Prediction of gradient microstructure distribution of grinding metamorphic layer in 8Cr4Mo4V steel by colourful oxide films

Grinding-induced thermal loads can alter the near-surface microstructure of hardened bearing steels. This study proposes a semi-quantitative post-process method for associating oxide-film interference colour with the grinding-induced metamorphic layer in 8Cr4Mo4V steel. Furnace specimens were oxidized for 30 s at 200°C–400°C under fixed imaging conditions. Oxide colour was recorded by CCD imaging, optical-equivalent film thickness was estimated using thin-film interference theory, and EDS was used to compare oxygen-enrichment trends. Grinding tests were conducted at depths of cut of 6, 12, 18 and 24  μ m, followed by cross-sectional CCD and FESEM observations and finite-element temperature analysis. The furnace colour changed from pale yellow to blue-grey, with calculated optical-equivalent thicknesses of approximately 125–605 nm. Increasing the grinding depth from 6 to 24  μ m increased the simulated peak surface temperature from approximately 450°C to 760°C and the observed colour-zone depth from approximately 7 to 23  μ m. The deeper colour zones were associated with a transition from predominantly acicular tempered martensite to a cryptocrystalline-martensite-dominated region and a mixed transition zone. A descriptive comparison gave R 2 = 0 . 997 . Because neither physical film thickness nor grinding temperature was measured directly, the proposed relationship represents quantitative trend consistency rather than direct validation. The method is intended for low-cost post-process screening rather than absolute temperature, film-thickness or phase-fraction measurement.