The Transformation of Carbides during Austenization and Its Effect on the Wear Resistance of High Speed Steel Rolls
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UCTION
RESEARCH into the use of high speed steel (HSS) for rolls was first started in Japan, with the first successful results being reported at the end of the 1980s.[1] Early trials in various rolling mills demonstrated the large potential of the use of HSS as a roll material. The HSS rolls used in the first stands of a hot strip mill showed 3 times higher wear resistance and an improved surface roughening resistance compared to the conventionally used high chromium cast iron rolls. Such roll properties are attractive to strip producers in order to fulfill the increasing demand for improved strip surface finish together with tight dimensional tolerances and high productivity.[1] In conventional high chromium cast iron rolls, a network of carbides formed along solidification cell boundaries results in good wear resistance. However, the carbide networks often initiate thermal fatigue cracks and act as easy crack propagation routes, thereby resulting in poor thermal fatigue properties.[2,3,4] These shortcomings are microstructurally alleviated in HSS rolls, where hard carbides are used to improve the hardness and the wear resistance.[5] Accordingly, HSS rolls contain strong carbide-forming eleXIAODAN ZHANG, Doctoral Candidate, WEI LIU and YOUGUO LI, Professors, are with the Advanced Materials Laboratory, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China. Contact e-mail: [email protected] DALE SUN, Research Professor, is with the Technology Centre, Baoshan Iron & Steel Co. Ltd, Baoshan, Shanghai 201900, People’s Republic of China. Manuscripted submitted December 26, 2005. Article published online March 27, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS A
ments such as V, Mo, and W, which help form hard carbides that significantly improve hardness and wear resistance.[6,7,8] However, a significant problem is that HSS’s are characterized by a long freezing range and a complex eutectic reaction, which results in segregation of alloying elements and formation of several different types of bulky carbides during casting and solidification. The traditional method used to break these carbides into smaller carbides is forging. This method is, however, not applicable for the fabrication of HSS rolls. Although a powder metallurgy route can be used to reduce both segregation of alloying elements and the size of primary carbides, this approach can only be used to make rolls of a limited size. It is important therefore to find a way to reduce the size of the primary carbides during heat treatment in order to provide HSS rolls that can be used in demanding conditions. In this study, we have investigated the transformation of carbides during austenization with annealing time and have identified the correlation between wear resistance and the type, morphology, volume fraction, and distribution of the carbides in a centrifugally cast HSS roll.
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EXPERIMENTAL PROCEDURE
The HSS material used in this study was taken from a HSS roll manufactured by a centrifugal casting
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