Effect of Microstructure on Chip Formation during Machining of Super Austenitic Stainless Steel

Effect of Microstructure on Chip Formation during Machining of Super Austenitic Stainless Steel

Mohanad Alabdullah (School of Engineering, Deakin University, Geelong, Australia), Ashwin Polishetty (School of Engineering, Deakin University, Geelong, Australia), Junior Nomani (School of Engineering, Deakin University, Geelong, Australia) and Guy Littlefair (School of Engineering, Deakin University, Geelong, Australia)
DOI: 10.4018/IJMFMP.2017010101
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The AL6XN Super Austenitic Stainless Steel alloy is a commonly used steel in corrosive environments and tough applications. This paper aims to investigate the execution of a machining process on the AL6XN alloy. A wet machining process has been executed to machine the alloy under a combination of various cutting conditions using an up milling approach. Two cutting speeds, two cutting depths and two feeds were used. The outputs obtained and listed in this paper are the microstructure analysis, surface microhardness and the chip morphology. The microstructure of the AL6XN alloy was revealed using Electron Microscope and Electron Backscatter Diffraction (EBSD). Work hardening layer was located in the subsurface of the machined alloy. EBSD data assured that no phase transformation was occurred within the deformed microstructure due to machining. The chip cross-section was revealed to identify the presence of the shear bands and to calculate the alloy serration degree.
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Austenite stainless steel (ASS) is distinguished from typical stainless steel by its chromium content, and has high ductility, extraordinary strength and toughness, and corrosion resistance (Abou-El-Hossein & Yahya, 2005; Fonda et al., 2007). The AL6XN alloy is a variety of ASS but it has been developed into a Super Austenitic Stainless Steel (SASS). The AL6XN SASS alloy is suitable for use in nuclear power and marine, food and chemical industry applications owing to its elevated resistance to corrosion (Koutsoukis et al., 2013). However, high percentages of alloying elements in the microstructure of the material affect its properties when applied to various manufacturing processes.

Machinability is the intrinsic ability of a material to deliver satisfactory performance when it is subjected to metal removal processes to form or shape a specific part that has a high surface finish obtained at a low cost (Lalbondre et al., 2013). The machinability of any work material depends on the properties of the material, the cutting tool geometry, the level of process parameters and the machining environment (i.e. dry or wet machining) (Kalpakjian & Schmid, 2014). A poor machinability feature of ASS is related to the work hardening layer produced that, along with its low thermal conductivity properties, produces segmented and serrated chips during the machining process. Vibrations owing to high cutting forces associated with cutting tool failure are a possible consequence when segmented chips are created. Surface microstructure and microhardness variations are utilised as quality indicators of the machined surfaces (Jang et al., 1996). Surface integrity, especially surface microhardness, is an essential factor when excessive mechanical and thermal loads are requested for special applications such as pumps and transformers (Grzesik et al., 2005).

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