Four types of temper embrittlement observed in the HAZ of 2 1/4Cr‐1Mo steel: Study of temper embrittlement in HAZ of Cr‐Mo steels (1st Report)

Summary

This paper describes a systematic investigation of the temper embrittlement occurring in the HAZ of 21/4 Cr‐lMo steel in the tempering temperature‐time range of 775–900 K × 0.2–10 000 hr. The transition temperature TrE of each specimen was measured, and the temper embrittlement is shown to increase with increasing TrE. The temper embrittlement is classified into four types. The first and second types of embrittlement respectively appear in the initial stage of tempering, such as at 775 K × 1 and 100 hr. They differ from the previously observed ‘low‐temperature temper embrittlement’. The secondary hardening occurring during tempering appears to play an important role in inducing these types of embrittlement. The third type appears during long‐term tempering (825 K × 100–10 000 hr). This type is equivalent to ‘high‐temperature temper embrittlement (reversible temper embrittlement)’. The fourth type occurs in the final stage of tempering (825 K × 1000–10 000 hr). Little work has been done to distinguish this type from the third type, and therefore little is known about its characteristics. A diagram shows the temperature‐time ranges in which these four types of temper embrittlement occur. The embrittlement‐free range is also indicated. Charpy impact tests conducted below the TrE temperature produce intergranular fracture in the specimen sustaining the third type of embrittlement and transgranular fracture in those sustaining the first, second, and fourth types of embrittlement.