Calculation of crack resistance of steel 20HL

Crack resistance of steel characterizes the reserve of technological strength at casting cooling as an exceeding the stresses of the temporary tear resistance at different cooling rates. Tests were performed on the complex for physical simulation of thermos-mechanical processes Gleeble 3800 at temperatures from to 1300 °C to 20 °C. At first, the sample was heated up to 1300 °C, then cooled to the given temperatures with various rates, after those tensile tests were conducted. Investigations were performed for steel 20HL at various cooling rates. We carried out dilatometry tests and found critical points of structural transformations at cooling of steel 20HL. We performed high-temperature tensile tests at the Gleeble System 3800 and defined the tensile strength and modulus of elasticity at temperature up to 1300 °C. Relaxation tests were carried out. The paper presents the results of the stress increase due to shrinkage at different cooling rates for the clamped sample, which simulates the condition of full deceleration of the shrinkage. Stresses decrease at the change of the size of the crystal grid in the transition zone during decomposition of austenite. Thus, dangerous temperatures for cracks formation are temperatures of the beginning of the shrinkage, the beginning of the ferrite-perlite transformation and the room temperature. According to this, the reserve of technological strength i.e. the crack resistance of steel increases in the transition zone and then decreases in ferrite-perlite zone. The highest values of stresses occur in samples of two-phase steel 20HL at two temperatures: temperature of the beginning of the phase transformation (tн(γ→α)) and the room temperature (20 °C). However, their level does not exceed the temporary tear resistance σв(t). A diagram of crack resistance of steel 20HL at high temperatures is obtained.