Effect of Alloying Elements on the Strength and Toughness of the Ultra-high Strength Maraging Steels

Abstract

As an ultra-high strength maraging steel which possesses the tensile strength of more than 2400 MPa, the 13Ni-15Co-10Mo steel is well known. However, it necessitates special treatments such as controlled rolling and repetition of rapid heating and cooling to improve ductility and toughness.
Systematic investigations regarding to the effect of alloying elements on the tensile strength and toughness of ultra-high strength maraging steels are hardly known.
For these reasons, effect of alloying elements, retained austenite and undissolved precipitates on the strength and toughness within the tensile strength range from 2000 MPa to 3000 MPa were investigated. Results are as follows.
1) Relation between the tensile strength and toughness was strongly dependent on the combination of the amount of Ni and Mo.
(a) Ni improved toughness remarkably.
(b) Mo increased tensile strength strongly without drastic decrease of toughness below 8%.
2) Ni and Mo definitely decreased Ms temperature. Decrease of tensile strength after aging due to the formation of retained austenite occured when the Ms temperature decreased below 140°C.
3) Excess addition of Mo increased the solution treatment temperature to dissolve the precipitates, and this made the austenite grain size coarse and decreased the toughness.
4) The 17.5Ni-6Mo steel was selected as the steel which showed the most suitable combination of strength and toughness.