Formation of a Heterophase Structure in Low-Alloyed Steel by Means of Application of Innovative Technology of Heat Treatment by ‘Quenching and Partitioning’

V. І. Zurnadzhy$^{1}$, V. G. Efremenko$^{1}$, V. G. Gavrilova$^{1}$, R. O. Kussa$^{1}$, А. V. Efremenko$^{1}$, V. V. Kudin$^{2}$, M. V. Pomazkov$^{1}$

$^{1}$State Higher Education Institute ‘Pryazovskyi State Technical University’, 7 Universytets’ka Str., UA-87555 Mariupol, Ukraine
$^{2}$Zaporizhzhya National Technical University, 64 Zhukovsky Str., UA-69063 Zaporizhzhya, Ukraine

Received: 18.04.2018; final version - 17.10.2018. Download: PDF

The article contains a description of the phase–structural composition and mechanical properties of low-alloyed steel 60Si2CrVА (0.53% С; 1.46% Si; 0.44% Mn; 0.95% Cr; 0.10% V; 0.016% S; 0.013% P) subjected to ‘Quenching and Partitioning’ (Q-n-P) heat treatment. This treatment is known for notable improving the complex of mechanical properties in low-alloyed steels that is beneficial for steel cost reducing. Treatment mode included: a) austenitization at 880°C; b) quenching to the temperature ‘Q’ (240, 200, 160°C) in the bath of Wood’s alloy melt; c) holding at ‘Partitioning’ temperature (270, 300°C) in a bath with a Pb–Sn alloy melt for 300–3600 s to partition the carbon from fresh martensite to austenite; d) final cooling in a quiescent air. The present work is carried out using SEM microscopy (Ultra-55, Carl Zeiss), TEM microscopy (JEM-100-C-XII, Jeol), x-ray diffraction (Pro-IV, Rigaku), mechanical testing (tensile testing, fracture toughness, hardness measurements). The volume fraction of quenched martensite is calculated according to Koistinen–Marburger equation. As established, the Q-n-P treatment results in the formation of multiphase structure consisting of the tempered martensite, carbide-free lower bainite, retained austenite, and dispersed vanadium carbides. The best combination of mechanical properties is achieved by quenching to 160–200°C with the formation of 50–70% of martensite and subsequent holding at 300°C for a time required to complete the bainite transformation ($\cong$ 300 s). In this case, the carbon concentration in the austenite rises to 0.95–1.05% that is accompanied by an increase in the volume fraction of retained austenite to 19%. According the data of TEM observations, the retained austenite is revealed as blocky or elongated areas adjacent to martensite as well as films of 20–60 nm width lying between the bainitic ferrite laths of 100–470 nm width. No sign of carbides (transition ones or cementite) is found on the TEM images. This is because of presence of 1.46% Si, which effectively inhibits the carbide precipitation from austenite and from martensite. The mentioned above treatment allows to achieve high strength (ultimate tensile strength of 2000–2100 MPa) and bulk hardness of 52–54 HRC combined with the increased ductility (elongation of 4–6%, reduction of 4–19%) and impact toughness (KCU$_{20}$ = 59–67 J/cm$^2$) with PSE values of 10.6–12 GPa. The fracture of Q-n-P-treated steel under dynamic loading occurs through the predominantly ductile mechanism that combines the quasi-cleavage fracture with the void coalescence and dimples formation. The quenching at 240°C results in the lower content of quenched martensite (18%) that leads to decrease of the retained-austenite volume fraction after partitioning. The increase in the soaking time at the ‘Partitioning’ stage up to 1800–3600 s leads to degradation of mechanical properties, which is ascribed to a decrease in the content of retained austenite down to 11–12%, presumably, because of prolonged transformation of the enriched austenite into the bainite via bainitic reaction. This is accompanied by an increase in the carbon content in the retained austenite up to 1.28–1.32%. The presumable scenarios of structure transformations under Q-n-P heat treatment are discussed.

Key words: Q-n-P treatment, strength, ductility, martensite, austenite, carbide-free bainite.



PACS: 61.72.Ff, 62.20.M-, 62.20.Qp, 64.75.Nx, 81.05.Bx, 81.30.Kf, 81.40.Gh

Citation: V. І. Zurnadzhy, V. G. Efremenko, V. G. Gavrilova, R. O. Kussa, А. V. Efremenko, V. V. Kudin, and M. V. Pomazkov, Formation of a Heterophase Structure in Low-Alloyed Steel by Means of Application of Innovative Technology of Heat Treatment by ‘Quenching and Partitioning’, Metallofiz. Noveishie Tekhnol., 40, No. 12: 1603—1624 (2018) (in Russian)

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