Annual Review
W. A. LUCE J. A. PEACOCK
Ferrous Alloys Technical progress focuses on control of corrosion, extension of usefulness of existing alloy systems, and deuelopment of new alloys ast year production was a t an all-time peak in many L industries and was accentuated by increased demands on materials of construction. These periods of high production in the chemical and related industries often result in more severe environments. Stainless steels are prominent in satisfying many of the demands in corrosive applications but even so, the very definite trend to higher temperatures and pressures in these same media necessitates additional exploration of existing alloy systems as well as development of new alloys. Predictability of equipment life is essential in minimizing costly downtime. I t is in this category that stainless steels are sometimes vulnerable because of premature failure due to localized corrosion. However, even many of these failures can be minimized by utilizing existing technology to the fullest. These annual reviews pertaining to stainless steels cover a period of approximately one calendar year and this one encompasses the year 1965. Many publications and periodicals pertaining to stainless and related alloys are extensively reviewed ; these are combined with expert comment from technical meetings and personal contact. Items of most significance to the chemical industry are then used as the basis for manuscript. Corrosion
Studies on corrosion control continue in the same vein as in recent years. Prevention of localized breakdown of passive films with resultant catastrophic failure of AUTHORS W. A . Luce is Chief Metallurgist and J . H. Peacock is Senior Materials Engineer with the Duriron Co., Dayton, Ohio. They have presented IMEC’s annual review of progress in thisjeld since 1957.
plant equipment receives the most emphasis. The surface potential developed during the formation of these films provides an indication of their protective characteristics and these potential measurements are being compared to actual corrosion rates to provide a practical evaluation of the technique. Once the general trends are established, it will then be possible to utilize potential studies alone and thus eliminate the more costly corrosion tests. The obvious goal is to determine the point a t which films resist localized breakdown in various media and, on this basis, the resistance of a particular stainless alloy to its environment might be readily established prior to specifying it in equipment. This procedure would be ideal for existing operations where the service conditions are well established. Proper evaluation of materials in the laboratory or pilot plant is often difficult because operating conditions may change, but potential measurements still offer a distinct advantage since they will reflect environmental changes more readily than long term corrosion tests. Stress corrosion cracking continues to receive the most attention because it remains the most serious problem facing the chemical industry. The amount of research devoted to this problem has continued to increase. I t has been mentioned that the relatively few failures have been blown out of all proportion but it is felt the current effort is justified. While much has been learned in recent years pertaining to the mechanism of stress corrosion cracking, no relatively simple means of controlling it has been developed. The three prerequisites for failure are the presence of chlorides (and possibly other halides), a relatively high stress, and appreciable temperature. Blocking any one of the three will essentially eliminate the problem. Peening critical areas to minimize tensile stresses, painting of metal surfaces to prevent concentration of chlorides, and minimizing the amount of applied stress by controlling cold working, all have been found helpful in specific instances but do not provide the universal approach required. Although cathodic protection is an established way to prevent stress corrosion cracking under many conditions, VOL. 5 8
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the chemical industry has been slow to develop the inherent potential of this procedure. I t is realized that cathodic protection would be useful only under completely submerged conditions but the fact that it will solve many problems in addition to stress corrosion cracking makes it appear promising. I t is well accepted that stress corrosion failures are caused by a combination of electrochemical and mechanical action, but the influence of the metal surface in initiating cracking is still in doubt. I t has been stated that cracks in the passive film are not enough to initiate cracking, but the imperfections beneath the film may well be critical. Dislocations in the base metal may be logical points for crack initiation, particularly since high energy levels at stacking faults are additive with other stresses to magnify the problem. The influence of chlorides in stress corrosion cracking is well established and one investigation (44) resulted in the conclusion that certain passive films transmit chlorides more readily than others under similar conditions of stress. Anodic dissolution of the metal underneath may largely depend on the amount of chloride transmitted through the film. The influence of various critical elements on resistance of stainless steels to stress corrosion cracking is also being studied. No definite trends have been established from this work, although certain copper-containing alloys have been found ( 2 A ) susceptible to this form of corrosion in sulfuric acid. Copper dissolved from the surface and then redeposited on the surface is considered to play a significant role in this process. Work also continues further to evaluate the beneficial effects of ferrite in the resistance of essentially austenitic stainless alloys. Intergranular corrosion continues to receive attention and, while failures can be controlled by careful specification of materials and realistic fabrication techniques, these procedures often seem difficult to accomplish on a practical basis. No production shortcuts should be attempted by the fabricator, and the consumer should take a realistic approach in specifying both materials and procedures. Thus, proper communication between supplier and user can eliminate this problem. More attention is being given to the tendency for various industrial environments to produce intergranular corrosion. Thorough evaluation of various media is now under way. Auld ( I A ) compared laboratory results with experience in numerous plants and determined that susceptibility of austenitic stainless steels to intergranular attack can be largely predicted by the 65y0 boiling nitric acid test. I t is expected that information of this type will enable this company more closely to supervise the fabrication of equipment in critical services and to be less restrictive where some leeway appears available. Additional attention is also being given to various tests to determine susceptibility to intergranular attack. The boiling nitric acid test is slow, and the development of other more rapid procedures will make the entire approach more universal. Other miscellaneous problems received some attention. Pitting corrosion still remains one of the most insidious forms of attack and while it can normally be con58
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trolled by the correct specification of materials, the stainless steels are still vulnerable. I t would be ideal if resistance to pitting attack could be anticipated by potential measurement of the metal surface in the exact service environment. Anodic protection still holds promise in critical environments where the utilization of stainless steels is questionable. Galvanic corrosion of valve stems and pump shafts by packing is still common in plants. The incorporation of inhibitors in the packing offers one way to minimize this problem when graphite or similar materials must be included. I t was also interesting to note that stainless steels were not heretofore suspected of being susceptible to hydrogen embrittlement, but new evidence suggests that this problem does exist. Industrial Application
The application of stainless steels to particular areas of the chemical industry receives much attention each year and usually typifies the trends with these important alloys. The seriousness of stress corrosion cracking is attested by the relatively large number of articles relating to field problems. The petrochemical and petroleum industries received special attention probably because chlorides are available to these processes representative of many other chemical applications as well. The handling of liquid metals such as lead, zinc, and sodium received considerable attention, and, while stainless steels are usually attacked by the liquid phase, liquid metals do find application in the vapor phase. Interest in seawater applications is particularly important, and it is expected that research efforts will be intensified to develop stainless steels with good corrosion resistance to this aggressive environment. All too frequently the failure of critical plant equipment can be traced to poor design. Groves (3A) provided a particularly timely article which considered many critical aspects known to be important. Stress corrosion cracking problems have focused the attention of industry on design, but other forms of corrosion can also be influenced by poor design. Proper design may be the single most important aspect in corrosion control, once the proper selection of materials has been made. All too frequently specialists in effects of corrosive environments on design are not given the opportunity to provide essential details to the designer and fabricator. High Temperature
The properties of stainless alloys at temperatures in excess of 1200" F. are receiving increased attention in the literature. Operating temperatures are continually increasing which means that existing alloys already operating beyond their optimum design range are being strained even further. I n many instances the environment is considered to be a cross between high temperature and corrosion such that,qtrength at temperature is no longer the only consideration for acceptable service. Thus the so-called corrosion grades of stainless steels are given increased attention in many applications. The influence of minor constituents on resistance of
various high alloys to high temperatures is important and probably will be the subject of more research. These trace elements may have beneficial or detrimental effects both in strength and resistance to high temperature conditions, but such effects must be known. There is a continuing search for new alloys to operate successfully a t the very high temperatures (in excess of 2000’ F.), since the trend is in this direction. I t is expected that even more attention will be given to this area in the future-materials may be required to withstand continuous temperatures of 2500’ F. Alloy Development
From year to year, noteworthy developments in stainless steel technology seldom have direct bearing on the chemical and related industries. This is particularly true in recent years when concentrated effort is being expended on stainless steels and other alloys for space age applications. T h e importance of the space effort is acknowledged, but the extensive expansions and rapidly broadening technology in the “less spectacular” chemical industry are also creating demands where new or improved stainless alloys could be utilized. I t is entirely possible that many of the existing alloys would have broader applicability if corrosion resistance were given more emphasis rather than the stressing of mechanical and physical aspects of alloys, as is now the case. Without question, chemical plant managers would welcome comprehensive and detailed information on basic corrosion resistance and potential areas of equipment application. Of particular interest are corrosion data on the precipitation of hardened stainless steels as compared to standard alloys such as AIS1 Types 304 and 316. The mechanical properties of many of these alloys are outstanding but development work should include corrosion data, if such alloys are to assume a position of prominence in the chemical industry. Of even greater importance is the desirability of extensive testing under plant conditions since this provides more searching results. This can be quite difficult to accomplish owing to the natural reluctance of the chemical industry to release detailed corrosion studies that would reveal confidential data pertaining to their processes. More and more of the newer process solutions now involve solids necessitating alloys resistant to erosion, in addition to corrosion. Once again the precipitationhardened stainless steels could come to the forefront if more were known about their basic corrosion resistance. The optimum in wear resistance is normally obtained from alloys possessing a combination of strength, toughness, and hardness. I t follows that maximum erosioncorrosion resistance would be obtained from the same type alloy if it had the required corrosion resistance. Many of the precipitation-hardened stainless alloys could become important alloys to the chemical industry, if the corrosion resistance were emphasized. Of practical significance is the influence of ferrite content on the properties of stainless steel castings. Beck and others ( I B ) showed that by controlling the ferrite content of the CF-3, CF-8, CF-3M, CF-8M, and CF-8C
alloys (18y0 chromium-8% nickel types), the tensile strength, yield strength, ductility, and impact strength can be controlled a t levels substantially higher than the equivalent wrought alloy. Also, a duplex structure of ferrite and austenite results in improved corrosion resistance especially under conditions of stress. Controlling the ferrite content in an alloy can be accomplished, on a theoretical basis, by balancing the major and minor constituents to give a calculated ferrite content. The theoretical basis is emphasized because the actual castings may not contain the same ferrite content throughout because of section size, rate of cooling, placement of risers, and other production variables. Furthermore, there is no nondestructive tool that can accurately measure. the ferrite content to the extent that destructive examination is capable of measuring. However, by realizing the limitations imposed owing to the lack of a good nondestructive measuring tool for actual castings and by making small adjustments, most of the advantages of controlled ferrite can be obtained. Stainless steel extrusions are relatively new in the United States, but continued development is broadening the market. Increased knowledge of glass lubrication, of extrusion die design, of production and finishing, and of improved die steels has placed extrusions in a much more attractive economic position. Incorporation of the importance of heat treatment into production procedures has also produced economies. A paper by Lennartz (5B) is of considerable practical interest. I t deals with the molybdenum-containing stainless steels and their stability with and without the stabilizing elements of titanium and niobium. T h e molybdenum in present commercial molybdenumcontaining stainless steels can, under certain circumstances, lead to precipitation of intermetallic compounds that will result in deterioration of corrosion resistance and mechanical properties. Manufacturing and Fabrication
Accelerated emphasis on all phases of stainless steel manufacture continues to be of utmost importance. Quality control a t all stages of operation is becoming commonplace as compared to even a few years ago. Such practices are of interest to all industries in general, but of particular interest to the chemical industry because of the extended equipment life that can result. Continuous casting of stainless steels and related alloys is a relatively new method of production of these alloys. Even in its short life span, noted improvements have been made by directing attention to quality aspects. These include melting practice, pouring temperature, pouring rate, cleanliness, design, deoxidation practice, gas content, and general overall practices. With technology advancing rapidly in all fields of endeavor, aspects that a t one time assumed a relatively minor role are now of major importance. The fact that vacuum treatment of stainless alloys has become a standard means of production in just a few years attests to the advancements that have been made. The effect of hydrogen, nitrogen, oxygen, nonmetallic inclusions, and V O L . 5 8 NO. 8 A U G U S T 1 9 6 6
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means to analyze these have all contributed to better stainless alloys from both a mechanical, physical, and corrosion point of view. No longer the era of, “It looks good so it’s OK.” Today quality assurance and means to attain it characterize all phases of manufacture. Surface condition is also receiving attention. Stability, roughness, defects, passivation, and pickling all are realized as very important aspects. To the chemical industry they are particularly important because the surface condition has an immediate effect on t4e corrosion resistance of the alloy, whether wrought or cast. Stainless steel castings and forgings have benefited from increased emphasis on quality in that new markets have been opened to them. Utilizing computers to study production variables in a foundry points to some of the many refinements that are being sought and attained. Technical advances in the production of stainless forgings have opened new fields in this industry to the extent that in some instances stainless forgings have been able to replace fabrications or castings. In other cases the reverse is true. Such advances demand even closer looks at quality and production techniques in an effort to remain competitive and provide the optimum product. Much has been said over the years regarding the relative merits of producing stainless steel parts by various forming operations such as stamping, deep drawing, and the like, as opposed to other production methods which necessitate more extensive utilization of machining techniques. Both approaches have their advantages but from the chemical industry aspect the situation will usually resolve itself to one of economics and whether any elements added to the stainless alloys to benefit the forming or machining operation will have any deleterious effect on the corrosion resistance. This point cannot be overemphasized because small alloy additions can have a pronounced effect on corrosion resistance. A review by Klingensmith (4B) emphasizing machining characteristics also outlines various other aspects. Included is a selector chart showing composition, machinability, surface finish, cleanliness, corrosion resistance, tensile strength, availability, and price. Over any given period of time, there undoubtedly is more published pertaining to welding of stainless steels than any other production or manufacturing procedure associated with stainless steels. This likewise could be a good indication of the importance associated with welding because all of the quality control and advanced production techniques are for naught if extreme caution is not exercised to assure sound high quality welds. Strength, ductility, cleanliness, corrosion resistance, technique, heat treatment, and many other items are extremely important aspects contributing to the success or failure of any stainless weldment in a corrosive use. I n the Alloy Development section, comments were provided concerning the influence of ferrite in stainless steel castings but the importance of this second phase in otherwise austenitic alloys is being recognized by wrought producers also. An article by Williams (8B) concerns itself with the ferrite content of AIS1 Type 321 stainless subjected to flash-butt welding. The author concludes 60
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the amount of ferrite formed at the weld interface is controlled by: (1) heat input, ( 2 ) displacement during upsetting, (3) mobility of the grain boundaries, and (4) material composition. The conclusions also state that cracking during welding is associated with low hot ductility due to excessive amounts of ferrite, but that modification of the welding technique can markedly reduce those amounts. In many areas, porosity also has been a problem with stainless steel and other nickel alloy weld metal. Data presented by Cresswell (2B) show that the addition of 10-20y0 hydrogen to the argon shielding gas in tungsten inert gas welding has reduced this tendency toward porosity. This article provides interesting data on hydrogen-argon shielding gases. A general article by Van Tromp (7B) details the metallurgical consideration in welding austenitic stainless steels. Included are carbide precipitation, knifeline cbrrosion, stress corrosion cracking, and ferrite and sigma phase formation. Miscellaneous Iron Base Alloys
Iron base alloys other than stainless steels will always have widespread application in many areas in the chemical and related industries. However, except for a few alloys such as high silicon irons and austenitic cast irons, these iron base alloys are not applied in corrosive media. With this in mind very little significant data were published in 1965 concerning these type alloys in corrosive media. A publication by Roberts (6B) pertained to recent research on high-strength steels. The work covers carbon steels, alloy steel, maraging steel, and precipitation hardened stainless steels. In this group tensile strengths as high as 404,000 p.s.i. were attained. Corresponding yield strengths of 363,000 p.s.i. and elongations of 6% were developed. Such outstanding properties could someday be a significant factor in the design of equipment in chemical plants. Dean and Copson (3B) conducted a program to evaluate the stress corrosion cracking behavior of the nickel maraging steels in seawater, industrial atmospheres, and marine atmospheres. The strength and toughness of these alloys make them attractive for many applications. The conclusions indicate these alloys can be applied in the type environments described, but careful consideration must be given to the composition and thermal history of the alloy if any significant benefits are to be derived in the resistance of the alloys to stress corrosion cracking. REFERENCES Corrosion-Industrial Applications (1A) Auid, J. R., ASTMSpecial Tech. Pub. ,VG.369, 183-9 (1965) (2A) Grafen, H., Werkstqfe Korrorion 1 6 , 876-9 (1965). (3A) Groves, N.D., March Design 37 (8), 118-22 (3965). (4A) Thomas, K. C.,A!lio, R. J., Nature 206 (4979), 82-3 (1965). Alloy Development-Fabrication (1B) Beck, F. H., Schoefer: E . A . , Flowers, J. W., Fontana, M. G., A S T M Special Tech. Pub. N o . 369, 159-74 ( 1 9 6 5 ) . (2B) Cresswell, R . A , , Second Commonwealth \%‘eldingConf. (London), 1 9 6 5 . (3B) Dean, S. W., Copson, H. R., Corroszon 21,95-103 (1965). (4B) Frederick, P. H., Klingensmith, D. G., Product EnE. 36 (ll), 56-63 (1965). (SB) Lennartz, G., DEW Tech. Ber. 5 ( 3 ) , 93-105 (1 965). (6B) Roberts, D. A,, DMZC Rep. 273, Battelle Memorial Institute, May 1965. (7B) V a n T r o m p , N., Steel Timer 190 (50341, 48-55 (1965). (8B) Williams, li. T., Brit. Welu’zng J . 12,435-41 (1965).
