Materials For Slurry And Abrasive Services

The construction materials for pumps that handle high concentrations of suspended solids are often based upon high bulk hardness. In many applications, coatings, hard liners, and weld overlays are used to specifically increase the surface hardness of the internal wetted portions within the pump. However, many of the slurry applications use non-metallics that do not have high bulk hardness because of their unique qualities.

Nonmetallics Contrary to "harder is better," a good number of slurry pumps use non-metallic materials, such as rubber, that absorb the kinetic energy of the solid particles through a large elastic deformation of the surface. Natural rubber is the most commonly used material since it provides good wear resistance with abrasive particles less than approximately 0.25 to 0.38 in (6 to 9 mm). Rubber linings pose a problem in the bonding of this outer shell to a metallic substrate. This is particularly true for cut water areas of a casing and, of course, attachment to metal skeletons of an impeller. Care must be taken also in considering the liquid phase of the slurry and the temperature of the application, both of which can degrade the rubber.

Other factors to consider when using elastomeric liners include pressure in the flow passage versus the pressure between the liner and the wall, and temperature, since rubber softens around 240°F (115°C). It is also important to consider the size of the particles greater than 0.2 in (5 mm) if they are dull or greater than 0.08 in (2 mm) if they are sharp, and the head per stage.

Metals In mildly abrasive services, carburized steels are sometimes used to increase the wear life of components. Carbon is diffused into the surface of carbon steel, which, after a hardening heat treatment, can achieve a surface hardness of 60 Rc. This gas diffusion heat treatment can produce high hardness layers that penetrate the outside surface of the pump component to a depth of approximately 0.080 to 0.090 in (2.0 to 2.3 mm). However, after carburization, the materials are impossible to weld-repair without cracking. Using a special process, usually a vacuum furnace, carburizing has been employed in the surface hardening of the 12% chromium stainless steels, such as CA15, for abrasive services where mild corrosion is expected.

The most commonly used materials for severe slurry services are the abrasion-resistant cast irons found in ASTM A532. Essentially, three main classes and several types of alloys are covered in this specification. The most widely employed material in slurry applications is the Class III hard irons. A brief description of this class of abrasion-resistant iron is as follows:

Composition (Cr, Ni, and Mo) Hardness (BHN)

ASTM A532 Grade

Class I: Type A (Ni-Hard) Class II: Type A, B, C, D, E

Class III: Type A (26% chrome iron, original trade name of HC-250)

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