Defining the Post-Machined Sub-surface in Austenitic Stainless Steels
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COMPONENTS produced from austenitic stainless steel often require machining; either to obtain the desired dimensions and/or to attain appropriate surface finish. Austenitic stainless steels generally possess excellent corrosion resistance. However, machining has the potential to be detrimental to corrosion performance.[1–4] Therefore, the governing processes of machining require a systematic study. Higher surface roughness is shown to deteriorate resistance to corrosion.[4–8] While machining does change surface roughness,[1,2,4,9] it has also been shown to alter the sub-surface layer by forming a nano-crystalline grain N. SRINIVASAN is with the IITB-Monash Research Academy, IIT Bombay, Powai, Mumbai 400076, India. B. SUNIL KUMAR and V. KAIN are with the Materials Processing & Corrosion Engineering Division, Bhabha Atomic Research Centre, Mumbai 400085, India. N. BIRBILIS is with the Department of Materials Science and Engineering, Monash University, Clayton, VIC 3800 Australia, S.S. JOSHI is with the Department of Mechanical Engineering, IIT Bombay, Mumbai 400076, India, P.V. SIVAPRASAD is with the Sandvik Group R&D, Sandvik Asia Pvt. Ltd, Dapodi, Pune 411012. India. G. CHAI is with the Sandvik Materials Technology, 811 81 Sandviken, Sweden. A. DURGAPRASAD and I. SAMAJDAR are with the Department of Metallurgical Engineering & Materials Science, IIT Bombay, Mumbai 400076, India. Contact e-mail: [email protected] S. BHATTACHARYA is with Technical Physics, Division Bhabha Atomic Research Centre, Mumbai 400085, India. Manuscript submitted August 25, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
structure,[1,3,7,10] enforcing strain hardening,[1,7,11] and introducing tensile residual stress.[2,4,12–15] Naturally, the topic has attracted academic and applied research, with such interests broadly classified into two categories: (a) interests originating from the mechanics of machining[15] and (b) interests related to microstructural developments.[2,7,8] Both the interests are, however, linked to the performance (mechanical as well as electrochemical) of machined surfaces and sub-surfaces. The demands of industry have led to incorporation of higher speeds in machining processes.[16,17] Higher machining speeds are expected to provide increase in strain rates, and hence enhanced plastic deformation of the sub-surface layer. The higher machining speed may also cause higher resultant surface temperature.[14] These counter-balancing effects, strain rate and temperature on the effective strain hardening, contribute to the performance of the machined surface and sub-surface. Development of sub-surface microstructure is critical in determining the nature and stability of passive Cr2O3 film. The stability of the passive film and the extent of introduced residual stress would affect its susceptibility to stress corrosion cracking.[7,18,19] In this paper, the response of three grades of stainless steels to machining at different speeds was studied. The effects of machining speed were explored by characterizing surface roughness, resid
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