Critical temperature range

Martensite is a hard, nonductile microconstituent formed when steel is heated above its critical temperature and cooled rapidly. In the case of steel of the composition conventionally used for rope wire, martensite can be formed if the wire surface is heated to a temperature near or somewhat in excess of 1400°F (760°C), and then cooled at a comparatively rapid rate. The presence of a martensite film at the surface of the outer wires of a rope that has been in service is evidence that sufficient frictional heat has been generated on the crown of the rope wires to momentarily raise the wire surface temperature to a point above the critical temperature range of the steel. The heated surface is then rapidly cooled by the adjacent cold metal within the wire and the rope structure, and an effective quenching results. 

Quenching through the critical temperature range (if necessary after heat treating well above 1120K to dissolve any existing chromium carbides). 

In principle it is also possible to eliminate the effects of sensitisation by prolonged heat treatment within the critical temperature range to allow diffusion of chromium from the grain interiors to level out and eliminate the region of chromium depletion adjacent to the grain boundaries. In practice, however, the times involved (many hundreds of hours) are too long. 

Data on the critical temperature of deposition of silver on glass are unsatisfactory. Knudsen (47) gives the critical temperature as above 575°. Cockcroft, however, reports critical temperatures ranging from —90 to —15°, depending on the filament temperature, for deposition of silver on mica. 

The critical properties, that are essential basic data if a cubic equation of state is used, can be evaluated using group contribution methods but the numerical values obtained depend on the method used. In particular, this fact represents a problem for multifunctional components that are generally involved in processing natural products and/or pharmaceuticals. As an example, depending on the prediction method used, a critical temperature ranging from 817.8 to 1254.0 K can be obtained for cholesterol [60]. 

The specified minimum of three test pieces is commonly employed, but in a study of test precision Spetz [11] advocates the use of five test pieces to improve test reproducibility. He also draws attention to the importance of thickness measurement and calibration. Care too should be taken to ensure test piece cutters are fully maintained. Some tolerance in cutter sharpness can often be accepted in tests at standard laboratory temperature, but a slightly blunt cutter can seriously impair tensile strength measured at elevated temperatures [12]. Cutter flaws not only reduce the temperature at which strength begins to fall rapidly but also increase the temperature band over which the fall occurs. With some vulcanized rubbers, the critical temperature range can embrace the laboratory temperature and so lead to significant scatter in results. 

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