Cryogenic processing (or cryogenic treatment or ‘cryotreatment’) includes a range of batch heat treatment processes conducted at temperatures below 193K (-80°C). These processes have been industrially applied since the first half of the Twentieth Century and commercially available for around forty years in the United States and Europe. During this time remarkable improvements (of up to 1257%) in the wear resistance of tool steels have been reported, along with smaller but significant improvements in hardness and other mechanical properties.
While martensitic tool steels have been the focus of the bulk of published research in this field, substantial effects have also been reported in other important engineering materials. However, coherent findings backed up by sound experimental results, analyses and appropriate metallurgical investigations have so far proved elusive. Currently, a significant portion of cryogenic treatment services are applied to automotive brake rotors and industrial cutting tools. Therefore a range of materials used in these applications were subjected to a combination of tribological testing and microstructural analyses to evaluate the effects of deep cryogenic treatment (93K).
Deep cryogenic treatment (DCT) was determined to improve the sliding wear resistance of EN10083 C50R pearlitic carbon steel, and AISI A2, D6 and M2 austenitic (as-cast) tool steels. Qualitative observations suggested that improvements in these tool steels were due to an increase in <100nm carbides following DCT. In the case of pearlitic carbon steel, however, no such observations were made, even following further characterisation of the material. It was theorised that the precipitation of nano carbides, along grain boundaries that were unable to be thoroughly investigated, were instead responsible. Mixed changes in the wear resistance of SAE J431 G10 grey cast iron are reported, thought to be as a result of the degradation of graphite flakes, but with similar beneficial changes as theorised to occur in C50R steel thought likely.
Furthermore, DCT was determined to have improved the abrasive wear resistance of SHM H13A cobalt-bonded tungsten carbide (WC-Co) turning inserts, when used to machine AISI 1045 steel, although indications of reduced toughness were also observed. DCT was determined to primarily effect the Co-binder phase, resulting in greater resistance to WC grain removal, but greater vulnerability to crack propagation.
In discussing these results, methodologies necessary for understanding the effects of cryogenic treatments are reviewed from an industrial or ‘applications-based’ and a scientific or ‘materials-based’ perspective. Finally, the significance of these findings was critically assessed, with a range of improvements to methodologies suggested.
