Alternative Heat Treatments for Complex-Alloyed High-Cr Cast Iron Before Machining
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-CHROMIUM cast irons (HCCIs) are widely used in many industrial areas (such as crushing, grinding, milling, and pumping or transportation) where advanced abrasive/erosive resistance is vital for machine parts and components.[1,2] In general, HCCIs are used in the as-cast shape, however, in some cases, machining is required to fit the casting into a machine or to provide the required shape/surface roughness (mill rolls, knives, slurry pump impellers and sheaths etc.).[3] HCCIs are difficult to be machined, machining may shorten the tool life because of the hard carbide M7C3, lower thermal conductivity of cast iron, and due to austenitic matrix which becomes harder under machining.[3–7] Several strategies have been developed to facilitate the
V.G. EFREMENKO and YU.G. CHABAK are with the Priazovskyi State Technical University, Mariupol, 87555, Ukraine. Contact e-mail: [email protected] K.M. WU and O.B. ISAYEV are with the The State Key Laboratory of Refractories and Metallurgy, Hubei Province Key Laboratory of Systems Science on Metallurgical Processing, International Research Institute for Steel Technology, Wuhan University of Science and Technology, Wuhan, 430080, China. K. SHIMIZU is with the Muroran Institute of Technology, Muroran, 050-8585 Japan. V.V. KUDIN is with the Zaporozhye National Technical University, Zaporozhye, 69061, Ukraine. Manuscript submitted December 22, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
machining of HCCIs for example (a) using expensive PCBN or ceramic-coated hard carbide tools,[6–11] (b) heating the ingot [up to 573 K (300 C)] during machining to soften the ingot,[12] (c) alloying the HCCIs with lead (up to 0.2 wt pct) and/or sulfur (up to 0.4 wt pct) to enable chip formation[13,14] and (d) applying heat treatment for preliminary HCCI softening.[15–17] The latter strategy is often adopted because of its simplicity and accessibility. Heat treatment increases the HCCI machinability by reducing its hardness to a suitable level (below 40 HRC[16]) and providing a ‘‘ferrite + granular carbides’’ microstructure that is optimal for high-carbon alloys before tool machining.[18] The commonest heat treatment for the enhancement of the machinability of HCCIs is one-step continuous annealing.[16,17,19] This treatment consists of isothermal holding in the range of 1123 K to 1273 K (850 C to 1000 C) for 5 to 10 hours with subsequent continuous cooling at 30 to 100 K/h.[14,16,17] The isothermal holding is aimed at destabilizing primary austenite by secondary-carbide precipitation (termed ‘‘destabilization’’[20–22]). The austenite depletion with carbon and chromium reportedly accelerates the austenite transformation into a ‘‘ferrite + granular carbides’’ structure.[16] Continuous annealing is applicable to cast irons that are alloyed with 12 to 20 wt pct Cr and up to 1 wt pct Ni (Mn); it provides a hardness of 250 to 320 HB[16,17,19] and eliminates austenite in the structure. The elimination is important, as austenite decreases the HCCI
machinability because of its work-hardening under de
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