Surface strengthening of St3 steel by electrospark alloying in combined media of different aggregate states

Abstract

The article presents the results of the St3 steel surface zone study after electric-spark alloying (ESA) by chromium, titanium and zirconium in combined saturating environments. The peculiarity of such treatment is the combination of environments of different chemical composition and aggregate state within the framework of one technological process. The treatment is carried out in two stages: ESA in liquid nitrogen and ESA in glycerin. The liquid nitrogen environment allows to rapidly reduce the temperature of the alloying electrode and cathode, which contributes to an increase in the penetration depth of anode metal atoms deep into the steel base. The glycerin environment acts as a source of carbon, which appears as a result of partial dissociation at the boiling temperature under the action of a spark discharge in local volumes of the liquid.

The choice of anode materials was determined by their ability to form carbides and nitrides. Extreme conditions of the ESA process, under which chromium, titanium and zirconium interact with elements of the environment (nitrogen and carbon), contribute to the formation of coatings enriched with different types of penetration phases.

Microstructural analysis revealed that by ESA with a change of environments in the sequence liquid nitrogen – glycerin, coatings with a thickness of 30 – 40 μm are formed. The surface alloyed layers composition on St3 steel is multiphase and depends on the material of the alloying electrode (Cr, Ti, Zr).

The microhardness of the studied coatings on St3 steel is 4.3 – 5.7 GPa due to the presence of carbides, nitrides of transition metals, as well as intermetallics and solid solutions with the base material. The distribution of microhardness deep into the steel base showed its increased values ​​​​up to 70 μm, which may be caused by the complex effect of temperature and of the environment density. Temperature gradients during ESA in liquid nitrogen contribute to a rapid increase in the penetration depth of anode metal atoms deep into the samples. Further alloying in glycerin causes diffusion processes of saturation of coatings and the base with non-metallic inclusions from the liquid environment, which lead to the appearance of carbide particles on clusters of dislocations formed at the previous stage of processing.