Changes in the Process of Solidification and in the Microstructure of High Speed Tool Steels as a Result of Rapid Cooling in the Production of Powders from the Melt.

In the production of metal powders from the melt, solidification rates of from 10**2 to 10**4 degree C/sec are reached. At such high cooling rates the solidification process in the high speed tool steels S 6-5-2 and S 12-1-4-5 is modified. Microscopic examination shows that with increasing cooling rate the formation of hexagonal M//2C and cubic MC is suppressed, and the peritectic transformation of the ferrite first solidifying from the melt is inhibited. In this case the residual melt crystallizes as highly supersaturated metastable austenite, and ferrite is present in the microstructure to an increasing extent as metastable phase. Very rapidly solidified powder particles S 6-5-2 thus consist of metastable ferrite, metastable highly supersaturated austenite, and only a small amount of austenite of normal composition, some of which transforms to martensite on cooling to room temperature. There are no carbides in these powder particles. Other structural characteristics, depending on process variations, are discussed.