Effect of Cryogenic Treatment on the Abrasive Wear Resistance of Engineering Alloys

Deep cryogenic treatment (DCT) is a heat treatment that utilises low temperatures (below 113 K) and is usually applied in conjunction with conventional heat treatments. Since this heat treatment is relatively new commercially, there are few studies about it in the literature, so a better understanding of its effects in the abrasive wear resistance of engineering alloys can be beneficial for its broader utilisation in industry.
The work presented in this thesis aims to investigate the effect of DCT on the mechanical performance of a selected range of engineering alloys, especially in abrasive wear resistance. To achieve this, alloys with broad application and different characteristics were selected: two types of austenitic stainless steel (AISI 304L and AISI 316L), a type of martensitic stainless steel (AISI 440C), a low alloy pressure vessel steel (SA508 Gr 4N), a titanium alloy (Ti-6Al-4V) and an additively manufactured titanium alloy (Ti-6Al-4V obtained by electron-beam melting). The changes in mechanical performance were assessed through Vickers hardness and microhardness tests, abrasive wear test, and microstructural characterisation. Analyses of the wear scar and its surface were performed using advanced microscopy techniques and by 3D profilometry of the surfaces.
DCT was performed on those alloys and its effects were analysed. Its beneficial effects were found in the increased hardness of AISI 440C and AISI 304L, the former due to the transformation of retained austenite into martensite and the latter due to the nucleation of nano-martensite particles. For the AISI 304L, the DCT also presented a very significant increase in the wear resistance, to the extent of changing the wear morphology of the wear scar. The SA508 Gr 4N specimens showed an increase in their corrosion resistance, an effect not previously reported. All the mechanical effects, wear scar features and microstructure characteristics are presented and thoroughly analysed from a tribological perspective.