Electric-spark alloying of steel with tungsten-free hard metals

Abstract

A study was made of the ESA of steel with tungsten and tungsten-free TN type hard metals in different units and under different conditions, with and without anode dressing. It has been established that in alloying under “finishing” conditions the erosion of a dressed TN-20 alloy anode is greater than that of an undressed one; this is due to the formation of stable oxide films on the electrode surfaces. In contrast to this, in treatment under “rough” conditions the erosion of an undressed anode is greater because the oxide films cannot withstand the higher thermal stress generated during alloying. In ESA with tungsten alloys maximum erosion under both “finishing” and “rough” conditions is observed with undressed specimens. This is attributable to the formation of a defective zone promoting periodic brittle disintegration in the course of treatment. Removing the defective zone decreases the erosion of the material. The most favorable conditions for the formation of a reinforced layer are created in the alloying of a dressed cathode surface, and consequently it is best to perform ESA with a specific time of not more than 1 min/cm2. Use of treatment conditions with large thermal loads increases the thickness and hardness of the reinforced layer. Reinforcement with TK type alloys sets up a stress which is greater on the specimen surface and extends to a greater depth than the stress generated in alloying with TN type hard metals. Electrodes made of the tungsten-free TN-20 hard metal can be used instead of tungsten alloy electrodes for the ESA of processes V and VI. The resultant reinforced layer is sufficiently thick and continuous.