Deterioration of 21/4Cr-1Mo steel ofter long-term service at elevated temperatures. 2.

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An outstanding decrease in low temperature toughness was observed from the examination of 21/4 Cr-1 Mo steel tubes after long-term service at elevated temperatures. Various forms of heat treatments were conducted on the samples to investigate the effect of thermal history of raw material on embrittlement, the relation between the accelerated short-time embrittlement tested in the laboratory and that encountered in the actual plant, and the effect of heat treatment for restoration of toughness.At the same time, a large decrease in high temperature strength was found in the materials embrittled after long-term service at elevated temperatures.Moreover, the restoration of not only toughness but also high temperature strength was found after de-embrittlement heat treatment. The strength then decreased again after reembrittlement heat treatment.In our previous paper, it was pointed out that some restrictions should be introduced with regard to the temperature range when the degree of embrittlement was evaluated by the time-temperature equivalent parameter method.The results of this investigation suggest the necessity of similar restrictions with regard to the time-temperature equivalent method in evaluating high temperature tensile strength and creep rupture strength.Up to this time, a comparison of creep strengths of used and unused material was made using the Larson-Miller parameter, and it was found that there existed some biased phenomena in many cases which could not be explained by means of conventional data.The unconditional parameter method was considered to be one of the reasons for this. Namely, as is evident from this study, the reversibility of high temperature tensile strength of a material, which decreased in a difinite range of temperatures, indicates that it is essential to give sufficient consideration to the application of the parameter method, which has been used as an indication related to the deterioration of the material.