Behaviour of Transformation Induced Plasticity of 18%Ni Maraging Steels

Abstract

The behaviour of transformation induced plasticity under tensile test has been experimentally investigated at various temperatures between 300°C and room temperature, and at various strain rates for 18% Ni maraging steels. The results obtained are as follows:
(1) Total elongation up to about 60% is obtained at the optimum condition (strain rate of below 0.03/min and temperature of 225°C for the 210 kg/ mm² class maraging steel or temperature of 275°C for the 245 kg/mm² class one).
(2) The “n value” (work hardening exponent) is not constant with increase in strain, and have two peaks at the optimum condition mentioned above. The first peak indicates the characteristic of work hardening of austenite and the second one depends on the work hardening by transformation.
(3) Total elongation is constant (about 60%) at strain rate of below 0.03/min, decreases with increase in strain rate in the range of 0.03-0.3/min and is constant (about 30%) over 0.3/min at above mentioned temperature. Decrease of total elongation in high strain rate depends on the heat of deformation.
(4) In the double stage tensile test, the optimum temperature in the second stage to get maximum total elongation for the 210 kg/ mm² class maraging steel, which is 20% prestrained at 300°C in the first stage, is 20 degrees lower than that of single stage tensile test, and the range of temperature to get more than 40% total elongation is 6 times of that of sigle stage tensile test.
(5) The tensile strength and optical microstructure at room temperature are not significantly affected by the transformation induced plasticity.