Decomposition Reaction of Metastable M 2 C Carbide in a Multi-Component Semi-High-Speed Steel
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igh-speed steels (semi-HSSs) are defined as lean HSSs offering a better combination of fracture toughness and fatigue resistance.[1] The convenient HSSs contain a high amount of carbide-forming elements such as Cr, Mo, V, or W and are widely used as work rolls for hot strip mill.[2–4] Recently, lean alloying becomes an important subject on developing advanced roller steels
LEI CHEN, Associate Professor, JIANMING PEI, FEI LI and YINGJIE ZHANG, Masters, and XIAOCONG MA, Assistant Researcher, are with the National Engineering Research Center for Equipment and Technology of Cold Strip Rolling, Yanshan University, Qinhuangdao 066004, P.R.China. Contact e-mail: chenlei@ ysu.edu.cn MINGJIA WANG, Professor, is with the Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, P.R. China. Manuscript submitted January 5, 2016. Article published online October 20, 2016 5662—VOLUME 47A, DECEMBER 2016
in response to the skyrocketing raw material cost. In this regard, the lean semi-HSS in which C content and total alloying content are reduced has been developed for candidates of roughing stand work roll materials.[5,6] As is well known, the dominance of better mechanical properties of casting HSS or/and semi-HSS roll is greatly affected by the carbides, especially primary or eutectic carbides obtained during solidification route. In addition, the carbide chemistry is important since carbide evolution (including dissolution and precipitation) causing improved mechanical properties usually depends on the thermal stability of eutectic carbides, and the thermal stability of carbides is mainly related to the composition of alloy and the cooling rate during solidification.[6–9] Obviously, the nature of eutectic carbides is a very important factor to be considered when attempting to produce tailored microstructures that can lead to necessary properties of semi-HSS rolls. Although semi-HSS contains a low alloying level, it still belongs to the multi-component system Fe-C-X, where X represents carbide-forming elements (Cr, Mo, V, W, etc). The fraction of total carbides in semi-HSS reduces due to such chemical components.[6] Eutectic carbides in semi-HSSs generally have a finer size than those in convenient HSSs and their morphology is similar to their crystallographic network as they precipitate directly from the liquid.[5,10] It is the case for a type of carbide known as metastable eutectic M2C. According to literature, the eutectic M2C-type carbide mainly distributed at prior c grain boundaries in most Cr-Mo (W)-V type semi-HSSs.[5,6] For the casting semi-HSS rolls, since they are not subjected to forging after solidification, the intergranular M2C carbides represent a preferential path for thermal cracks,[11] and hence the nature of the M2C carbide is essentially considered for improving thermal fatigue resistance. It is known that the metastable M2C carbide can decompose into various secondary carbides such as MC and M6C,[12,13] M7C3 and M6C,[6,14] or even M23C6 and M6C[15] under the effect of tem
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