Abstract
Effects of alloy compositions on the delayed-failure susceptibility have been studied on Ni-Co-Mo-Ti-Al maraging steels by means of tensile tests with varied strain rates, electron microscopy and fractography.
The results are as follows: Strengthening only by Ti and/or Al additions causes the enhancement of the delayed-failure susceptibility, while the addition of Mo is effective in suppressing the susceptibility by preventing the harmful precipitation along prior austenite grain boundaries. Co also acts as an inhibiter for the delayed-failure when strengthened by Ti and/or Al and Mo, while the addition of Co is not necessary for the tensile strength below 200kgf/mm2. Ni is most effecitive in suppressing the delayed-failure as well as strengthening.
A Co-free 18Ni-2Mo-2Ti maraging steel exhibited the tensile strength as high as 200kgf/mm2 and very low delayed-failure susceptibility.
All fracture surfaces of tensile specimens with high delayed-susceptibility were intergranular at the crack initiation site near the surface of the specimen.
