The effect of dynamic strain aging on fatigue property for 316L stainless steel
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The effect of strain range on dynamic strain aging (DSA) is discussed based on the low cycle fatigue tests for different strain ranges conducted for 316L stainless steel at strain rate of 1 103 s1. The variations of stress drop and hardening ratio are both compared for different strain ranges. The variations of stress drop are attributed to the dependence of vacancy concentration on strain range. The hardening ratio is higher at 600 °C than those at 20 °C and secondary hardening behavior occurs for larger strain range. The dependence of DSA on the number of cycles and the wave type for different stages are analyzed. Obvious DSA is observed in first few cycles, followed by weakening serrated yielding. However, the serrated yielding occurs again before fatigue failure. The difference of serrated yielding can be explained by the types of atom atmospheres at different cycles. A serrated wave is observed for smaller strain ranges, however, A, B, A 1 B, C, and B 1 C serrated waves can be found at different cycles for larger strain range. Finally, the crack nucleation and propagation on fracture surfaces are characterized by scanning electron microscope (SEM).
I. INTRODUCTION
316L stainless steel is a promising material for application in reactor vessels and piping systems in nuclear power plants owing to a good combination of its excellent creep strength, corrosion resistance, and high sensitization resistance.1 Generally, the service temperature range for 316L stainless steel is from 450 to 600 °C, which is right in the regime of its dynamic strain aging (DSA) effect.2 DSA, which depends on strain rate and temperature, is the phenomenon of interactions between diffusing solute atoms and mobile dislocations during plastic deformation. DSA is manifested as serrations on a stress–strain curve, which is one of the most prominent examples of plastic instabilities known as serrated yielding (PortevinLe Chatelier effect).3 Scholars worldwide have paid more attention to the essence and occurrence condition of DSA and proposed different physical models, for example, Cottrell model,4 McComick model,5 and Van den Beukel model.6 These models have held that the serrated flow results from the repeated pinning and unpinning of mobile dislocations with the presence of solute atoms. Schoeck et al.7 further extended the original DSA theory by referring the collective behavior of dislocations. Considering the integral behavior of dislocations and the interaction between solute atoms and dislocations, Xiao et al.8 proposed a physical model to determinate the Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.49 J. Mater. Res., Vol. 31, No. 5, Mar 14, 2016
critical strain corresponding to the appearance and disappearance of serration and discussed the strain ratetemperature ranges when serration occurred. Recently, some studies have been conducted on DSA for stainless steel. The results showed that the dislocation structure of austenitic stainless steels invol
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