Influence of tellurium addition on drilling of microalloyed steel (DIN 38MnS6)

Purpose - This paper seeks to evaluate the influence of tellurium content on the machinability of the microalloyed pearlitic steel (DIN 38MnS6). Two grades of steels were used, one with high (27 times greater) tellurium content and one with a low tellurium content. Machinability of the steel was determined by the number of holes drilled by the tool before undergoing severe deformation. The drilling test matrix was prepared using a fractional factorial design with five input variables studied at two levels (25-1). Other variables investigated include cutting speed (45 and 60 m/min), feed rate (0.15 and 0.25 mm/rev), geometry of the twist drills and use of minimum quantity lubrication (MQL) at the flow rates of 30 and 100 ml/h. Statistical analysis of the results revealed that composition of the work material was most influential on tool performance. Addition of tellurium to the steel significantly improved machinability, increasing the number of drilled holes by over 100 per cent. The MQL flow rate was the least influential as increase in the flow from 30 to 100 ml/h reduced drill life only by about 9 per cent. Design/methodology/approach - The drilling tests were carried out in the vertical position, up-down, without pre-holes (full drilling). Cutting speeds of 45 and 60 m/min and feed rates of 0.15 and 0.25 mm/rev were employed. Drills with two sharpening types were tested. Cutting fluid used was vegetable based and applied using the MQL technique at flow rates of 30 and 100 ml/h. The rejection criterion adopted was severe deformation of the drills and the number of machined holes was used to measure the machinability of the material. Findings - Of all the variables investigated in this study, the least influential on drill performance is the MQL flow rate. Increase in the flow rate from 30 to 100 ml/h reduced drill performance by 9 per cent, contrary to expectation. This is a result of the cooling-lubricant action balance promoted by the cutting fluid applied in low quantities (MQL). The most influential variable on drill performance is addition of Te to the work material which gave over twofold (103 per cent) improvement in drill performance at the cutting conditions investigated. The Te particles act at the chip-tool interface, reducing the work necessary to shear the material during chip formation. Increase in both the cutting speed and the feed rate both lowered drill performance during machining due to associated increase in cutting temperature which tended to accelerate thermally related wear mechanisms. Originality/value - This work was conducted to evaluate the machinability of a novel alloyed steel employed in the automobile industry. Drilling was, considered as most automobile components especially the engine block is designed with many holes which require drilling process.