The effect of preexisting VC carbides on the bainite transformation in medium-carbon high-alloy steel
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The effect of preexisting VC carbides on the bainite transformation in medium-carbon high-alloy steel Xiaoli Zhao1, Lizhan Han2, Chuanwei Li1, Jianfeng Gu3,4,a) 1
Institute of Materials Modification and Modeling, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China Shanghai Key Laboratory of Materials Laser Processing and Modification, Shanghai Jiao Tong University, Shanghai 200240, China 3 Collaborative Innovation Center for Advanced Ship and Deep Sea Exploration, Shanghai Jiao Tong University, Shanghai 200240, China 4 Materials Genome Initiative Center, Shanghai Jiao Tong University, Shanghai 200240, China a) Address all correspondence to this author. e-mail: [email protected] 2
Received: 7 November 2018; accepted: 4 January 2019
Bainite transformation in steels is influenced by various factors. In the present work, bainite transformation in medium carbon high alloyed steel was investigated focusing on the influence of preexisting VC carbides on the morphology and transformation kinetics of the subsequently formed bainite. Hot-work die steels were held at 950 °C for various times to precipitate VC carbides, then rapidly cooled from 950 to 350 °C and held at this temperature for the bainite transformation. It is found that the bainite transformation was obviously accelerated by the preexisting VC carbides precipitated at the austenite region. The precipitation of carbides leads to a decrease in carbon concentration in the matrix, which decreases the effective activation energy and increases the highest temperature for the nucleation of bainite. Besides, bainite was observed to grow beside the VC carbides. It suggests that the VC carbides in the matrix act as nucleation sites for the bainite transformation. In the specimens, the bainite transformation is accelerated, and a higher fraction of bainite is formed when there are carbides in the matrix.
Introduction Medium carbon high alloyed steel has been widely used as the casting mold material for a long time due to a combination of excellent high-temperature strength, impact toughness, hot cracking resistance, and thermal fatigue [1]. To further improve the properties of the steels, extensive efforts have been made to investigate the effect of heat treatment on the microstructure and mechanical properties [2, 3, 4, 5]. As a matter of fact, it is the bainite transformation that plays an important role in industrial applications in that the bainitic microstructure has an optimal combination of strength and toughness [6, 7, 8, 9]. From this point of view, a thorough understanding of bainite transformation kinetics is of significance to control the microstructure and properties of materials. Earlier, Bhadeshia et al. [10] reviewed the bainite transformation in steels including a detailed account of microstructure, chemistry, and crystallography of bainitic ferrite, referring to the effect factors of the bainite transformation such as dislocation density and chemical composition. However, the formation mechanism of the bainite
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