Nanoscale clusters in secondary hardening ultra-high strength steels with 1 and 3 wt% Mo: An atom probe investigation
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Nanoscale clusters in secondary hardening ultra-high strength steels with 1 and 3 wt% Mo: An atom probe investigation R. Veerababu1,2,a), R. Balamuralikrishnan1, S. Karthikeyan2 1 Special Steels Group, Directorate of Special Melting and Processing Technologies, Defence Metallurgical Research Laboratory (DMRL), Kanchanbagh, Hyderabad 500 058, India 2 Department of Materials Engineering, Indian Institute of Science, Bangalore 560 012, India a) Address all correspondence to this author. e-mail: [email protected]
Received: 7 April 2020; accepted: 26 May 2020
In the present study, 3D atom probe was used to study the effect of increased Mo (1–3 wt%) on clustering in secondary hardening ultra-high strength steels. Clusters have been classified into three categories, namely, Type I, Type II, and Type III with the (Cr + Mo)/C ratio of 3.25, respectively. Cluster evolution suggests that size and volume fraction (Vf ) of Type II clusters increase continuously from as-quenched to aged samples, while the number density (Nv) increases in 400 °C aged sample and decreases in 450 and 500 °C samples. On the other hand, Nv and Vf of both Type I and Type III clusters decrease on aging. This work clearly suggests that on aging, Type II clusters, which are close to M2C stoichiometry, become most stable, which may eventually either become M2C precipitates upon prolonged aging or act as potential nuclei for the precipitation of equilibrium M2C precipitates.
INTRODUCTION Secondary hardening ultra-high strength (SHUHS) steels form an important class of structural materials for aircraft landing gear and armor owing to their outstanding combination of high yield strength and fracture toughness along with good stress corrosion cracking resistance. Typical examples of these steels include HY180, AF1410, and Aermet100 [1, 2, 3, 4]. These are quench and temper steels and derive their good combination of mechanical properties from the presence of fine M2C carbides in a highly dislocated lath martensitic structure. The typical heat treatment for these steels consists of austenitizing in the range of 810–900 °C, followed by oil quenching to room temperature and cryogenic treatment using liquid nitrogen. Subsequently, these steels are tempered between 480 and 510 °C for 5 h depending on the composition, which represents a slightly over-aged condition. The typical microstructure of these steels, in the as-quenched (AQ) condition, consists of lath martensitic structure along with small amounts of undissolved primary carbides such as M23C6 and M6C [2]. During tempering at lower temperatures of 425 °C, Widmanstätten cementite would be formed on {110} planes
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of martensite. When tempered in the range of 450–480 °C, the cementite gets coarsened and replaced by fine M2C carbides nucleated predominantly at dislocations, lath, and grain boundaries. The microstructure of the steel in the over-aged conditions (i.e., tempered at temperatures >510 °C) consists of coarsened M2C carbides in a recovered lath martensitic micros
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