Effect of carbon on the corrosion behavior of thermally hardened rolled products

In this work, we studied the corrosion behavior of hot-rolled and heat-hardened rolled products made of carbon steel with different carbon content 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 processes of general corrosion with hydrogen depolarization. The studies were carried out on samples of carbon steel (0.14-0.8% C) in hot-rolled and heat-hardened states. It has been established that the corrosion rate (K) in specimens made of steels in the quenched state is lower than in the hot-rolled state; however, even in this state, with an increase in the carbon content within the studied limits, it increases 40 times. An increase in the corrosion rate with an increase in the carbon content in hot-rolled steel is explained by an increase in the amount of pearlite and an increase in the length of the ferite-cementite interphase boundaries. It was determined that in quenched hardened steel in the carbon content range from 0.14 to 0.80 % C, an increase in the corrosion rate is apparently due to an increase in microstresses (accumulation of excess internal energy) associated with an increase in the tetragonality of the martensite lattice and the appearance of concentration inhomogeneity in the solid solution. It was found that during the course of corrosion with hydrogen depolarization, the dependences of the general corrosion rate of thermally hardened structural steels on the tempering temperatures are described by extreme dependences with a maximum corrosivity in the temperature range of 350-500 oC, while, with an increase in the carbon content in steel, rental increases. It was determined that at the tempering temperature corresponding to the maximum K, the corrosion rate is 1,5-3,6 times higher than in the quenched state, and 1,4-2,6 times higher than in hot-rolled, and with an increase in the carbon content this difference is growing. An increase in the rate of corrosion with hydrogen depolarization in the tempering temperature range of 350-450 oC is apparently associated with the process of carbide formation and the appearance of coherent stresses during the release of dispersed particles. An increase in corrosion losses in this interval with an increase in carbon content is due to an increase in the volume fraction of fine particles of the carbide phase and an increase in the level of elastic microdistortions of the crystal lattice (Δa/a).