W. A. LUCE J. H. PEACOCK
annual review
Ferrous Alloys
Corrosion control research, applications engineering, and new a l l y development seek to provide stainless steel and otherferrous alloys for increas.
c
i n g b seuere process conditions
xtensive expansion in the chemical industry during
E 1964 had a significant influence on the increased
consumption of nickel and stainless steels. During 1964, nickel consumption in the free world reached another peak and stainless steels accounted for the largest gain. Estimates indicate that 35y0 of the total nickel consumed last year was for production of stainless alloys. This more than exceeds,the combined total of the next two highest fields of application. No estimate is available pertaining to the percent or quantity of stainless steel applied in the chemical industry. All facets of stainless steels continue to be explored in an effort to satisfy the many demands made on these alloys. A trend toward increased studies in the generally less recognized areas appears to be developing. I t is likely that better techniques and more advanced instrumentation are contributing to such areas of investigation. Searching studies concerned with localized corrosion always have been and will continue to be of particular interest to chemical industry personnel. However, increasing interest is developing pertaining to aspects other than corrosion resistance because of the broader scope of applicability of stainless steels in the chemical industry. Corrosion
Corrosion control is the primary reason for the existence of stainless steels, but it has become increasingly
apparent that the full potential of these alloys will not be realized until certain serious drawbacks are eliminated. Stainless alloys are often subject to selective attack in certain critical media. This is particularly the case when a strong state of passivity is not maintained on the surface, and improving this situation has been the object of much research in recent years. Selective attack usually results in rapid and unexpected failure ; thus, improved predictability is the main objective of current technology. Potentiostatic studies are among the fundamental approaches used to provide the degree of passivity needed on stainless alloys for acceptable resistance in various media. Schwaab and Schwenk (5A) combined such electrochemical techniques with use of the electron microscope to study conditions under which passive films form on an 18% Cr-lOyo Ni alloy. They found that the surface films formed in various potential ranges differ in thickness, degree of crystallization, and composition. When both time and potential are increased, the films eventually become visible and are seen to have a crystalline structure. The influence of surface active agents in improving the degree of passivity of stainless steels is also receiving continued attention because this offers one of the best possible means for maintaining a steady film which resists localized breakdown. Stress corrosion cracking continues to be the most serious problem involving stainless steel equipment. The problem is undoubtedly related to passivation, but despite increased research emphasis during the past few years it continues to plague the chemical industry with large financial losses. There is no relatively simple remedy in sight. Much excellent work of a fundamental type is being accomplished on this problem by use of the electron microscope and by other means. It is still evident that chlorides are a requirement for failure, and Nielsen (3A) indicates that these chlorides diffuse down “tunnels” or “pipes” which are extensions of dislocations in the steel. This action results in local passivity breakdown by anodic action rather than by mechanical VOL. 4
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Important factors in ferrous alloy utilization are proper material
failure. Investigators are beginning to develop important clues, from which it is hoped that a solution to this problem can soon be developed. Certain practical studies are also an important part of this stress corrosion picture. Temperature (6A) is even more important than chloride ion content in the stress corrosion susceptibility of Type 304 stainless steel. Other variables which increase the rate of cracking are higher oxygen content and the presence of mercury salts. However, sodium dichromate inhibits attack. Proper thermal treatment is also important in minimizing stress corrosion cracking of stainless fabrications, but Sangdahl ( 4 A ) showed that there is no single treatment required for all situations. He provided an excellent summary of recommended treatments for various alloys operating under different degrees of service severity. This indicates that much of the problem can be eradicated by careful specification of materials and thermal treatments and by making certain that these requirements are strictly followed by the fabricator. Perhaps the safest approach is to utilize materials which are less subject to stress corrosion cracking, despite a slightly higher initial cost. The previous review (2A) emphasized that increasing the ferrite content of essentially austenitic 18% Cr-8% Ni alloys improved stress corrosion resistance. Bourrat ( I A ) also showed that alloys with high silicon contents were surprisingly resistant to this condition. More attention is being paid to alloy development as a possible means for controlling this problem. While intergranular corrosion can definitely be controlled by careful specification of materials and fabrication techniques, this approach is often quite costly and as such is by-passed for the sake of economy. This is fine when the decision is based on reliable data, but all too often this is not the case. A survey undertaken by the Welding Research Council ( 7 A ) is designed to shed light on this situation and thus will allow users of stainless equipment to predict when they are vulnerable to intergranular corrosion failure. This study will determine the extent of intergranular problems and will include such variables as the type of stainless steel, the effect of welding, and the influence of heat treatment on the many solutions common to the chemical process industry. Thus, if it is known beforehand that intergranular corrosion is a strong possibility, then the design engineer can insist on proper precautions to eliminate a costly failure. Pitting corrosion still causes extensive damage in specific environments and is the subject of continued study. However, this form of attack is also associated with passivity reactions, and techniques which will control this problem will undoubtedly be adaptable to stress corrosion cracking, intergranular corrosion, and similar conditions. 90
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Industrial Applications
The widespread application of stainless steels in the chemical process industries is emphasized by the numerous articles published each year. Fertilizer production is showing tremendous growth, and one review (2B) emphasized the importance of stainless steel not only in production but also for equipment used to apply the liquid chemicals on the farm itself. Another paper (3B) emphasized the use of stainless steels for petrochemical applications and illustrated the various pitfalls if care is not taken in the proper selection of the alloy. Stainless steels are also used in handling hydrofluoric acid and various fluorides which are among the most corrosive chemicals encountered. Seastrom (4B) studied the passivity of the 18% Cr-870 S i alloys in various concentrations of hydrofluoric acid alone and in combination with other acids. While the rate of corrosion may be appreciable under standard exposure, it is possible to use anodic protection to provide satisfactory resistance. Sea water handling provides many problems for the conventional stainless steels and these are discussed in one publication ( I B ) . Any means for improving the resistance of these alloys to this important corrosive would be noteworthy, but to date their application is limited. Certain of the highly alloyed austenitic stainless steels already find application and can be used with confidence. High Temperature
High-temperature applications are becoming . increasingly severe and more comprehensive data are required to allow engineers to satisfactorily design equipment with a predictable life. The common corrosion grades, containing relatively low carbon, are now of interest at temperatures of 1000" F. or more because of the conditions being handled. However, these alloys are generally susceptible to carbide precipitation and sigma formation in this range. Thus, they h a y have only limited usefulness. This situation may require additional research for alloys with a combination of stability in the carbideand sigma-forming ranges and with superior resistance to surface attack. The higher carbon alloys primarily designed for more elevated temperatures are being studied for services approaching 2000" F., and additional research is under way for alloys to be applied in excess of 2000" F. Cast alloys are usually superior to wrought alloys of a similar composition.
W . A . Luce is Chief Metallurgist and J . H. Peacock i s Senior Materials Engineer with the Duriron Co., Dayton, Ohio. They have prepared IGIEC's annual review of progress in this.field for several years. AUTHORS
selection and thermal treatment as well as careful mechanical design
Alloy Development
Minor adjustments in composition and more thorough investigation of highly technical aspects of existing stainless steels continue to demand specialized attention. The chemical and related industries are constantly searching fol; new and better alloys to minimize down time. I n past years, corrosion resistance was a primary criterion for new alloys or modifications of existing alloys, and though it may still be the primary motivating force, increased emphasis is being placed on other characteristics. These include mechanical and physical properties a t ambient, subzero, and elevated temperatures; weldability ; and, perhaps most important, quality. New alloys cannot be developed as rapidly as process changes occur in the chemical industry, but more searching studies of existing alloys have led to interesting developments. Stainless alloys of the 18y0 Cr-870 Ni type have been exploited rather thoroughly as far as modification of the major elements is concerned. Present-day studies emphasize the effects of elements such as titanium, aluminum, molybdenum, copper, columbium, boron, zirconium, nitrogen, vanadium, and arsenic, as well as carbides and intermetallic compounds. When a specific metallurgical characteristic is desired, then developmental work with alloy additions is minimized. But if more than one unique property is needed much more extensive work must ensue, and in most cases the latter situation is typical. Only small additions of the foregoing elements are usually made, but they can have significant effects. These may involve increased mechanical properties, improved corrosion resistance, better weldability, or higher quality. I n many cases, major changes can be predicted by microstructural examination. Many of the precipitation-hardened stainless alloys develop outstanding mechanical properties in the hardened condition, but unfortunately some corrosion resistance is usually sacrificed to attain these mechanical properties. Continued effort is being expended on these alloys in an attempt to maintain the exceptional mechanical properties without sacrificing corrosion resistance. A number of years ago, a cast, austenitic, high alloy stainless steel, commonly referred to as alloy 20, was introduced primarily for applications involving sulfuric acid. Shortly thereafter, its wrought counterpart became available. Both alloys had a nominal composition of 2970 Ni, 20% Cr, 2% Mo (min.), 3% Cu (min.), 0.07% C (max.), and a small columbium addition for the wrought alloy. A modification of the wrought alloy was announced in 1964, which concerned increasing the nominal nickel content from 29 to 34y0 while the other nonferrous elements remained unchanged. The original alloy performed satisfactorily in most sulfuric acid applications, but some difficulty was
encountered with stress corrosion cracking of wrought equipment. 'The particular area of difficulty involved 10 to 4oy0 sulfuric acid at high-metal temperatures such as are encountered with heat exchanger equipment. Increasing the nickel content is claimed to minimize and/ or eliminate the stress corrosion cracking encountered in these severe environments and is considered a significant breakthrough. A particularly timely article by Richter (5C) emphasizes new developments pertaining to the use and application of stainless steel screws. Unfortunately, little consideration is given to the fasteners used on stainless steel equipment. I n many instances the fasteners are as important as the vessel or major item itself, and proper selection of the stainless fasteners is required for proper operation. Use of stainless steel screws is expanding more rapidly than use of stainless steel as a whole and is a good indication of the importance of this aspect. Developments in this area can contribute much to the serviceability of equipment in the chemical industry. Vacuum technology, as related to stainless steels, has taken tremendous strides in the past few years. Many consider vacuum melting, pouring, and degassing as standard methods of production for many alloys, and in some instances this is entirely correct. However, present demands on stainless steels will bring many additional developments in this area. Manufacture and Fabrication. The interest in and demands for precipitation-hardened alloys are high. Though most of the tonnage is in the wrought forms, Dvorak and Fritz (3C) discussed production of 17-4 p H stainless steel castings. This alloy in the cast form is attracting increased attention for heat-resistant applications. I n addition to having good tensile properties u p to 900' F., its corrosion resistance is superior to that of Type 410 stainless and, in many respects, comparable to that of Type 302 or 304 stainless. To obtain optimum results, the composition must be closely controlled: it must be poured below 3000" F., and it must be double-solution annealed a t 1900' F. before the precipitation hardening step. Processing in such a manner will result in optimum corrosion resistance. The following are average mechanical data of 18 specimens heat-treated as outlined above : -tensile strength, 193,000 p.s.i. -yield strength, 166,000 p.s.i. -elongation, 13.6yo -reduction of area, 36.7y0 Increased emphasis continues to be expended on production of stainless steel castings whether they are produced by shell, investment, continuous, or green sand techniques. Such increased interest can undoubtedly be attributed to the desire and the demands for a general up-grading of casting quality by improved production VOL. 5 7
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technique and more stringent quality control measures. Stainless steel and higher alloy producers are realizing the need for more exacting controls to assure quality and are equipping their foundries with modernized equipment that will enable them to improve over-all casting quality. Forming or processing operations continue to be a major item of interest to producers of stainless steels. Such techniques as explosive, electrohydraulic, electromagnetic, and gas types of high-energy rate forming are being thoroughly investigated. While the chemical industry may not be interested directly in such operations, it must have an indirect interest. I t is imperative to know whether any such forming operation would tend to detract from the corrosion resistance of the alloy. Electrochemical, spark erosion, abrasive, and hightemperature machining are being employed with stainless steels. While design and type of equipment may demand these more advanced techniques, it is equally important to make certain that such procedures have no harmful effect on the mechanical properties or corrosion resistance of stainless steels. The addition of alloying elements such as sulfur, selenium, phosphorus, lead, and aluminum to stainless steels is common practice to improve machineability. A review by Blott (IC)considers the machining characteristics of many of the stainless steels. While alloying elements can prove highly beneficial, they can have adverse effects, such as lower transverse ductility and reduced corrosion resistance. Thus, it is important to consider the nature of the particular corrosive when use of a free machining stainless steel is contemplated. Welding is one of the most important single items contributing to the success or failure of stainless steel equipment in the chemical industry. When intergranular, stress cracking, galvanic, concentration-cell, or pitting corrosion are more than remote possibilities, extreme care in fabrication is not enough. A thorough knowledge of the fabrication history is as important as an understanding of the corrosive characteristics of the solution to which the equipment is exposed. Nuclear, submarine, and aircraft equipment require superquality weldments and this is promoting new welding techniques. These newer procedures appear encouraging from many aspects, but the increasingly stringent demands of the chemical industry dictate that any new technique be thoroughly evaluated in aqueous media to minimize the possibility of corrosion failures. Because of the broadening scope of stainless steel applications, more and more attention is being directed to the highly technical aspects of welding operations and their resultant effects. In this category are included pre- and postweld heat treatment, crack susceptibility in the weldment and adjacent areas, low- and high-temperature properties, low carbon alloys, susceptibility to selective types of corrosion, and welding of both cast and wrought stainless steels. An excellent series of reviews by Wilcox (6C-8C) covers the many aspects of the AIS1 200, 300, and 400 series stainless steels. Thorough consideration is given to all aspects of welding. 92
INDUSTRIAL A N D ENGINEERING C H E M I S T R Y
Miscellaneous Iron Base Alloys. During the past year many articles were published pertaining to various iron base alloys other than stainless steels, but few had direct or indirect bearing to applications of particular concern in the chemical industry. One area of interest would be 9y0 nickel steel. This alloy is excellent for application at subzero temperatures. I t possesses good notch toughness and impact properties at the low temperatures and can be used for pressure vessels because it is weldable. It has been accepted by the ASME Boiler and Pressure Vessel Committee under Code Case 1308 for use without stress relief in welded structures up to 11/4-in.thick at temperatures to -320" F. Renewed interest appears to be developing in the austenitic manganese steel. This alloy would not be applied in the chemical industry for its corrosion resistance, but alloy additions and modifications may dictate its use in noncorrosive applications where its mechanical properties are needed. The austenitic cast irons probably will never be used extensively in the chemical industry for corrosive applications, but because of the versatility of the various flake graphite and spheroidal graphite grades, their variety of properties could well be used to economic advantage in many less corrosive environments. A review by Kickel (4C) outlines briefly the history and development of these alloys. Detailed data are provided on the various characteristics of these alloys, including the most recent development for low-temperature applications. The 18% Xi maraging steels, not known for their corrosion resistance, are receiving widespread interest in all industries because of their outstanding mechanical properties. I t had been established previously that these alloys were susceptible to stress corrosion cracking by chlorides or from hydrogen embrittlement. However, modifications have been developed to improve their resistance to stress corrosion cracking. Many articles were published on these alloys in 1964, but one by Campbell, Barone, and Moon (ZC)provides detailed data on important mechanical properties. LITERATURE C I T E D Corrosion Bourrat, J., Hochmann, J., Aciers Speciaux, hfonographies Techniques 9, 8 (1964). Luce, W. A , : Peacock, J. H . , IND.ENG.CHEM.56 (8), 51-4 (1964). Sielsen, N. A , , Corrosion 20 (3), 104t-109t (1964). Sangdahl, G. S.,Metai Progr. 86 ( 2 ) , 100-4 (1964). ( S A ) Schwaab, P.; Schwenk, W., Z. M e t a l l k . 5 5 , 321-7 (1964). (6.4) Thomas, K. C., Ferrari, H. M., Allio, R . J., Corrosion 20 (3), 89t-92t (1964). (7.4) W'elding Research Council, Bull. 93, 25 (January 1964). (1A) (2A) (3A) (4.4)
Industrial Applications (1B) (2B) (3B) (4B)
International Nickel Co., Ltd., Publ. 2830, 8 (1964). Progr. 86, 78-99 (1964). Renshaw, W. G., Chem. Eng. Progr. 60, 98-105 (1964). Seastrom, C. C., Corrorion 20 (6), 179-83 (1964). Metal
Alloy Development (1C) Blott, Deo M., Metal Progr. 86 ( 2 ) , 119-24 (1964). (2C) Campbell, J. E.: Barone, F. J.: Moon, D. P., Defense Metals Information Center, Batklle Memorial Znrtitule Re#. 198, 122 (Feb. 24, 1964). (3C) Dvorak, Richard J., Fritz, John C., M e t a l Progr. 86 ( Z ) , 174, 176, 178, 180 (1 964). (4C) Kickel, O., Giesserei 51, 509-18 (1964). (SC) Richter, E., Wire 70, 45-53 (1964). (6C) Wilcox, Wayne L., M e t a l Progr. 85 (61, 96-101 (1964). (7C) Ibid., 8 6 ( l ) , 121-4, 126-30. (8C) Zbid., (Z), 140, 142, 148, 150.
