On the microstructural evolution pattern of 300 M steel subjected to surface cryogenic grinding treatment

In the present work, an ultrahigh strength 300 M steel was treated by the surface plastic deformation with surface cryogenic grinding treatment (SCGT) technique. The SCGT was carried out by rotational grinding wheel embedded with numerous zirconium corundum abrasive grits, which contributes to a decreasing distribution of strain, strain rate and strain gradient from the treated surface to the matrix. The depth-dependent microstructural evolution was systematically evaluated from micrometer to nanoscale and in particular, the deformation mechanisms and texture alternation were addressed. It was identified that the microstructural evolution starts from the formation of dislocation structures, that is subsequently followed by the distorted microstructures featured as the origination and evolution of ultrafine laminates and ultrafine grains in the form of dislocation activities. Nanocrystalline was generated in the topmost surface region and nanoparticle precipitates induced by the SCGT process occur within a few microns of the treated surface. In SCGT, a very high strain rate of over 10(6) s(-1) and a strain gradient of over 0.2 mu m(- 1) were induced on the topmost surface of 300 M steel sample, which impels the martensite-matrix lamella extended into the elongated structure and finally transformed into the nanosized grains during processing. A dislocation slip-dominated process that was supplemented by the mechanical twinning governs the microstructural evolution pattern during the SCGT process, the nucleus of dynamic recrystallization initiated from the pre-existing dislocation cells were observed, and the transformation from martensite to austenite was effectively suppressed. The increase of grain size from nanoscale to submicron and finally microscale contributes to a decrease of microhardness by 25.6% (from 7.8 GPa at the treated surface to 5.8 GPa at a depth of over 100 mu m from the processed surface).

Automated summary

In this study, an ultrahigh strength 300 M steel was subjected to surface plastic deformation treatment using surface cryogenic grinding treatment (SCGT) technique. The microstructural evolution began with the formation of dislocation structures, followed by the development of ultrafine laminates and grains. SCGT induced a very high strain rate and strain gradient, leading to the transformation of martensite-matrix lamella into nanosized grains. The increase in grain size from nanoscale to microscale resulted in a significant decrease in microhardness.