Melting and crystallization behaviour of W‐V‐Si high‐speed steels

Abstract

Differential thermal analysis was used to determine the effects of variable Si, W and C contents on the behaviour of W‐V‐Si highspeed steels during melting and crystallization and to develop certain fragments of pseudobinary phase diagrams corresponding to melting and crystallization of these materials. Melting of S 9‐0‐2 steel involves the following sequence of transformations: reverse three‐phase eutectic reaction, direct melting of austenite grains, reverse three‐phase and then four‐phase peritectic reactions, and melting of high‐temperature ferrite. In steels containing more than 2% Si reverse four‐phase peritectic reaction occurs immediately after partial completion of reverse eutectic reaction and in steels with more than 4% Si there is also direct melting of carbides. A decrease in tungsten content from 9.5 to 7 % in steels containing 2 % Si and 1.05 or 0.9 % C enhances direct melting of austenite grains; an increase in tungsten content to 12% inhibits this reaction completely. A decrease in carbon content from 1.05 to 0.9 % in steels containing W 7‐12, V 2.5 and Si 2 % does not change the nature of transformations on melting. Crystallization of conventional S 9‐0‐2 steel involves the following sequence of events: formation of high‐temperature ferrite crystals from the liquid, three‐phase peritectic reaction, direct crystallization of austenite from the liquid, and three‐phase eutectic reaction. With silicon contents exceeding 3% direct crystallization of austenite and three‐phase eutectic reactions occur simultaneously. This sequence of reactions is not affected by reduction of tungsten content from 9.5 to 7 % in steels containing 2% Si and 1.05 or 0.9% C. An increase in tungsten content to 12% in such materials results in complete inhibition of direct crystallization of austenite from the liquid and in 1.05% C steel there is an additional change involving simultaneous occurrence of three‐phase peritectic and eutectic reactions. A decrease in carbon content from 1.05 to 0.9% in steels containing 7–12% W and 2% Si does not change the nature of transformations during cooling but does enhance partially simultaneous occurrence of three‐phase peritectic and eutectic reactions.