Design of Online Spheroidization Process for 1.0C-1.5Cr Bearing Steel and Microstructure Analysis
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ODUCTION
DUE to the high strength and excellent fatigue property, 1.0C-1.5Cr steel is widely used in the bearing industry.[1,2] As a hypereutectoid steel, its hot rolled microstructure is composed of lamellar pearlite and a small amount of proeutectoid carbide (grain boundary cementite), which is unfavorable for the subsequent cold working. In order to improve the cold deformability, it is necessary to conduct spheroidization annealing, which can produce a softer microstructure consisting of spherical carbides and ferrite matrix. High carbon bearing steels are commonly spheroidized through the intercritical annealing, in which the steels are heated to a certain intercritical temperature for austenitizing, and then slowly cooled to room temperature.[3,4] Due to the existence of lots of globular undissolved carbide particles in the microstructure at the intercritical temperature, divorced eutectoid transformation will occur during the controlled cooling stage.
ZHEN-XING LI, CHANG-SHENG LI, JIN-YI REN, and BIN-ZHOU LI are with the State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, P.R. China. Contact e-mail: [email protected] DONG-WOO SUH is with the Graduate Institute of Ferrous Technology, POSTECH, Pohang, 790-784 Korea. Manuscript submitted May 8, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
That is to say, accompanying the austenite-to-ferrite transformation, carbides will directly nucleate on the existing undissolved carbides upon controlled cooling, which will result in the formation of spheroidized microstructure.[5–7] At present, there are two typical intercritical spheroidization annealing processes for 1.0C-1.5Cr bearing steel: one is austenitizing at about 1073 K (800C), and then cooling to 1023 K (750 C) at a rate of larger than 15 K/h, and further cooling to around 953 K (680 C) at a rate of smaller than 10 K/h, and finally air cooling to room temperature; the other is directly cooling at a rate of around 120 K/h from the intercritical temperature to 993 K (720 C), and then isothermal holding for 2 hours, and further cooling to around 923 K (650 C) at a rate of 30 K/h.[8–10] The practical annealing parameters differ to some extent, which depend on many factors such as the furnace type, size, and initial microstructure of billets. Although intercritical spheroidization annealing is widely used in the industrial production, there are still challenges, especially on the excessively long annealing time. Several methods for accelerating the spheroidization process of pearlite steels have been proposed. Nam and Lee[11] reported that the drawing at 973 K (700 C) was beneficial to reducing the subsequent annealing time. Zhang et al.[12] reported that when the high carbon steel was cooled from 1423 K (1150 C) and substantially deformed at 973 K (700 C), the spheroidized microstructure could be directly obtained during the
subsequent slow cooling process. However, the need of additional equipment and the difficulty of deformation around 973 K (700 C) have been th
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