Effect of shot peening on the residual stress and microstructure of duplex stainless steel
Introduction
Duplex stainless steel (DSS) consisting of approximately equal amounts of ferrite (α-phase) and austenite (γ-phase) often combines the best features of α-phase and γ-phase stainless steel. Generally, it has good mechanical properties, such as high strength and ductility, good corrosion resistance, and it has found a growing use in mechanically loaded constructions [1], [2]. As DSS is subjected to cyclic loading in many applications, the improvement of its fatigue life is necessary. Armas et al. [3], [4] have studied the cyclic behavior of DSS of different generations. Balbi et al. [5] have studied the surface damage evolution in the standard DSS with the internal dislocation structure in order to determine the mechanisms of microcrack nucleation and propagation. Alvarez-Armas et al. [6], [7] showed that the microcracks initiate in the α-phase either on slip planes with the highest Schmid factor or at the α/α grain boundary, while the ductile γ-phase has the capacity to accumulate plastic deformation and thus, it delays the formation of the favorable conditions to allow crack propagation. These results might be explained with the amount of residual microstresses in the material. Residual microstresses are always presented in DSS due to the difference in coefficient of thermal expansion between the two phases [2]. In order to avoid the initiation and growth of cracks, mechanical treatments such as shot peening (SP) at surface layers are employed to improve fatigue strength and life [8], [9], [10]. By introducing fine domain, high dislocation density and compressive residual stress (CRS) into the near surface region, SP can considerably improve the fatigue strength and fatigue life of cyclically loaded metallic components [11], [12], [13]. Microstructure variations after SP are important for the improvement of component properties. Therefore, the investigation on microstructure and residual stresses of DSS after SP is significant.
X-ray diffraction (XRD) line profile analysis evaluates microstructural parameters in a statistical manner, moreover, the analysis is easy, reliable, quick and non-destructive to specimen. Rietveld's XRD structure refinement based on simultaneous structure and microstructure refinement [14] can be adopted for routine analysis of microstructural parameters such as domain size, microstrain, changes in lattice and atomic parameters etc., and this refinement is suitable for materials containing a large number of overlapping reflections. Although Rietveld analysis method has been reported [15], [16], [17], [18], [19], this method has seldom been applied to investigate SP effect on DSS. Therefore, using Rietveld software Maud to study the microstructure of DSS S32205 after SP in the present work, the effect of SP on the mechanical properties of DSS is investigated.
Section snippets
Sample preparation
DSS S32205 was provided by Shanghai Baosteel Group Corporation. Continuous casting followed by a solution annealing treatment at 1050 °C and quenching in water was carried out on this material in order to avoid the precipitation of secondary phases. In this work, all samples were cut into 20 mm in diameter and 2 mm in thickness directly from ingots and then polished. The chemical composition is C (0.029), S (0.006), Si (0.42), Mn (1.27), Cr (22.10), Ni (5.17), Mo (3.10), N (0.18), P (0.021), and
Analysis of phase transformation
Under SP intensity of 0.30 + 0.15 mmA, γ-phase weight fraction obtained by Maud software is 0.485 ± 0.008 at the depth of 15 μm, as shown in Fig. 1. And it ranges from 0.479 ± 0.007 to 0.511 ± 0.009 with the increasing depth to matrix from the top surface, indicating little decrease of γ-phase weight fraction. However, many researchers reported that γ-phase translates into martensite after SP [24]. In fact, austenitic–martensitic transformation depends strongly on alloying elements, and this was indeed
Conclusion
With the method of XRD line profile analysis, the surface layer characteristics of shot-peened DSS are investigated. It is concluded that for in the near-top surface layers, SP has stronger influence on CRS and microstructure of γ-phase than on α-phase under the same SP condition. In α-phase MCRS locates at the material surface. However, for γ-phase with higher hardness, high residual stress is present at the surface and MCRS is found below the surface. Lattice parameters of both phases in
Acknowledgment
The authors are grateful to Shanghai Bao-steel Corporation for supporting this work.
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