Atom-Probe Tomographic Investigation of Austenite Stability and Carbide Precipitation in a TRIP-Assisted 10 Wt Pct Ni Steel and Its Weld Heat-Affected Zones

• Published in: Metallurgical and materials transactions. A, Physical metallurgy and materials science Physical metallurgy and materials science, 2018-04, Vol.49 (4), p.1031-1043
• Main Authors: Jain, Divya, Seidman, David N., Barrick, Erin J., DuPont, John N.
• Format: Article
• Language: English
• Subjects: Austenite; Carbides; Characterization and Evaluation of Materials; Chemical precipitation; Chemistry and Materials Science; Chromium; Heat affected zone; Heat treating; Manganese; Martensite; Martensitic stainless steels; Martensitic transformations; Materials Science; Mathematical analysis; Mechanical properties; Metallic Materials; Metallurgy; Microhardness; Microstructure; Molybdenum; Nanotechnology; Nickel; Precipitates; Structural Materials; Surfaces and Interfaces; Thermal resistance; Thermal stability; Thin Films; TRIP steels
• ISSN: 1073-5623; 1543-1940
• DOI: 10.1007/s11661-018-4470-1

Newly developed low-carbon 10 wt pct Ni-Mo-Cr-V martensitic steels rely on the Ni-enriched, thermally stable austenite [formed via multistep intercritical Quench-Lamellarization-Tempering ( QLT )-treatment] for their superior mechanical properties, specifically ballistic resistance. Critical to the thermal stability of austenite is its composition, which can be severely affected in the weld heat-affected zones (HAZs) and thus needs investigations. This article represents the first study of the nanoscale redistributions of C, Ni, and Mn in single-pass HAZ microstructures of QLT -treated 10 wt pct Ni steels. Local compositions of Ni-rich regions (representative of austenite compositions) in the HAZs are determined using site-specific 3-D atom-probe tomography (APT). Martensite-start temperatures are then calculated for these compositions, employing the Ghosh-Olson thermodynamic and kinetics approach. These calculations predict that austenite (present at high temperatures) in the HAZs is susceptible to a martensitic transformation upon cooling to room temperature, unlike the austenite in the QLT -treated base-metal. While C in the QLT -treated base-metal is consumed primarily in MC and M 2 C-type carbide precipitates (M is Mo, Cr, V), its higher concentration in the Ni-rich regions in the HAZs indicates the dissolution of carbide precipitates, particularly M 2 C carbide precipitates. The role of M 2 C carbide precipitates and austenite stability is discussed in relation to the increase in microhardness values observed in the HAZs, relative to the QLT -treated base-metal. Insights gained from this research on austenite stability and carbide precipitation in the single-pass HAZ microstructures will assist in designing multiple weld cycles for these novel 10 wt pct Ni steels.