Recovery Kinetics of Cold-Deformed Cr-Mo Steels
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IN many industrial processes, steel products are cold deformed in order to obtain the required shape. In some cases, the deformation process may be inhomogeneous so a strain profile and residual stress field remain in the material after deformation. As the presence of dislocations and residual stresses highly affects different steel properties such as toughness, corrosion resistance, and sulfide stress cracking susceptibility, an after deformation heat treatment is commonly used to recover the original properties of the steel. During treatment, two metallurgical phenomena occur, creep and recovery. Creep produces the relaxation of the elastic stresses,[1,2] while recovery gives rise to a decrease of the mechanical properties to values before cold deformation.[3] Recovery involves the annihilation of point defects and dislocations and the arrangement of the latter into lower energy configurations. During recovery, the changes in the microstructure occur in a very small scale and cannot be easily observed by conventional optical or scanning electron microscopy. Usually, the recovery process is evaluated indirectly, following the change in some bulk property. Measurements of yield strength,[4] hardness,[5] X-ray diffraction (XRD) line broadening,[6] and coercive field[7] have been successfully used so far. M.A. VICENTE ALVAREZ, PhD, Researcher, Metallurgy Department, and T. PEREZ, Metallurgy Department Head, are with Tenaris Siderca R&D, Argentina. Contact e-mail: mvicentealvarez@ tenaris.com M. MARCHENA, formerly Researcher, Tenaris Siderca R&D, is with the UNSAM CNEA, Argentina. Manuscript submitted October 12, 2007. Article published online October 28, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
Despite the industrial relevance of the problem, little work has been done to study the recovery kinetics of steel in the last years. On the experimental side, the early work of Michalak and Paxton[4] studies recovery in zone-refined iron predeformed at 5 and 15 pct. More recently, Mukunthan and Hawbolt[6] measured the recovery and recrystallization kinetics of low-carbon steels by XRD peak resolution measurement. A more recent work was performed by Martinez de Guerenu et al.[7] on extra-low-carbon steel cold rolled 84 pct, where the recovery kinetics was monitored through the measurement of the coercive field of isothermally annealed probes. As far as we know, no work has been reported on Cr-Mo steels. On the theoretical side, several empirical relations have been proposed to predict the kinetics of recovery. The most commonly used equations for isothermal annealing have a logarithmic or an exponential time dependence.[3] On a more fundamental basis, it was proposed that recovery is controlled by several thermally activated concurrent and consecutive atomistic mechanisms each with its own kinetics, including dislocations glide, climb, and cross-slip.[8–10] This leads to an apparent activation energy Q, which is assumed to depend on the internal stress.[8,9,11] The rate of recovery measured as the evolution of the internal
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