Stainless-Clad Steels

Generally speaking, all economy is lost if more than 50 per centof the mass is solid stainless steel, and economy is the chief justification for stain...
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STAINLESS-CLAD STEELS T. S, FITCH Composite Steel Division, Jessop Steel Company, Washington, Penna.

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conductor of heat than solid stainless steel, clad has superior LATE8 and sheets of mild steel faced with stainless steel have been available for several years. Practically any heat conductivity, provided the weld is good. Frying pans, for example, have proved definitely superior when made of grade of stainless steel may be used as the cladding, and a14 clad stainless steel. percentages from 5 to 50 per cent are offered commercially, It is obvious from the patent literature that the commercial The customary product comprises 20 per cent stainless steel significance of clad metals has been recognized for sereral and 80 per cent mild steel; there is an increasing demand for decades. It is only within the last ten years, however, that 5 and 10 per cent clad, especially in the heavier plate gages such materials have been made available to the trade, and the such as l/z inch or thicker. Generally speaking, all economy last five have shown the major advance. It is still evident is lost if more than 50 per cent of the mass is solid stainless that clad has not had universal consideration, and the next steel, and economy is the chief justification for stainless-clad five years should show an impressive increase in tonnage as steels. Some work has been done on clad strip, clad tubing, well as applications. Considerable harm was done t o the cause and clad bar stock, but no significant results have been anof clad steels during the first few years, chiefly due to the lack nounced. It is possible that all of these may in time become important offerings. of sufficient bond between the components. There are still many who recall disappointing experiences a t the start, and I n any application where corrosion or oxidation resistance some have hesitated to try again. The various stainless-clad of one side only is important, the stainless-clad steels permit steel? now available, however, are almost entirely free from of savings as high as 45 per cent in many cases as compared such difficulty; in fact, most of them are guaranteed from to solid stainless steels. It is also timely to note that a wider failure of the union. use of clad is a means of economizing on vital alloys, which The purpose of this article is to describe briefly the types would become even more significant in the event of our of stainless-clad steels available and to enumerate some precaucountry engaging in a war in the Far East. More attention tions which should be observed to obtain optimum results. is also being given to the use of clad in place of mild steel, or other relatively inexpensive materials. Since the cost of fabrication is practically equivalent on clad as compared to Successful Systems mild steel, the ultimate cost of a clad tank mag be cheaper CAST’ING. B pair of stainless steel plates, separated by an than two mild steel tanks, in spite of the fact that the mainert compound and arc-welded peripherally, is centered in terial cost in the clad tank may be as much as six times as an ingot mold (Figure 1); mild steel is then cast around it. great as the material cost in a single mild steel tank. For The resultant composite ingot example, a tank weighing 1500 is heated t o approximately pounds, on which the fabrica2300” F. and rolled to twice tion cost was $200 and the the desired finish gage. The material cost $50 if made of mild edges are sheared off into the steel, would probably cost about separated plane, and two clad $225 to fabricate in clad steel, and the stainless-clad would be plates are the result. The product of this method is one of worth about $275 so that the what might be called the “homonet value would be $6500. Thus, geneous” stainless-clad steels. if the stainless-clad tank lasted ASSEMBLY OR SANDWICH. On only twice as long, it would still the outside surfaces of two be a good investment in spite of stainless steel plates about 0.015 the substantial difference in material cost. There are the inch of pure iron is electromatters of down time, additional deposited. The plates are then engineering, additional office placed face to face with an work, etc., which are also reinert compound between them (Figure 1). This pair is sandduced by the use of the single wiched between two relatively stainless-clad vessel. There are thicker and larger mild steel also many cases where there is plates, the edges are completely no particular corrosion but where sealed, and the assembly is clad is used to ensure protection heated t o approximately 2200” of foodstuffs, better color of F., rolled to double the desired liquids, or simply for appearfinish gage, and sheared into the ance. In a few instances where separated plane; two clad plates heat transfer properties are imresult. As in the casting system, portant, clad is preferred over PRESSURE VESSEL, FABRICATED FROhl 20 P E R the product of this procedure solid stainless steel even if the CENTSTAINLESS-CLAD STEEL,FOR A UNITPROCmay be termed one of the price is the same. Since the ESS IN THE BEVERAGE INDUSTRY WHERECON“homogeneous” stainless-clad majority of the metal is mild TAMINATION WOULDRE DELETERIOUS TO THE steels. FINALPRODUCT steel, and mild steel is a better so2

INDUSTRIAL AND ENGINEERING CHEMISTRY

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FIQTJRE 1. SEYENCOMMERCIAL METHODS OF MAKINQ STAINLEIBS-CLAD &EEL

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FIGURE2. RESISTANCE T O PENETRATION OF PORTIOS OF CLAD STEEL

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STAINLESS

SPOTOR RESIS~ANCE WELDING. As the name implies, a sheet of stainless steel is joined to a relatively thicker plate of mild steel by spot welding on about */,-inch centers (Figure 1). The product is an intermittently welded, stainless-clad steel. STREAM WELDING. In this process a stainless steel sheet 1s also spot-welded to a relatively thick, mild steel plate, except that the spot welds are made to overlap one another, t o produce in effect a stream of spot welds (Figure 1). The adjoining streams may or may not overlap and the product may thus be homogeneous stainless-clad steel, while in other cases it may fall in the general class of intermittently welded stainless-clad steels. In some cases stainless sheets are attached likewise to the mild steel plate, one upon the other. INTERMELTING. A refractory jacket is built around one face of a mild steel slab; in this jacketed space is introduced stainless steel by a procedure which is a combination of electric-arc welding and electric furnace melting (Figure 1). An intermelting of the stainless steel and the superficial mild steel takes place. The resultant “armored” slab is then heated and rolled to gage in the usual manner. The product of this method is another homogeneous stainless-clad steel. FUSION MELTING. A mixture of ferroalloys, nickel, iron, and suitable slag, all in granular form, is placed on the top face of a mild steel slab (Figure I). An arc is passed along the face causing a coalescence of the essentials. The resultant composite slab is then rolled in the usual manner. To date, this method has not had substantial commercial application, a t least in competition with the others described. ARC WELDING. Overlapping beads of weld metal may be deposited by electric arc, oxyacetylene, or atomic hydrogen welding onto one face of a mild steel slab (Figure 1). The resultant clad slab is then ground, heated, and rolled in the customary manner. This product may be included in the homogeneous group.

Precautions in Application In applying any material, especially a new one, certain precautions are advisable if the optimum results are t o be

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expected. Stainless-clad steel is no exception to this rule; in fact, some of the hesitancy as to its use is due to insufficient consideration of these precautions in its earlier history. SINGLE-SIDEEXPOSURE. Obviously, stainless-clad steel should be utilized only in cases where the advantages of stainless steel are necessary on one side alone. The question does arise as to the applicability of clad in cases where fumes from a contained liquid may escape and attack the mild steel. The noncontacting surfaces may be painted safely any number of times without the possibility of contaminating or otherwise harming the material in process. Clad is therefore applicable in such cases. Superficial protective coatings such as chrome plating, spraying, and impregnation by heat treatment may be measured in a few thousandths of an inch. The stainless portion of a clad plate or sheet has relatively greater mass and thus affords greater protection. This is especially significant in processes where scraping or picking is used as a cleaning method. Resistance to penetration is shown in Figure 2. The most common question in regard to the application of stainless-clad steel is, what do we do about the “raw edges”? Much depends on the application-that is, whether or not the corrosion condition is severe. Where stainless-clad is used for protection only, it is usually satisfactory simply t o paint the edges (Figure 3). Up to and including 14 gage, the edges may be crimped. Closed tanks require no special consideration since the joints are almost invariably butt welds. Heavier gages may be beveled and then brazed or covered with stainless steel by any convenient means of welding. Often a stainless-clad flange is incorporated into the design which affords the desired protection. This matter of exposed edges was a definite problem in those instances where strip was contemplated, but a t the moment strip is not commercially significant. WELDING. It is in the welding of stainless-clad steel that real damage can be done. One must face the fact that dissimilar metals are present, one of which is corrodible (Figure 4); thus if any of the corrodible steel is mixed with the corrosion-resisting steel, there will be a consequent reduction in the stainless qualities of the cladding. A further possibility is that a brittle zone can be developed as a result of the admixture of the stainless and the mild steels during fusion; the brittleness is attributable to the martensite which is formed. In the case of straight 18-8 stainless-clad, welding can also promote carbide precipitation a t the grain boundaries which reduces appreciably the corrosion resistance of the cladding. The fact remains that hundreds of tons of stainless-clad steel have been welded in such a manner that the weld zone had corrosion resistance equivalent to the rest of the cladding (Figure 5 ) . Fully satisfactory physical properties in the weld zone are also being obtained. Since the subject of proper welding of stainless-clad is a volume unto itself, it is suggested that anyone contemplating the use of the material contact any of the accredited producers or fabricators for complete information. HEATTREATMEXT. The proper heat treatment of stainless-clad at each step is sometimes underestimated; this is particularly true of nonstabiliaed 18-8 stainless-clad steel. So far as the condition of the cladding is concerned, the columbium- and titanium-bearing types need not be annealed, but it is customary fully t o anneal nonstabilized 18-8 clad by air cooling from 1925” F. -4pparently the molybdenum-bearing types (316 and 317) are also reasonably satisfactory without annealing, as far as the condition of the cladding itself is concerned. However, all clad is easier to machine if it is fully annealed when supplied to the fabricator. Figure 6 indicates that temperatures between 1700’ and 2000” F. are permissible for full annealing; obviously time is an important element as well as temperature. On the basis of 1 hour per

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inch of thickness, one may readily calculate the approximate time at which the material should be a t temperature; it is wise to keep the time as short as possible commensurate with uniform heating because grain growth takes place in the backing steel, and it is desirable to minimize this grain growth in the backing as much as possible. This phenomenon (grain growth in the backing steel) has not proved so serious as was a t first supposed. In our experience, no piece of equipment has ever failed because of it. However, if it is absolutely essential that the structure of the mild steel backing be the same as the structure of an as-rolled mild steel plate, the following procedure may be followed by the producer: The plate or sheet of stainless-clad steel is heated to 1800" F., cooled by a water spray t o 900" F., and then allowed to cool normally to room temperature. The straight chromium types of clad must be furnacecooled in the same manner as solid straight chromium steels, except that a fair normalizing effect can be produced by cooling clad in still air from 1450" F. The above comments refer primarily to heat treatment of clad by the producers prior to fabrication. The heat treatment given to clad material by the producer does have an effect upon the material from the point of view of the fabricator. A fully annealed clad plate is easier to form, easier to machine, and not so susceptible to carbide precipitation when nonstabilized 18-8 is the cladding. Carbides may be precipitated in straight 18-8 a t all temperatures from 1250" to 1800" F. (Figure 6), but the most dangerous range is between 1250" and 1500" F. Thus, this range must be avoided in any heat treatment given the clad material by the fabricator. An annealing temperature in excess of 1800" F. can be used to eliminate possible carbide precipitation in nonstabilized 18-8 clad, but that is not usually contemplated because of possible distortion in the fabricated equipment. Stabilized clad may be stress-relieved a t the usual temperatures; nonstabilized clad must not be stress-relieved a t a temperature in excess of 1250" F.; in fact, it is inadvisable to exceed 1225" F. By increasing the time a t temperature, it is possible to ralieve welding and

MIXING TANKWITH WALLSMADEOF 0.086 I N C H O F STBINLESS-STEEL CLADDING INSEP.4RABLT UNITED TO 0.351 INCH OF MILD STEEL

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FIGURE 3. CROSS-SECTIONAL VIEWS OF METHODS FOR PROTECTING RAWEDGES

machining stresses in the backing a t temperatures as low as 1100" F. Distortion due to the presence of dissimilar metals has received attention by producers and users alike ever since the introduction of stainless-clad steels. It is difficult to establish by laboratory methods, but observations over a period of several years in production, in application, and in the laboratory are as follows: Up to 400" F. any stresses due to differences in coefficients of expansion seem to be absorbed within the mass with no noticeable or measurable change of shape. This appears to be comparable to steel rails which are now being welded in great lengths with no expansion joints. I n the course of heating clad, any distortion that may take place seems to be self-corrective both in flat plates and in welded construction. In cooling there is no evidence of distortion until 600" F. is reached. From 600" down to 400" F. there is a change of shape both in flat plates and welded equipment. I n the case of the welded equipment this change of shape may be held within commercial limits simply by cooling through this range slowly. I n the case of units which, in operation, pass through this range frequently and often quickly, change does take place, but the design may well be such that a slight change in shape will have no effect on the efficiency of the operation. There has been no evidence that continuous heating and cooling result in cumulative changes of shape sufficient to render the unit inoperative. Further, this change of shape is not so extensive when the maximum temperature is relatively low. For example, a unit operating between room temperature and 900" F. will not change shape so much as a unit operating between 1700" F. and room temperature.

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As far as the effect of heat on the union of the components is concerned] all of the recognized stainless-clad steels are satisfactory. No separation should take place. In nonstabilized 18-8 clad the heat of electric-arc welding is apparently sufficient to cause carbide precipitation; therefore, if annealing is not contemplated] it is wise to specify stabilized alloy cladding. M A C H I K I N GI. n t h e machining of any stainlessclad steel, especially those which are not fully annealed] a cobalt type highspeed tool steel has been found to yield the most economical results in spite of its higher original cost. Such operations as punching] shearing, etc., may be performed in the same manner as on solid stainless. Fully annealed clad CH/P G.QOOV& behaves in fabrication more like ordinary boiler plate than like solid stainless. TYPE O F CLADDINQ. The choice of the proper type of cladding is primarily determined by the operating requirements. Generally speaking] one FIGURE4. W E L D I K G O F chooses the type of cladSTAIKLESS-CLAD STEEL ding that would be used if solid stainless steel were contemplated or had been used. If the problem is one of protection only and no corrosion is involved] straight 18-8 should be used because it is the cheapest. Vhere there is some corrosion, careful consideration should be given to the choice, as this is the “twilight zone”. Since welding is more apt t o cause carbide precipitation in clad than in solid, trouble might ocrur with a straight 18-8 clad where it would not with solid 18-8. Although stabilized 18-8 clad is more expensive than nonstabiliaed 18-8 clad, it is still substantially less than solid 18-8 and therefore does not negate the economy of using clad. Wherever the corrosion problem is severe and one of the higher alloyed stainless steels is indicated] we may safely use the same type of clad RS we would in solid form; observation shows that the fabrication procedure is most carefully considered and executed when the more expensive metals are utilized. Stainless clad is a material that must always be handled carefully in fabrication. The surface condition of the cladding is directly comparable to that of equivalent solid stainless steel. If a polished finish is required on solid stainless, a polished finish is likewise required on clad, If a No. 1 finish (hot-rolled] annealed, and pickled) is satisfactory on solid, it will likewise suffice on clad. I n those cases where clad is used in place of mild steel, the No. 1 finish is generally good enough. Those stainless-clad steels made by the assembly or casting methods tend to have a superior No. 1 finish because the stainless steel is totally enclosed and does not touch the rolls; thus it is usually smoother and more free of pits than the customary Yo. 1 finish on solid stainless steel. The surface condition of the backing steel is usually not so good as the finish on an ordinary mild steel plate, especially where a 1900” F. anneal has been used for conditioning the stainless cladding. However, such scale as persists after roller leveling and further handling is usually fairly easy to remove, and the surface takes paint well and so appears acceptable. Some cold rolling of stain-

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less-clad is being done, primarily for improving the condition of the backing steel and for affecting a uniform appearance on the clad side. Flame cleaning is an excellent means of cleaning the mild steel side of stainless-clad steel plates. Polishing the clad side is usually easier than polishing solid stainless steel because the cladding is slightly softer (due apparently t o a lesser superficial hardening effect in hot rolling). More speed is also possible because the mild steel portion acts as a chill plate. It must, however, be kept in mind that the stainless steel is merely superficial and excessive spotting may penetrate the cladding] in which case the corrosion resistance is gone. DEGREEOF CLADDING.The degree, or percentage] of cladding is a matter which should receive closer attention in the future. Because the usual product is 20 per cent stainless steel on 80 per cent mild steel, many have specified it whether or not it is the ideal. For example, a 20 per cent half-inchthick clad plate has 0.100 inch of stainless steel whereas 0.050 inch, or 10 per cent clad, might be ample. On the other hand, a 10-gage clad plate has 0.025 inch of cladding whereas it might be wise to have 0.050 inch or 40 per cent clad plate. On very heavy plates (in excess of 1 inch thick) 10 per cent is usually ample, and there are many cases where even 3 per cent may be sufficient. This matter of lighter claddings has been recognized and acted upon primarily by those who produce the so-called intermittently welded types. In fact, these products are rarely defined on a percentage basis; they usually state the thickness of the stainless steel in fractions and the thickness of the mild steel likewise. HOMOGEKEOUS US. INTERNITTENFLY JYELDED CLAD. The choice between homogeneous and intermittently welded stainless clad is essentially a matter of cost comparison. For clad plates in excess of 1 inch thick, the intermittently welded products are generally more economical. For clad plates less than inch thick and for clad sheets the homogeneous

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resistance of the cladding. It is during heating operations such as annealing, heating for flanging and dishing, and stress relieving that migration supposedly may take place. So far as we are able to ascertain, there has never been a failure which has been definitely or even presumably ascribed to the migration phenomenon. Since there is evidence from well-conducted tests to prove that elements can migrate (for example, carbon in carburizing of low-carbon steel), we must assume that in the manufacture and fabrication of stainlessclad we do not approach the necessary conditions for migration, or at least the conditions are not right for a sufficient length of time to result in a damaging degree of migration. I n a few rare cases we have observed apparent migration, but it has been impressive chiefly because it has progressed t o such a slight extent. I n short, the migration phenomenon does not appear to have any commercial bearing in stainlessclad steels. This discussion refers to the stainless-clad sheet or plate itself and does not pertain to migration which may take place during the welding of one clad plate or sheet to another clad plate or sheet. I n that case actual admixture takes place which is essentially a different consideration; the paragraphs on “Welding”, page 504, cover this phase. The matter of raw or exposed edges is another of the bugaboos. It is very rare indeed, except possibly in the case of some strip applications, that the additional cost of suitably protecting the edges will offset the economy of using clad as compared to solid.

PROCESSKETTLEFABRICATED FROM 10 PER CENTSTAINLESS-CLAD STEEL

clad products are most economical. I n the range of over 1/21 inch thick, they are fairly even; in some cases one may prove cheaper than the other, in another case the opposite may be true. The intermittently welded products are not usually sold simply as clad plates but rather as fabricated lined vessels; thus all the costs of fabricating enter into the picture. A relatively simple lined vessel can usually be built more cheaply by a system of resistance welding, whereas a complicated vessel can usually be made up more cheaply in the end by starting with one of the homogeneous clad steels. In the case of vessels involving l/rl inch clad plates, it is well to get bids based on both types-that is, on homogeneous stainless-clad and intermittently welded clad. As far as quality is concerned, both the intermittently welded and homogeneous clad steels have amply proved themselves. EXPANSION RATE AND MIGRATION OF ELEMENTS. Certain “bugaboos” have developed along with stainless-clad steel, most of which have been found to be either nonexistent or greatly overemphasized. Although there is a difference in the rates of expansion during heating and cooling of the stainless steel and the mild steel, it does not result in “horrible” distortions. Another bugaboo concerns itself with migration of elements from one component to another. For example, if the carbon in the backing steel is 0.27 per cent and the carbon in the cladding is 0.07 per cent, can some of the carbon from the backing steel migrate into the stainless and thus lower the corrosion resistance? The question has also been asked as to whether any of the chrome in the stainless cladding can migrate into the backing; presumably this might tend to emhrittle the mild steel and also slightly reduce the corrosion

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FIGURE6. HBAT TREATMBNT CHARTFOR 18-8 CLADS ~ B L S

For a time some buyers, especially the larger ones, felt there was not an ample source of supply. Judging by orders now placed and pending, it would appear that the source is ample, a t least for the time being. It would also appear that the accredited producers are prepared to consider greater capacity as soon as a greater demand is signified. CONTINUITY OF CLADDING. One last precaution may well be mentioned: Be certain that the continuity of stainless is maintained. When any kind of fitting, such as pipe, an

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outlet, or the like, is incorporated, be sure it is welded in such manner that none of the corroding substance can reach the backing steel. If a removable head is attached by steel bolts through a flange on a shell and the fit is not perfect, the steel bolt may be eaten out; rust may settle on the stainless surfaces and start electrolytic action. Thus, it is well to use a corrosion-resistant packing ring; in some cases a ring of solid stainless steel is inserted between the removable head and the flange, or stainless steel bolts may be preferred. There are so many places where continuity of cladding is essential that we cannot attempt to cover them in any detail or even indicate the main ones. The number of cases where this important precaution had not been observed is amazing.

Conclusions The precautions outlined are indicative rather than complete, There may be others that are known and observed by users and fabricators alike. I n any case, it is assumed that anyone using such material for the first time will seek the counsel of an accredited supplier. Certain precautions have to be observed with any material, especially a relatively new one. None of the precautions exceed reasonable commercial practice, and the fact that substantial quantities are being fabricated by all kinds of shops indicates that proper handling is neither impossible nor burdensome. In fact, many of the suggested precautions are self-evident and are mentioned merely to make this article more complete. Precise figures as t o the amount of stainless-clad steel now in service are not available, but as closely as we can estimate about 21,500 tons are giving satisfactory service. Yet those who produce stainless-clad and some of the larger users believe that this is a mere beginning. One of the producers spoke of utilizing clad for everything from “tin whistles t o battleships”. Perhaps there is some exaggeration in this broad coverage, but it is undoubtedly true that only a few of many applications have been developed. Whereas the present tonnage for the most part goes into the process industries, the

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decorative, building, transportation fields, etc., are to be considered as well as places where stainless-clad may be used as a substitute for various materials other than solid stainless steel. The first and most obvious place to look for valid applications is where solid stainless steel is now used; by no means have all of these possibilities been exhausted. It is suggested that purchasing and engineering or research departments might, to their mutual advantage, cooperate more closely on stainless-clad steel. The purchasing men are attracted by the potential savings but often hesitate to involve themselves in negotiations with their engineers on design changes; the engineers, who are always busy, tend to resist a change in material where the unit is giving satisfactory service. It may mean some redesigning, but savings in material cost of as much as 45 per cent are not often obtainable. It has also been observed that some purchasers have resisted stainless-clad because of possible past difficulties in their own plants or because someone they knew had trouble; in most of these cases the engineers of the buyer and the consultants of the supplier may not have given the matter thorough consideration. Or the material, a t the time offered, may not have been comparable to that offered today. A good consultant for any of the accredited producers of stainless-clad can undoubtedly walk through a processing plant with an engineer and find a variety of potential applications; often the engineer may not have considered some of them. We do not mean to infer that stainless-clad is a cureall; nor is it true that every plant is sure to have a potential use. But its possibilities have not been exhausted nor has more than a good start been made except in a few isolated cases. It would seem self-evident that the plant which takes advantage of the savings made possible by using stainlessclad will have a competitive advantage on a cost basis. Those who make use of the material t o yield a better product, when substituting i t for something other than solid stainless steel, should also have a competitive advantage.

Rotarv Cooler for Ammoniated Fertilizers E. F. HARFORD AND F. G. KEENEN E. I. du Pont de Nemours & Company, Inc., Wilmington, Del.

which are strongly exothermic, are completed in a few seconds rather than days. These ammoniation reactions liberate heat in proportion t o the weight of ammonia added. Other slower reactions which may occur between the solid components of the mixture generate much less heat. Fertilizer manufacturers have long realized that to take full advantage of the rapid reactions a simple low-cost method of dissipating the heat developed would go a long way toward helping solve a number of problems.

Existing Cooling Operations

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RIOR to 1928, practically all commercial fertilizers containing superphosphates were mixed and stored in bulk piles to complete the slow reactions between components. If this curing period was not completed, the fertilizer was probably in poor physical condition when received by the farmer. With the widespread use of the low-cost ammoniating solutions since 1928, some of these reactions, particularly those

For cooling solids, the process industries have used such equipment as water-jacketed screw conveyors, conveying screen coolers, and water-tube rotary coolers. Hot foundry sand has been cooled in a horizontal shell by controlled addition of water ( 3 ) . Calcined materials have been cooled in a rotary drum by transferring heat to the atmosphere through the shell or in a conveyor by spraying the material with water until the temperature reaches 212” F. None of these methods appeared adaptable to fertilizer cooling.