Mechanism of Secondary Hardening in Rapid Tempering of Dual-Phase Steel

• Published in: Metallurgical and materials transactions. A, Physical metallurgy and materials science Vol. 45; no. 13; pp. 6153 - 6162
• Main Authors: Saha, Dulal Chandra, Nayak, Sashank S., Biro, Elliot, Gerlich, Adrian P., Zhou, Y.
• Format: Journal Article
• Language: English
• Published: Boston Springer US 01.12.2014; Springer; Springer Nature B.V
• Subjects: Applied sciences; Bainite; Carbides; Carbon; Characterization and Evaluation of Materials; Chemistry and Materials Science; Decomposition; Dual phase steels; Ductility; Exact sciences and technology; Ferrite; Lasers; Materials Science; Mechanical properties; Metallic Materials; Metals. Metallurgy; Microstructure; Nanotechnology; Secondary hardening; Steel; Steels; Structural Materials; Surfaces and Interfaces; Temperature; Tempering; Thin Films; Transmission electron microscopy; Canada; Carbide; Bainite; Select Area Diffraction Pattern; Martensite; Cementite; Hardening; Secondary hardening; Heat treatment; Tempering; Dual phase steel; Mechanism
• ISSN: 1073-5623; 1543-1940
• DOI: 10.1007/s11661-014-2591-8

Dual-phase steel with ferrite-martensite-bainite microstructure exhibited secondary hardening in the subcritical heat affected zone during fiber laser welding. Rapid isothermal tempering conducted in a Gleeble simulator also indicated occurrence of secondary hardening at 773 K (500 °C), as confirmed by plotting the tempered hardness against the Holloman–Jaffe parameter. Isothermally tempered specimens were characterized by analytic transmission electron microscopy and high-angle annular dark-field imaging. The cementite (Fe 3 C) and TiC located in the bainite phase of DP steel decomposed upon rapid tempering to form needle-shaped Mo 2 C (aspect ratio ranging from 10 to 25) and plate-shaped M 4 C 3 carbides giving rise to secondary hardening. Precipitation of these thermodynamically stable and coherent carbides promoted the hardening phenomenon. However, complex carbides were only seen in the tempered bainite and were not detected in the tempered martensite. The martensite phase decomposed into ferrite and spherical Fe 3 C, and interlath-retained austenite decomposed into ferrite and elongated carbide.