Characterization of the microstructure evolution and subsurface hardness of graded stainless steel produced by different mechanical or thermochemical surface treatments

The work presented in this paper is dedicated to a multi-scale characterization of plastically graded materials obtained with mechanical (ultrasonic shot peening) and thermo-chemical (nitriding followed by over-carburizing, nitrogen ion implantation, and low-temperature carburizing) surface treatments. The benefits of several characterization techniques (scanning electron microscopy with electron backscattering diffraction, spectrometry (glow discharge optical emission spectrometry and energy-dispersive X-ray spectrometry), instrumented indentation, and X-ray diffraction), at different scales, and the possible existence of a signature of the studied surface treatments on the material are examined. It is found that nitriding followed by over-carburizing has the largest microstructure signature: there is a clear correlation between changes in the grain sizes, the volume fraction of precipitates, the carbon content and the hardness profile. In contrast, specimens subjected to low-temperature carburizing and ion implantation show no changes at the microstructure scale. However, for both treatments, hardness varies over the same depth as the chemical composition. Specimens subjected to ultrasonic shot peening present little or no microstructural changes. For the specimen treated with the highest intensities, the depth of the area showing high misorientations is different from the depth over which hardness varies. Thus, the sole observation of the microstructure changes leads to the identification of a depth impacted by the treatment smaller than the one identified using the misorientations. (C) 2016 Elsevier B.V. All rights reserved.