Crack Healing in a Low-Carbon Steel Under Hot Plastic Deformation
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INTRODUCTION
PROPAGATION and healing of cracks are two contrary processes that occur in materials. Since Griffith proposed the brittle fracture model, the propagation of cracks in materials[1,2] along with the macroscopic, microscopic, and nanoscale aspects of fracture mechanics have attracted considerable research interest. However, the process of crack healing in materials has only been investigated relatively recently. Some investigations have been carried out on crack healing in different materials.[3–5] However, studies on crack healing in steel are comparatively rare, especially under the conditions of metal forming. Internal cracks in metallic materials can heal without plastic deformation (the voids can gradually disappear) under electric current pulse treatment[6,7] and heat treatment.[8,9] Heat treatment has been employed to restore the plasticity margin destroyed during cold plastic deformation and to avoid damage,[10] and it could be concluded from the experimental data that healing of microdamage in metals had occurred. Some experiments on crack healing in pure iron samples at high temperature were carried out by Zhang et al.[11] Micron-sized internal cracks were introduced into the pure iron samples by application of low-cycle fatigue
HAILIANG YU, Vice-chancellor’s Research Fellow, is with the School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, Wollongong NSW 2500, Australia and also Professor, with the School of Mechanical Engineering, Shenyang University, Shenyang 110044, P.R. China. Contact e-mails: hailiang@ uow.edu.au, [email protected] XIANGHUA LIU, Professor, is with the State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110004, P.R. China. XINWEN LI, Master Student, formerly with the State Key Laboratory of Rolling and Automation, Northeastern University, is now with the Baosteel, Shanghai, P.R. China. AJIT GODBOLE, Senior Research Fellow, is with the School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong. Manuscript submitted November 8, 2012. Article published online October 12, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
load. Cracks on grain boundaries were found to have a regular penny-like shape. The fatigued specimens were annealed for 7 hours in vacuum at 1173 K (900 °C). Scanning electron microscope (SEM) observations showed that the microcracks initially changed to a wavy shape after 2 hours, then evolved into an array of circular voids after 5 hours, and after 7 hours, isolated voids were scarcely observable. Compared with the results of crack healing with heat treatment and no plastic deformation, crack healing with plastic deformation could occur rapidly. Wei et al.[12] carried out a series of experiments on preset internal crack healing in 20MnMo and 20# steel samples using the ‘‘HIGHMULTI 5000’’ hot-press equipment. Experimental results showed that the internal cracks could heal completely and only ferrite was discovered in the crack healing zone. Based on the measured stre
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