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INDUSTRIAL AKD ENGINEERING CHEMISTRY
and 60.00 to 80.00 per cent nickel are used in the food and dairy industries. They have good resistance to oxidation u p to 2000" F. and are also used for electrical resistance material for high-temperature service. It has been demonstrated that the addition of chromium t o nickel makes it free from tarnishing and corrosion under the very conditions for which nickel itself is unsuited. Chromium in combination with tungsten and cobalt is a well-known alloy, commercially mailable for use where corrosion and oxidation resistance are required along with resistance t o abrasion. This alloy has many of the same corrosion-resisting properties as chromium iron alloys but to a higher degree. It is being used for the trim of valves handling hot, high-sulfur crude oils. It is not attacked b y acetic, citric, formic, lactic, nitric, and cold sulfuric acid, but it is not recommended for use with hydrochloric, hydrofluoric, or concentrated boiling sulfuric acid. For resistance of hydrochloric acid there is a group of alloys containing essentially nickel, molybdenum, and iron. They are resistant t o hydrochloric acid in practically any concentration and at any temperature u p t o t h e boiling point, although the rate of attack increases with the temperature. These same alloys are resistant t o sulfuric acid u p to about 1300" F. They are not, however, resistant t o nitric acid and their use is not recommended where free chlorine is present. When 20.00 per cent chromium is added t o the nickelmolybdenum alloy just mentioned, it becomes resistant to strong oxidizing agents such as nitric acid, free chlorine, and acid solutions containing ferric and cupric salts. With the addition of chromium this alloy retains its resistance t o hydrochloric acid and becomes somewhat more resktant to sulfuric acid than the alloy without chromium. With the chromium addition, the alloy is resistant to acetic, formic, and sulfurous acids. These alloys are used commercially for catalyst tubes, kettles, mixer parts, pickling boxes and equipment, centrifugal pumps, shafts, sludge burners, and such parts as may be in contact with solutions containing any of the acids mentioned.
Conclusion In this brief discussion of the use of chromium and its alloys in the chemical industry, statements made represent conditions as they generally exist in present industrial practice. Slight variations in chemical compounds change their characteristics, sometimes very materially, and the difference in concentration, variation in temperature or pressure, or the contamination of the chemical compound b y t h e presence of a foreign substance, may cause a given type of stainless steel to fail in service whereas i t might have been entirely satisfactory under different conditions. It therefore follows that laboratory tests are not sufficient t o qualify a particular type of stainless steel for commercial application. T h e performance of a given metal should be tried out under circumstances as nearly similar as possible t o practical operating Conditions which are peculiar t o the individual plant. Small pilot operating units are frequently built to t r y out new metals, thus saving time, expense, and trouble that might arise if experimentation were done in a full-size operating unit. It is also desirable to select stainless steel of such compositions as are available commercially, both for economic reasons as well as for t h e advantage of obtaining a product with which the steel maker and the fabricator are familiar. The question of melting the different grades of stainless steel, methods of working and forming, the matter of finish on the surface, and the means of fabricating are important items which have been worked out from past experience, and the chemical engineer should not lose the benefit of this experience when specifying new equipment. RECPITEDSeptember 12, 19361
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Discussion V. B. BROWNE Allegheny Steel Company, Brackenridge, Pa.
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NVESTIGATIOX and service experience teaches that often a difference of only a small percentage of the alloying elements present determines the success or failure of the alloy for a given application. It therefore becomes highly essential, for each application, to know all of the conditions to which the alloy will be subjected in order to select the most suitable composition. Two alloys which should prove of interest and value t o the chemist and engineer will be briefly discussed as examples. The first contains a maximum of 0.12 per cent carbon, 11.5-14 per cent chromium, and 2-3 per cent tungsten. The 11.5-14 per cent chromium steel, without the tungsten, has long been used for such exacting service as buckets for steam turbines operating at temperatures up to about 700' F. Its resistance t o corrosion, erosion, and shock, its high elastic limit in the heat-treated condition, and its creep strength have made it an ideal material for service of this kind. This alloy is, undoubtedly, the best structural alloy of all the stainless steels. I t was not surprising, therefore, when it was found that the addition of tungsten to this alloy further enhanced its strength a t high temperatures. The temperature a t which the alloy lost its temper was raised from about 750" to 900" F. At 1050' F. the creep strength was increased over 400 per cent. While these changes in mechanical properties might have been expected, investigation showed the unusual and unexpected result that the addition of tungsten materially increased its resistance to oxidation and that up to 1500' F. it was twice as good as the regular 18 per cent chromium8 per cent nickel alloy. This alloy is being used successfully for high-temperature service up to 1500' F., where strength, resistance to creep, and oxidation are essential. Its properties are such that it should prove a valuable structural alloy for hightemperature service in the chemical industry. The second of these alloys contains a maximum of 12 per cent carbon, approximately 20 per cent, chromium, and approximately 10 per cent nickel. In this alloy the corrosion resistance of 18-8 steel has been enhanced to a greater degree than would be expected from the increase in the chromium and nickel contents. It has been found to be an excellent alloy for the sulfite pulp industry. Under these conditions its corrosion rate is only about one-sixth that of the regular 18-8 steel and approximately onehalf that of the 18-8 steel containing 2 per cent molybdenum. I n this application cellulose products are being made which are practically free from metallic contamination. Although the chromium-nickel alloys within the 18-8 range are not generally resistant to dilute sulfuric acid, there is one case on record where oxidation of the sulfite liquor caused the formation of an appreciable amount of sulfuric acid. Under these conditions the 18-8 alloy was badly attacked whereas the 20-10 alloy showed no visible effect. Therefore, for applications of this kind, the 20-10 alloy has considerable advantage. Where welding is involved, carbide precipitation in the area adjacent to the weld is sufficiently sluggish in the 20-10 alloy so that for most applications, annealing of the finished vessel is unnecessary. This is an extremely desirable property which greatly simplifies many fabrication problems. The properties of the 20-10 alloy are such that it is worthy of consideration by the chemist for many applications where not only corrosion resistance is of importance, but where the product is to be kept free from contamination with metallic salts. The surface condition of an alloy is often as important as its composition. It is not enough t o specify to the steel manufacturer the desired surface finish and expect that tanks and vessels fabricated from this steel will give the desired result. It should
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be remembered that, in the fabrication, iron and steel tools and iron supports and braces are dragged over the surface, leaving particles of iron and steel which would be attacked later in service, thus contaminating the product. In many cases this iron would form salts which would either cause electrolysis or catalyze other chemical reactions, the products of which may cause rapid local attack a t the spot where they are in contact with the corrosion-resisting steel. These chemical reactions are often regenerative so that the cycle may continue until failure results. Therefore, it becomes necessary in the use of these corrosion-resisting alloys to give careful consideration to the condition of service of the finished piece of apparatus. A pickling treatment or surface grind followed by a pickling treatment is often neeessary to obtain surface conditions that will permit the inherent corrosion-resisting properties of the alloy to be effective. It is not necessary for all conditions to resort to an elaborate finish. The kind of finish and final treatment will depend upon the service involved, but the condition of the surface should always be considered since considerable trouble has resulted because of improper final surface treatment of the finished apparatus. Several excellent pickling solutions have been developed by the makers of these corrosion-resisting steels for putting the finished surface in good condition. The future looks extremely bright for stainless steel; indeed, the existence of these steels has rendered the future of many industries most promising. The ability to obtain steels that will not contaminate products will enable the organic chemist to develop future processes of extreme value which would not have been possible except by the use of noble metals such as platinum and gold which would have rendered the cost prohibitive. RECEIVED October 9, 1936.
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Discussion of Chromium-Bearing Steels in Pressure Vessel Construction R. K. HOPKINS M. W. Kellogg Company, Jersey City,N. J.
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HE use of chromium in steels for pressure vessels has been growing for the past several years and is now accepted practice. This element is not used for increasing strength and hardenability as is the case in the structural and automotive industries, but rather for retarding and in some cases eliminating the chemical industry’s worst enemy, corrosion. Where corrosion is not a serious factor, steels containing 5 per cent and in some cases as low as 2 per cent chromium (with 0.5 per cent molybdenum) are being utilized. The use of these steels has been almost entirely confined to tubes and headers for cracking furnaces and t o hot oil lines in and around the oil refinery. Both the chemical and oil industries are using a considerable tonnage of straight chromium steels for pressure vessels with chromium in the range of 12 to 14 per cent. This grade of material is corrosion resistant to a large number of media and in addition possesses the quality of being heat-treatable. The latter characteristic enables the fabricator to put the completed vessel into its best physical condition after welding, and the user can take full advantage of these optimum physical properties. The 16-18 per cent chromium steels are somewhat more resistant to certain kinds of corrosion than those containing 12-14 per cent chromium. They therefore fill certain specific needs which are peculiar to the chemical industry. This steel, like the 23 and 27 per cent chromium steele, is not susceptible to heat treatment and therefore must be used with discretion. These ferritic steels suffer from serious grain growth in and near the weld which causes a tremendous drop in impact resistance at these points; since these grains cannot be broken down by heat
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treatment, they must remain in the hished vessel. It is therefore recommended that vessels made of this type of steel not be put into service where impact is a factor. (The use of nitrogen in these steels alleviates this condition somewhat.) In the fabrication of such materials, these important rules should be followed with all straight chromium steels: Preheat before welding, keep hot during welding, and keep hot until a t least a stress-relieving treatment (better still, an annealing treatment) can be applied. Where corrosion is most severe, the chromium-nickel steels are used. The three varieties, 18-8, 25-20, and 25-12, are those generally accepted, but countless other formulas are being used under various service conditions. These three types of steels probably have better welding qualities than any other known alloy. They require, however, a rather difficult heat treatment to restore their best qualities after welding. The use of stabilizers such as titanium or columbium lowers the required temperature for heat treatment and therefore somewhat simplifies the fabricator’s problems in this respect. Although a large tonnage of these steels has been used in pressure vessel construction, it has been confined to the smaller, lighter weight vessels because of cost and the difficulties of obtaining large area and thick plate of solid alloy. The oil industry requires extremely large and heavy vessels, upwards of 200 tons. Obviously it would be uneconomical as well as almost impossible to build such vessels of solid alloy material today. For this reason the r e h e r has resorted to many expedients to retard or eliminate corrosion inside ordinary carbonsteel vessels. These remarks will be confined to those expedients where the element chromium has been used. Straight chromium and chromium-nickel alloys have been applied to the inside of pressure vessels by the metal spray process. However, such applications have not been highly successful so far. The application of aluminum by this process has been somewhat more satisfactory but does not stand up under extreme conditions. One company has spent a great deal of money trying to develop an electroplating process for depositing pure chromium on the inner wall of pressure vessels. This process has been carried beyond the experimental stage, and full-size pressure vessels have been chromium-plated. This work was done five or six years ago, and the data available indicate that it has not been successful. ONE type of material that shows a great deal of promise for this service is the bimetal plate; one side is covered with a corrosion-resistant material such as a straight chromium or chromium-nickel steel and the other with regular carbon-steel base metal. Two methods have been used in this country for the past several years for applying the corrosion-resistant materials to the carbon steel. One method consists in applying a relatively thick plate of corrosion-resistant metal to the specially prepared surface of a steel slab. Two such units are made up, and then a refractory lubricant is spread on the top surface of one stainless plate and the other unit is placed on top of this, making a sandwich. In order to exclude the air from the inner faces of these four pieces of material, they are welded together around the periphery a t the meeting edges of the slabs. The air is exhausted in some suitable manner and the composite sandwich is then heated in a soaking pit and subsequently rolled to any desired thickness within the range of the process. After rolling, the resulting plate is sheared on its edges and the sandwich opened, producing two bimetal plates. The other process which has been used in this country is similar t o the above except that, instead of attaching the alloy plates to slabs of carbon steel, the plates are suspended in a mold and the carbon steel cast around them. The rolling and shearing are carried out in the same manner as has been described above. A new process was recently put on the market which is quite similar to the one first described: the difference is that special
