The Influence of Plasma Treatment Regimes on the Distribution of Nonmetallic Inclusions in Steel

Abstract

This article is devoted to the problem of transformation of nonmetallic inclusions in welded joints under the action of a plasma arc. The possibility of using a plasma arc to improve the quality characteristics of a welded seam is considered. The quantity, shape, size and distribution of nonmetallic inclusions in the weld metal before and after plasma treatment by the methods of quantitative metallography, as well as redistribution of nonmetallic inclusions over the thermal exposure zone are investigated.A significant difference was noted between the volume fraction of nonmetallic inclusions in the weld zone and in the zone of thermal action during plasma treatment of 16G2AF steel. In 100% of the samples examined in the thermal exposure zone, the total amount of nonmetallic inclusions is 20-30% higher than the number of inclusions in the weld zone, and this difference increases with increasing running energy. It is established that when the polarity of the plasma treatment changes from direct to reverse, an increase in the volume fraction of inclusions is observed, both in the weld zone and in the zone of thermal action. It is noted that irrespective of the polarity and protective gas with increasing current to 200 A, the volume fraction of nonmetallic inclusions decreases. And with a current strength of more than 200 A, the number of inclusions again increases.When carbon dioxide is used as the protective gas, a predominantly globular (rounded) form of inclusions characteristic of oxides and oxysulfides is observed. When using nitrogen as a protective gas in the weld metal, the appearance of faceted particles of complex shape -nitrides and carbonitrides -is observed.Non-metallic inclusions larger than 10 µm can be serious stress concentrators -the stress level near the inclusion may exceed the yield strength of steel and even reach the ultimate strength.The rate of floating of non-metallic inclusions depends on their size, the viscosity of the liquid metal, the difference in the specific gravity of the particle and the metal. The velocity of the particles ascending is the greater, the bigger the particle, the less its density. The rate of emergence of slag particles is significantly affected by the presence of convective flows in the metal, the release of bubbles from the metal that mix the metal and entrain the slag particles to the surface of the metal bath.Keywords: nonmetallic inclusions of the investigated samples, welded seam, heat affected zone, plasma treatment, direct polarity, reverse polarity, shielding gas, deformation development, particle emergence rate.