High Wear Resistance of White Cast Iron Treated by Novel Process: Principle and Mechanism
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NTRODUCTION
HIGH-CHROMIUM white irons are usually hypoeutectic ones, and they are often applied in heavy abrasion condition, such as mineral processing, cement production and steel manufacturing industries, and so on.[1–5] Their excellent wear resistance is mainly attributed to the type, amount, morphology and distribution of the carbides and also to the matrix microstructure.[1,4–7] The addition of chromium can form considerable carbides in casting irons.[4] Additionally, some carbide-forming elements such as cerium, vanadium, boron, titanium, and tungsten can generate the refined secondary carbides, thereby to enhance the abrasion resistance.[8–12] The usual production route of high-chromium white irons is usually casting. In the as-cast condition, the network distribution of coarse eutectic M7C3 type carbides is inevitably formed during solidification and leads to poor fracture properties and relatively high wear rate.[5,13,14] Moreover, the network eutectic carbides are very stable so that they are not easily eliminated by heat treatment.[3,15] Therefore, the microstructure of high-chromium white irons is improved by various heat treatment methods, such as destabilization heat treatment and sub-critical heat XIAOSHUAI JIA and YU LIU, PhD Students, XUNWEI ZUO, Engineer, and NAILU CHEN and YONGHUA RONG, Professors, are with the School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China. Contact e-mail: [email protected] Manuscript submitted February 8, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
treatment.[5,16–19] Destabilizing heat treatment with relative low austenitizing temperature is used to cause coarse secondary carbides to precipitate,[15] which lowers the alloy content in austenite and leads to the martensitic transformation at room temperatures if cooling is fast enough.[5] While sub-critical heat treatment below austenitizing temperature is to reduce the retained austenite amount and obtain martensitic matrix by the precipitation of considerable fine secondary carbides.[18,20] To obtain more martensite, the sub-critical treatment following deep cryogenic treatment was sometime also used.[20] However, sub-critical treatment needs long holding time to sufficiently precipitate secondary carbides from austenite so that maximum hardness can be obtained.[19,21] The retained austenite has a high intrinsic fracture toughness[22] and transformation-induced plasticity (TRIP) by strain-induced martensitic transformation,[16,17] such as wear induced martensitic transformation.[18] However, it is easy to cause high phase-transformation stress and cracking during the transformation induced by wear.[19] It has been verified that the secondary carbides play an important role in determining the mechanical properties and wear resistance of high-chromium white irons.[3,23–25] For the above heat treatments, rapid cooling is necessary for white cast irons to avoid the formation of pearlite as possible, which is unfavorable for the wear resistance. However, because white cast irons h
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