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| THE TEMPERING OF HIGH SPEED STEEL |
| HS TSU-YAO (Shanghai Chiao Tung University) |
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Cite this article:
HS TSU-YAO (Shanghai Chiao Tung University). THE TEMPERING OF HIGH SPEED STEEL. Acta Metall Sin, 1965, 8(4): 443-454.
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Abstract The paper discusses the conditioning and stabilization of austenite during tempering, and the suggested new tempering process based upon the results of research on high speed steels. The effect of conditioning of austenite, expressed in term of the M' temperature at which the retained austenite transforms to martensite during cooling, depends upon the temperature and duration of tempering. On step tempering, i.e., holding at a higher temperature followed by a lower one (but above a certain temperature T_c), the effect of conditioning is additive and sometimes equals or exceeds that of tempering only at the higher temperature with identical duration. The amount of retained austenite transformed after optimum step tempering (e. g., 560℃ holding followed by 550℃ holding) is similar to that after multiple tempering process, but the tempering time is shortened. The M' temperature falls with the increasing amount of retained austenite and varies with the carbon content of austenite. Holding at temperatures below T_c and above M' causes a marked effect on the stabilization of austenite. The degree of stabilization and the initial rate of stabilization decrease with the increasing holding temperature. But the maximum amount of austenite stabilized increases with the holding temperature when the holding temperature is below M'. It is suggested that the stabilization of austenite during tempering is chiefly due to the reduction of stresses that promote the growth of martensite embryos. The conditioning may be regarded as a reversed process of stabilization and the following changes are suggested: the rearrangement of random dislocations, the formation of dislocation loops at α-γ phase boundaries and the removal of atmospheres formed from vacancies and solute atoms. A new tempering process consisting of step tempering followed by a normal tempering is suggested, e.g., holding at 590℃ for 25 minutes, then at 550°C for 25 minutes, and finally tempering at 560℃ for 60 minutes. The amount of austenite transformation, the mechanical properties and the cutting abilities of the high speed steel treated with the new process are comparable with those obtained by traditional multiple tempering. An optimum tempering process for reducing the deformation of tools can also be designed by the application of the stabilization of austenite. The total tempering durations are shortened in both cases.
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Received: 18 April 1965
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