White Layer In Machining Of H13 Steel And Its Electrochemical Characteristics

Hardened steel has been widely used in industry such as mold and bearing due to its high strength and high hardness. The common machining methods for hardened steel are hard cutting, grinding and electrical discharge machining, surface integrity directly influence the performances of machined parts. White layer is a kind of special microstructures existing in the surface layer of workpiece, and it formed in the machining process of hardened steel. White layer has distinctive characteristics, such as fine-grain structure, high hardness and high brittleness, and it has important effects on the performances of parts. The influencing factors of white layer are so many that it is difficult to control and predict the generation of white layer. So far, only traditional metallographic observation method can be used to get information in the surface layer of parts. This method not only has poor efficiency but also damage the test specimen, so, nearly nondestructive or nondestructive detection methods of white layer need to be developed. As the development of electrochemical technology, some electrochemical measurement methods not only can evaluate the performances of materials, but also have little damage to the test specimen, so, electrochemical methods have the potential of being applied in the detection of white layer. In this paper, AISI H13 steel was used as experimental material, the detection method of white layer using electrochemical methods was proposed on the basis of studying the white layer and its electrochemical properties.The microstructure evolution of surface layer as tool wear in the process of hard milling was studied. The extrusion and friction effect between the tool and the workpiece intensify with the increase of tool wear, surface layer material may undergo phase change and microstructural change, forming dark layer and white layer in sequence.Microscopic experiments, in which metallurgical microscopy, SEM et al. are used, are conducted to study the characteristics of white layer produced in the hardened steel machining process. Microstructural analysis shows that, white layer is composed of sub-micron grain, grain boundary is not obvious, and white layer in hard milling condition is a severe plastic deformation area with high density of dislocation; compared with bulk, the chemical elements contents in white layer change significantly, the carbon and oxygen contents increase distinctly in had milling white layer.The influence rules of subsurface microstructures on the impedance of double electrode layer between workpiece electrode and electrolyte are studied. The impedance curves of specimens with different subsurface microstructures locate in different areas of EIS. According to the results of equivalent circuit fitting, the appearance of white layer significantly reduced the charge transfer resistance Rct of double layer between electrode and solution interface. In other words, white layer decreases the corrosion resistance of specimen. The electrochemical experimental results also manifest that white layer can be detected by comparing the electrochemical parameters of specimens.The effects of electrochemical measurement methods and electrolytes on the effectiveness of white layer detection were analyzed. By observation, potentiodynamic polarization measurement damaged the test specimen, surface material of the specimen was corroded seriously. While EIS measurement not only can distinguish specimens with/without white layer, but also has little negative influence on the test specimen. Compared with 3.5wf% NaCl solution, lmol/L NaOH solution can make a better distinction between specimens with and without white layer in EIS measurement. Therefore, NaOH solution of low solubility suits as the electrolyte used for the white layer detection.