Hardness Tests at High Temperatures

Hardness determinations were made at every fifty degrees up to 600° on sixty cylindrical test pieces, 20 mm. long and 20 mm. in diameter. The pressure used was 500 kg. 

The results are shown in Fig. 60, which should be considered side by side with those obtained under the same conditions for aluminium and casting alloys. 

—High Temperature Hardness Tests (600 Kg.) on Duralumin quenched from 475°. 

The cupro-aluminiums considered, from an industrial standpoint, are those in which the respective amounts of the constituents are limited to the part of Curry s diagram lying between 88 % and 92 % of copper, or 12 % and 8 % of aluminium, though the presence of other constituents, such as manganese, iron, or nickel, may cause variations in these amounts. 

The typical alloy, i.e. the alloy containing 90 % of copper and 10 % of aluminium, was studied in a very thorough manner by H. St. Claire Deville, more than sixty years ago, at which time it was still a precious metal, whose cost price was about 32 francs per kilogramme (11s. 9d. per lb.). 

---

In the other casts, cylindrical bars were made for hardness tests at high temperatures. These were carried out, using a 10 mm. ball and loads of 500 and 1000 kg. 

The results of the hardness tests at high temperatures are shown in Eig. 31. 

The results of the hardness tests at high temperatures are summarised in Fig. 35, which shows the rapid falling off in hardness as the temperature is increased. The hardness at ordinary temperatures, however, is greater than that of the majority of other casting alloys. 

Hardness tests at high temperatures were carried out on cylinders, 2 cm. in diameter, and 2 cm. high, as in the case of the light alloys of great strength. 

The hardness tests at high temperatures were carried out under the same conditions as for Type I, and the results are shown in Fig. 786. 

Another way to achieve the strongly time-dependent hardness range, preceding the achievement of the constant time independent minimum value of hardness predicted by equation (4.18), is to make hardness tests at elevated temperatures. As the test temperature increases above where T is the melting point of the sample, the hardness rapidly decreases and becomes time dependent until a temperature is reached where the dislocation mobility becomes high, the yield strength becomes very low, and hardness approaches the constant, time-independent value. It may not be easy with some ceramics to achieve the constant hardness zone because elevated temperature may permit diffusional creep, when once again the hardness will become obviously time dependent, and hardness values lower than that predicted by equation (4.18) will be achieved. 

It is hardly surprising that the preparation of surfaces of plain specimens for stress-corrosion tests can sometimes exert a marked influence upon results. Heat treatments carried out on specimens after their preparation is otherwise completed can produce barely perceptible changes in surface composition, e.g. decarburisation of steels or dezincification of brasses, that promote quite dramatic changes in stress-corrosion resistance. Similarly, oxide films, especially if formed at high temperatures during heat treatment or working, may influence results, especially through their effects upon the corrosion potential. 

Vertical media with very high coercivities can be produced by plating into alumina pores [112], Some of these media are too hard to be easily written with present heads. Tailoring of the pore size can be used to obtain structures with the desired Hc [115, 116], however. Recording characteristics of disks have been determined [112-114, 116] such media show excellent promise as vertical recording media. In addition, structures with electrodeposited Fe in the pores were tested in life-tests at elevated temperatures and humidity and in corrosive atmospheres. They were found to perform satisfactorily. 

A series of mechanical property determinations was made to determine the reproducibility and spread of the tests results when materials of various levels of crystallinity were tested at cryogenic temperatures. It was found that there was a definite correlation between the crystallinity (as determined by the hardness test) of the samples and their low-temperature mechanical properties. As an example, samples of relatively low crystallinity were much stronger and more ductile at cryogenic temperatures than the highly crystalline samples. As a result of these determinations, it was found that a reasonably accurate set of low-temperature mechanical properties could be assigned to any fluoroplastic by performing this simple non-destructive room temperature hardness test. 

Low Temperature Properties. Medium hardness compounds of average methyl acrylate, ie, VAMAC G, without a plasticizer typically survive 180° flex tests at —40° C. Such performance is good for a heat-resistant polymer. Low temperature properties can be greatly enhanced by the use of ester plasticizers (10). Careful selection of the plasticizer is necessary to preserve the heat resistance performance of the polymer. Plasticized high methyl acrylate grades lose only a few °C in flexibiUty, compared to grades with average methyl acrylate levels. 

The crystalliza tion resistance of vulcaniza tes can be measured by following hardness or compression set at low temperature over a period of time. The stress in a compression set test accelerates crystallization. Often the curve of compression set with time has an S shape, exhibiting a period of nucleation followed by rapid crystallization (Fig. 3). The mercaptan modified homopolymer, Du Pont Type W, is the fastest crystallizing, a sulfur modified homopolymer, GN, somewhat slower, and a sulfur modified low 2,3-dichlorobutadiene copolymer, GRT, and a mercaptan modified high dichlorobutadiene copolymer, WRT, are the slowest. The test is often mn near the temperature of maximum crystallization rate of —12° C (99). Crystallization is accelerated by polyester plasticizers and delayed with hydrocarbon oil plasticizers. Blending with hydrocarbon diene mbbers may retard crystallization and improve low temperature britdeness (100). 

---