DECEMBER, 1936
INDUSTRIAL AND ENGINEERING CHEMISTRY
precautions are taken to prevent oxidation from taking place between the carbon steel and alloy plate by electrolytically depositing a thin skin of iron on the surface of the alloy plate before applying it to the steel slab. The purpose of this electrolytic iron is to prevent the formation of highly refractory chromic oxide on the stainless slab and thereby increase the weldability of the two materials. Since this product is new, no service data are available. One of the main difficulties of eacah of these processes is that t,hey depend upon the combination of heat and roll pressure to form the bond between the two materials; obviously it would be almost impossible to prevent oxidation completely during and even before the heating process. In the case of the first two methods described, considerable trouble has been experienced in the field because of localized failure of this weld. Where vessels of this material are subject'ed t'o alternate heating and cooling, they almost invariably develop large blisters. Another drawback to the first two methods, and one which will also probably apply to the third method, is that they have been limited to size and thickness of the plates obtainable. Thus the thickness obtainable from a given mill would be just half the thickness of a carbon-steel plate, because of the fact that these composite plates are rolled as a sandwich. Another disadvantage of this type of material is its extremely high cost.
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Discussion E. C. WRIGHT National Tube Company, Ellwood City, Pa.
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ANOTHER method used to retard the rate of corrosion in preswre vessels is to line the inside with a separate sheet of corrosion-resistant material. Most of this work has been done with 18 per cent chromium-8 per cent nickel steel and with straight 12 to 14 per cent chromium steel. These sheets, which are usually approximately 6/84 inch thick, have been applied by one of two processes. In one process the sheets are perforated and subsequently plug-welded with an alloy rod to the base metal. In either process the sheet may be welded to the base metal either before or after bending. Special precautions must be taken to see that cracks do not develop either in the welds or in the sheets adjacent to the welds. Air testing and magnetic testing are usually used to determine whether or not such defects exist. Another method somewhat similar to the two just described was put on the market in the latter part of 1935. The alloy steel is welded to the carbon-steel base plate by continuous arc-welded bands. These bands are laid down in a circumferential direction and extend around the girth of the vessel. They are spaced on close centers usually 2.5 to 3 inches. The welding is carried on with a rod whose alloy content is sufficiently high to take care of dilution from the carbon steel, so that the weld deposit has substantially the same analysis as the alloy sheet. The welding is carried on under complete protection from the arc. The alloy sheet is usually attached to the carbon-steel plate before the latter is bent into a cylinder. Before bending, the excess deposited weld metal is chipped and ground off. The same methods of testing as were described above are applied to this type of material. The two advantages offered by this method over those described are: (1) The vessel is separated into small cells which, in case of a leak, would prevent the material from collecting in the bottom of a section or in the bottom of a tower, and thereby ultimately pushing off a large area of the lining; (2) the presence of the alloy welds a t the surface of the lining makes it possible to weld clips, angles, trays, etc., direct to the welds rather than hanging these attachments on the liner sheet. This method of lining vessels shows a great deal of promise. There is still a need, which will undoubtedly be met within a relatively short time, for a carbon-steel plate with an integral corrosion-resistant facing on one side, which can be produced in large sizes and heavy gage a t a price low enough to make it commercially practicable.
HE low-chromium steels as grouped in Priestley's paper up to 3.5 per cent chromium are becoming increasingly important and are finding wider application. During the last few years a large quantity of a chromium-silicon-copper steel has been used, particularly for resistance to atmospheric corrosion. This steel contains only about 1to 1.5 per cent chromium and very little carbon. I t is also coming into use for tanks and piping where corrosion is severe on regular steel but mild enough to give increased life in this special analysis. In the high-temperature and high-pressure field considerable quantities of steels containing from 1.25 to 2 per cent chromium have been used. All of these steels also contain 0.5 to 1 per cent molybdenum and the same percentages of silicon. The latter alloys are added, respectively, for higher high-temperature strength and better resistance to oxidation. A steel recently introduced for nitrogen fixation work in ammonia plants contains 2.5 to 2.75 per cent chromium, 0.5 per cent molybdenum, and 0.5 per cent vanadium. Chromium steels in this range are superior to ordinary steel in resisting the decarburizing effect of hydrogen a t high temperatures, but recently the chromium-molybdenum-vanadium steel has been favored for ammonia plants. I n the group of steels varying from 3.5 t o 9 per cent chromium, the 4 to 6 per cent chromium type is by far the most important. This also represents a family of steels with modifications for various purposes. The original 4 t o 6 per cent chromium steel without other alloys is almost obsolete, but 4 to 6 per cent chromium with 0.5 per cent molybdenum has found wide application in ammonia plants, gasoline refining, and high-pressure steam service. The high-temperature strength of the 4 t o 6 per cent c h r o m i u m 4 . 5 per cent molybdenum steels is about the same as that of the 1 to 2 per cent chromium-molybdenum steels or slightly lower, but its resistance to oxidation and corrosion a t temperatures up to 1200' F. is considerably better. Steels in the range of 6 to 9 per cent chromium have found very little application up to the present. All of these steels containing from 1 to 6 per cent chromium are susceptible to air-hardening, and the intensity of this effect is proportional in general to the chromium content. This feature has been detrimental in the use of these steels, particularly for welding and fabricating operations. Metallurgists have overcome this situation, particularly in the 4 to 6 per cent chromium steels, by making additions of columbium or titanium. These addition alloys have the property of seizing the carbon in the steel and reducing or eliminating the air-hardening almost completely. This increases not only the ease of fabricating the alloys into complicated structures but also the corrosion resistance of the steel, since none of the chromium is combined with the carbon. The steels containing 16 t o 20 per cent chromium have been largely used in nitric acid plants and chemical processes involving nitration reactions. The corrosion resistance of this type of steel in this service is generally similar to the corrosion resistance against nitric acid of the 18 per cent chromium-8 per cent nickel type, but the fabricating properties of the plain chromium steels are greatly inferior to those of the chromium-nickel alloys. The steels containing over 20 per cent chromium have largely been used for high-temperature service where resistance to scaling or oxidation is required. All of these plain chromium steels have low high-temperature strength and are, therefore, not generally applicable to high-pressure equipment. The use of nitrogen additions pointed out by Priestley is increasing in this series of steels since it increases the ductility and reduces the tendency to grain growth which has been so detrimental in this series.
RECEIVED October 9, 1936.
RECEIVED October 9, 1930.
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