Hardness, Structural Alloys

After a few years of operation there was a significant increase in radiation fields from the primary circuit piping in the Douglas Point generating station. Other water-cooled reactors around the world experienced similar effects. The principal source of the radioactivity was traced to cobalt-60, formed by neutron absorption in the natural cobalt-59 which arose from hard-facing alloys and was also present as an impurity in boiler materials such as HMDnel, and in carbon steel and other structural materials. The mechanism of this radioactivity transport was found to be corrosion of the cobalt bearing materials, transport... 

When chromium is alloyed with iron, tough, hard steels or steels that are corrosion-resistant are formed. Chromium is also alloyed with other transition metals to produce structural alloys for use in jet engines that must withstand high temperatures. A self-protective metal, chromium is often plated onto other materials to protect them from corrosion. 

In alloyed air-hardened steels (13 CrMo44), thorough tempering at 750°C is needed after welding to eliminate the hardness structure. Only when the hardness is below 220V are these steels resistant to stress corrosion cracking. 

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. 

We next consider liquid alloys which, though metallic in character, have thermodynamic properties which suggest a fair degree of local order. The total X-ray pattern for such liquids appears to have rather more structure than for hard sphere alloys and is frequently characterized by a split main peak. One example is liquid CueSns and the X-ray data for this Uquid is given at Figure 7.16. 

Iron is hard, brittle, fairly fusible, and is used to produce other alloys, including steel. Wrought iron contains only a few tenths of a percent of carbon, is tough, malleable, less fusible, and has usually a "fibrous" structure. 

UNS C 81500 (chromium—copper alloy) is used structurally where strength and hardness are required and UNS C 81700 (beryllium—copper alloy) is used stmcturaEy where high strength and hardness are required. 

Poor Weldability a. Underbead cracking, high hardness in heat-affected zone. b. Sensitization of nonstabilized austenitic stainless steels. a. Any welded structure. b. Same a. Steel with high carbon equivalents (3), sufficiently high alloy contents. b. Nonstabilized austenitic steels are subject to sensitization. a. High carbon equivalents (3), alloy contents, segregations of carbon and alloys. b. Precipitation of chromium carbides in grain boundaries and depletion of Cr in adjacent areas. a. Use steels with acceptable carbon equivalents (3) preheat and postheat when necessary stress relieve the unit b. Use stabilized austenitic or ELC stainless steels. 

Nickel normally crystallises in the f.c.c. structure it undergoes a magnetic transformation at 357°C and is ferromagnetic below that temperature. In all the alloys shown in Table 4.21 the f.c.c. (austenitic) structure is substantially retained, and in consequence most of the alloys possess the combination of properties required of materials for widespread industrial acceptability, i.e. tensile strength, ductility, impact strength, hardness, hot and cold workability, machinability and fabrication. 

Steel may have some merit SSCC of weld repairs in well-head alloys was investigated by Watkins and Rosenberg who found that the repairs were susceptible to this problem because of the hard HAZs developed by welding. Post-weld heat treatment was an essential but not complete cure compared with unrepaired castings. In the case of hydrogen-assisted cracking of welded structural steels, composition is more important than mechanical properties and the carbon equivalent should be... 

Homogeneous alloys of metals with atoms of similar radius are substitutional alloys. For example, in brass, zinc atoms readily replace copper atoms in the crystalline lattice, because they are nearly the same size (Fig. 16.41). However, the presence of the substituted atoms changes the lattice parameters and distorts the local electronic structure. This distortion lowers the electrical and thermal conductivity of the host metal, but it also increases hardness and strength. Coinage alloys are usually substitutional alloys. They are selected for durability—a coin must last for at least 3 years—and electrical resistance so that genuine coins can be identified by vending machines. 

Metals are crystalline in structure and the individual crystals contain positive metal ions. The outer valency electrons appear to be so loosely held that they are largely interspersed amongst the positive ions forming an electron cloud which holds the positive ions together. The mobility of this electron cloud accounts for the electrical conductivity. The crystal structure also explains the hardness and mechanical strength of metals whereas the elasticity is explained by the ability of the atoms and ions to slide easily over each other. Metals can be blended with other metals to produce alloys with specific properties and applications. Examples include ... 

The work on carbon nitride solids is strongly related to research on diamondlike carbon (DLC) materials [5, 6]. DLC materials are thin film amorphous metastable carbon-based solids, pure or alloyed with hydrogen, which have properties similar to that of crystalline diamond (high hardness, low friction coefficient, high resistance to wear and chemical attack). This resemblance to diamond is due to the DLC structure, which is characterized by a high fraction of highly cross-linked sp -hybridized carbon atoms. To obtain this diamond-like structure... 

The structures of electroplated hard alloys have been less extensively studied than those of similar electrolessly deposited materials. Sallo and co-workers [118-120] have investigated the relationship between the structure and the magnetic properties of CoP and CoNiP electrodeposits. The structures and domain patterns were different for deposits with different ranges of coercivity. The lower-f/c materials formed lamellar structures with the easy axis of magnetization in the plane of the film. The high-Hc deposits, on the other hand, had a rod-like structure, and shape anisotropy may have contributed to the high coercivity. The platelets and rods are presumed to be isolated by a thin layer of a nonmagnetic material. 

---