Features of the structure and properties of high-speed steels after laser treatment

Abstract

1. Depending upon the nature of the processes occurring in high-speed steels in laser treatment, their hardness may be higher or lower than the hardness obtained both directly after through hardening and after the standard heat treatment (harden and triple temper at 560°C). 2. In the zone formed as the result of rapid laser tempering the hardness of the high-speed steels decreases from 890–930 H to 700–770 H. In contrast to through tempering in the 560°C-Ac1 temperature range, when the reduction in hardness is caused both by decomposition of the martensite and by coagulation of the carbide phases, in tempering under the action of laser irradiation the loss of hardness occurs only as the result of decomposition of the solid solution. 3. In those cases when laser treatment is done with fusion the microhardness of the structural constituents formed in crystallization from liquid is lower than after the standard heat treatment and does not exceed 800–850 H. 4. Specimens with a martensitic-carbide structure formed during secondary hardening from temperature close to the solidus temperature but not exceeding it have the highest hardness after laser treatment (1120–1150 H). 5. In high-speed steels after laser hardening, practically no residual austenite is observed and their structure is microcrystalline martensite containing 0.4% C and eutectic secondary carbides not capable of dissolving during the time of action of the pulse. The absence of residual austenite in the structure of steels after laser hardening may explain the fact that their hardness, in contrast to the hardness in through hardening, does not decrease in cooling from temperatures close to the solidus temperature. 6. In laser hardening structural heredity, including restoration of the original grain size in reheating under the action of irradiation in the Ac1-TS temperature range, is revealed. 7. To harden high-speed steels with the use of laser treatment it must be done without fusion and so that a zone of secondary hardening of the maximum possible thickness is obtained in the surface layer. This is obtained with a laser irradiation pulse power density of 310–330 MW/m2.