Tempered stresses

There are actually four groups of tempering stress levels in the glass industry as defined by the American Society for Testing Materials (ASTM). 

The upper use temperature for annealed ware is below the temperature at which the glass begins to soften and flow (about Pa-s or 10 P). The maximum use temperature of tempered ware is even lower, because of the phenomenon of stress release through viscous flow. Glass used to its extreme limit is vulnerable to thermal shock, and tests should be made before adapting final designs to any use. Table 4 Hsts the normal and extreme temperature limits for annealed and tempered glass. These data ate approximate and assume that the product is not subject to stresses from thermal shock. 

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. 

Most wrought alloys are provided in conditions that have been strengthened by various amounts of cold work or heat treatment. Cold worked tempers are the result of cold rolling or drawing by prescribed amounts of plastic deformation from the annealed condition. Alloys that respond to strengthening by heat treatment are referred to as precipitation or age hardenable. Cold worked conditions can also be thermally treated at relatively low temperatures to affect a slight decrease in strength (stress rehef annealed) to benefit other properties, such as corrosion resistance and formabiUty. 

Eig. 10. Stress relaxation behavior at 105°C of the phosphor bron2e alloy C521, in the A, reHef annealed HR04 and roUed B, H04, tempers compared to C,... 

For stress-relieved tempers (T3.51, T3510, T3.511, T451, T4510, T4511, T651, T 510, T6511) stress values for material in the listed temper shall be used. 

The maximum operating temperature is arbitrarily set at 260 C (500 F) because harder temper adversely affects design stress in the creep-rnptnre-temperatnre ranges. 

Pipe produced to tliis specification is not intended for high-temperatnre service. The stress values apply to eitlier nonexpanded or cold-expanded material in the as-rolled, normalized, or normalized and tempered condition. 

The strength of glass under constant loading also increases with decrease in temperature. Since failure occurs at a lower stress when the glass surface contains surface defects, the strength can be improved by tempering the surface. 

In water solutions containing hydrogen sulfide, austenitic steels fail by stress corrosion cracking when they are quenched and tempered to high strength and hardness (above about Rockwell C24). 

Fig. 8.12 Effect of applied stress intensity upon crack velocity for high-strength (180 GN/m UTS) quenched and tempered steel (AFC 77) in distilled water (after Spicdel )...

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