Effect of cutting on surface hardness and residual stresses for 12Mn austenitic steel

Purpose: Austenitic steels are known for their high impact toughness and resistance against abrasive wear, yet their machining is difficult and limits their application. Since surface conditions resulting from production strongly affect the performance of finished products, any information linking the machining process to the mechanical properties of the surface is useful not only in production but also in the design phase of the product. Design/methodology/approach: The state of the cutting zone was researched using a quick-stop device for suddenly stopping the cutting process and so retaining the mechanical conditions developed during machining. Residual stresses were measured using X-ray diffractometry, while the standard Vickers micro-indentation hardness test was used for determining the material hardness in the cutting zone. Findings: It was confirmed that the analysed material hardens substantially during machining and that the wear of cutting tools can be related both to this phenomenon and to the material structure after heat treatment. Furthermore, it was found that inadequate machining conditions can lead to tensile stresses that alone can initiate cracks in the surface layer even before the material is additionally loaded. Research limitations/implications: Since measurements of temperature in the cutting zone were not performed, the effect of temperature on the final mechanical properties of the surface can only be estimated. The dislocation theory of hardening is briefly explained, while actual research of dislocations is limited. Since the orthogonal cutting process involves substantial plastic deformation, the study of dislocation motion in the cutting area could be of interest. Practical implications: The main reasons why highly hardening materials require an accurate assessment of the cutting conditions are outlined. It is shown that an apt choice of cutting conditions has a favourable influence both on the condition of the surface after cutting and on the tool life. Originality/value: This paper presents an account of some of the difficulties that are associated with machining austenitic and other highly hardening materials. Since the detailed composition of the material and all the important machining parameters are listed, the results presented can also be useful for checking or calibrating numerical models of the cutting process.