Effect of milling on the microstructure and mechanical property of austenite stainless steel

A milling induced deformation layer of Z3CN20.09M, 304L and 316L austenite stainless steel (SS) was investigated by electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and a nanoindenter. The results indicated that the deformation layer was formed with a depth of about 200 μm, including a nanocrystalline layer within the range of 3 μm at the subsurface and followed by a large amount of persistent slip bands (PSBs). The significant plastic deformation was observed on the cross section of deformation layer with a range of about 80 μm for Z3CN20.09M and 304L SS, while being only about 30 μm for 316L SS. The highest residual stress tested on the milled surface reached about 1000 MPa, which can be attributed to the fact that a deformed martensite phase was formed at the surface during the milling operation. The nanohardness increased by 20-60 % on the cross section of the deformation layer as compared to the matrix.

New Approach to Produce a Nanocrystalline Layer on Surface of a Large Size Pure Titanium Plate

It was demonstrated that the mechanical shot peening (MSP) technique was a viable way to obtain a nanocrystalline layer on a large size pure titanium plate due to the MSP provided for severe plastic deformation (SPD) of surface high velocity balls impacting. The MSP effects of various durations in producing the surface nanocrystalline layer was characterized by optical microscope (OM), X-ray diffraction (XRD), transmission electron microscope (TEM), and Vickers micro-hardness tester. The results showed that the thickness of the SPD layer gradually increased with the MSP processing time increase, but saturated at 230 μm after 30 min. The average grain size was refined to about 18.48 nm in the nanocrystalline layer. There was equiaxed grain morphology with random crystallographic orientation in the topmost surface. By comparing with the nanocrystalline layer, acquired by surface mechanical attrition treatment (SMAT), the microstructure and properties of the nanocrystalline layer acquired by MSP was evidently superior to that of the SMAT, but the production time was cut to about a quarter of the time used for the SMAT method.

On an Elastoplastic Sliding Model for a Coated Single Asperity

In this study, a sliding friction model for coated single asperity contacts is proposed. A displacement-driven layered contact algorithm is firstly introduced and verified by the finite element method. Then, this algorithm is applied to simulate the contact between two semispherical asperities. The full sliding contact process is discretized into a series of transient steps, and each of these steps are calculated by the displacement-driven contact algorithm. The effects of the interference depth and the properties of, respectively, the tribofilm (thickness, elastic modulus, and yield strength) and the nanocrystalline layer on the sliding coefficient of friction are investigated. The results suggest that when surface adhesion and asperity damage are ignored, the plastic deformation of the tribofilm is the main source of the sliding friction. Greater interference depth, tribofilm with greater thickness, higher elastic modulus or lower yield strength, and the presence of a nanocrystalline layer will lead to a higher coefficient of friction in single asperity sliding.