Hisashi Maeda1, Toshio Kawabe1 and Yoshihito Endo1
1TôyôKogyo Co., Ltd., Hiroshima
It is generally said that the toughness of quenched and tempered steel depends upon the austenitic grain size of steel iust before quenching. However, we have compared, on the structural steel in case of insufficient quenching, the toughness and the fatigue strength after tempering between ordinary quenched steel with fine austenitic grain and forge-quenched steel with coarse austenitic grain, and also investigated the effects of the forging ratio and the final forging temperature on the toughness of the forge-quenched steel. The above study has lead us to the following conclusions. (1) In case of the absence of complete quenching, the toughness of quenched and tempered structural steel is more heavily affected by the degree of insufficient quenching rather than by the size of austenitic grain. Thus, the impact value of a tempered medium carbon steel which has been forge-quenched is doubled as compared with that of ordinary quenched steel. (2) Since the heating temperature of the die forging process is generally held at about 1,300°C in applying the forge-quenching method, the toughness of forge-quenched steel is hardly influenced by the forging ratio and the finishing temperature of forging. (3) In respect of the fatigue strength, the forge-quenched steel is equal or superior to the ordinary quenched steel. (4) In view of the above results, it may be concluded that application of the forge-quenching method often makes it possible to replace alloy steel parts with carbon steel parts or to reduce the weight of carbon steel parts.
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