in Vol. 9 - July Issue - Year 2008
Surface Integrity After Shot Peening Applied To A Precipitation-Hardened Aluminium Alloy
Author Prof. Dr. Janez Grum
Figure 1: Influences on material degradation under different operating conditions of machine components.
Figure 2: Importance of residual stresses in loaded machine component on actual stress variation.
Part 1: General on surfaces and processing (Part 2 September Issue)
As to machine components in operation, one should be aware that there surfaces are subjected to the strongest loads as well as to environmental influences in a mutual contact. Fig. 1 shows various requirements for machine components in operation. In addition to mechanical and thermal stresses, environmental influences and influences of other components, which are, during their operation, subjected to sliding or rolling contacts, should be taken into account. At the extreme right side of the scheme there are listed phenomena which can be found in machine components due to operating circumstances, including fatigue, creep of materials, wear and corrosion. In order to provide an adequate life of a machine component, a suitable material, an adequate construction and processing, particularly surface processing, should be selected. Presuming that the material and construction chosen are the right ones, correct material processing, particularly of a surface layer, should be provided to ensure an adequate life of a machine component.
In surfaces of machine components being most stressed, all attention should be paid to treatment processes, with which the size and variation of residual stresses can be influenced. If the influence of mechanical stresses on material wear or development of stress corrosion is to be reduced, then a suitable size and variation of residual stresses in a surface layer of a machine component shall be provided .
Highly exacting machine components for the most exacting applications resulted, in the early 1960s in the development of a new scientific discipline, i.e. Surface Integrity [2, 3]. Surface integrity takes into account influences of manufacturing processes and machining conditions on the quality of a surface and a thin surface layer with particular regard to a microstructure, hardness variation, i.e. microhardness, and residual stresses as well as influences on material fatigue and corrosion.
The scientific discipline of surface integrity describes only surface properties after different mechanical and thermal treatment processes. Professionals started wondering what is to be done to ensure adequate quality of individual machine components and what is to be taken into account when assessing the quality of a surface or of a thin surface layer to fulfil operating requirements.
Figure 2 shows a total of influences exerted by residual stresses and load stresses on the actual stress in the thin surface layer. In order to provide favourable total stresses in a material during loading of a machine component, experts on manufacturing technologies should be well acquainted with influences of manufacturing conditions on microstructure changes and, in turn, variations of microhardness and residual stresses in the surface layer with individual machining processes. As to material fatigue from the viewpoint of residual stresses, suitable surface heat-treatment processes such as surface hardening, case hardening, nitriding etc., can influence the occurrence of compressive residual stresses. In surface-hardening and case-hardening processes account should be taken that the quenching process may produce high internal stresses during heat treatment, which are due to temperature stresses occurring during cooling, i.e. quenching, which are, in a certain time interval, accompanied also by stresses due to phase transformations. High internal stresses during quenching may lead to high residual stresses that are related to volume changes and/or distortion of machine components. Since 1980 there has been a scientific discipline of Surface Engineering (1980) that has systematically studied various procedures for the improvement of surface properties [4,5].
One should be aware that every material treatment produces certain residual-stress variations and that every machining process will also change residual-stress variations. Control of volume changes and distortion of machine components and knowledge of residual stresses in them are becoming increasingly important in practical applications to high-tech products, particularly from the viewpoint of more adequate operating conditions and economical reasons. The first start of organized treatment of distortion of machine components in steel quenching was a thematic conference in Chicago (1992) titled »Quenching and Control of Distortion« under the auspices of the American Society for Metals and organized by Prof. G.E. Totten. He further organized three more thematic conferences on »Quenching and Control of Distortion«, i.e. in Indianapolis (1996), Prague (1999), and Beijing (2003) [6-9]. With the aim of quality control of manufacturing of machine components, a Collaborative Research Center was founded at the Bremen University in 2001, under a support of the "Deutsche Forschungsgemeinschaft" (DFG), including a new integral approach to product quality. A new scientific field within the project was called Distortion Engineering (2001). It has been directing research to a reduction of distortion and, consequently, cost-effective manufacture of components.
Results of relevant studies on integral quality control of products were presented at the 1st Int. Conf. on Distortion Engineering 2005 (IDE) in Bremen  and at the 5th Conference on Quenching & Control of Distortion in Berlin, which was organized under the auspices of numerous domestic and international associations in the field of materials, heat treatment and surface engineering .
Cold working hardening is a hardening process occurring in cold. Consequently, these processes are easier to control and the size and variation of residual stresses can be efficiently predicted.
Cold working hardening of materials proceeds at a temperature below their recrystallisation annealing temperature. To this group of surface treatment processes belong: shot peening, deep rolling, roto peening or flap peening, and laser shock processing. Here also belongs laser shock processing, although in the interaction between laser light and workpiece surface in the thin surface layer, evaporation of material or absorber can be observed, which, however, depends on the preparation of a machine component, i.e. workpiece material or the type of absorbent applied to the machine-component surface [12-14].
The investigations conducted on numerous machine components in the last decades confirm a major importance of surface quality assurance since the machine components are subjected to major stresses at their surface. At first attention was mainly paid to the manufacture in terms of influences of tools and processing conditions on surface quality of different material types (Surface Integrity – 1961). At a later date various heat treatment and thermal-chemical surface treatments were included, which could additionally substantially improve the surface integrity as far as material fatigue is concerned (Surface Engineering – 1980).
After 1992, considering the conferences organized by Prof. Totten, increased attention has been paid to an influence of steel quenching on machine-component distortion (Quenching and Control of Distortion). Finally the process is treated integrally. All machining processes are optimized with a view to controlling residual stresses and distortion after each production phase and to obtain desired residual stresses and reduction of machine-component distortion (Engineering Distortion-2001).
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14. Schulze V., Niku-Lari A.: Proceedings of the 9th Int. Conf. on Shot Peening, Paris 2005, IITT International, Noisy-le-grand, 2005.
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