Iron, Mild Steels, and Low-Alloy Steels C. P. LARRABEE AND B. J. KELLY United States Steel Co., Pittsburgh, Pa. i
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T h e general trend in the literature during the past year on the use of iron, mild steels, and low-alloy steels as chemical engineering materials of construction has been toward the discussion of corrosion. The use of cathodic protection for solving the complex problems of corrosionis one of the many methods reviewed herein. The review also points out that spheroidal cast iron has continued to find increased u s e as an engineering material. From the performance data gathered to date it is expected that nodular graphite cast iron will replace other grades for many uses.
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HIS paper summarizes information published since the previous articles were written (84, 36, 37, 58).
A new type of electrolytic iron powder, being produced on a pilot plant scale, is reported (6) to resemble Swedish sponge iron physically, but to have a purity of 99.8%. The compacting properties of this material are reportedly improved if contamination with silica is prevented. MILD STEELS
IRONS AND IRON POWDER
In previous papers (S4,36), itttention was called to the increasing use of spheroidal cast iron. Evidence of continued interest in this product is shown by a list (6) of 54 companies in the United States who are licensed to manufacture ductile iron by the magnesium process. Performance data accumulated to date presage a strong demand for nodular graphite cast iron as an engineering material. I n this connection, 10 United States companies (9) indicate successful application of this product. Vennerholm (46) revietl.s the production, heat treatment, castability, machinability, wear resistance, applications, and cost of nodular iron. A comparison of the engineering properties of nodular iron with those of other types of cast iron and with those of two typical cast steels is also made. Galloway ($3) describes ductile iron castings weighing up to 30 tons. He states that composition and heat treatment are the keys to the broad range of properties obtainable. Oscillograph recordings of special vibration dampening tests illustrate how this versatile material fills the gap between cast iron and cast steel. Kahles and Goldhoff (SI)state that the machinability and mechanical properties of ductile iron can be adjusted by slight changes in silicon and manganese contents. Ductile iron ip reported (6) to have good resistance to wear and galling. The long service life occasionally experienced with unpainted cast iron in outdoor or underground exposures is attributed by Acock ( 1 ) to the formation on the material of a protective film containing silicon, carbon, and phosphorus. Westwood (47) reports the results obtained from comparative corrosion tests of cast iron, Ni-Resist, and phosphor bronze in 38 types of solutions. Cast iron was not superior to the other materials in any of the solutions, although it was equal to the others in a few solutions. He states that in an ammonium sulfate solution containing 5% sulfuric acid, cast iron corrodes 424 times as much as Ni-Resist. Another report (7) of a comparo, tive test states that Ni-Resist pumps are standing up better than cast iron in handling pulp mill liquors. According to Herington (27),the size and shape of the graphite in cast iron, and hence certain mechanical properties, can be controlled by the method employed in the manufacture of Meehanite iron castings. Physical structures that offer a wide range of properties are obtained by various heat treatments. Lynch and Snodgrass (36) state that high-strength gears made of iron powder impregnated with copper have replaced malleable iron castings in the gear train of an automatic washer. Savings of 60 to 70% in manufacturing costs were realized.
The type of attack on steam boilers which wm formerly called caustic embrittlement still receives considerable attention. The results ( 2 ) of nearly 40 years of continuing research have shown that the preventive measures involving control of the ratio of the sodium sulfate-sodium hydroxide contents of boiler water are inadequate. As a substitute for this method of control, the following procedures are recommended: 1. Complete investigation of boiler feed waten with special reference to alkaline content and the presence of natural inhibitors 2. Installation of described embrittlement detectors on boilers operating in districts which have water containing sodium carbonate or sodium hydroxide 3. Pro erly supervised and controlled use of sodium nitrate or lignin in akaline waters 4. Proper workmanship, resulting in tight m m 8 in all boiler construction
Since stress-corrosion failures are sometimes found in steel equipment containing caustic soda, Berk and Waldeck ( l a )conducted a series of tests with a U-bend specimen to determine the temperatures and concentrations of the soda at which stressed steel was most likely to crack. Severe cracking of the specimens occurred a t concentrations between 17 and 40% sodium hydroxide and a t temperatures above 200’ F. Weir (46) also reports the results of strewcorrosion tests of carbon steel in sodium hydroxide solutions at elevated temperatures. He found that (1) failures closely comparable with those occurring in practice may be produced under controlled Iaboratory conditions; (2) failures were obtained with solutions containing less than 10% sodium hydroxide, although more concentrated solutions were more harmful; (3) the presence of silica in the solution did not affect the number of failures; (4) addition of tannin to the solutions did not prevent cracking but did decrease the frequency of failures in some cases; (5) anodic polarization may stimulate failure, whereas cathodic polarization decreased the corrosive attack; and (6) fine-grained low-carbon steel was no more resistant than coarse-grained steel of the same carbon content, although steels with higher carbon contents showed improved resistance. Stressed mild steel wire in an ammonium nitrate solution a t 100’ C. was used by Cubicciotti and Boyer (17) to study stresscorrosion cracking. The factors studied included the effect of heat-treating times and temperatures and the effect of heat treating in vacuum and in atmospheres of air, nitrogen, hydrogen, and helium. No cracking occurred unless the wires were heat treated; varying the type of atmosphere appeared to have little effect on the “breaking time” of the wires.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 43, No. 10
an addition of 1.270 lead. Dipping galvanized steel in an Corrosion in high-pressure steam generators for ships n as reacidified solution of 1% sodium or potassium dichromate for 5 ported ( I O ) to be caused by air in the feed Rater and contaminaseconds decreases the rate of formation of the white rust. tion with sea water. Strict control of the pH value of the feed In the use of mortar and concrete in conjunction with steel, water and regular additions of caustic soda and trisodium phoscorrosion of the steel must be prevented or extensive spalling of phate served to suppress corrosion. the concrete will result. Friedland (88)states that the composiGardner, Clothier, and Coryell (24)report that ammonia in tion of the mortar used is more important than the depth of coverconcentrations as Ion- as 0.4% in crude oil systems containing 2% ing. He reports the results of teets made on steel sections coated hydrogen sulfide by volume is effective in controlling vapor zone with various thicltnesses of mortar which contained a highercorrosion. Field tests in 55,000-barrel storage tanks showed that than-normal proportion of cement. With a 0.4-inch thick coatthe protection afforded the steel nas gradually increased from ing of mortar, only 1% of the steel surface was rusted after 2 years IS%, when an inhibitor injection rate of 1.4 pounds per tank per of atmospheric exposure, and with 2.4 inches of mortar, only 0.07% day was employed, to almost loo%, when 12 to 15 pounds per of the steel surface was rusted. day were added to the tank vapor zone. The uninhibited corThe waters of Lake Maracaibo, Venezuela, are reported to be rosion rate averaged 0.053 inch per year for the test period. so corrosive that 4.5-inch bare steel pipe (wall thickness approxiKetting agents and, less frequently, inhibitors are used to mately inch) must be replaced every 5 years. Concrete coatdiminish corrosive attack on steel equipment. Fontana (20) ings on piling are stated by Zauner (49)to give satisfactory prodescribes experiments which show that the addition of wetting tection in these waters. agents to dilute acidic and basic solutions decreased the rate of The protection of steel ships by paints has been investigated corrosion of low-carbon steel and increased the rate of corrosion of extensively. The surface preparation for painting is of greatest lead in contact with the solutions. importance, states Ffield ( 1 9 ) ,became the presence of large areas Foster (21) states that rusting of ordnance items of ferrous of inill scale adjacent to small areas of exposed steel may cause materials is prevented by enclosing the equipment in air-tight seiious corrosion even under some paints. Removal of the mill envelopes containing dicyclohe\ylamine nitrite or related comscale by pickling or sand blasting are the surest methods, but repounds. One advantage of this method of packaging is that moval by adequate weathering is more satisfactory than nonweapons shipped to combat zones can be used immediately upon removal. “Stray-current corrosion,” in the form of pits, is frearrival. quently encountered on ships. Such attack is believed to be An inhibitor in stick form decreased corrosion in a Louisiana caused by inadequate return n-ires to on-shore welding generators gas well according to Harper (226). Weight losses of steel coupons employed in making ship repairs. decreased from 2.03 grams to 0.02 gram when the inhibitor was present. No information is given regarding the composition of C4THODIC PROTECTION the inhibitor. Cathodic protection of iron and steel installations has become The use of metallic, organic, and inorganic coatings has been an important tool in the campaign to increase the service life of one of the most common methods of decreasing the corrosion of equipment. This protective method is discussed in an elemensteel. Zinc coatings have long been successfully used in many tary manner by Stewart, Palmer, and Braunholtz (44). The types of environment. Haclterman and Shock (85) describe complexity of corrison problems is illustrated by the case history coatings which contain zinc, magnesium, and aluminum and of a tank that %vasa common connector of several distribution which were reported to be more resistant than zinc coatings in lines in different soils. Some of the connecting pipes were anodic field tests of steel tubes used in the Frasch process for mining sulto others and these fur. Difficulties in obtaining adherent coatfailed, after which the ings, however, have tank itself b e c a m e thus far imposed defia n o d i c a n d failed. nite limitations on the Other case h i s t o r i e s useful alloy range. are described. Under certain conLaQue (33)describes ditions of expo s u r e , the cathodic protection “white rust” forms on system employed on zinc surfaces. It has off-shore oil d r i l l i n g been stated (I) that rigs. He recommends this flocculent oxide is cathodic protection beproduced only in the low the low tide zone, presence of l i q u i d Monel sheathing in the water :on the zinc surtidal zone and in the face. Losses as high splash zone, and paints a s 0.065 o u n c e p e r above the splash square foot per week zone. have been experienced. Zauner (49) states L a r g e concentrations that cathodic protecof carbon dioxide tend to reduce the amount tion is given all coated of white rust formed on pipes immersed in the zinc. The rate of corwaters of Lake hlararosion of the zinc surcaibo, but that the cost faces on g a l v a n i z e d of protecting uncoated steel is also decreased pipes is very high. b y t h e a d d i t i o n of In an oil refinery a COURTESY THE INTERNATIONAL NICKEL CO. 0.05 % ’ aluminum to net saving of 30% in Ducti!e Iron Pump Castings and Impellers the s p e l t e r , b u t t h e maintenance costs of attack is increased by Cast, machined, and heat treated to a Brinell hardness of 400 to 450
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INDUSTRIAL AND ENGINEERING CHEMISTRY
pipe lines and storage tanks by cathodic protection is reported by Holsteyn (28). A bibliography on cathodic protection containing 436 chronological references grouped into five categories has been prepared by Rohrman (43). LaQue (32) reviews the various methods of preventing corrosion other than by the use of protective coatings. Control of environments, use of cathodic protection, attention to details of design, and selection of the proper composition of material are discussed. References to 77 articles and 14 books are given. Riesenfeld and Blohm (41) state that the corrosion of heat interchangers is reduced by lowering the solution temperatures and the degree of saturation of the liquid, or by the elimination of oxygen. They advocate the stress relieving of all major vessels. If stress relieving is not practicable, the use of more corrosion-resistant alloys is recommended. An extensive bibliography of experiences with various materials for lining sulfate digesters has been prepared by Collins (16). The use of cast iron, carbon steel, alloy steel, stainless steel, brick, and other materials is considered. Moore (39) states that keeping the stresses to a low value aids in preventing corrosion fatigue of drill collars. Also, cathodic protection by a magnesium ring in the drill collar box will prevent corrosion fatigue. The apparent economies possible by the use of steel castings as a substitute for rolled steel sections, when irregular shapes are required for chemical equipment, are discussed by Boldgett (IS). It is pointed out, however, that maximum economy frequently requires that equipment be designed so that it can be fabricated from standard rolled sections. ALLOY STEELS
The results of experimental tests and operating data are summarized by Nelson (40) to support his statement that practical operating limits may be established for carbon and alloy steels for all degrees of severity of service in hydrogenation plants. It is reported (8) that the performance of carbon steel is unsatisfactory in the presence of hydrogen a t high pressures or high temperatures. At pressures of 2140 pounds per square inch, 1% chromium-molybdenum or 1% chromium-vanadium steels have given good service; and a t pressures of 6000 pounds per square inch, 2% chromium-molybdenum and 6% chromium-molybdenum are used. When both pressure and temperature are high, 18-8 stainless steel may be required to obtain satisfactory service. Tests by Ihrig (29) indicate that a t pressures of 13,000 to 15,000 pounds per square inch and temperatures of 204' to 593' C., steels are seriously attacked by hydrogen-nitrogen mixtures. Carbon and low-alloy steels lose carbon and absorb nitrogen throughout their cross sections and the formation of numerous cracks and fissures impairs the strength and ductility of the material. Austenitic stainless steels have relatively good resistance to attack by the mixture, but the surface layers of metal become nitrogenized. Carbide-forming elements do not prevent attack on intermediate chromium steels as has been reported by some investigators working with lower pressures. The corrosion of twenty digesters in similar service in a sulfate mill is described by Johnson (30). Some digesters failed after 3 to 5 years of service, whereas others lasted 12 to 14 years. The additional cost of carbon-brick lining is justified if the useful life of the digester is increased by 5 to 7 years and the use of stainless-steel linings is warranted by an increase of 10 years in service. Buchan (14) reviews the problem of corrosion in condensate and in high-pressure wells producing sweet oil. Joint designs, thread lubricants, and tube installation practices are discussed, along with the use and relative values of low-carbon steel tubing, 9% nickel steel tubing, internally nickel-plated steel tubing, and plastic-coated steel tubing. Methods of determining corrosive conditions and the efficiency of methods used to prevent corrosion are summarized.
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Data on the corrosion of a new distillation unit handling lowsulfur (0.20%) crude oil are presented by Burns (16). Excessive corrosion of the overhead condenser shells of the atmospheric pressure tower was eliminated by injection of ammonia into the overhead stream. Alloy-steel tubes (7% chromium-0.50jo molybdenum) were substituted for carbon-steel tubes in the radiant section of the furnace to decrease corrosion. The substitution of manganese-molybdenum steel for carbon steel resulted in an appreciable decrease in maintenance costs a t an aggregate plant handling crushed granite (3). Armstrong and Greene (11) report that a nickel-chromiummolybdenum steel valve casting, which had been in continuous operation for 53,810 hours (6.14 years) a t 900' F. and at an average pressure of 841 pounds per square inch, was still in excellent condition. Investigation of the steel showed that no loss of strength, no embrittlement, and no graphitization had occurred during this service. The valve in question was a 4-inch, 900pound valve with stellitefaced seat rings. Comparative data on the performance of carbon steel and SAE-4340 steel churn drill bits are presented by Dewey and Armstrong (18). Performance records indicate that SAE-4340 is considerably superior to C-1065 in drilling various types of rock. It is suggested that the 4340 grade should contain a t least 0.40% carbon and be heat-treated as follows: heat to 1650' F., hold 8 hours, air-cool; reheat to 1525' F., hold 8 hours, air-cool; reheat to 1100 to 1200" F., hold 8 hours, air-cool. Ripling (42)reporta that the yield strength and tensile strength of SAE-1340 steel are found to decrease continuously with increasing tempering temperature and/or increasing testing temperature. Static-load tension tests were made a t temperatures of 70°, -110', -220', and -321' F. on steel that had been tempered at various temperatures. Tempering a t 600' F. resulted in embrittlement of the steel. HIGH-STRENGTH LOW-ALLOY STEELS
Whitney and Spar (48) discuss the metallurgical problems encountered in drilling of oil wells and production of petroleum. It is stated that ASTM A-94 (structural silicon) steel has replaced A-7 (structural carbon) steel in many applications. The low-alloy steels are now replacing A-94 steel because of increased strength and greater corrosion resistance. The use of low-alloy steels in derricks, power transmission equipment (clutches, shafts, bearings, and sprockets), slushing pumps, drill pipe and joints, casing, sucker rods, pumps, and many special types of equipment is described. Data presented by Acock (1) show the superior atmospheric corrosion resistance of chromium-copper-silicon-phosphorus lowalloy steel when compared to mild steel or copper steel. It is suggested that the useful life of low-alloy steel, copper steel, or mild steel in contact with fresh waters will be increased by deaeration and softening of the water. LlTERATURE CITED
(1) Acock, G. P., Brit. Cast Iron Research Assoc., J . Research and Development, 3, 539 (1950). (2) Anon., Am. Ry. Eng. Assoc. Bull., 52, 223-4 (November 1950). (3) Anon., Eng. News-Record, 145, 40-1 (September 1950). (4) Anon., J . I n s t . Metals, 78, 47 (1950). (5) Anon., Muterials and Methods, 32, No. 3, 136, 138, 140 (1950). (6) Anon., Nickel Topics, 4, N o . 1, 2 (1951). (7) Anon., Paper Trade J.,131, 23, 24, 26 (Sept. 21, 1950). (8) Anon., Petroleum Refiner, 29, 104-10 (1950). (9) Anon., Steel, 127, 85-9, 100 (1950). (10) Anon., 2. Ver. deut. Ing.,92, 560-1 (1950). (11) Armstrong, T. N., and Greene, R. J., Steel., 127, Sect. 25, 66 (1950). (12) Berk, A. A., and Waldeck, W. F., Chem. Eng., 57, 235-6, 238 (June 1950). (13) Boldgett, Omer. Iron Age, 166, 110-11 (September 1950). (14) Buchan, R. C..Corrosion,6, 178-85 (1950). (15) Burns, D. L., Ibid., 6 , 169-77 (1950).
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Collins, 1'.'I?., Jr., POPWTradr .I., 130,No. 21, 32-4, 3 6 , 38, N o .
22, 20-2 (1950). Cubicciotti, D.! and Royer, \V.,Welding J., 29, 1403-58 (1950). Dewey, J. H., and i2riiistrong, .'I N., Eng. Mining J . , 151, 64--(i (July 1950). Ffield, Paul, Marine Isrig. r r l i d Shipping RaG-~.ro, 55, 6,17 (Deceniber 1950). Fontana, b9. G., IND. Ids
