A Study On Cryogenic Mechanical Properties And An Application Of High Strength Steels

This dissertation is focused upon a study of the cryogenic mechanical properties andconstitutive relationship of the high strength40CrNi2Mo and42CrMo steels, with the effects ofheat treatment, testing temperature and strain rate on metal flow and mechanical properties aswell as the physical mechanism investigated. Meanwhile, the problem of the excavator circulartrack failure is also solved.The author studies the effects of initial microstructures and heat treatment on cryogenicmechanical properties of40CrNi2Mo steel. The results show that the mechanical properties ofthe steel treated, respectively, by quenching+tempering and sub-critical quenching are close atroom or lower temperatures. With decrease of temperature, the strength of the steel is increasedwhile the plasticity is decreased. The strength has a relationship with temperature by an indexfunction, but a linear relationship as the steel is tempered at680C. The impact toughness isrelated to temperature by S-type curve with the Charpy energy for sub-critical quenched steel ishigher than quenched and tempered one, indicating that for the steel tempered at highertemperatures sub-critical quenching is more advantageous than over-critical quenching.SHPB method is applied to experimentally investigate the flow performance of40CrNi2Mosteel at low temperatures and strain rates. The yield strength is little dependent on the strain rateat the ambient temperature, whereas it tends to decrease with decrease of temperature at identicalstrain rates.Unidirectional tension of42CrMo steel is carried out in the range of–150to700C, bywhich a sharp increase of stress on the stress-strain curves for the case of low strain rates can befound, showing the minimum value of the stress increment in a temperature range of0to100C.Both of the flow stress and strength of the steel decrease depending on increase of thetemperature, but a stress platform (temperature independence) appears in the range of0to200C, which is resulted from dynamic strain aging.A constitutive model of metal flow is established based on dislocation dynamics, and isapplied in simulating the plastic deformation of42CrMo and HSLA-65steels. The flow stress isconsiderably influenced by athermal and thermal-activated factors, among which the thermally-activated stress exerts as the temperature is below a critical value. The effect of viscous stress isnotable only at high strain rates. The initial microstructure greatly influences the microstructures and mechanical propertiesof40CrNi2Mo steel heat-treated by medium frequency induction heating and quenching. As aprior heat treatment, quenching-tempering is one of efficient ways to improve the strength,toughness and wear resistance of steels.40CrNi2Mo steel is used to manufacture circular tracksfor excavators, which is heat-treated by Q&T+medium frequency induction surface hardening,leading to an improvement in the performances and prevention of early failure of the circulartracks.