The kinetics of dynamic recrystallization of 42CrMo steel
Introduction
Material flow behavior during hot formation process is often complex. The work hardening (WH), dynamic recovery (DRV) and dynamic recrystallization (DRX) often occur in the metals and alloys with low stacking fault energy during high temperature deformation [1], [2], [3], [4], [5], [6]. Especially, the effects of DRX behavior in metals and alloys on the flow stress and microstructural evolutions are significant. Therefore, understanding of the relationship between thermo-mechanical parameters and DRX behavior of metals and alloys under hot deformation condition is of great importance for designers of metal forming processes (hot rolling, forging and extrusion).
A considerable amount of researches have been done on the DRX behavior for various materials in recent years. Imbert and McQueen [7] investigated the DRX of A2 and M2 tool steels by continuous hot torsion tests and found that the M2 steel experiences the early critical strain and fast DRX kinetics because of its greater volume fraction of carbides and smaller grain size. Kim and Yoo [8] studied the DRX of AISI 304 stainless steel with torsion tests. The DRX volume fraction was established as a function of processing variables, such as strain rate, temperature, and strain. Sitdikov et al. [9] investigated the grain refinement in a commercial Al–Mg–Sc alloy under hot ECAP conditions and concluded that the grain refinement occurs in accordance with deformation-induced continuous reactions. Chen et al. [10] simulated the initial microstructure and DRX of 30Cr2Ni4MoV rotor steel using the cellular automaton (CA) method. Qin et al. [11] investigated the flow stress behavior of ZK60 alloy at elevated temperature and proposed a model to predict the flow stress based on the DRX mechanism. Momeni and Dehghani [12] studied the hot deformation behavior of AISI 410 martensitic stainless steel by hot compression tests, and a model based on the Avrami equation was developed to estimate the DRX kinetics for different deformation conditions. Zeng et al. [13] investigated DRX behavior of a heat-resistant martensitic stainless steel 403 Nb during hot deformation. The mathematical models of peak strain and kinetic equation for DRX of 403 Nb steel were established. Yue et al. [14] developed four 3D finite element models to simulate the whole rod rolling process of GCr15 steel, and a FORTRAN program was developed to predict the evolution of recrystallization behavior and austenite grain size in rolled piece. Wang et al. [15] investigated the DRX behavior of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy in β-forging process, and the effects of plastic deformation strain, strain rate and deformation temperature on the DRX behavior of Ti-alloy were systematically investigated. He et al. [16] investigated the strain hardening and DRX behavior of ZK60 magnesium alloy during hot deformation.
42CrMo (American grade: AISI 4140) is one of the representative medium carbon and low alloy steel. It is widely used for many general purpose parts, such as automotive crankshaft, rams, and spindles, due to its good balance of strength, toughness and wear resistance. Up to now, some investigations have been carried out on the behaviors of 42CrMo steel [17]. Smoljan [18] found that it is possible in effective way to improve a steel strength and toughness by the combined cyclic heat treatment. The increasing of both yield strength and Charpy-V notch toughness is achieved due to structure refinement. Lin et al. [19] developed an artificial neural network (ANN) model to predict the constitutive flow behaviors of 42CrMo steel during single-pass hot deformation. Furthermore, Lin et al. [20] proposed a revised model to describe the relationships of the flow stress, strain rate and temperature of 42CrMo steel at elevated temperatures by compensation of strain and strain rate. Besides, the static recrystallization behavior (SRX) and metadynamic recrystallization behavior (MDRX) in 42CrMo steel were investigated [21], [22], [23], [24]. The DRX kinetics of 42CrMo was also studied by Quan et al. [25]. In their study, the effects of deformation temperature and strain rate on the DRX kinetics were considered. However, a large amount of work [26], [27], [28], [29] showed that the characteristics of DRX behavior depend on not only the deformation temperature and strain rate but also the initial grain size for the metals or alloys. Thus, the DRX kinetics in the hot deformed 42CrMo steel still need further investigation by considering the effects of initial grain size on the DRX behavior.
In this study, the DRX behavior in 42CrMo steel was investigated by isothermal hot compression tests. The effects of deformation temperature, strain rate, and initial austenite grain size on the dynamic recrystallization behavior were discussed. The kinetics equations of DRX in the hot deformed 42CrMo steel were developed to predict the softening behaviors induced by DRX. The validity of the proposed DRX kinetics equations was confirmed.
Section snippets
Materials, specimens and experimental equipment
The material used in this study was the commercial 42CrMo high-strength steel, and its chemical composition (wt.%) is 0.450C–0.280Si–0.960Cr–0.630Mn–0.190Mo–0.016 P–0.012 S–0.014Cu–(bal.)Fe. Cylindrical specimens were machined with a diameter of 10 mm and a height of 12 mm. In order to minimize the frictions between the specimens and die during hot deformation, the flat ends of the specimen were recessed to a depth of 0.1 mm deep to entrap the lubricant of graphite mixed with machine oil. The hot
Analysis of true strain-stress curves
The typical true strain-stress curves for 42CrMo steel under different deformation temperatures and strain rates are shown in Fig. 3. It can be found that the strain for peak flow stress, called peak strain (εP), is sensitive to the deformation temperature, strain rate and initial austenitic grain size. First, the peak strain decreases with the increase of the deformation temperature. Generally, the critical strain for the onset of DRX, called critical strain (εc), is equal to (0.6–0.85) εP.
Conclusions
In this study, the effects of deformation parameters on the DRX behavior in 42CrMo steel were discussed. The kinetic equations were developed to predict the DRX volume fraction. The DRX behavior in 42CrMo steel is not only sensitive to the deformation temperature and strain rate, but also to the initial austenite grain size. The DRX in 42CrMo steel easily occurs at high deformation temperature, low strain rates and fine initial austenitic grain. A good agreement between the experimental and
Acknowledgments
This work was supported by Program for New Century Excellent Talents in University (No. NCET-10-0838), and Sheng-hua Yu-ying Program of Central South University and the Postdoctoral Science Foundation of Central South University (No. 117327), China.
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