Hardness of alloys

Small amounts of tin (3—5%) are added to leaded red brass and semired brasses to increase the strength and hardness of alloys. For example, alloy UNS C 94700 (88% Cu, 5% Sn, 5% Ni, and 2% Zn) deoxidi2ed with phosphoms, is heat-treatable to provide high strength. 

The dependence of Vickers hardness of alloys under investigation vs. Zr content is demonstrated in Fig. 2. It is seen that Vickers hardness of as-cast Ti-3Al-2Si alloy decreases with Zr additions. For hypoeutectic Ti-3Al-4Si and Ti-3Al-6Si alloys a non-monotonous dependence is observed. 

The various chromium carbides are relatively hard and brittle. They significantly increase the hardness and pyrophoric stability of carbon rich hard materials. These compounds are known as Stellite . The hardness of alloyed steels [9] results from several chromium-iron double carbides of compositions (Fe, Cr)3C2, (Cr, Fe)23C6, and (Fe, Cr)yC3. These mixed carbides crystallize all in the structures of the respective pure chromium carbides with a mixed occupancy of the chromium positions by chromium and iron atoms. 

And] Andersen, A G.H., Jette, E.R., Notes on Mierostrueture and Hardness of Alloys Consisting Essentially of Iron, Chromium and SiUeon , Trans. Amer. Inst. Min. Met. Eng., 131,318-326 (1938) (Phase Relations, Experimental, Meehan. Prop., 9)... 

There is hardly a metal that cannot, or has not, been joined by some welding process. From a practical standpoint, however, the range of alloy systems that may be welded is more restricted. The term weldability specifies the capacity of a metal, or combination of metals, to be welded under fabrication conditions into a suitable stmcture that provides satisfactory service. It is not a precisely defined concept, but encompasses a range of conditions, eg, base- and filler-metal combinations, type of process, procedures, surface conditions, and joint geometries of the base metals (12). A number of tests have been developed to measure weldabiHty. These tests generally are intended to determine the susceptibiHty of welds to cracking. 

In the electronics industry, gold is used as fine wires or thin film coatings and frequendy in the form of alloys to economize on gold consumption and to impart properties such as hardness. Gold has properties that satisfy specific requirements not achievable with less expensive metals (see Electrical connectors Electronics coatings Thin films). 

Refiner Plates. The refiner plates have a constmction of the type shown in Figure 12. The plates are constmcted of hard steel alloys. Alloys of... 

Sodium—lead alloys that contain other metals, eg, the alkaline-earth metals, are hard even at high temperatures, and are thus suitable as beating metals. Tempered lead, for example, is a beating alloy that contains 1.3 wt % sodium, 0.12 wt % antimony, 0.08 wt % tin, and the remainder lead. The German BahnmetaH, which was used ia axle beatings on railroad engines and cars, contains 0.6 wt % sodium, 0.04 wt % lithium, 0.6 wt % calcium, and the remainder lead, and has a Brinell hardness of 34 (see Bearing MATERIALS). 

The greatest use of cubic boron nitride is as an abrasive under the name Bora2on, in the form of small crystals, 1—500 p.m in si2e. Usually these crystals are incorporated in abrasive wheels and used to grind hard ferrous and nickel-based alloys, ranging from high speed steel tools and chilled cast-iron to gas turbine parts. The extreme hardness of the crystals and their resistance to attack by air and hot metal make the wheels very durable, and close tolerances can be maintained on the workpieces. 

Carbides of the Iron Group Metals. The carbides of iron, nickel, cobalt, and manganese have lower melting points, lower hardness, and different stmctures than the hard metallic materials. Nonetheless, these carbides, particularly iron carbide and the double carbides with other transition metals, are of great technical importance as hardening components of alloy steels and cast iron. 

The abrasion resistance of cobalt-base alloys generally depends on the hardness of the carbide phases and/or the metal matrix. For the complex mechanisms of soHd-particle and slurry erosion, however, generalizations cannot be made, although for the soHd-particle erosion, ductihty may be a factor. For hquid-droplet or cavitation erosion the performance of a material is largely dependent on abiUty to absorb the shock (stress) waves without microscopic fracture occurring. In cobalt-base wear alloys, it has been found that carbide volume fraction, hence, bulk hardness, has Httie effect on resistance to Hquid-droplet and cavitation erosion (32). Much more important are the properties of the matrix. 

The desked balance of ductility and strength can be obtained in age-hard-enable alloys, such as beryllium copper, by controlling the amount of precipitate. For higher strength, aging is conducted to provide a critical size dispersion. Greater amounts of precipitate are obtained by increasing the beryllium content of the alloy. 

Table 9 Hsts select properties of Co—Cr alloys. It is generally conceded that the casting shrinkage of the cobalt—chromium alloys is greater than that of the gold alloys. The lower density of the base metal alloys provides a weight advantage over the higher-density gold alloys in certain types of bulky restorations. Cobalt—chromium alloys have Knoop hardnesses of 310—415.

Weld overlays of stainless steel or cobalt-based wear-resistant and hard-facing alloys such as Stellite may salvage damaged equipment. In addition, weld overlays incorporated into susceptible zones of new equipment may provide cost-effective resistance to cavitation damage. 

The second approach, that of surface coating, is more difficult, and that means more expensive. But it is often worth it. Hard, corrosion resistant layers of alloys rich in tungsten, cobalt, chromium or nickel can be sprayed onto surfaces, but a refinishing process is almost always necessary to restore the dimensional tolerances. Hard ceramic coatings such as AbO, Cr203, TiC, or TiN can be deposited by plasma methods and these not only give wear resistance but resistance to oxidation and... 

Table 17.2 Normalised hardness of pure metals, alloys and ceramics...

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