Low-chrome steels

Which Alloy to Use. Unalloyed mild steel parts have been known to corrode at rates as high as 800 mils per year. The low-chrome steels, through 9-Cr, are sometimes much more resistant than mild steel. No corrosion has been reported, with both 2%-Cr and 5-Cr furnace tubes, whereas carbon steel tubes in the same service suffered severe coiTosion. The 12-Cr stainless steels are scarcely, if any, better than the low-chromes. But the 18-8 Cr-Ni steels, without molybdenum, are often quite resistant under conditions of low velocity although they are sometimes subject to severe pitting. 

However, the convective-section finned tubes are not intended or designed to withstand such high temperatures, and so the tubes themselves are often made from low-temperature-rated carbon steel, whereas the fins, which are not much cooled by the process flow, are often made from low-chrome steel. As it is much easier to make finned tubes from just one type of metal instead of two, though, the furnace manufacturers will often choose to make the finned convective-section tubes entirely out of low-chrome steel in which case one could expect... 

Some success has been obtained, however, with Mo applied as a coating on low-chrome steel. The process was developed by the Vitro Corporation and is described in detail in KLX-10009. Essentially, the coating is applied by first electrophoretically depositing 80% Mo - - 20% M0O3 on the steel. The coating is then pressed, reduced in a H2 atmosphere, repres.sed and sintered in an H2 - - HCl atmosphere. 

The experience in handling salt with larger-sized equipment is quite limited. A small loop built of 347 stainless steel has been operated satisfactorily for a fairly short time. A much larger loop, loop "N, is now being constructed at BNL. This will contact the chloride salt and the bismuth fuel. The. salt part of the loop is constructed of 347 stainless. steel. The bismuth fuel section of the unit is constructed of 2 % Cr-1% Mo steel. The actual contacting units are constructed of both 347 and the low-chrome steels. This pilot plant, when placed in operation, should furnish considerable information on the corrosion characteristics of the molten chloride. salt. 

Low-chrome steels. Several methods have been used for cleaning metals of this type. One method is descaibed [30] for PbBi systems in which boiling detergent solution is used to remove dirt and scale, followed by a distilled water rinse and drying under conditions of heat and vacuum. The same reference describes the following cleaning procedure ... 

The higher boiling phenols, present in considerable amounts in CVR and low temperature tars, are corrosive to mild steel, especially above 300°C. Cast iron, chrome steel, and stainless steel are more resistant. Furnace tubes, the insides of fractionating columns, and the rotors of pumps handling hot pitch and base tar are generally constmcted of these metals. Nevertheless, to ensure satisfactory furnace tube life, particularly in plants processing CVR or low temperature tars, the tube temperature should be kept to a minimum. 

Shafts are made of material ranging from medium carbon to low alloy steel and are usually heat treated. Shafts were originally made of forgings for the compressors in process service. But because of the availability ot high quality material, hot rolled bar stock has been used for shafts up to S inches in diameter. Bar stock shafts are given the same heat treatment and quality control as forgings. Many of the process users prefer a low alloy, chrome-moly-nickel material for shafting, particularly for compressors in critical service. 

Rotor material in all cases is low alloy steel with an appropriate heat treatment to match the stresses imposed by the blades and rotor weight The rotor is generally manufactured from a forging with the material being a chrome-molybdenum alloy such as AISI 4140 or AISI 4340. 

Nuclear stations tend to employ ferritic chrome steels, stainless steels and nickel based alloys in their boilers. Turbines contain a variety of steels while condensers are usually of brass or, increasingly, of titanium. Low pressure feed heaters have traditionally been made of brass also, but increasingly steels are used. 

A typical process heater tube diameter is 4 to 10 in. Tube thickness is usually between V4 and V2 in. Heater tubes are often constructed out of chrome steel. A high chrome content is 13 percent. The chrome content increases the heat resistance of the tube. A tube with a 11 to 13 percent chrome content can normally withstand a skin temperature of up to 1300 to 1350°F. A low-chrome-content tube of perhaps 3 percent may be limited to 1200°F tube metal temperature. Naturally, the pressure, thickness, and diameter of the tube all affect its maximum skin temperature limitations. 

Weights are based on low-carbon steel with a density of 0.2836 Ib/cu. in. For other metals multiply by the following factors aluminum, 0.35 titanium, 0.58 A.I.S.I. 400 Series S/steels, 0.99 A.I.S.I. 300 Series S/steels, 1.02 aluminum bronze, 1.04 aluminum brass, 1.06 nickel-chrome-iron, 1.07 Admiralty, 1.09 nickel, 1.13 nickel-... 

Low alloy steels, also called weathering steels, contain small amounts of copper, chrome, nickel, phosphorus, silicon, and magnesium (< 1 %, typically). Their resistance to atmospheric corrosion generally exceeds that of carbon steel. Indeed, when exposed to environments that are not too strongly polluted a dark brown patina forms over some years that slows down the corrosion rate. On buildings this natural layer can thus replace a paint coating. For this reason, alloyed steels find numerous applications in architecture. 

The initial wall activity diminished as less active carbon builds upon the walls decreasing available active sites. The initial activity of nickel was clearly lower than that of low carbon steel, but higher than that of stainless steel. The results are in general agreement with the conclusions of Tamai, et al. (1968) and Buell and Weber (1950). The former indicated that nickel had a lower "affinity to olefins than iron, while the latter concluded that the nickel content in austenitic steel alloys is primarily responsible for their activity (carbon formation) when compared to the less active chrome steel alloys. The carbon-conditioned nickel walls were less active than those of low carbon steel reactor probably because the catalytic activity of the base metal did not penetrate through the carbon layer as effectively as it did with low carbon steel. 

The best and most accurate service in low-chrome or stainless-steel systems is obtained by welding the thermocouple junction directly to the outside of the pipe wall. The difference between the temperature on the pipe wall and the bulk bismuth at 930°F is no greater than 10°F. If required, thermocouples located in w ells have also been used in bismuth systems. 

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