Influence of Cryoprocessing on Mechanism of Carbide Development in Cobalt-Bearing High-Speed Steel (M35)
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HIGH-SPEED steel containing cobalt is extensively used for industrial application. Silva et al.[1] have conducted experiments on cryogenically treated M2 high-speed steel tools and conventionally treated ones. The cryogenically treated specimens showed a fraction very close to 0 pct of retained austenite. This means that practically 25 pct in volume of the retained austenite observed in the untreated sample was transformed into martensite by the cryogenic treatment. The gain observed during drilling steels adopting catastrophic failure as the end of tool life criterion varied from 65 to 343 pct depending on the cutting conditions used. Huang et al.[2] performed by soaking the sample in the liquid nitrogen for one week on M2. Both the cryogenictreated and noncryogenic-treated samples were tempered at 473 K (200 °C) in nitrogen atmosphere for 24 hours. They confirmed that cryogenic treatment not only can facilitate the carbide formation and increase the volume fraction of carbide in martensite matrix but can also make the carbide distribution more homogeneous. The increases in carbide density and volume fraction are responsible for the improvement in wear resistance. Alexandru et al.[3] have quantified the precipitated particles in M2 and verified their influence on the material properties. Their research involved seven steel samples, each of them submitted to different heating and cooling cycles up to 203 K (70 °C). The results confirmed an increase in carbide precipitation N.B. DHOKEY, Professor, A.R. HAKE, M.Tech. Student, and S. KADU and I. BHOSKAR, B.Tech. Students, are with the Department of Metallurgical Engineering, Govt. College of Engineering, Shivaji Nagar, Pune 411005, India. Contact e-mail: [email protected] G.K. DEY, Scientist, is with the Material Science Division, Bhabha Atomic Research Centre, Trombay, Mumbai, India. Manuscript submitted February 3, 2013. Article published online October 17, 2013 1508—VOLUME 45A, MARCH 2014
(from 6.9 to 17.4 pct), a reduction of the retained austenite (from 42.6 to 0.9 pct), and an increase in the martensite content (from 66 to 81.7 pct). The machining tests carried out with bits in turning AISI 1050 steels showed a significant increase in tool lives of cryogenically treated tools. Meng et al.[4] carried out cryotreatment on 12CrMoV1.4C tool steel and have reported that real improvement in the wear resistance is observed as a result of precipitation of g-carbides in the martensitic matrix as revealed in the high-resolution TEM study. The size of g-carbides ranges from 5 to 10 nm. Yang et al.[5] have conducted treatment on tool steel (2.78 pctC, 12.70 pctCr, and 1.6 pctV) at cryogenictreatment cycle at 77 K (196 °C) for 3 hours, followed by warming up to 373 K (100 °C), and then air cooling to room temperature. It is reported that cryotreatment steel gives homogeneous distribution of secondary carbides compared with cast tool steel. Kelkar et al.[6] studied fundamental aspect of M2 tool steel by subjecting one lot of steel to hardening, cryotreatment at 78 K (1
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