The grindability of titanium alloys that are classified as hard-to-machine materials is studied in high-speed cylindrical grinding using a cubic boron nitride (CBN) wheel. The investigation is concerned with residual surface stresses, including the construction of its empirical model, orthogonal experiments with a CBN grinding wheel at a speed of 45-150 m/s, and prediction with the back propagation (BP) network. The results of residual surface stress measurements obtained in grinding experiments and simulation analysis for five sets of grinding conditions are compared, it can be seen that the empirical model is partially applicable to a Ti-6Al-4V titanium alloy (TC4) under examined grinding conditions. Generally, the calculation results with the empirical model exhibit a significant deviation from the data of actual measurements in some cases. The BP network possesses the function of complex nonlinear mapping and adaptive learning. So the BP network is adopted to predict the relation between residual surface stresses and three key grinding conditions accurately enough. The accuracy of the network is verified, which lays the foundation for its in practical application.Introduction. Titanium alloys are difficult to grind due to their poor thermal properties and high chemical activity. The primary challenges in grinding titanium alloys are: the high specific energy, the high temperature in grinding area as well as the heavy grinding wheel adhesion and wear. Grinding is a common finishing method for titanium material, in order to obtain a precise surface quality. The surface residual stress is one of the main evaluations indices of the surface quality, which has a high impact on the performance [1, 2] of the workpiece. Lin and Lee [3] studied the effect of tool flank wear on the surface residual stresses of the machined surface. Ee et al. [4] used the finite element method to investigate the impact of sequential cuts, cutting conditions, etc., on the residual stresses induced by orthogonal machining. Vosough et al. [5] investigated the impact of high-pressure water-jet on the surface residual stress and concluded that the high-pressure jet increases the level of residual compression stresses in both cutting and feed directions, and thus the high-pressure water jet-assisted machining of titanium alloys is beneficial. Kang and Ren [6] analyzed the causes of the grinding residual stress of Ti-alloy specimen and optimized the grinding residual stresses of titanium alloy by selecting the reasonable grinding conditions, CBN grinding wheel and using the high-quality grinding fluid with extreme-pressure (EP) agent. Hu and Yuan [7] analyzed the forming mechanism of residual stresses produced in the grinding process and concluded that higher constraints to the deformation in grinding process generate lower residual stresses, while the wheel material can diffuse into the surface layer of the workpiece to increase the compressive residual stresses.

Exposure and slow strain rate tensile (SSRT) tests were conducted in a simulated pressurized water reactor (PWR) primary water to investigate the oxidation resistance and SCC susceptibility of 308L and 309L stainless steel (SS) cladding layers. A double-layer structure oxide layer grown on 308L SS and 309L SS contained the Cr-enriched nanocrystalline internal layer and the Fe-enriched spinel oxide in the external layer. Ni-enrichment at the matrix/oxide (M/O) boundary was observed. The internal oxide film on 309L SS was thicker and had a lower Cr content than that on 308L SS. Preferential dissolution of inclusions led to pits on 308L SS and 309L SS surfaces during the exposure tests. More inclusions in 309L would decrease its SCC resistance due to the pits can act as the SCC initiation site. 308L SS had a lower susceptibility of SCC than 309L SS in PWR primary water. Lower ferrite content, higher strength/hardness reduced the oxidation and SCC resistance of 309L SS cladding. The effect of ferrite on oxidation and SCC of the SS claddings was discussed.

The microstructure and the oxide films formed on Ni-base alloy cladding in simulated pressurized water reactor (PWR) primary water environments at 325 o C were characterized. There was a steep drop of iron content and increase of Ni and Cr content in the Ni-base alloy cladding near the fusion line, and a relatively mild drop of iron content and the resultant increase of Ni and Cr contents at the location about 3.8 mm away from the fusion line. The surface oxide morphologies were observed by scanning electron microscopy and the cross-section morphologies and chemical composition of the oxide films were measured by transmission electron microscopy (TEM) and energy dispersive spectrometer. After 146 h immersion in PWR primary water, there were more oxide particles on the cladding alloy at the location closer to the cladding fusion line. After 1860 h immersion in PWR primary water, fine oxide particles on the specimen surface were covered with sparsely distributed large oxide particles. The oxide film after 1860h immersion exhibited a duplex structure, which was thicker at the location closer to the fusion zone, according to TEM results. The outer layer was rich in iron and the inner layer was rich in chromium. Both the inner and the outer oxide films were depleted in Ni, despite of the high Ni content in the alloy matrix.