Creep Evaluation of Solution Annealed and Aged Maraging 300 Steel

Maraging steels are well-known for their outstanding combination of strength, fracture toughness and machinability in the solution annealed condition, minimum distortion during subsequent aging. These steels are used in industrial applications that demand these properties,  such as in aerospace technology, in the manufacturing of aircraft structural components, lading gear, fasteners, rocket motor cases, missile casings, and others. Data available on the mechanical properties of maraging steel at elevated temperature are very few and most of the studies were confined to tensile properties or stress-rupture studies up to 550 ºC. Specialized applications of the steel occasionally demand short-time exposures at unusually high temperatures and it is desirable to have data on the creep behaviour of the material during such service conditions. In the present investigation, maraging steel (300 grade) was solution annealed at 820 °C during 1 hour, followed by aging at 480 °C during 3 hours. Microstructural changes are examined with the help of optical microscope, scanning electron microscope (SEM) and X-ray diffraction (XRD), showing a typical 18Ni martensite microstructure. Microhardness after the aging treatment was improved to 604 ± 18 HV compared with the 331 ± 5 HV of the material only solution annealed. Creep tests of the material solution annealed and aged were performed under load creep tests on a standard creep machine at temperature range of 550 ºC to 650 ºC and stress range of 200 to 500 MPa. Creep parameters, such as state creep rate (ε·_s), time to rupture (t_r),  stress exponent from secondary creep (n), activation energy (Q_c), final strain (ε_f) and reduction of area (RA) are determined. Analysis of the steady-state creep are consistent with the creep mechanisms of dislocation climb process. Fracture surfaces of specimens after creep tests were analyzed by SEM showing ductile fracture in the tertiary stage.