Microstructure and properties evolution of Nb-bearing medium Cr wear-resistant cast steel during heat treatment
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ORIGINAL PAPER
Microstructure and properties evolution of Nb-bearing medium Cr wear-resistant cast steel during heat treatment Jia-cheng Zhang1 • Tuo Zhang1 • Yi-tao Yang1 Received: 30 March 2020 / Revised: 6 May 2020 / Accepted: 7 May 2020 Ó China Iron and Steel Research Institute Group 2020
Abstract The effect of quenching temperature and cooling conditions on the microstructure and mechanical properties of a 0.2%Nb medium chromium wear-resistant cast steel was investigated. The microstructure, carbides and volume fraction of retained austenite were characterized using the optical microscope, scanning electron microscope, transmission electron microscope and X-ray diffraction. The influence of heat treatment on hardness, impact toughness and tensile properties of test steel was studied. It can be observed that lath martensite can be obtained under the condition of air cooling and oil cooling upon austenitizing in the range of 900–1020 °C. Total carbide content of 0.2–1.1 wt.% under air cooling is more than that under oil cooling due to the lower cooling rate. Nb6C5, M23C6 and M7C3 were found at lower austenitizing temperature, of which niobium carbide mostly located at grain boundaries, while chromium carbides were uniformly distributed in the matrix with the size of 20–50 nm. The chromium carbides are basically dissolved into the matrix in test steel austenitized at 1020 °C. Meanwhile, the negligible growth of prior austenite grain size is achieved. Specimen austenitized at 1020 °C and cooled in air ? tempered at 200 °C has a best combination of hardness, plasticity and tensile strength due to fine grain size and more amount of retained austenite. Under this condition, the hardness is 58 HRC, the impact toughness is 22.92 J/cm2, and the tensile strength is 1136.9 MPa. Keywords Wear-resistant steel Quenching temperature Cooling condition Carbide Retained austenite Strengthening Toughening
1 Introduction Ball mill liner is an important wear-resistant part of the ball mill. As the bottom of the liner often suffers from the falling abrasive, the employed alloys are required to exhibit an adequate combination of wear resistance and impact toughness [1, 2]. In addition, the production cost induced by replacement of liner cannot be ignored [3]. Various alloys such as austenitic manganese steels, high chromium cast irons, Ni-hard cast irons and Cr–Mo steels are frequently applied in manufacturing mill liners [4–6]. Austenitic manganese steel is only useful in high stress state, which exhibits insufficient work hardening ability under the application for mill liners. High chromium cast irons & Yi-tao Yang [email protected] 1
School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China
and Ni-hard cast irons usually have sufficient wear resistance, but their toughness is not enough and not competitive in price due to relatively high alloying content. In the working condition of mill liners, martensitic alloy steel exhibits better
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