Tempered residual stress

Measurements of stress relaxation on tempering indicate that, in a plain carbon steel, residual stresses are significantly lowered by heating to temperatures as low as 150°C, but that temperatures of 480°C and above are required to reduce these stresses to adequately low values. The times and temperatures required for stress reUef depend on the high temperature yield strength of the steel, because stress reUef results from the localized plastic flow that occurs when the steel is heated to a temperature where its yield strength is less than the internal stress. This phenomenon may be affected markedly by composition, and particularly by alloy additions. 

Tempered glasses are useful because failure normally occurs under an applied tensile stress, and failure in ceramics and glasses is almost always initiated at the surface. When a permanent compressive stress, called a residual compressive stress, is placed on a surface, either through thermal or chemical means, the applied stress must first overcome this residual compression before the surface is brought into tension under which failure cau occur (see Figure 5.53). Notice that the residual stress is compressive in nature at the surface of the plate and is tensile in the center (shaded areas). When... 

Figure 5.53 Residual stress, applied stress, and resultant stress distribution for transverse loading of a tempered glass plate. From W. D. Kingery, H. K. Bowen, and D. R. Uhknann, Introduction to Ceramics. Copyright 1976 by John Wiley Sons, Inc. This material is used by permission of John Wiley Sons, Inc.

Post quench hardening and tempering of D2/TiN does not influence the coating residual stresses significantly (Figure 16a). This is due to the almost equal specific substrate volumes... 

Fig. 16 (a) Residual stresses of D2/CVD TiN and D2/CVD TiN/quench hardened -i- tempered and (b) Residual stresses of L3/CVDTiN and L3/CVD TiN/quench hardened -i-tempered, measured by x-ray diffraction (sin xiz-method). 

Figure 3. Residual stresses in automotive steel crown gears (a) net-shaped and submitted to nitrocarburation surface treatment machined by standard technology and submitted to (b) multi-frequency induction tempering, (c) quenching and nitriding, (d) case-hardening.

Residual stresses are generally deleterious to the mechanical properties and should be avoided. This is especially true if a part is to be subjected to thermal cycling. In some situations, however, residual stresses can be used to advantage. A case in point is the transformation toughening of zirconia discussed in Chap. 11, and another excellent example is the tempering of glass discussed in the next section. 

Because of the transparency and chemical inertness of inorganic glasses, their uses in everyday life are ubiquitous. However, for many applications, especially where safety is concerned, as manufactured, glass is deemed to be too weak and brittle. Fortunately, glass can be significantly strengthened by a process referred to as thermal tempering, which introduces a state of compressive residual stresses on the surface (see Sec. 11.3.3). 

If properly introduced, thermal residual stresses can be beneficial, as in the case of tempered glass. 

Qualitatively explain how the following parameters would affect the final value of the residual stresses in a tempered glass pane (a) thickness of glass, (b) thermal conductivity of glass, (c) quench temperature, (d) quench rate. 

Heat treatment given after welding to reduce residual stresses [stress relief heat treatment] and/or to temper hard regions, usually in the HAZ. The temperatures are usually between about 550 and 750 °C,... 

Figure 8. Residual stress distribution in the base of a container ass. For historical reasons, the measured values are usually displayed in the unit of apparent or real temper number...

Temper. The residual stress in annealed glass-ware as measured by comparison with STRAIN DISKS (q.v.). See also TEMPERING. 

J. C. Conway Jr, and J. J. Mecholsky Jr, Use of Crack Branching Data for Measuring Near-Surface Residual Stresses in Tempered Glass, ). Am. Ceram. Soc., 72 1584-1587 (1989). 

They afford low residual stress through controlled stretching processes, and thus manufacturing possibilities of warp-free mold plates. A post-treatment, such as hardening and tempering, is not necessary. 

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