Effect of Indirect Transformation of Retained Austenite During Tempering on the Charpy Impact Toughness of a Low-Alloy C
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Effect of Indirect Transformation of Retained Austenite During Tempering on the Charpy Impact Toughness of a Low‑Alloy Cr–Mo–V Steel Yong‑Han Li1 · Zhong‑Hua Jiang1 · Zhen‑Dan Yang1 · Jue‑Shun Zhu1 Received: 12 December 2019 / Revised: 9 February 2020 © The Chinese Society for Metals (CSM) and Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract A modified tempering treatment has been designed in order to avoid the direct transformation of retained austenite (Ar) during tempering of a low-alloy Cr–Mo–V steel. Instead of the direct transformation of Ar into ferrite and M23C6 carbides during conventional tempering at 700 °C, transformation into aggregate of ferrite and cementite has been forced by a pre-tempering at 455 °C before conventional tempering. Experiments have been performed on specimens quenched with cooling rates 1.5, 3 and 12 °C/s, providing different types of Ar within the as-quenched microstructures. The results show that the tempering modification does not improve the Charpy impact toughness at the highest quenching rate of 12 °C/s, where the specimens cannot incur cleavage cracking induced from fine and discontinuous M23C6 carbides along lath interfaces. For the lowest quenching rate 1.5 °C/s, the Charpy impact toughness can be improved, and the failure is dominated by carbide aggregates, which originate from the decomposed products of blocky Ar. This is because the tempering modification effectively suppresses the formation of coarse M23C6 carbides at interfaces between the carbide aggregate and bainitic matrix, thereby resulting in a relatively homogeneous distribution of M23C6 carbides inside carbide aggregates. Therefore, the tempering modification is recommended for large-scale forgings, in which relatively high quenching rates are difficult to achieve. Keywords Bainite steel · Retained austenite · Impact toughness
1 Introduction Many pressure vessels for petrochemical industry applications are made of low-alloy Cr–Mo–V heat-resistant steels, which are complex iron-base alloys containing C, Cr, Mo, V, and a small amount of microalloying elements such as Nb, Ti and B [1, 2]. Usually, the classical heat treatment of low-alloy Cr–Mo–V bainitic steels used in heavy forgings consists of a quenching treatment followed by a high-temperature tempering. The microstructure after quenching is a fully bainite-like microstructure, which in principle, can be produced by a relatively rapid quenching treatment. However, practically, this is particularly difficult to achieve [3]. A Available online at http://link.springer.com/journal/40195. * Zhong‑Hua Jiang [email protected] 1
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China
real microstructure additionally consists of different amounts and types of retained austenite (Ar) which are depended on quenching rate. A few of Ar have been reported to negatively influence the material properties during tempering at high temperature [4]. In or
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