Abstract Hardened layers are commonly required for automotive components after their production using a machining process in order to enhance the service life of these components. This study investigates the possibility of producing a nanostructured machined surface which can increase the hardness of the machined surface by varying the machining parameters under cryogenic conditions in end milling of AISI 4340. The end milling experiments were performed using multi-layered TiAlN- and AlCrN-coated carbide. Prior to the experiment, a finite element method (FEM) was used to simulate the cutting temperature generated and it had been found that at cutting speed of 200–300 m/min, feed rate of 0.15–0.3 mm/tooth, axial depth of cut of 0.3–0.5 mm, and radial depth of cut of 0.2–0.35 mm, the temperature generated can be sufficiently high to cause austenitic transformation. A field emission scanning electron microscope (FESEM) equipped with angle selective backscattered (AsB) detection analysis was used to investigate the microstructure and machined-affected layers of the machined surfaces. The crystallographic orientation/phase change and nano-hardness were analysed through X-ray diffraction (XRD) and a nano-hardness testing machine. The results showed that the cryogenic machining had significantly affected the surface integrity characteristics of the AISI 4340 alloy due to refined microstructure, favourable phase structure, and higher hardness near the surface layer. The results of this study may be useful in providing an insight into a potential technological shift from conventional surface case hardening processes to the present technique. Graphical abstract

中文翻译：

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AISI 4340 纳米结构加工表面的低温铣削和形成

摘要 汽车部件在使用机加工工艺生产后通常需要硬化层，以提高这些部件的使用寿命。本研究调查了通过在低温条件下改变 AISI 4340 端铣加工参数来提高加工表面硬度的纳米结构加工表面的可能性。端铣实验使用多层 TiAlN- 和 AlCrN- 进行涂层硬质合金。在实验之前，使用有限元方法（FEM）模拟产生的切削温度，发现切削速度为 200-300 m/min，进给速度为 0.15-0.3 mm/齿，轴向深度为切削0.3-0.5 mm，径向切削深度0.2-0.35 mm，产生的温度可以足够高以引起奥氏体转变。配备有角度选择性背散射 (AsB) 检测分析的场发射扫描电子显微镜 (FESEM) 用于研究加工表面的微观结构和加工影响层。通过X射线衍射（XRD）和纳米硬度试验机分析晶体取向/相变和纳米硬度。结果表明，低温加工显着影响了 AISI 4340 合金的表面完整性特性，因为其微观结构细化，相结构良好，表层附近硬度更高。这项研究的结果可能有助于深入了解从传统表面表面硬化工艺到当前技术的潜在技术转变。图形概要