Effect of Austenitic Grain Size and Cold Rolling Prior to Aging on Strain Rate Sensitivity of Tensile Properties of 245 kg/mm² Grade Maraging Steel

Abstract

An investigation has been made to clarify the effects of austenitic grain size and cold rolling prior to aging on the strain rate sensitivity in the tensile properties of a 245 kg/mm² grade maraging steel. Austenitic grain size numbers were varied from ASTM NO. 4 to NO. 12. The temperature range in aging was from 475°C to 525°C. For the purpose of clarifying the effect of cold rolling on the strain rate sensitivity in tensile properties, fine grained (ASTM NO. 1112) plates and coarse grained (ASTM NO. 45) plates were cold rolled to 60% reduction in thickness and then aged at 475°C and 500°C. The cross head speeds in a tensile test were 1 mm/min and 0.005 mm/min.
In aged specimens, it is found that the strain rate sensitivity in elongation is increased with increasing their austenitic grain size. This fact is considered to be due to the enhancement of stress concentrations piled up at grain boundaries in coarse grained specimens in plastic deformation.
In cold rolled and aged specimens whose initial grain size is fine (ASTM NO. 1112), its strain rate sensitivity in elongation is higher than that of specimens with the same grain size aged without cold rolling. On the other hand, in cold rolled and aged specimens whose initial grain size is coarse (ASTM NO. 45), its strain rate sensitivity in elongation is lower than that of specimens with the same grain size aged without cold rolling.