Abstract
The authors have pointed out in preceding papers that four types of temper embrittlement arise in the temperature range below 900 K when the heat affected zone of 2 1/4Cr-lMo steel is reheated. The causes of these temper embrittlements were examined in the present paper.
The first type of embrittlement which arises at 775 K was compared with "the low temperture temper embrittlement" which has been known to arise at 575 K and to be caused by the fine precipitates of cementite. Experimental results on a series of specimens which were tempered at each temperature from 575 K to 775 K showed that both types of temper embrittlement are identical.
The second type of embrittlement, which arises at 825 K in the short term range and produces cleavage fracture by the impact test, was found to be caused by the hardening of ferrite which was induced by the fine precipitates of Mo2C type carbide.
The thirdd type of embrittlement arises at temperatures around 825 K and produces intergranular fracture by the impact test. A new technique for analyzing the grain boundary revealed that the segregation of phosphorus caused this embrittlement.
The fourth type of embrittlement, which arises at 875 K in the long term range and produces cleavage fracture, was found to be caused by the large sized particles of Mo2C type and Cr, C, type carbide. In the impact test, fracture occurred preferentially along the planes in which carbide particles of large size exist.
The proper terms of the first to the fourth types of embrittlement were proposed by the authors : the low-temperature-, the short-term-, the intergranular- and the long-term temper embrittlement.
