Paper 4: Surface Integrity—A New Requirement for Surfaces Generated by Material-Removal Methods

Abstract

The design of structures for modern industry has created stringent service, reliability, and safety requirements. Present manufacturing specifications for surfaces are concerned primarily with geometry, which includes surface roughness and accuracy. The need is growing to consider not only surface geometry per se, but also the nature of alterations in the surface layer. Typical surface layer modifications, as a result of temperature, stress, and environment during material removal, include phase transformations, plastic deformation and fracture, and chemical changes. These various changes in the surface layer become increasingly significant with the more extensive application of materials such as high-temperature alloys, high-strength steels (including the maraging steels), refractory alloys, titanium, and beryllium. Some of the non-conventional machining methods, such as electrical discharge machining and grinding, electrochemical machining and grinding, and laser beam machining, can generate surface layer characteristics quite different from those produced by conventional methods. Surface alterations may also affect the residual stress, and with it the accuracy of thin section hardware, as well as decrease some of the significant mechanical and physical properties, such as fatigue strength, stress corrosion resistance, and fracture propagation rate. Control of surface layer characteristics, consistent with design requirements, develops ‘surface integrity’ which, in turn, leads to reliable service performance. A general review of previous work on the subject of surface integrity is presented. Typical microstructural surface alterations are illustrated, and the residual stress induced in the surface and its correlation with dimensional accuracy is demonstrated. A survey is presented relating machining methods to fatigue strength and stress corrosion characteristics. Finally, some guidelines are suggested for the practical implementation of surface integrity by means of processing specifications. Some consideration is given to the cost of implementing surface integrity requirements and their effect on production rates, including precautions concerning needless over-emphasis for certain manufacturing requirements. The data presented in this paper manifest that the metrologist's technical jurisdiction should be extended in depth from the Ångström range to the millimetre subsurface level.