Phase Evolution During Mechanical Milling of Pre-alloyed Gas Atomized Maraging Steel Powders and Magnetic Characterization

Abstract

Maraging steels are an important class of high strength steels that exhibit an exciting combination of magnetic and mechanical properties. Past research work, specifically on the magnetic properties, focused on improving the magnetic properties of the bulk form of the steel, fabricated by conventional materials processing and manufacturing. With the recent focus towards additive manufacturing, it is imperative to investigate the structure and magnetic properties of the maraging steel powder and the influence of temperature. In this thesis work, firstly, the structural and magnetic characterization of a commercially available pre-alloyed gas atomized powder was investigated. It comprised of primarily the martensite phase (α) and a small amount of austenite (γ). The powder particle size characteristics, D90 of the as-received powder was estimated as ~21 μm. The saturation magnetization (MS), intrinsic coercivity (HCI), and remanent magnetization (MR) of the as-received powder, at ambient temperature, was ~176 Am2/kg, ~3 kA/m, and ~1.4 Am2/kg, respectively. Thermal treatment of the powder up to 900 K for ~1 h showed an inappreciable change in MS, while the coercivity decreased, suggesting good magnetic properties and promising opportunities to reuse the powder. Subsequently, phase evolution during mechanical milling of the pre-alloyed powder was investigated. Powder milled from 3 h to 8 h comprised nanocrystalline martensitic phase. The estimated grain size was as low as ~20 nm. The MS, HCI, and MR ranged between ~164 Am2/kg and ~169 Am2/kg, ~4.9 kA/m and ~6.7 kA/m, and ~3.4 Am2/kg to ~3.9 Am2/kg, respectively. Milling more than 8 h resulted in the formation of austenite and extraneous intermetallic phases, resulting in the reduction of MS and increase in HCI. At cryogenic temperatures (60 K-300 K), MS (0) (MS at 0 K) and maximum magnetic moment per atom (μH) of the nanocrystalline milled maraging powders were ~ 178 Am2/kg and ~ 1.83 μB, respectively. The thermally treated maraging steel powders retained the nanostructure, and their MS and HCI were comparable to as-received powder.Master of Science in EngineeringMechanical Engineering, College of Engineering & Computer ScienceUniversity of Michigan-Dearbornhttps://deepblue.lib.umich.edu/bitstream/2027.42/150650/1/Ganesh Varma Thotakura - Final Thesis.pdfDescription of Ganesh Varma Thotakura - Final Thesis.pdf : Thesi