450 (annealed for machining) 600 (hardened) 400-600 depending on desired properties

All three classes contain a martensitic matrix with secondary hard phases of chrome and iron carbides that increase the wear resistance. The molybdenum in class II increases the material's hardenability for thicker cross sections.

In general, it should be stressed that machining and welding these three classes of material is impossible. Another important consideration is the role of carbon content on corrosion, erosion, and fracture resistance. High-carbon contents reduce corrosion resistance because any chromium tied up as chrome carbide is no longer available to form a protective chrome oxide layer. Although beneficial with regard to erosion and abrasion resistance, high-carbon content increases the susceptibility to breakage by thermal and mechanical shocks.

To counteract this problem, a number of precautionary measures must be adopted to enhance the serviceability of this class of material. First, slow warm-up cycles must be instituted, typically around 100 to 150°F per hour.14 Another strategy is to lower the hardness from about 600 to 400 Brinell by a partial anneal. This measure reduces brittleness, but at the expense of erosion resistance.

Linings, Inserts, and Coatings The low ductility and toughness of A532 cast irons do not permit their usage with primary pressure boundaries according to ASME code and API regulations. This restricts the use of hard irons to internal wetted parts. Therefore, it is necessary to use steel pressure casings with hard materials as liners. A common slurry pump consists of ASTM A532, Class III, Type A (HC-250) impellers and replaceable HC-250 wear liners for the volute and for both the inlet and outlet ends of the pump casing.

Since pump erosion is often quite localized, in some instances it is more practical to install replaceable, mechanically attached inserts at high wear areas, such as the cut water. These are typically made of sintered tungsten carbide or some other hard material. Newer materials, such as ceramic composites and toughened ceramics, should perform better than the "cermets" used in the past.

Several problems, however, can occur with mechanically attached inserts. One problem is protecting the fastening device against erosive wear. Another problem is the insert's tendency to act as a turbulence riser due to an imperfect fit or erosion-induced crevices and offsets. Limited success has been achieved with weld-applied overlays of Stellite and other hardfacing materials. Weld overlays are extensively utilized, but several problems may occur. These include a propensity for cracking, debonding resulting from preferential corrosion of the bond line, dilution of the hardfacing material with the substrate, and potential uneven thickness after machining.

Thermal spray coatings as well as diffusion surface treatments have been used in pump applications for fluids containing high concentrations of suspended solids. Spray coatings are restricted to areas within the pump, accessible by the line of sight. Diffusion-produced coatings are not limited by this constraint. A disadvantage of diffusion processes is that they are performed at high temperatures that can negatively influence the base material properties. Diffusion coatings can range from traditional gas carburizing to the diffusion of high chromium alloys. These coatings increase the sur face hardness of the component and, depending upon the process, can increase the material's surface hardness to values in excess of 60 Rc. Diffusion layers can be produced to a depth of approximately 0.100 in (2.54 mm). One item of caution: these coatings usually render the material unweldable after application. For this reason, steps must be taken to protect areas of anticipated welding, such as attachment piping. Future weld repairs are not possible unless the coating is completely worn off or removed.

Through the years, developments in thermal spray equipment have enhanced the acceptability of this surface modification process. Thermal spray processes employ the transfer of a material onto another by raising the temperature of the hard-facing material, usually in powder form, and projecting it against the component that requires the additional erosion resistance. The bond strength between the hard-faced material and the substrate material is directly influenced by the maximum velocity that the particles of molten material achieve in a given thermal spray process. The greatest bond strength is achieved by the highest velocity process. The typical thermally sprayed materials used in pumps to resist solid particle erosion damage are as follows:

• Nickel chromium boride coatings

• Cobalt-based hardfacing coatings

• Tungsten carbide coatings

• Solid particle tungsten carbide loaded (1) or (2)

The processes used to apply the above hardfacing materials are as follows:15

Process Typical Particle Velocity

Plasma spray 800 ft/s (244 m/s)

D-Gun (Union Carbide tradename for 800 ft/s (244 m/s)

detonation gun process)

HVOF (high velocity oxy-fuel) 3000 ft/s (915 m/s)

The severity of the service usually dictates the process. In the past, thermal spray coatings, for the most part, were tungsten carbide and the diffusion coatings were high in borides. It was found that for spray coatings an increased performance could be achieved by applying them over erosion-resistant substrates. This is a challenge because the high-chromium, carbon, abrasion-resistant materials are thermal-crack-sensitive. Overlay coatings, if applied several times, are thicker but are more prone to cracking, chipping, and spalling. An alternative is to use carburized carbon steel or 12% chromium stainless steel centrifugal pump components for mildly corrosive environments. Coatings are frequently used to increase the life of plungers in reciprocating pumps for slurry services.

Another process for applying hard-faced materials is by laser consolidation. This process can be accomplished in two different ways. The first case is one in which a laser beam is used to melt a thermally sprayed coating applied upon a substrate. The other process is to simultaneously melt the substrate while applying a hard-faced material. In either case, the principle is to use the hard-faced material as a consumable in a laser-welding operation. Since laser welding is a rapid process, very little dilution of the hard facing material is produced. This allows for much thinner coatings that are less prone to thermally induced cracking during an operation. In addition, since there is little dilution, the hardness and chemistry of the coating are very consistent. This provides for uniform erosion resistance throughout the entire coating thickness.

Survival Treasure

Survival Treasure

This is a collection of 3 guides all about survival. Within this collection you find the following titles: Outdoor Survival Skills, Survival Basics and The Wilderness Survival Guide.

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