RESISTANT METALS
LL metals are resistant to something. We have been following a tendency for some years, however, which will eventually relegate plain steel to the background and credit its resistance less and less. Coupled with the rapid increase in the use of metals known generally as “alloys,” there has arisen a grave problem pertaining to allowable as well as sensible tolerances in design and construction. The “alloy” classification now in almost daily use embraces all metals not falling under the classification of mild steel. Not very long ago an alloy was known to the trade, other than the specialists, as plain steel with small percentages of seldom heard-of elements which, by virtue of secret sleighbof-hand manipulations, resulted in a fairy-tale metal possessing astonishing characteristics. Today the layman and the general trade are alloy-conscious and are aware of the tremendous effect the trend will have upon everyday life.
Design and Construction The stride, however, has been so rapid that in many instances design and c o n s t r u c t i o n have suffered. Alloys of nickel, chromium, copper, aluminum, etc., are expensive as compared to the more common materials such as cast iron, cast steel, and rolled steel, and to substitute in any form or d e g r e e seemingly r e p r e s e n t s an exorbitant expenditure. Even
after considering the obvious advantages, there has been a strong tendency to give more thought to the elasticity of the formulas than to the limitations of the metals themselves. However, this statement is not universally true, for many users and manufacturers have maintained established standards and in many instances have been cautious beyond known and accepted regulatory measures. In this connection we should emphasize the necessity of utilizing, where possible, existing 0% modified endorsed procedures to overcome the dangers resulting from designing and manufacturing without regard to the differences in physical and chemical make-up of the materials. That which is sound practice for one metal may be unsafe for another. Why have specialized metals advanced to such an extent? The answer could not be adequately stated without the d e s c r i p t i v e w o r d - r es i s t a n c e. Under the proper classifications, we ascertain t h a t r e s i s t a n c e t o corrosion, elevated temperatures, scaling, contamination, abrasion, etc., is readily obtained through the use of pure nickel, Monel metal, many grades of chromium alloys, combinations of chromium and nickel, copper, aluminum, manganese, tantalum, silver, vanadium, and other commercially perfect materials. Such properties make for economies in first costs and/or tremendous savings during operation-the former by virtue of lesser weights and simplified construction details, the FIGITRE 2. WELDEDALLOYVALVE 389
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WELnmn ALWK P ~ P ~HnwN E IN
FIGURE
3
0
FIOURE3. Wmmn ALLOY PIPE SHAPES
latter through longer life, decreased maintenance, freedom from contamination, process procedures not otherwise possible, increased efficiencies, etc. The inherent properties under discussion function collectively as well as singly, in that resistance to temperature, corrosion, and pressure are often simultaneously required.
I n view of the fact that base costs are considerably over those of the more common material?, it is not surprising to note the stress placed upon price comparisons, the resultant effort exerted to lessen manufacturing costs, and the subse quent radical departures in the design of minor as well as major details.
Price Factors
Welding
Although ultimate manufactured costs often range from five to twenty times those of mild steel, the investment returns are of varying percentages which frequently reach complete write-offs in a few weeks. Where circumstances permit, such high costs may be modified considerably by taking advantage of the characteristics of the particular material to be employed. However, i t is vitally important to consider all of the physical properties and the effects which they may have individually and collectively upon the final determinations for thickness and construction. Equipment far operation under high temperatures and/or high pressnrcs and projects off the beaten track have received merited considerution, but a considerable number of the general run of everyday low-pressure tanks, vacuum units, jacketed apparatus including small autoclaves, and many high-temperature installations have been installed with very little or no regard for any physical property other than tensile strength. Prom an economical viewpoint, construction along such lincs may react advantageously or disadvantageously, depending upon the application and upon the importance of the characteristics which have been ignored. Study must be given intelligently to yield, endurance, creep, fatigue, and impact.
Under the cloak of welding we have committed many sins, but had it not been for this great advancement in the art of fabrication many of the widely employed alloys would have remained dormant as compared to their present wide indnttrial value. The various methods of welding possess economic advantages as governed by their individual adaptabilities; two of them (gas and arc) are so valuable that they could not be divorced from this industry without its complete collapse. Disregarding, for the time being, elaborate formulas and their argumentary aspects, we find that castings in thousands of types, shapes, and analyses, special rolled shapes, structural forms, machinery and the bases for it, pipe and tubing, valves and fittings, etc., can be weld-fabricated from resistant metals and to such standards that they are commercially acceptable and highly desirable in view of their lightness, strength, flexibility, ease of attachment, resistance, and price. Figure 1shows stainless-steel &inch i. p. s. pipe, Van Stone type welded tank nozzles from 1.5 inches to 18 inches in diameter, reducing fittings, fabricated elbows from 3 to 10 inches in size, and 20-inch manhole fittings. These ar0 representative of the possibilities of weld-fabricated fittings which are widely applicable and extremely economical
Without flanges such items may be welded in place making for uniform uninterrupted flow. Flanged or Van Stoned fittings are utilized extensively in connection with equipment proper and the piping for it. Wall thicknesses are governed by corrosion, pressure, abrasion, or other individual operating requirements and can be varied from 20 gage to 1.5 inches. The saving in weight, where permissible, is an important factor. Uniform metal thicknesses without concentrated points of stress or radical shape differences are made possible by fabricated fittings. Figure 2 shows the details of a weld-fabricated alloy reducing valve, 5 X 3 inches in size and entirely devoid of castings except for the handwheel. The savings in weight and price through construction of this nature are obvious. With a maximum carbon content of 0.06 per cent, this stainlesssteel valve possesses excellent corrosion resistance and is entirely free from pits or blowholes. Second only in importance to equipment falling under the classification of tanks we find piping in all forms, including tubing. Pure nickel, together with its alloys and the others
of the “ r e s i s t a n t ” g r o u p , h a v e made p o s s i b l e the use of huge tonnages of process piping, their physical characteristics permitting unusually light wall and flange construction. Light wall prefabrication coupled with the elimination of castings and the utilization of loose steel-backed Van Stone type joints permits a reduction in budget, field erection, labor, and maintenance charges. Seamless pipe and tubing cannot be procured over the entire range of alloys and only in certain sizes for those which are pierceable. Practically the entire range, however, is weldable by several processes, and the limitations as to diameter and length are in most instances only the limits of the raw material. In keeping with regulations pertaining to allowable pressures, contingent of course upon the individual metal peculiarities, welded pipe will prove to be as uniformly resistant as the parent metal, will possess uniform wall thickness, can be obtained in nonstandard diameters, and will withstand drastic breakdown tests. Figures 3 and 4 illustrate the ability of 6-inch i. p. s. welded alloy pipe to withstand crushing tests. Figure 5 shows 6-inch, high-silicon and -titanium tubes. High-temperature, high-pressure 8-inch piping is shown in Figure 6, Only an alloy steel was obtainable for resistance to the conditions under which these lines were subjected. Monel metal and stainless-steel coolers similar to that indicated in Figure 7 have many advantages, including weight, strength, freedom from contamination, decreased friction loss, cleanliness, constant heat transfer rate, low maintenance, and resistance.
T
Alloy Tanks .FIGURE6. WELDEDJOINTIN %INCH HIGH-PRESSURE HIGH-TEMPERATURE ALLOYSTEELPIPE
With mild-steel nonpressure tanks costing only several cents per pound more than the material itself, it is difficult to do more than mention the comparative initial costs of alloy or resistant tanks. However, physical properties of the latter permit radical modifications in design. Utilizing less expensive reinforcing members properly placed and with reason behind their design, it is possible t o construct light wall equipment possessing resistant features but strong enough to serve as closed- or open-top storage tanks, vats, kettles, and a multitude of other purposes not
e
TANKWITH STEELREINFORCEMENT FIGURE 8. PURENICKEL
FIGURE7. ALLOY STEELCOOLER COILS 391
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methods have advantages, and in actual service both have proved satisfactory. It is obvious that the nature of the service should determine the advisable procedure. Constructions along these lines are not in accordance with code design, but the thought is offered that proper design and workmanship will make for serviceable units representing material savings. An outstanding example of everyday utilization of the above principles is the modern milk truck tank seen on all highways. Many of these tanks are built of No. 12 gage stainless steel or Monel metal, reinforced with steel shapes, lagged, and sheathed. Continuous service of this nature is indicative of the fact that the inner tank metal possesses attractive physical properties, for it must resist corrosion, contamination, stress, fatigue, impact, and the abrasive action of daily cleaning. Many of these units are emptied under a pressure of 20 pounds per square inch, in which case we have a contradiction of earlier statements. Reports show, howFIGURE9. TANKBUILTOF 20 PERCENTSTAINLESS-CLAD ever, that many of the pioneer tanks failed to varying degrees; STEEL the present ones are the result of considerable study of proper design, position, and method of attaching the steel strengthening members.
Tank Liners The next attempt to substitute economical light-gage resistant metals was the move to line steel and wood tanks. A great number of methods have been developed, some patented, some not. A few of each have been successful under favorable operating conditions, and a very few have withstood corrosion, elevated temperature and pressure, or vacuum. The latter involve complicated fundamentals and are used mostly in connection with extremely heavy-walled steel vessels. Nonpressure, nonrigid l i n e r s for application t o m o r e understandable problems are included in the group just mentioned and can p r o v e s a t isf a c t o r i 1y resistant, provided (‘cheap” construction is not confused w i t h “economical” c o n s t r u c t i o n Paperweight sheets of metal are inexpensive; solder is ine x p e n s i v e ; nails and rivets, even though they are of nickel, Monel metal, or stainless FIGURE 10. NOZZLES AND MANHOLE BUILTON THE TANKCOVER BY WELDING steel, are inexpensive; talk and brawn are inexpensive. Yet they do not often combine to make for an economical or pleasing reaction involving pressure or too much temperature variation. Tank Into this type of liner must be designed flexibility, strength, liquid tightness, freedom from distortion, cleanliness, endurunits up to 21 feet in diameter, 20 feet high, and possibly ance, and long life. larger, with seven years’ service to their credit, have been so manufactured and are still in operation with a complete absence of tendency toward failure. The body proper should be made from the least number of sheets or plates, with all seams butted and double-welded. Thicknesses greater than No. 14 gage should be machine-beveled prior to welding. The minimum allowable working stress of the material between reinforcing members should be used and greater care taken to ensure against the shell assuming too much of the load. Accurate fitting is essential to guard against undesirable space between the shell and strength members. If plain-steel reinforcement is objectionable, the substitution of nickel, Monel metal, or alloy shapes still permits a saving over a heavier wall self-supporting unit. Figures 8 and 9 illustrate tanks of the type just described. The reinforcing members shown in Figure 8 are fabricated as a n independent cage with complete absence of welding to the tank proper; in the stainless-clad unit the channels and angles are stagger-stitch-welded to the shell and heads. Both
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Other Considerations Manufacturing codes will of necessity be formulated and adhered to in order to meet fairly the requirements of many industries. Procedures must be modified or amplified to provide for adequate and intelligent selection of the proper materials and fabricating methods. Much equipment is produced without knowledge of, or thought given to, its physical or chemical life. The required wall thickness to meet pressure conditions should be supplemented with the expected loss due to corrosion. This old rule is often side-
FIGURE11. COMPARISON
OF
1M-
MA DE WELD (Left) WITH ONE MADE AcPROPERLY
CORDING TO BEST PRACTICE, A FTER EXPOSURE TO CORROSION
FOR
HOURS
3
tracked. Substitutions for metal removal in heads under pressure should be given as much thought when designing for resistant metals as for steel, Localized stresses should be avoided or counteracted. The tremendous cost of nozzles, manholes, sight glasses, stuffing boxes, agitators, and other tank appurtenances if made from resistant materials in the conventional manner would affect the use of these metals more than we realize. Consequently it has been necessary to fabricate all such items from plates and bars and yet abide by certain fundamental regulations. Much ingenuity has resulted. A simple and common example is illustrated in Figure 10. The welded construction incorporated here is clearly shown, the loose steel backing flanges having not yet been positioned. Process units of all diameters, thicknesses, and lengths are being daily installed to meet extreme operating conditions ranging from -100" to f2100" F. with varying degrees of corrosion. The requisite understanding of the numerous resistant metals, together with their peculiar physical and chemical properties plus their individual fabricating characteristics, presents a problem to the designer, the purchaser, the manufacturer, and the operator. Many of the chromium and chromium-nickel alloys must be heated through close ranges during shop handling. Some are air-hardening, some are extremely susceptible to grain growth, others require stress-relieving or heat-treating to produce
maximum resistance to corrosion and/or maximum d e v e l o p ment of d e s i r a b l e physical properties. Omission of, or improper heat-treating in c o n n e c t i o n with, finished stainless tanks sometimes results in complete failure in a few days; yet instances not requiring heat-treating of the finished product are numerous. Here again resistant metals are economical or costly depending upon the degree of i n t e l l i g e n t thought given to the entire problem, including operating conditions. Rolling, hot- and cold-forming, forging, grinding, welding, riveting, as well as heat-treating, often detrimentally affect the properties of the sensitive metals. The limitations must be respected in order to produce equipment approaching expectations. Thousands of pages have been published 0x1 the subject of welding. One of the greatest obstacles to a more rapid advance in the industrial use of the resistant metals has been, and still is, the vast amount of study, research, experience, and high degree of workmanship necessary. The most perfect material is useless for the majority of uses without means of joining. The weld and adjacent area can be the weak link in the chain or can be equal to, or stronger than, the parent metal. Figure 11 shows two identical pieces of material welded a t the same time and subjected to the same operating conditions for 3 hours. Thought must be given to the characteristics of the weld metal proper as well as to sections of base plate and weld together. Economical c o n s t r u c t i o n will, in most i n s t a n c e s , r e s u l t in lower first costs, but "cheap" c o n s t r u c t i o n is not economical, The advance in the field under d i s c u s s i o n will be in direct proportion to the intelligent thought given to the above and to many other unmentioned phases. RECEIVED February 18, 1937.
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STEELTANKWITH HOMOGENEOUS LEADLINING,USED FOR OIL REFINING (See Mantius and Freiherr, page 373.
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