Resistance to High-Temperature Oxidation and Wear of Various Ferrous Alloys Used in Rolling Mills
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THROUGH several improvements of rolling technology, rolls have always been a critical part of the process.[1] The work rolls used in hot roll mills are submitted to severe in-service conditions and demand optimum properties in terms of wear resistance and toughness. Moreover, they must resist both mechanical and thermal solicitations.[2] In the first finishing stands, they are submitted at least to abrasion, oxidation, adhesion, and thermal fatigue.[3] Usually, work rolls consist of a hard shell made of cast iron or steel and a soft core of gray/nodular cast iron.[4] The shell matrix presents high wear resistance, stability in shape, and good surface finish. The softer core ensures good mechanical properties and resistance to thermal and mechanical loads during mill operation.[3,5] It is then very important to choose the right quality of rolls for desirable rolling to produce flat products.[1] In the middle of the twentieth century, work rolls used in all the finishing stands of hot rolling mills were FABIENNE DELAUNOIS and VICTOR IOAN STANCIU are with the Service de Me´tallurgie, Faculte´ Polytechnique, Universite´ de Mons, rue de l’Epargne, 56, 7000 Mons, Belgium. Contact e-mail: [email protected] MARIO SINNAEVE is with the Marichal Ketin, Rue Ernest Solvay 372, 4000 Lie´ge, Belgium. Manuscript submitted August 21, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS A
usually made of ICDP (Indefinite Chill Double Pour) cast iron.[6,7] ICDP cast iron presents good wear and thermal fatigue resistance[4,5] due to its specific microstructure obtained by a double precipitation: both graphite (between 1 and 3 pct) and coarse carbides M3C type-Fe3C (cementite) (between 35 and 45 pct) are present in a mixed martensite/bainite matrix.[8] However, this grade suffers for a poor wear resistance due to the low hardness of its carbides.[9,10] Moreover, microstructure of those rolls varies with the roll radius: the bigger the radius, the lower the free graphite and the amount of carbides. Thus, the decrease of shell hardness with roll wear leads to rapid wear and to regular roll change during production, resulting in a reduction of productivity and an increase of consumables costs.[8] To improve the durability of the work rolls located in the first stands of finishing mills (F1 to F4[8,11,12]), new materials were developed such as high-chromium white cast iron (Hi-Cr iron: 2-3 pct C and 15-20 pct Cr) and high-chromium steel (Hi-Cr steel: 1-2 pct C and 1520 pct Cr).[12] They contain chromium-rich M7C3 carbides that are harder than the cementite present in the ICDP.[9,10] The Cr content of Hi-Cr cast irons and steels (around 18 wt pct) gives them good resistance to thermal oxidation and good corrosion resistance.[9,10,13] Unfortunately, due to the high chromium content, the roll surface doesn’t oxidize enough in practice due to their low oxidation kinetics[9,10] and sticking appears in
the last stages of the rolling process because of a lack of lubrication.[14] In 1985 HSS (High Speed Steels) were introduced.[6] HSS workin
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