Effects of Aging Conditions and Strain Rate on Elongation of 245kg/mm² Grade and 210kg/mm² Grade Maraging Steels

Abstract

Tensile strength and elongation of 18% Ni-type maraging steels, the nominal strength levels of which are 245kg/mm² and 210kg/mm², have been investigated as a function of aging time. The temperature range in aging was from 575°C to 400°C. A cross-head speed in the tensile test was varied from 1.0mm/min to 0.005mm/min. The change in electrical resistivity at liquid nitrogen temperature has also been measured.
Brittle fracture which is caused by decreasing the cross-head speed is observed in the 245kg/mm² grade maraging steel in the early stage of aging in the temperature range from 500°C to 400°C. The decrease in elongation is also observed in the tensile test of the cross-head speed of 0.005mm/min in the 210kg/mm² grade maraging steel in the early stage of aging from 425°C to 400°C. The brittle behavior in low strain rates is reduced by prolonging aging time. Activation energy is calculated to be about 80 kcal/mol for the 245kg/mm² grade maraging steel from the relation between reciprocal absolute aging temperatures and times giving the removal of the brittle behavior at the cross-head speed of 0.005mm/min. It is noted that aging temperature and time are important for the removal of the brittle behavior in low strain rates.