Cast steel, erosion

Liquid chlorine is generally stored in vessels made from nonalloyed carbon steel or cast steel. Fine grain steel with a limited tensile strength is used to ensure proper conditions for welding these containers. Erosion of protective layers on steel is prevented by maintaining the flow velocities to less than 2 m/s. Although organic materials, zinc, tin, aluminum, and titanium are not acceptable for liquid chlorine use, copper, silver, lead, and tantalum are acceptable for some equipment. 

Materials of Construction. The principal materials of construction are ferrous throughout, but different grades are used for the different components. Casings usually are in cast iron or cast steel, and impellers may be in the same materials or in fabricated steel. Early designs of centrifugal compressors used riveted assembly for impellers castings or welded construetions are preferred today. Shafts usually are of carbon steel with hardened or stainless steel sleeves at points where erosion is likely. The selection of materials for smaller components that may be subject to friction is particularly important because of the danger of comlnistion. 

The pipework system should be kept simple with the minimum number of valves installed so that the lines are self-cleaning. Fluid velocity through pipework should be restricted to minimise erosion problems. A velocity of not more than 0.5 m/s is preferable. Sharp bends increase turbulence and should be avoided wherever possible. Cast steel plug valves with fluon sleeve or cast steel ball valves with fluon seal should be used. Centrifugal pumps with special mechanical seals with either reinforced PTFE or ceramic seat are suitable for pumping oleum 65 and 20. Instruments in contact with oleum should normally be fabricated from stainless steel. 

For erosive wear. Rockwell or Brinell hardness is likely to show an inverse relation with carbon and low alloy steels. If they contain over about 0.55 percent carbon, they can be hardened to a high level. However, at the same or even at lower hardness, certain martensitic cast irons (HC 250 and Ni-Hard) can out perform carbon and low alloy steel considerably. For simplification, each of these alloys can be considered a mixture of hard carbide and hardened steel. The usual hardness tests tend to reflect chiefly the steel portion, indicating perhaps from 500 to 650 BHN. Even the Rockwell diamond cone indenter is too large to measure the hardness of the carbides a sharp diamond point with a light load must be used. The Vickers diamond pyramid indenter provides this, giving values around 1,100 for the iron carbide in Ni-Hard and 1,700 for the chromium carbide in HC 250. (These numbers have the same mathematical basis as the more common Brinell hardness numbers.) The microscopically revealed differences in carbide hardness accounts for the superior erosion resistance of these cast irons versus the hardened steels. 

The austenitic cast irons are in widespread use in many industries (food, pharmaceutical, petroleum, chemical, petrochemical, pulp and paper, etc.) in mildly corrosive and erosive situations where the life of unalloyed or low-alloy cast iron or steel is short, but the high cost of stainless steel and nonferrous alloys cannot be justified. 

Material Shell carbon steel with 4-5 in. (10-12 cm) thick heavy weight, single-layer, cast-vibrated refractory with needles. Internals 304H stainless steel for temperature >1.200°F (650°C) and Grade H, % chrome for < ,200°F. Internal components exposed to catalyst should be refractory-lined for erosion resistance. Sliding surfaces should be hard-faced, minimum thickness in. (3 mm). 

The liners are fabricated in stainless steel with a cast stellite inlet for improved erosion resistance. All liner components are readily replaceable on an individual basis, and with no moving pans, maintenance should be relatively simple with long periods between subsequent inspections once the erosion resistance for a particular application has been confirmed. 

Figure 15.4 Mapping electrochemical material loss against mechanical erosion rates for a nonpassivating surface carbon steel (AISI1020) along with two potentially passivating surfaces of nickel aluminum bronze (NAB) one that has been thermally sprayed by high-velocity oxy-fuel deposition as a coating on carbon steel ( j and another which has been cast (A.). These results were obtained from jet impingement erosion-corrosion tests. Reprinted from Ref. [7]. Copyright (2007) with permission from Elsevier.

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