Surface Integrity in Machining AISI 4340 Steel

The effect of cutting speed and tool wear land length on the surface integrity of quenched and tempered AISI 4340 steel machined under dry, unlubricated orthogonal conditions is determined. The surface region of machined test pieces is examined using optical microscopy, scanning electron microscopy, X-ray microprobe analysis, microhardness measurements, and profilometry. In addition, tool forces are measured and tool temperatures calculated. The results of the investigation show that during machining a damaged surface region is produced which is quite different from the bulk of the material. It is found when cutting at low speeds with sharp cutting tools that the damage is restricted to a variety of geometrical defects associated with the surface. It is found when cutting at high speeds or with tools having large artificially controlled wear lands that considerable subsurface damage involving changes in metallurgical structure and hardness is produced. The results are interpreted in terms of the type of chip produced during machining, the temperatures generated during machining, and the interaction between the tool nose region and workpiece. It is suggested that observations based on scanning electron microscopy are more indicative of the true surface condition than surface roughness measurements.