Impact of a Multi-step Heat Treatment on Different Manufacturing Routes of 18CrNiMo7-6 Steel
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THE case-hardened 18CrNiMo7-6 steel is widely used in the field of transportation, energy generation, and general mechanical engineering due to its three key characteristics—high wear resistance, good fatigue strength, and cost-efficiency. The typical applications include gears of all kinds, camshafts, heavy-duty axles and arbors, bushings, wear pins, bearings, sprockets, metal rolling equipment, machine tools, universal joints, link components, etc.[1,2] This steel is typically supplied in the annealed condition followed by a multi-step heat treatment that develops a hard wear-resistant case (hardness of up to 60 HRC) on a relatively soft core material. Alloying the base material with chromium, molybdenum or nickel enhances the hardenability and impact toughness of the core material, whereas the quenching and tempering treatments increase the strength of core material by developing a martensitic microstructure. Additionally, the heat treatment can be
PARANJAYEE MANDAL, LAURIE DA SILVA, and HIMANSHU LALVANI are with the Advanced Forming Research Centre, University of Strathclyde, 85 Inchinnan Drive, Inchinnan, Renifrew PA4 9LJ, UK. MAIDER OLASOLO is with the Advanced Forming Research Centre, University of Strathclyde, 85 Inchinnan Drive, Inchinnan, Renifrew PA4 9LJ, UK and also with the Advanced Materials Research Laboratory, University of Strathclyde, 75 Montrose Street, Glasgow, G1 1XJ, UK. Contact e-mail: [email protected] Manuscript submitted August 8, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
selected in such a way that it imparts a homogenous microstructure with fine grain size in the core material. This microstructural homogeneity of any case-hardened steels is affected by macro-segregation and micro-segregation. Macro-segregation occurs due to the difference in chemical composition in the material, whereas micro-segregation appears because of the difference between the alloying elements during the solidification process.[3,4] During heat treatment, the micro-segregated areas respond differently and often transform into different phases depending on the heat treatment and cooling rate. For example, austenization followed by slow cooling of low and medium carbon steels results in micro-segregation during solidification, i.e., forms a banded microstructure consisted of alternating areas of ferrite and pearlite. Micro-segregation can also be observed in the carburized layer in alternating areas of martensite and bainite or of retained austenite and martensite depending on the chemical composition of the steel and the cooling rate. Both retained austenite and bainite are much softer than martensite, thus the banded microstructure shows large differences in strength and hardness along the longitudinal and transverse directions.[3] Grain size is an another important factor influencing the core properties. Forging often develops large grains and unwanted structural components such as bainite and carbides in the steel microstructure due to overheating that adversely influences the impact strength, fatig
