Surface Integrity Analysis in Machining of Hardened AISI 4140 Steel

Abstract

This study aimed to analyze the residual stresses and roughness in finishing milling of AISI 4140 steel, quenched and tempered up to hardness of 58 HRC. Machining operations were performed with the use of CBN inserts and by varying three basic cutting parameters (cutting speed, feed per tooth and cutting depth). Hardened materials are typically machined by abrasive processes, which in turn are more expensive and complex to be studied due to the undefined cutting geometry of the grinding wheel. A series of machining tests with milling process and CBN tools was implemented in order to study the resultant condition of the specimen´s surface. An experimental design was used and the results were statistically treated, enabling the generation of a model that aims to obtain roughness values due to the optimization of three adopted cutting parameters. The roughness values found in the range of Ra 0,16 to 0,4 µm indicate that it is possible to use the milling process with CBN tools for finishing, reducing machining time and the cost of the machined part. The generated residual stresses were compressive and the feed per tooth parameter showed greater influence in this result. The research was limited to test only one type of CBN insert, which was constantly replaced, preventing the influence of tool wear on responses. The geometry of the tool as well as the use of cutting fluid were not considered. Milling process with CBN inserts is confirmed as a possibility for replacing grinding process for finishing machining leading to significant gains in machining time. An optimized model was derived to predict the value of the roughness and three optimizations were made to specify the best cutting parameters to desirable answers such as better roughness, higher compressive residual stresses and low cutting forces, for example.