Effect of Microstructures on the Ductility and Toughness of Ultra-high Strength Maraging Steels

Abstract

A maraging steel of 17.5%Ni-6%Mo-12.5%Co-1%Ti shows superior ductility and toughness at ultrahigh tensile strength level after conventional heat treatment (solution treatment and aging). In steels with higher nickel content from 19% to 24% to the above mentioned base composition, austenite is retained at room temperature after solution treatment due to lowering of Ms temperature. The high nickel maraging steels with retained austenite show various microstructures (lath martensite, lenticular martensite and untransformed austenite) after cold rolling or sub-zero treatment. Relation between ductility, toughness and various microstructures is discussed.
Principal results are as follows :
1) Retained austenite, formed owing to the increase of nickel content, transformed to lath martensite after cold rolling at room temperature. In alloys with 100% lath martensite structure obtained from above treatment, the beneficial effect of nickel on the increase of strength and toughness was confirmed up to 20%.
2) Retained austenite had no effect on the ratio of notched tensile strength to tensile strength after aging.
3) Lenticular martensite was observed in the steels with the nickel content of more than 20% after sub-zero treatment. Lenticular martensite was ascertained to decrease toughness remarkably after aging.
4) Cold rolling in the range of reduction of area from 30% to 60% was effective to increase toughness. The maraging steel with 20% nickel, cold rolled with the reduction of area of 60%, had the tensile strength of 2 700 MPa with the ratio of notched tensile strength to tensile strength of more than 1.0.