ASSESSMENT OF THE INFLUENCE OF MANGANESE CONTENT ON THE CORROSION OF THERMALLY HARDENED METAL

Abstract

In this work, we studied the corrosion behavior of hot-rolled and heat-hardened rolled products made of low-carbon steel with different manganese content in it in order to select the optimal conditions for its heat treatment to obtain a satisfactory combination of strength and corrosion resistance during the development of general corrosion processes with hydrogen polarization. A promising way to save metal in construction, mechanical engineering and transport, as well as to reduce the consumption of alloying elements and deoxidizers as a result of replacing, in some cases, low-alloy structural steels with simpler and less costly low-carbon ones, is the use of hardening heat treatment, especially using the heat of rolling heating. However, the widespread use of thermally hardened rolled products of thinner thickness in order to save metal in structures and structures exposed to the surrounding, as a rule, corrosive environments of various nature and intensity, comes across a completely insufficient study of the corrosion behavior of such a material as an energetically more unstable system than a conventional hot rolled steel products. It is the hot-rolled state that is attributed to the scientific data on the corrosion behavior of carbon and low-alloy structural steels, which are far from systematic and not unanimous in their assessments, and the data on the corrosion of heat-hardened rolled steel, as a metastable system with increased free energy, are limited and contradictory. Therefore, the study of the laws governing the behavior of thermally hardened rolled metal as a metastable metal system is of great practical importance for the choice of processing technology and materials that would provide a satisfactory combination of strength and ductility of thermally hardened rolled metal. The studies were carried out on samples of low-carbon steel of laboratory heats, having in their chemical composition approximately the same content of C, Si, S, P, Cu and different content of Mn (0,66, 1,00 and 1,68 % by mass) in hot-rolled and thermally hardened conditions (quenching from 880-900 oC, and then tempering was carried out in the temperature range 200-700 oC every 25-50 oC for 1 hour, followed by cooling in calm air). Corrosion tests were carried out on polished and degreased samples constantly immersed in 1 N H2SO4 solution at room temperature for 72 h. It was found that alloying low-carbon steel with manganese (up to 2 %) leads to an increase in the corrosivity of the metal in a thermally hardened state, shifts the peak of corrosivity during tempering towards a lower temperature (from 475 to 375 oC) and increases corrosion losses in the region of the peak in 1, 6 times. It is shown that after quenching and high tempering (at temperatures > 600 oC), on the contrary, in steel with an increased manganese content, corrosion losses, for example, in an acidic environment, are significantly reduced due to the fact that manganese accelerates the processes of coagulation of carbide particles. It is recommended to carry out heat hardening treatment of structural steel with an increased (1,5 %) manganese content while satisfying the technical requirements for a combination of strength and satisfactory corrosion durability; it is advisable to prescribe a tempering (self-tempering) of at least 600 oC for heat hardening. In this case, with a significant gain in strength (increase by 20-25 %), it is possible to achieve a higher corrosion resistance of the metal in comparison with the hot-rolled (unhardened) state.