Jinxing Kong

The plastic deformation produced in the machining process can cause residual stress. As an effective way to control the residual stresses, the cryogenic process is used in modern industries such as aerospace, automobile, and shipping industry. Focusing on the minimization of surface stress in the cryogenic process of pure iron, the Taguchi design is used in this paper. The effect of cryogenic temperature (77–193 K), holding time (8–24 H), cooling rate (2–6 K/min), and warming-up rate (0.5–1.5 K/min) on surface residual stress is discussed and the optimal combination of cryogenic parameters is obtained using signal-to-noise (S/N) ratios. To overcome the weakness of the Taguchi method that cannot calculate stresses, an exponential model to predict residual stresses considering cryogenic parameters is developed. The coefficients of this mathematical model are obtained using multilinear regressive analysis based on the database of the Taguchi experiment. After this, the optimization process is conducted with this model using the genetic algorithm (GA). The optimized results using both ways coincide with each other. The optimal cryogenic parameters are obtained, i.e., cryogenic temperature of 193 K, holding time of 24 h, cooling rate of 2 K/min, and warming-up rate of 0.5 K/min. When the optimum cryogenic parameters are used, the surface residual stress is reduced by 42.9% in the cutting direction and 46.2% in the feeding direction. The method can be applied to the actual machining engineering to realize the low-stress control on the cutting surface.

To reduce the influence of internal residual stress on the processing deformation of thin-walled hydrogen-resistant steel components, combined aging cryogenic and high-temperature treatment was used to eliminate the residual stress, and the effect of cryogenic process parameters on the initial residual stress of the specimens was compared and analyzed based on the contour method. X-ray diffraction, electron backscatter diffraction, and transmission electron microscopy were used to research the mechanism of the effect of cryogenic treatment on the internal residual stress of the specimen. After forging, the internal residual stress distribution of the hydrogen-resistant steel specimens without aging was characterized by tensile stress on the core and compressive stress on both sides, with a stress amplitude of −350–270 MPa. After compound treatment of −130 °C for 10 h and 350 °C for 2 h, the internal residual stress distribution remained unchanged, and the stresses decreased to −150–100 MPa. The internal residual stresses were reduced by 57–63% compared with the untreated specimens. The cryogenic treatment did not cause phase transformation and carbide precipitation of the hydrogen-resistant steel material. Instead, grain refinement and dislocation density depletion were the main reasons for the reduction in internal residual stresses in the specimens.

A thin-walled spherical shell made of pure iron material is a key part of precision physical experiments. Tool wear is an important factor restricting its geometric accuracy. And tool wear characteristics of curved surface parts are significantly different from those of single-point turning due to the movement of the contact point between the tool and the workpiece. Hence, this article takes the pure iron spherical shell as the research object, which is aimed at investigating the formation mechanisms of flank wear land. Tool wear characteristics are compared between spherical shell turning and end face turning. The results show that uniform flank wear land and notable notch wear occur when turning end face, but notch wear disappears and only flank wear land exists when turning spherical shell. Based on major notch position and minor notch position, a mathematical model is developed to explain formation mechanisms of flank wear land during turning spherical shell of pure iron materials. Theoretical and experimental results show that flank wear land results from the major and minor notch movement. Spherical shell turning and end face turning have the same wear mechanisms, mainly composed of adhesive wear, diffusion wear and oxidation wear.