Scientists increased wear resistance and hardness of products from stainless steel

Newswise — Scientists improved the method of plasma electrolytic treatment, in frames of which on the surface of sample from stainless steel they formed oxide coating. They used solutions that contained compounds of nitrogen, boron and carbon, that led to formation of corresponding modified layer under oxide coating.  Thus, nitridation increased wear resistance of material by a factor of 100, and their hardness – fivefold. Addition of carbon also increased wear resistance of sample by a factor of 100, and hardness – in 2.3 times. However, after addition of boron a surface didn’t become harder. The obtained results widen the spere of usage of stainless steel. Results of the research are published in Metals journal.

Products from stainless steel in great amount are used in food and chemical industry, heat power engineering. Such materials are highly plastic, thermal and corrosion resistant, but have low hardness and solidity. It is possible to improve characteristics of stainless steel with the help of plasma electrolyte treatment. In the frames of this method under the influence of electrolytic plasma modified layer, that increases hardness and wear resistance of the material, is formed on the surface of a sample. However existing methods of treatment are laborious and can’t be properly regulated, that significantly complicates their industrial usage.

Scientists from Kostroma State University (Kostroma) and Saint-Petersburgh Academic University (Saint-Petersburg) studied new variants of plasma electrolytic treatment in order to increase hardness and wear resistance of surface of stainless steel. Authors tested saturation with nitrogen, boron and carbon. In the process of treatment scientists used water solutions that carry electrical current and contain ammonium chloride (NH4Cl) with addition ammonia (NH3), boracic acid (H3BO3) and glycerin (C3H5(OH)3). Gradually under the influence of current on the surface of materials formed steam gas covering. The sample was covered with oxide coating under which formed modified layer, that contained compounds of iron with nitrogen, boron and carbon. Treatment was conducted under the temperatures from 650oC to 950oC.

Each type of diffusive impregnation has its own peculiarities. Thus, during nitriding with increasing of temperature the surface became more homogeneous, and roughness decreased in 3.7 times. By this hardness of nitride layers increased fivefold. This increased wear resistance of material by a factor of 100.

Boriding didn’t increase hardness of surface, however, it became less rough and more wear resistant. After carburising samples became in 2.3 times harder than untreated analogues, by this a great number of oxides in the outer surface tightly bonded it with underlying layers. As a result, wear resistance of carburised samples also increased by a factor of 100.

“Surface treatment of alloy steels, for example, stainless steel, demands the use of multi-faceted approach. It is necessary not only to improve one or some characteristics, but also not to worsen other characteristics of materials. The suggested approach showed the efficiency of hardening of products from stainless steel without worsening of other important operating parameters. We are planning to introduce this technology into production in the nearest future”, - tells Sergei Kusmanov, professor, director of department of mathematical and natural sciences of Kostroma State University.

The material has been prepared with the financial support of Ministry of Education of Russia within the federal project «Popularization of science and technology».