Characterization of Aging Behavior in M250 Grade Maraging Steel Using Ultrasonic Measurements
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M250 grade maraging steel, by virtue of its excellent mechanical properties, i.e., ultrahigh yield strength combined with good fracture toughness,[1] is the most preferred structural material for critical applications in advanced technologies. In addition to the aforementioned properties, its high strength to weight ratio, good weldability, and easy machinability in the solutionannealed condition and dimensional stability during aging make this material an ideal choice for critical rocket motor casing applications in aerospace industries. The aging behavior of maraging steels has been extensively studied.[2–12] It has been reported that depending upon the aging duration, the steel undergoes systematic characteristic microstructural changes. The early aging period is characterized by recovery of martensitic structure and hardening due to precipitation of hexagonal Ni3Ti intermetallic precipitates. The formation of Ni3Ti takes place rapidly due to fast diffusion of titanium atoms.[3,4,5] The intermediate aging period is characterized by reversion of austenite accompanied by precipitation of hexagonal Fe2Mo intermetallic phase. These two processes, occurring at the intermediate aging
K.V. RAJKUMAR and A. KUMAR, Scientific Officers ‘‘E,’’, T. JAYAKUMAR, Head, NDE Division and B. RAJ, Director, are with the Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India. Contact e-mail: [email protected] K.K. RAY, Professor, is with the the Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur 721 302, India. Manuscript submitted June 14, 2006. 236—VOLUME 38A, FEBRUARY 2007
period, affect hardening in the opposite manner; thus, overall hardening levels off after reaching a maximum. Recent studies revealed that the maximum hardening of 18 wt pct nickel maraging steel is due to the combined presence of Ni3Ti and Fe2Mo intermetallics.[3] A decrease in hardening, observed during longer aging durations, is attributed essentially to the formation of reverted austenite rather than precipitate coarsening. The amount of reverted austenite has been reported to increase with an increase in aging temperature and time.[2] M250 maraging steel components (casings) are subjected to solution annealing at 1093 K for 1 hour followed by aging at 755 K for 3 hours. This heat treatment results in the best combination of mechanical properties, i.e., ultrahigh strength coupled with good fracture toughness due to precipitation of intermetallic phases in low-carbon soft martensitic matrix.[5–12] Coarsening of the intermetallic precipitates in addition to reversion of martensite to austenite at longer aging periods affects both the tensile and fracture properties of these steels. While elastic properties of most of the structural materials differ very marginally, the aging of maraging steel exhibits variation of the elastic modulus by about 10 pct.[13] As propagation of ultrasonic wave depends on the elastic properties of a material, ultrasonic velocities would be suitable for characterizing the aging be
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