Effetto del trattamento preliminare sulle proprietà dell'acciaio AISI M2 sottoposto a trattamento criogenico

Cryogenic treatment is widely used to enhance mechanical and physical properties of tool steels, hot work steels and high carbon steels. The application of cryogenic treatment on cutting tools improves wear resistance, hardness, dimensional stability, cutting tool durability, tool life and it reduces tool consumption, leading to a general production cost reduction. These benefits are achieved by deep cryogenic treatment because it decreases retained austenite content and it promotes the precipitation of fine carbides uniformly dispersed in martensite matrix. Retained austenite is a soft and unstable phase that reduces steel hardness and could be converted into martensite in working conditions and under stress, forming brittle (not tempered) martensite, with an increase of volume of 4%, inducing local stresses. Cryogenic treatment, by transforming retained austenite to martensite, improves dimensional stability. In addition to the transformation of retained austenite to martensite, secondary and fine carbides are formed in the structure, increasing mechanical properties, toughness and wear resistance. Cryogenic treatment generate an high internal stress state due to thermal stresses and transformation of martensite into austenite. Furthermore, thermal stresses increase the number of structural defects and the martensite, carbon-supersaturated, becomes unstable. Carbon atoms move towards the new structural defects created, martensite is decomposed and carbide precipitation takes place during the warming up phase to room temperature, producing a reduction of residual stress and resulting in an homogeneously dispersed network of tiny carbides. the greatest improvement in properties is obtained by carrying out the deep cryogenic treatment between quenching and tempering. However, in case of tool steels, an improvement can be obtained even performing cryogenic treatment at the end of the usual heat treatment cycle, i.e. to treat the finished tools. This last solution is more flexible than the previous one and can extend the use of the treatment to many practical applications. Cryogenic treatment barely changes the tensile mechanical properties and hardness of tool steel and hot work steel. But it is worth noting that cryogenic treatment notably improved fracture toughness of such steels because a fine, homogeneously dispersed carbide precipitation and a tougher martensite matrix are formed (with lower carbon content). In this framework, toughness measurement is an important tool to assess the effectiveness of the cryogenic treatment on such steels, but standard methods require careful sample preparation and dedicated equipment, while a simpler technique could be easily adopted as a quality control tool, as an alternative to ASTM E399 e BS 5447 standard method. Akono et al. in [14,15] proposed an alternative novel technique to measure the fracture toughness by scratch testing. Hence, aim of the current study is to investigate the effect of post-tempering cryogenic treatment on the microstructure and mechanical properties of AISI M2 steel. The samples were treated with two different cycles before cryogenic treatment: 1. quenching, cooling at-80°C, three tempering; 2. quenching, three tempering. All the samples were cryogenically cooled for 4h, 12h and 24 h to investigate also the influence of the time at-196°C. The results of microstructure, wear tests, X-ray diffraction, instrumented microhardness and scratch tests shows that posttempering cryogenic treatment promotes the precipitation of fine and homogeneously dispersed carbides, improves wear resistance and fracture toughness, without decreasing hardness. Also the heat treatment cycle before cryogenic cooling influences the wear and mechanical properties of the steel.