Inhomogeneity of plastic deformation in austenitic stainless steel after surface mechanical attrition treatment
Introduction
Over the past decades, nanostructured and ultrafine-grained metallic materials have attracted considerable interest due to their remarkable improvement of mechanical strength [1], [2], [3], [4], [5], [6]. For this reason, they will probably find many applications in future structural engineering.
A disadvantage of such high strength materials is their lowered plasticity. However, this problem can be solved using recently developed ultrasonic-assisted surface mechanical attrition treatment (SMAT) [7], [8], [9], [10]. It allows to refine the microstructure only at the surface, preserving the plasticity of the core.
In ultrasonic-assisted SMAT, spherical balls (steel, glass or ceramic) of relatively high hardness are placed in a vibrating chamber, which vibrates with a frequency from 20 Hz to 20 kHz. A sample, which surface is going to be treated, is placed at the upper side of the chamber and impacted by a large amount of flying balls. The kinetic energy of the impacting balls induces high plastic deformation at the specimen's surface resulting in grain refinement down to the nanometric scale without significant contamination or porosity [11], [12]. Moreover, as opposed to conventional shot peening [13], [14], in ultrasonic-assisted SMAT random directional impacts of the balls onto the sample surface facilitate the grain refinement process [7].
A nanocrystallised structure can be observed up to several tens of micrometers below the surface and, as the depth increases, a hardened transition zone is observed at the layer of about 200 μm depth characterised by a grain size gradient [8], [15], [16], [17]. Such limited volume fraction of the refined grains restricts thickness of the sample that is going to be processed by SMAT to value of the order of 1 mm. A greater thickness of the sample would result in obtaining the relatively low volume ratio of strengthened after SMAT and initial microstructure. As a consequence, it would cause only minor strength enhancement. Subsequently, plates subjected to SMAT can be joined together using any suitable thermo-mechanical process, for instance, accumulative roll bonding (ARB) [18], [19], [20]. Such duplex technique allows to obtain multilayered materials for structural applications with enhanced yield and ultimate strength, while conserving an acceptable elongation to failure. However, the metallic samples limited to about 1 mm thickness exhibit significant non-uniform change in shape during SMAT as a result of large residual stresses introduced by impacting balls (Fig. 1). In the earlier reported investigations of SMAT, such deformation inhomogeneity has not been taken into account [9], [12], [16], [18], [20], [21], [22], [23], [24], [25]. However, as it is showed in this work, it has substantial impact on microstructure of the treated material that will inevitably be reflected in the obtained mechanical properties. This inhomogeneity may also have meaningful influence on the bonding mechanisms during subsequent thermo-mechanical processing affecting the quality of the final multilayered metallic material. In this paper, the microstructure evolution accounting for the inhomogeneity of plastic deformation during ultrasonic-assisted SMAT is studied.
A special emphasis was placed on the study of the strain-induced α′-martensite, as it increases the work-hardening capacity and has substantial influence on ductility of steels [26], [27], [28], [29]. In order to evaluate the possible microstructural changes during thermo-mechanical joining process, annealing studies of SMATed plates were performed at similar temperatures which are usually used in the second step of duplex technique. It allowed for observation of the changes in α′-martensite volume fraction depending on the heat treatment parameters and the area of the sample under consideration. The amount of α′-martensite significantly increased after annealing at relatively high temperatures proving high thermal stability of this phase in the SMATed material. The thermal stability may influence the bonding mechanisms during thermo-mechanical joining process affecting quality of the multilayered metallic structural materials.
The aim of this work is to investigate the conditions leading to inhomogeneity of plastic deformation in austenitic stainless steel after ultrasonic-assisted SMAT that can significantly affect the microstructure and mechanical properties of the treated material. The influence of non-uniform deformation on the volume content of α′-martensite has been studied. In order to investigate the stress distribution within the steel sample due to ball impact during SMAT, a finite element (FE) analysis was performed.
Section snippets
Experimental procedure
The cold rolled 1 mm thickness sheet of AISI 316L austenitic stainless steel with initial grain size of 10 to 50 μm was used in the investigation. Its chemical composition is shown in Table 1. The temperature, at which 50% of the austenite transforms to martensite with 30% true strain Md30, the martensite start temperature Ms and the stacking fault energy (SFE) values for the considered material calculated according to equations based on the chemical composition are presented in Table 2. The SFE
Mechanism of grain refinement in austenitic stainless steel
The mechanism of surface nanocrystallization has been studied for various austenitic stainless steel grades. One of the most studied grades is 304 SS, for which the grain refinement process is based on formation of the planar dislocations arrays and twins; on twin-twin intersections leading to grain subdivision and martensite transformation; and on formation of randomly oriented crystallites [44]. The SFE of 304 SS is rather low, hence the twinning based mechanism is likely to be dominant. The
The cross-sectional morphology
SEM observations of the steel morphology were made using the specimen cross-sections. Fig. 2a–d illustrates the obtained microstructures under A1, B1, A2 and B2 surfaces subjected to SMAT for 30 min. Microstructures under A3 and B3 surfaces are very similar, hence they were not shown. It can be seen, that there are no significant differences in the cross-sectional morphology between each of the surfaces of the treated steel sample. The noticeable changes of microstructure occur at areas situated
Conclusions
Inhomogeneity of plastic deformation in cold-rolled 316L austenitic stainless steel after ultrasonic-assisted SMAT has been studied. In this work, a textured surface was taken into account. The detailed XRD investigation combining BB and GIXD geometries allowed for non-destructive studies carried out at different material depths. Evaluation of the α′-martensite volume fraction in different areas of the treated specimens was the central point in the present study.
SMAT in combination with
Acknowledgements
The support from the National Science Centre, Poland (grant no. DEC-2013/09/B/ST8/00141) is greatly appreciated. This research was supported in part by PLGrid Infrastructure.
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