1989 Volume 75 Issue 6 Pages 988-995
Effects of Ni and aging on strength, ductility and fracture toughness of Fe-0.02C-18Cr-(10-30)Ni alloys were investigated at cryogenic temperatures. The main results are as follows;
(1) The aging at 973K for 270ks caused M23C6 carbide to precipitate at grain boundary. The precipitates increased with Ni content.
(2) Tensile strength was affected greatly by Ni content but not by aging because amount of induced α' martensite corresponded to it.
(3) Yield strength at 4K was lowerd by induced martensitic transformation.
This trend was enhanced by aging.
(4) Elongation was explained by Md30. Aging lowered Md30 in metastable steels.
(5) In fracture toughness test, aging caused intergranular fracture and deteriorated KIC regardless of Ni content though KIC increased with Ni content. Its dependence in metastable steels was much larger than in stable steels. The 15Ni% steel had extremely high KIC because TRIP relaxed stress concentration at crack tip.
(6) It seemed to be possible to improve 4K fracture toughness of aged steels which included a number of grain boundary precipitate if stress concentration around precipitates was relieved.