in HAZ of 2 1/4Cr-1Mo steel was studied in the tempering temperature-time range of 774 to 900 K and 0.2 to 10000 hours. The transition temperature, TrE was measured on each specimen, and the temper embrittlement was detected as the rise of TrE. The temper embrittlement were classified into four types. The first and second types of embrittlement appear in the earlier stage of tempering, such as 775 K for 1 hour, and 100 hours, respectively. They differ from "the low-temperature temper embrittlement" which has ever been observed. The secondary hardening occurring during tempering seems to play an important role to induce those types of embrittlement. The third type appears in the tempering in a log time (825 K for 100 to 1000 hours). This type is identical with "the high-temperature temper embrittlement (reversible temper embrittlement) ". The fourth type arises in the final stage of tempering (825 K for 1000 to 10000 hours). Little work has been made to distinguish this type from the third type, and hence little is known about its characteristics. The temperature-time ranges in which those four types of embrittlement arise were shown in a diagram. The range where no embrittlement arises was also pointed out. Impact test carried out below the temperature of TrE produced an intergranular fracture in the specimen causing the third type of embrittlement, and a transgranular one in those for the first, second and fourth types of embrittlement.