A multiscale analysis of compressive stress-enhanced carbide precipitation in deep cryogenic treated martensitic...

It is well known that the deep cryogenic treatment (DCT) hardens a steel alloy by generating more martensite, yet we find that, for a martensitic steel, the cryogenic treatment has resulted in little more martensite but an improved strain hardening, better elongation, a slight decrease in hardness, and an enhanced abrasive wear resistance. This paper reports a mechanistic study into the rationale for these unconventional results. It is shown that the DCT at - 180 °C followed by tempering at 200 °C significantly altered the kinetics and pathways of carbide precipitation by accelerating carbon diffusion under compressive stresses, enabling η-carbide that preceded M₇C₃ and M₂₃C₆ carbides during tempering. Dilatometric analysis revealed a marked drop in the apparent activation energy for η-carbide formation, from 59 kJ mol⁻¹ in the pre-DCT to 12 kJ mol⁻¹ post-DCT, indicating enhanced nucleation possibly driven by lattice contraction and defect redistribution on-heating. X-ray diffraction, electron microscopy, and dynamic light scattering indicated the formation of η-carbide during DCT, which served as precursors for a refined dispersion of M₇C₃ and M₂₃C₆ carbides during tempering. Thermo-Calc calculations indicated that the driving force for η-carbide nucleation remains favorable after DCT and at low temperatures. Finite element simulations showed that the cryogenic thermal cycle generates compressive surface stresses, part of which is retained as residual stress after DCT. XRD measurements confirmed a reduction in surface stress, while EBSD analysis revealed decreased lattice strain and GND density after DCT, with a slight increase following low-temperature tempering. Therefore, the cryogenically induced carbon diffusion under compressive stresses may have enhanced fine carbide precipitations that result in an improved mechanical performance for a martensitic steel widely used in the mining industry.