Abstract
Plasma nitriding of a solution annealed and aged 300 grade maraging steel was studied aiming to increase the creep resistance. The surface microhardness reached 1,140 HV, producing 50 µm layer composed of e-Fe3N and γ'-Fe4N nitrides at the uppermost sample layer. The inner core remained unaltered presenting typical plate-like martensite microstructure of maraging steels with average microhardness of 604 HV. Surface RMS roughness in the nanometric scale increase from 52 nm to 71 nm. The continuous layer of iron nitrides seems to behave as a barrier for oxidation and for inward oxygen diffusion improving the creep resistance by reducing the steady-state creep rate (es) in 52-65% when compared with the literature results. Dominant creep mechanism is controlled by dislocations climb. Fracture surfaces of specimens presented ductile failure consisting of equiaxed and bi-modal dimples in the fibrous zone surrounded by 45o shear lip. Nitrided sample presented a reduced ductility, associated to the hard surface layer.
Highlights
The 18-percent nickel 300 grade maraging steels belong to a family of iron based alloys that are strengthened by a combination of martensite formation during a solution annealing followed by an aging treatment
The following concluding remarks could be drawn based on plasma nitrided and un-nitrided 300 grade maraging steel:
Creep resistance of nitrided samples improved from 52-65% when compared to un-nitrided samples, and it seems to be associated with the continuous layer of iron nitrides that behave as a diffusion barrier for inward oxygen diffusion into the alloy reducing the oxidation rate
Summary
The 18-percent nickel 300 grade maraging steels belong to a family of iron based alloys that are strengthened by a combination of martensite formation during a solution annealing followed by an aging treatment. Since 1959 when their development was announced, maraging steels evoked tremendous interest, especially in the aerospace world, due to the combination of structural strength and fracture toughness in a material that was, at the same time, readily weldable and easy to heat-treat. 300 grade maraging steel is a member of ironnickel based alloy family with yield strength of 300 ksi[1,2,3,4]. 15-18 In this study, the creep resistance and fracture surface of a plasma nitrided maraging 300 steel is evaluated and compared with the results obtained by Reis et al.[18] for the same material but without surface treatment Short-term Creep Properties and Fracture: Surface of 18 Ni (300) Maraging Steel Plasma Nitrided properties of maraging steel at elevated temperature under creep are scarce3, . 15-18 In this study, the creep resistance and fracture surface of a plasma nitrided maraging 300 steel is evaluated and compared with the results obtained by Reis et al.[18] for the same material but without surface treatment
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