Influence of Isothermal Hardening Temperature on Strengthening of High-Strength Irons during Deformation

The influence of isothermal hardening temperature on mechanical properties of ADI materials is investigated. Heat treatment of the experimental samples consisted of heating above the temperature of conversion of the ferrite component into austenite and isothermal hardening at temperatures from 280 to 380 °C. Liquid tin was used as the quenching medium. In the specified temperature ranges, the plastic characteristics and strength parameters of the samples depending on the heat treatment modes were studied. Particular attention is paid to the parameters of deformation hardening. It is established that at isothermal hardening temperatures in the range of 330…360 °C during plastic deformation, the TRIP effect appears, the appearance of which is accompanied by a high rate of hardening due to the conversion of residual austenite into martensite. Strengthening of samples at plastic deformation has 2 stages. At the first stage, the hardening takes place according to the traditional mechanism due to the plastic deformation of residual austenite. With increasing load, the basis of strengthening is the TRIP effect. Plastic characteristics change with increasing tempering temperature. At 280 °C, the strength and hardness of the metal is maximum, and plastic characteristics are minimal. The optimum mechanical properties, including fatigue resistance, is observed in the temperature range 330…360 °C. For parts of tillage machinery that work at low loads (cultivator legs) should be used hardening on the lower bainite. In this case, the hardness plays a more important role.