Residual stress compressive

There are many characteristics of hard cases that make their development desirable. One is wear resistance. Usually, the process is designed to develop high compressive residual stresses in the surface which counteract tensile stresses induced by the loading condition during use of the component (1) (Fig. lb). 

Figure 4 shows a typical hardness distribution (7). The case depth is considerably less than that for flame and induction hardening. The case has a high compressive residual stress, which improves the fatigue properties (8). 

The surface may gain a very high (eg, 1000 Vickers) hardness from this process. Surface deformation also produces a desired high compressive residual stress. Figure 9 illustrates the improvement in fatigue properties of a carburized surface that has been peened (18). 

Composite materials inherently develop residual stresses during processing. This happens because the two (or more) phases that constitute the composite behave differently when subjected to nonmechanical loading. For example, consider a reinforcing phase that has low thermal expansion characteristics embedded in a matrix phase with high thermal expansion characteristics. If the material is initially stress free and the temperature is decreased, then the matrix will try to shrink more than the reinforcement. This places the reinforcement in a state of compression (i.e. a compressive residual stress). If the phases are well bonded, then models can be developed to predict the residual stress field that is induced during processing. 

The so-called hypercompressors for the production of LDPE represent a special case. The ethylene is compressed in a primary piston compressor, with several stages up to around 200 to 300 bar the hypercompressor (or secondary compressor) brings the gas up from there to 3000 bar. The hypercompressors show pairwise-opposite-cylinders, and are built with up to fourteen cylinders in a multiplex arrangement. The components loaded by an internally pulsating pressure are either shrunk and/or autofrettage-treated in order to implement protective compressive residual stresses (Fig. 4.1-34). 

After the removal of the autofrettage pressure the part shows the compressive residual stresses required to reduce the operational stresses effectively (see Chapter 1 Introduction Fig. 1.4.10). Successful autofrettage treatment needs tough and sufficiently strong steels which should offer a certain potential for strain-hardening without unacceptable embrittlement. 

It should be understood that the positive effects of autofrettage are always based on two different sources compressive residual stresses, which permit a larger amplitude of the operational stress and, depending on the material, the improved local strength of the material owing to strain-hardening by local plastification. 

The successful application of hard coatings on cutting tool substrates is due to the combination of physical and mechanical properties of the coating. From a functional standpoint, chemical stability, hot hardness, and good adhesion to the substrate are essential optimum coating thickness, fine microstructures and compressive residual stresses can further enhance their performance. CVD A1203 and PVD TiAIN provide the necessary chemical inertness required to machine irons and steels. 

The compressive residual stress crrl in the outside layers of a laminate shields natural and artificial cracks in the layer. Therefore, the effective (apparent) fracture toughness of such a structure increases. The more compressive residual stress induced, the more shielding occurs. Another important factor that contributes to the apparent fracture toughness increase... 

However, during long exposures to medium-temperature operating conditions, e.g. 1000°C, spinel formation is certainly expected. Wang etal.60 demonstrated this for the Ni-alumina system, showing the diffusion of Ni atoms to the free surface of the nanocomposite, followed by the formation of a nickel spinel surface coating which then limits the kinetics of subsequent oxidation. In this case the formation of a spinel surface layer may be beneficial to mechanical properties, since the reaction results in a volume increase, and the formation of compressive residual stresses. An analogous behavior was reported for ceramic particle nanocomposites, where oxidation of SiC particles results in an increase in volume and compressive residual stresses.61... 

Shot peening and other mechanical processes that create compressive residual stresses at the surface are recommended. 

Carburizing + CVD offers the possibilities of strengthening the substrate surface and inducing compressive residual stresses in the substrate surface. Further, it can be done sequentially in one chamber just by changing the atmosphere from low pressure carburizing to CVD. Figure 27 shows an example of a... 

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