Influence of Prolonged Heating on Thermal Softening, Chemical Composition, and Evolution of the Nanocrystalline Structure Formed in Quenched High-Carbon Steel upon Friction Treatment

摘要

Methods of transmission and scanning electron microscopy, X-ray diffraction analysis, micro-hardness, and wavelength dispersive X-ray microanalysis have been used to investigate the changes (in the process of vacuum treatment for 10-1200 min at temperatures of 350-550°C) in the structure, microhard-ness, and chemical composition of the surface layers of steel U8 (0.83 wt % C) in the initial quenched state and after friction treatment in the argon medium under the conditions of sliding friction using a like spherical indenter-flat sample pair. It has been shown that the layer nanostuctured by friction treatment possesses an enhanced resistance to thermal softening compared with the undeformed quenched steel, not only upon relatively short heating (1-2 h), but also prolonged heating (to 20 h) at temperatures of 350,450, and 550°C. This is due to the retention of predominantly nanocrystalline structure in the deformed layer upon the prolonged heating to a temperature of 350°C by retarding the processes of the formation of carbide particles and recovery in the a phase and by decelerating the development of recrystallization, including the absence of anomalous growth of separate recrystallized grains upon prolonged high-temperature holdings. After vacuum annealing for 10-1200 min at a temperature of 350°C, an increase was revealed in the carbon concentration by 0.2-0.4 wt % in a layer up to 1 μm thick on the surface of quenched eutectoid steel deformed by friction action.