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INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 21, No. 12
Recent Developments in Corrosion-Resistant and Heat-Resistant Steels’ John A. Mathews CRUCIBLE STEELCOMPANY OF AMERICA, NEWYORK,N. Y.
ECENT events connected with heat-resistant and corrosion-resistant steels have brought chemists and metallurgists nearer together than they have been for a long time, for metallurgists have aided in solving a great many problems, particularly those arising in manufacturing chemistry, through the development of corrosionand heat-resistant steels, and they may, in fact, consider that they have contributed notably to the advances of chemical industry in this country since the war. Prior to this decade no one imagined that steel might be used to replace porcelain, earthenware, lead, and other metals for application involving extreme conditions of corrosion. Investigations which brought this about, however, were started in the previous decade, and centered in Germany around the work of Professor Strauss of the Krupp Works, and in America around the investigations of C. M. Johnson of our company in regard t o the development of austenitic chromium-nickel steels. We are familiar with the fact that Stainless Steel originated with Brearley of Sheffield, before the Great War, and is occupying a unique position in the manufacture of cutlery. I t s application in the field of chemical manufacture has not been very great; but since the development of Stainless Steel, which contains about 0.30 per cent carbon and 12.0 per cent chromium, there have been developed other chromium alloys which have found a place for themselves, particularly in nitric acid and atmospheric nitrogen work. These are, for the most part, lower in carbon and higher in chromium than the original Stainless Steel, and some of them show almost perfect resistance to nitric acid, weak or strong and hot or cold. It would be well if the term “stainless” were confined exclusively to its use as a trade name and to the product originally so designated-i. e., the 12 per cent chromium alloys. The term “non-corrosive steels” is also inaccurate, because for the most part the most non-corrosive steels are not 100 per cent perfect. The only truly accurate term to use is “corrosion-resistant,” since this term indicates a degree of resistance, and we are usually dealing with relative corrosion and not with perfect resistance. The rate of attack may be from one hundred to many thousand times less than that of ordinary steel; nevertheless, we cannot honestly say that the material is non-corrosive. We have suffered from extravagant claims on the part of producers and from too great expectations on the part of consumers. Under many conditions a moderate degree of staining or discoloring is of no particular concern, providing progressive corrosion and pitting do not occur and the rate of corrosion is sufficiently slow to warrant the difference in cost between other materials and the corrosion-resistant product. There is a lack of uniformity in reporting corrosion tests. Sometimes these are reported in terms of loss in weight in percentage, sometimes in milligrams per centimeter, or per square inch, and sometimes in terms of penetration per day, per month, or per year. Reports in terms of loss in perceritage of weight are misleading unless the tests are made with the same size and shape of test pieces. Obviously, a sphere
R
* Presented before the New York Section of the American Chemical Society, June 7, 1929.
and a thin sheet of metal may weigh the same and yet present very different amounts of surface to the corroding medium. Losses in milligrams per centimeter or per square inch are of no great value unless the time factor is given, and in any case they are very hard t o visualize. The best way of reporting losses, in the writer’s judgment, is in terms of penetration per unit of time. This, a t least, gives a concrete idea as to the life which may be expected from a structure or a vessel made of a given thickness. Again, the comparison of tests by different observers is difficult because the conditions of the tests are not minutely described. It makes a difference, in many cases, whether the test is made with the exclusion of air or with stirring with access of air, also it makes a difference whether or not the products of solution are allowed t o accumulate. In some cases the accumulation of salts, owing to the action of the corroding medium on the alloy, may either check corrosion altogether or may accelerate it. It is therefore highly important in comparing tests to know exactly all the conditions observed in ronnection with the test. Furthermore, the chemicals employed by different observers may not be of equal purity, and in some cases small amounts of impurities, which are frequently unknown, may exert a material influence upon results. Although we have now had several years of experience both in laboratory tests and in commercial applications, it is still true that whenever possible these should be supplemented, in the case of any application, with actual tests under operating conditions. This will help to avoid disappointing failures due to the fact that the laboratory tests usually cannot duplicate operating conditions. The present paper will discuss first, certain matters in regard to ordinary corrosion a t ordinary temperatures, and second, the type of corrosion more familiarly called oxidation or scaling, which occurs a t high temperatures. Frequently the two are combined in a single problem where the corrosive action of the products of combustion must be considered, or in some cases one side of the metal may be subjected to heat and scale and the other side of the same metal subjected to ordinary chemical attack. Corrosion Resistance Although there are many possible combinations of iron, carbon, chromium, and nickel, experience and research have shown that certain types are most desirable and useful. Table I gives type analyses of the various products most commonly in use. Manganese, sulfur, and phosphorus are also in normal amount, and the steels may also contain various other elements in small amounts, such as copper molybdenum, tungsten, etc. No. 1 represents the original Stainless Steel of Brearley, which was developed just prior to the Great War, but its commercial introduction was delayed for several years because of the necessity of using available chramium for other purposes. Nos. 2 to 4 represent more recent products that have been developed to meet conditions as they have arisen and to these three products the name “iron” is given merely as a distinguishing term to differentiate them from the steel, although as a matter of fact they are all steels. These three irons represent ascending amounts of chromium and all are
INDUSTRIAL A N D ENGINEERING CHEMISTRY
December, 1929
made with very low carbon. They do not require hardening to render them reasonably stainless or non-corrosive to ordinary atmospheric conditions and mild corroding media, such as food acids. Steels 1 to 4 are produced under the patents of the American Stainless Steel Company. They are of the type known as ferritic alloys, since they are all magnetic, and the members of this family below 15 to 16 per cent chromium respond to hardening and tempering in the ordinary way of the engineering alloy steels. After hardening, they show very high strength and hardness. Steel of the type containing above 16 per cent chromium is nonhardenable and, owing to the higher amount of chromium, hardening is unnecessary to produce corrosion resistance. Nos. 5 to 11 represent the principal types of austenitic chromium-nickel steels, first developed in this country by our company and covered by patents issued to C. M. Johnson. As most of them were developed during, and just after, the war period, we were not aware that similar steels were being produced a t the Krupp Works as a result of the researches of Benno Strauss, and Professor Strauss antedated us in the type represented by Resistal2-KA. The high silicon content was an original feature of the work of C. M. Johnson. As a matter of fact, what he had in mind primarily was r e sistance to scaling rather than resistance to ordinary corrosion and, as will be discussed later, silicon is of very great importance in retarding oxidation or scaling a t high temperatures. Table I-Types of TRADE NAME Stainless Steel Stainless Iron No. 12 Stainless Iron No. 18 Stainless Iron No. 24 Rezistal 2-KA Rezistal 2-C Rezistal 4 S Rezistal 7-K 9 Rezistal 2600 10 Rezistal 25542 11 Rezistal 355-C
No, 1 2 3 4 5 6 7
Corrosion- a n d Heat-Resistant Steels CARBON CHROMIUM NICKEL SILICON 0.30 0 . 1 2 max. 0 . 1 0 max. 0 . 2 0 max. 0 . 1 5 max. 0.20max. 0.20max. 0.25max. 0.40max. 0 . 2 5 max. 0 . 2 5 max.
12.0-14.0 12.0-14.0 17.0-19 .O 24.0-27 .O 17.0-19.0 17.0-10.0 16.0-18.0 24.0-26.0 8.0-10.0 10.0-12.0 10.0-12 . O
. .. . . . , .. .. .. .. ,. .. .. . . . .. . .
7.0-10.0 7.0-10.0 24.0-26.0 14.0-16.0 21.0-23.0 24.0-26.0 33.0-35.0
0 . 5 0 max. 0 . 5 0 max. 0 . 5 0 max. 0 . 5 0 max. 1 .O max. 2.0-3.0 2.5-3.0 1.5-2.0 1.2-1.7 4.5-5.5 4.5-5.5
A recently formed company known as Krupp-Nirosta has been assigned various patents formerly owned by the Crucible, Ludlum, and Krupp steel companies-all dealing with the type of steels here shown, and their heat treatment. Neither the American Stainless Steel Company nor the Krupp-Nirosta Company are operating companies but holding companies. Our benevolent Patent Office, particularly in the Alloy Division, issues patents overlapping and duplicating one another. As a result there are various conflicting patents which it would be wise to merge. Inasmuch as the subject is such a vast one and its technology so complex, it would be for the public interest to merge, not only the patents, but more particularly the experience and research work of the various companies which have been pioneers in this field. Other companies, in addition to those owning the original patents, have been or will be licensed, so that the public may have the advantage of the best metallurgical skill and research, both in the manufacture and heat treatment of these extremely interesting products. These steels are distinguished from those of the Stainless Steel and Iron group in that they are austenitic in character and almost totally non-magnetic. They may not be hardened by quenching but, on the other hand, are materially softened by quenching from high temperatures; in fact, up to temperatures of 1200" to 1300" C. the higher they are heated the softer and more ductile they become. They are considerably hardened by cold work, such as cold-drawing, forming, or pressing operations, and to a certain extent after fully softening by quenching they may be hardened by various tempering operations. I n this particular they are diametrically opposite in behavior to the ordinary carbon and alloy steels ( 2 ) .
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I n considering the use of corrosion-resistant steels, the problem must be considered from different angles, depending upon the application desired. In some cases the use depends merely upon their relative life and in others the question is whether the products of solution will interfere with processes or contaminate and prove deleterious to the product with which they come in contact. If it is merely a question of the life of the vessels or containers made from them, the corrosion test in terms of penetration per year is sufficient. If contamination of the product or interference with the process is concerned, then the problem is somewhat more intricate. In general, the chromium steels of 18 per cent chromium and upward are extremely resistant to nitric acid and mixtures of nitric and sulfuric acid. This has made possible their use in a large way in the construction of nitric acid plants, several thousand tons of these products having been employed. The chromium-nickel steels of the type represented by 2-KA are also extremely res'stant to nitric and nitro-sulfuric acid mixtures, when sufficiently low in carbon and suitably heat-treated. The same steels are also very resistant to sulfurous acid, either in the form of moist gas a t high temperatures and pressures or in the calcium bisulfite and sulfurous acid in pulp digesters, also a t high temperatures and pressures. For pure sulfuric acid, either dilute or strong, the chromium alloys are not good; in fact, Stainless Steel might have been discovered a good many years earlier than it was, had not a distinguished metallurgist made his test of certain high-chromium products with sulfuric acid and neglected to test them with nitric acid. The standard test for Stainless Steel cutlery when hardened and polished consists of testing either in 1.20 specific gravity nitric acid or in acid copper sulfate. With the latter reagent if the hardening is not properly done or if the blade has been scorched in the grinding, copper will be plated out from the solution. The drop test with vinegar is another test for cutlery. If the material is in its best condition none of these tests will make any spot or discoloration whatever on the polished blade. In connection with tests we must not be satisfied with a single test of short duration, for in some cases the rate of attack may be relatively strong for a short period of time, after which it ceases altogether. This is true of the nitro-sulfuric mixture on high-chromium steels. There may be a little attack for one or two hours only, after which the alloy becomes quite permanent. This is a case of rendering the surface passive as has been fully discussed ( I ) . As has already been pointed out, the accumulation of salts due to the action of the attacking chemical often exerts a marked influence on the progress of the attack, in some ca$es even prohibiting it altogether. It has been stated that chromium irons are not resistant to sulfuric acid. If copper sulfate happens to be present in sufficient quantities, however, these products will withstand sulfuric acid quite satisfactorily. The same is true in some cases when ferric sulfate is present in sufficient quantities. Matters of this kind make the subject very complicated in its practical application to industrial uses. It has frequently been pointed out that acids in food products, such as lactic acid in milk, citric acid in lemon juice, and acetic: acid in vinegar, are quite inactive toward Stainless Iron or Steel, but that the same strength of acid in pure aqueous solution may attack this product. It is necessary to accept with considerable reservation those long lists which have been published showing what may or may not be resisted by various steels. We do not mean to imply that the tests upon which such lists have been based have not been honestly and carefully made, but because slight variations in conditions themselves produce such remarkable differences in performance, they must be accepted
1160
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
tentatively and as guides rather than infallible information or standard data. Let us review briefly the outstanding characteristics of the types of corrosion-resistant steels shown in Table I. As regards the plain chromium-iron alloys, represented by the first four items, in general resistance to corrosion increases almost directly with the chromium content. The 12 per cent chromium alloys, after hardening, are especially useful to withstand ordinary atmospheric conditions and mild corroding media, such as those found in food products. They also show a high resistance to nitric acid attack, but not quite high enough to permit their use in nitric acid plant construction. For this use the Stainless Irons 18 and 24, particularly the former, have been developed and very largely used. This steel will withstand protracted boiling in concentrated nitric acid of 1.42 specific gravity. The loss in terms of penetration per year will hardly exceed 0.02 inch (0.51 mm.). The use of such steel has been approved for the construction of shipping containers for nitric acid, including tank cars. For the purpose of inspection, a test is made, using a 66 per cent nitric acid, plus 1 per cent sulfuric acid and 0.10 per cent hydrochloric acid. With this mixture the specification permits a loss of 0.0078 gram per square centimeter per month. Actual tests of six different heats with our material (No. 18) showed a loss of only 0.00164, or about one-fifth of what the specification permits; whIle the test with three heats of Rezistal 2 showed a loss of one-half as much or only one-tenth of what the specification permits. It was shown that the Stainless 18 samples were somewhat discolored in this test, while the Rezistal samples retained their original luster throughout the month. This test is made a t 38" C. (100" F.). Stainless 12, because of its ready response to heat treatment and development of high-strength characteristics and fair hardness, may be used in many machine parts, such as beater and Jordan bars and bed plate in paper-mill machinery. For this application it is a decided improvement over ordinary steel bars, which have been used for the lower grades of paper, and also over bronzes, which are largely used in the manufacture of the higher grades of paper. The chromium irons are all malleable and ductile and may be formed and fabricated, but for the most part riveting construction is used rather than welding. Although these materials may be welded, there is difficulty in producing welds without brittleness a t the weld. I n fact, the chromium irons of 16 per cent chromium and upward are rather low in their impact resistance and the welds are likely to be still lower than the plates themselves, so that for parts, such as shipping containers, subjected to abuse in handling, welding could hardly be recommended. The austenitic steels, Nos. 5 to 11 in the table, can all be welded, and the welds themselves have little tendency to become brittle, because the material in the weld is not air-hardening in any sense, but is likely to be quite as soft and ductile as the adjacent material of the plates welded. Rezistal 2-KA as a type is probably the most generally useful of all the corrosion-resistant steels. This material is also represented by the steels known as Enduro KA-2 made by the Ludlum Steel Company and the Central Alloys Steel Company. It withstands the cold nitric acid test almost perfectly, and this, coupled with its high impact resistance and the ease with which it may be deep-drawn, formed, and welded recommends i t for use where abusive handling is encountered. It is largely used for ornamental purposes as well as in the chemical industry, owing to its ability to be deepdrawn and formed and t o take a high degree of polish and a pleasing color, which will remain permanent indefinitely under ordinary atmospheric conditions. Rezistal ZKA with-
VOl. 21, No. 12
stands perfectly the action of boiling acetone of 5 per cent, 50 per cent, or c. P. strength. There is no evidence of attack on the metal either immersed in the solution or in the vapors above the solution. When exposed to the same strength of solution a t room temperature for a long period of time, no rust spots appeared nor was there any change in polish or luster on Rezistal 2, while Stainless 12 and 18, under the same conditions, developed local rust spots in the 5 and 50 per cent solutions but not with the c. P. acetone. Material of the Rezistal 2-KA type has given very excellent results in sulfite-pulp digester tests, where protracted boilinqs at temperatures about 130" C. are maintained. The solution consists of free sulfurous acid as well as calcium bisulfite. Tests of several hundred hours have shown from no loss a t all up to a loss of 0.007 inch (0.178 mm.) per year. A 30-day test with sulfur dioxide saturated with water vapor a t 70-80" C. and 50 to 60 pounds (3.2 to 4.4 kg. per sq. cm.) pressure left samples of this type with their original luster and finish, while many other ferrous and non-ferrous alloys were strongly attacked and discolored under the same conditions. One other material which met the test was chromium iron of the 24 per cent type. For weak or strong mixtures of nitric and sulfuric acid this steel gives a very remarkable resistance after a preliminary slight attack. The same is true of some of the higher chromium alloys and also Rezistal 7. Oxalic acid is fairly corrosive and even a 1 per cent solution in the cold attacks ordinary steel and the chromium alloys, but not Rezistal 2-KA. A 5 per cent solution seems to be less active than 1 per cent with many of the metals tested in the cold, but for either 1 or 5 per cent strength in the cold or a t 50-60" C. Rezistal 2-KA will prove quite satisfactory. Concentrated formic acid is very corrosive, but with a 90 per cent solution in two different tests there was a loss of not over 0.0155 mg. per square centimeter per day. Rezistal 2-C differs from Redstal 2-KA in carrying a high silicon content. It is, in general, quite as resistant as the 2-KA material, and in some cases more so, but its particular advantage is in the greater resistance to high temperatures and scaling. This material has been found to weld even more readily than the low-silicon variety, but it is not quite equal in its ability to withstand deep-drawing and forming operations. Its heat-resistant qualities will be discussed later. Rezistals 4 and 7-K are also especially good heat-resistant steels; the latter also shows unusually good resistance to phosphoric acid of all strengths up to 85 per cent. This applies to pure phosphoric acid solution and not to impure solution, such as occurs during the process of manufacture of the acid in which hydrofluoric is usually present to a considerable extent. In general, Rezistal 4 and 7-K seem to meet certain conditions more satisfactorily than 2-KA, and may therefore be considered as "special-purpose" corrosionresistant steels. The next three steels are not so well known to the public as those previously described. No. 9, or Rezistal2600, was originally introduced for marine use during the war and has been largely used for submarine parts that are exposed to salt water conditions. It has also been used for periscope tubes, replacing a copper-nickel alloy, which had been standard, for several very excellent reasons. It was found equally satisfactory as far as resistance to corrosion was concerned, and it possesses nearly the normal modulus of elasticity of ordinary steel, which is considerably higher than that of the non-ferrous alloy and of considerable importance since it is desired to maintain accurately the optical axis through a tube sometimes as much as 28 feet (8.5 meters) long. Although it is a higher
INDUSTRIAL A N D ENGINEERING CHEMISTRY
December, 1929
tensile product, it is nearly 10 per cent lighter in weight than the non-ferrous alloy formerly used. It was also found that this steel possessed very fair resistance to dilute sulfuric acid, in fact, almost up to the present time, it has been about the most resistant ferrous alloy for this type of use, such as pump shafts in oil refineries, which are subjected to dilute sulfuric acid , hydrogen sulfide, and other sulfur compounds. However, it was not considered good enough for more severe conditions involving sulfuric acid, and therefore we have been experimenting for some time to produce a still more resistant alloy and the result is Nos. 10 and 11, known as Rezistals 255-C and 355-C. The relative merits of these steels, particularly in contrast with other available material, are shown in Table 11. The data on Anka and V28 have been taken from Monypenny’s valuable book on “Stainless Iron and Steel;” Anka is a well-known English product and V2A a German product of the type of Rezistal 2, or the so-called 18 chromium-8 nickel group. Although Stainless Iron is not satisfactory to sulfuric acid conditions, some data regarding it, based partly upon Monypenny’s data and partly on our own, have been included. It will be noticed that with Stainless Iron the attack increases with the strength of sulfuric acid, while Rezistal 2600 is very much less attacked and the rate is almost uniform between 5 and 30 per cent acid. The improvement over 2600 in the newer steels (255-C and 355C.) can also be seen in the lower part of Table 11. Altogether this represents a great many tests by three or four observers. For moderate strength a t atmospheric temperatures there is not much difference between the two steels, but for a higher strength a t higher temperatures 355-C shows considerable improvement. It also possesses \ ery material resistance to dilute hydrochloric acid, although we do not offer it as being satisfactory for that reagent. of Sulfuric Acid on Various Steels
Table 11-Effect
TEM.
ACID PERASTRENGTH TURE 0
5 35 5 10 30 5 10 30 1 2 3 4
s
16 30 50 10 20 10 30 10 92 92
4.3
LESS
NO.
NO.
NO.
...
0,065 0.17
6.09 9.76 24.18
0.0108 0.0101 0.0093 0.0116 0.010s 0,0054 0.0071 0.0024
20 20
4.20 11.90
69
STAIN-
IRON 2600 25.542 355-C Milligrams pe7 square centrmeter per hour
c.
20 20 20 20 20 20 20 20 20 20 20 20 20 20
lo+
2% NaCl
SOFT STEEL “ANKA” V2A
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resistant steels. They are very malleable and ductile, can be machined with little difficulty, and are readily welded. I n this respect they offer a great advantage over certain cast products, which are unmachinable and necessarily very heavy in comparison with structures made of sheet or plate metal. In the preparation of Table I1 data from various sources and reported in various ways have been correlated by calculating to milligrams per square centimeter per hour. It might be well to state that 1 mg. per sq. cm. equals 0.0000492 inch of penetration per hour or 0.431 inch per year; also 1 mg. per square inch of surface per hour equals 0.00000763 inch penetration or 0.0668 inch per year, assuming a specific gravity of 8, although most ferrous alloys are slightly below this. So far as the writer knows, these newer steels represent a great advance over all previous ferrous alloys in their resistance to sulfuric acid conditions. There are many other applications of these materials where dilute sulfuric acid is involved-for example, in vessels for scrubbing gas to remove ammonia. We made tests of this kind starting with a solution obtained from an industrial plant, which as received showed about 5 per cent by weight of sulfuric acid saturated with ammonium sulfate. Two 4-hour tests a t 50” C. showed no loss whatever with Rezistal 2600, 255-C, or 355-C. The temperature was then raised to 75” C. for another 4 hours and still no loss resulted. The free acid was then increased to 12 per cent by weight, and another 4 hours a t 50” C. showed no loss, nor 4 hours at 75” C. The samples which originally had a fairly smooth ground finish showed some slight discoloration only. There are also processes in which a containing vessel is alternately exposed to acid and alkali conditions, as in certain processes in petroleum refineries. We therefore made tests where these steels were subjected alternately to 25 per cent sulfuric acid and 25 per cent caustic soda. Three boilings of 4 hours each in both acid and alkali showed an average loss of 0.186 mg. per sq. cm. for 255-C steel in the acid and no loss in the alkali treatment. The 355-C steel showed an average loss of 0.059 mg. per sq. cm. in the acid and no loss in the alkali treatment. These tests should be considered extremely satisfactory, considering the strength and the temperature of the acid solution. Steels 255-C and 355-C are still somewhat in the experimental stage, as there are some difficulties in their manufacture and fabrication still to be overcome. Their performance, however, encourages us to go ahead with the development of these products, which seem to offer so much promise of usefulness. Heat-Resisting Steels
0.054 0.031 0.155 0.062 0.062
0.054 0.077 . 0.0155 0.031
0.372 0.155
In the manufacture of steel the pickling operation in sulfuric acid is frequently used. Some bars of this material were placed in a pickling tank for several months. Most of the time they were subjected t o 10 to 15 per cent sulfuric acid a t temperatures from 60” to 80” C., and occasionally hydrochloric acid or common salt was added to the bath; yet after about 4 months the bars had shown no appreciable loss in diameter. Following this experiment a large pickling tank was made from this material, and this has been in use for 4 months with complete satisfaction, most of the time with sulfuric acid as described above and occasionally with some added hydrochloric acid. It is believed that these steels are a very great contribution to the family of corrosion-
There is very extensive literature upon corrosion and theories of corrosion. A recent book covers a bibliography of some four thousand titles dealing with this subject, for the most part with aqueous solutions or under atmospheric conditions. A particular form of corrosion resulting from oxidation a t high temperatures has been given relatively little attention. The mechanism of scaling has not received the attention it deserves. Scale or oxide on bars is not a simple structureless oxide of iron, but long-continued exposure of steel to high temperatures results in a scale which consists of three distinct layers. Pfeil (3) recently discussed the nature of these three layers consisting of an outer and an inner layer and the third layer attached to the unoxidized metal. The middle layer is definitely crystalline. The determination of composition of each of these layers showed that some very interesting reactions were going on. For example, in a 36 per cent nickel alloy the outer layer of scale contained but 1.46 per cent nickel, the middle layer 2.29, while in the
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
third layer there was a concentrabion of nickel to 52.08 per cent. Witli ordinary Stainless Steel of the 12 per cent chromium variety the oiiter layer contained 0.61 per cent chromium, the middle layer 1.12, and the inner layer 23.32 per cent. A similar concentration in the inner zone appears in tungsten, vanadium, and silicon steels. I n a commercial chromium-nickel engineering steel there was a concentration of both nickel and chromium on the inside layer. Manganese steel proved the only exception to the somewhat general rule and, strangely enough, the manganese content in none of the three layers was as high as that of the original steel.
Figure 1 S c a l i n g Test a t 1090O C. Top row left to risht: 4-7-25-172-173-114 Bottom row left io right: 2-15-1S---679-175--93
From a purely practical side of the use of materials a1 high temperatures, there are three conditions to be m e t (1) high resistance to scaling or the formation of an adherent scale which does not readily fall off, permitting re-oxidation of fresh surfaces; (2) steel which is not impaired physically by exposure to high temperatures and the products of combustion and the atmosphere or the various chemicals with which it may he in contact; (3) for most purposes the material should also retain relatively high physical properties a t elevated temperatures, particularly where high heat is to be resisted, such as in stills and boilers subjected to high gaseous pressures which may result in explosions. In general, so far as the first question is concerned, chromium-iron alloys are quite satisfactory. Stainless Iron has not much resistance to scaling, but the resistance increases with the addition of chromium, so that the Stainless 18 is a considerable improvement over Stainless 12, and irons coutaking 25 per cent or more chromium are extremely resistant to oxidation. I t has also been found that when m r y high silicon is present the amount of chromium may be reduced and still retain a fair resistance to scaling. For example, the well-known Silchrome valve steel contains about 9 per cent chromium and 3 per cent silicon and possesses very notable resistaiice at temperatures up to, say, 925" C. It is also true that tire higher the chromium the less silicon is needed to insure high resistance to scaling. The high-nickel alloys, sometimes called "ferro-nickels," have long been known as resistant to ordinary oxidation and rusting a t atmospheric temperatures, but they are not notably good at elevated temperatures in mium and chromium-silicon the fist of the three conditions mentioned above.' The nickel steel answers the second quite acceptably for more moderate temperatures, for they are not embrittled by long
Vol. 21, No. 12
exposures a t high temperatures. Neither the chromium nor the nickel alloys possess very high physical properties at elevated temperatures. The combination of nickel and chromium is much better, and in the combination of chromium, nickel, and silicon, as represented by the Rezistal steels developed by this company, we have the best combination to meet all three of the conditions described. These steels resist oxidation better than the chromium-nickel without silicon, as will he shown later, and they are not embrittled, but become tongher, after exposure a t high temperatures, aud their physical properties at high heats are notably better than the simpler alloys of chromium or chromium and silicon or high-nickel only. For this reason they have come into general use for supplying the needs of industry in connection with the construction of furnaces, heaktreak iug equipment, oil-cracking stills, and other high-temperature or high-pressure duties. The high chromium and chromium-silicon alloys are rather low in impact resistance even at ordinary temperatures, and they are rendered less resistant and, in fact, may become quite brittle owing to long exposure at high temperatures, say from 1000" to 1200" C . The chromium-nickel steels of the usual types are very tough and resistant to impact at ordinary temperatures and, in general, become more resistant after heating at high temperatures, and this quality is not decreased but rather increased by long holding; but they are not so resistant to scaling as are the chrominmnickel-silicon steels, which are also very highly resistant to impact at ordinary temperatures and after long subjection to high temperatures. They are very definitely more resistant to scaling than the same steels with normal silicon content. I n on0 series of experiments with steel of the 18 chromium8 nickel type, tests were made with three variations of silicon contenW.40, 1.10, and 2.14 per cent. After being exposed for 36 hours at 980' C. and all scale removed, the rate of penetration per year figxed 0.88, 0.49, and 0.043 inch (2.23, 1.21, and 0.10 cm.), respectively. I n other words, the 2.14 per cent silicon WLLStwenty-two times as resistant as the steel with 0.40 per cent and the steel with 1.10 per cent silicon wm twelve times as resistant.
Figure Z-Scallng Tesf--1200° C. left to right: 25-7-173-1724
In order to give some idea of how the high silicon functious in both the chromium and chromium-nickel steels to reduce scaling, some practical experiments will be described. Also from the impact resistance tests the fundamental difference in the nature of the chromium and chromium-silicon steels and the chromium-nickel austenitic steels will be apparent. The difference between the chromium-silicon and the chromiurn-nickelailicon steels will be seen from a study of their behavior under impact, after long holding at high - temceraturw but tested at room temperature.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
December, 1929
Tnhle 111 gives the actual aualyscs of steels used in certain of the sealing tests. They cover chromium and silicouchromium, also clironiiuin-nickel-silicon alloys, and one chromium-nickel steel, low in silicon (No. 679). Some of these steels are of the same type as mentioned in Table 1. Each type of steel has been, at one time or anotlier, described as a heat-resisting steel, but "heat resisting" is a relative term and, as will be shown later, the various products differ widely a.nlong themselves. However, to show how very much better they are than commercial carbon or alloy steels, Rezistal 4 was directly compared with the ordinary chromium-nickel gear steel. The test was conducted for 3 weeks at 900" * 25" C. It was carried out in a tube in an electrically heated furnace, open a t the ends and elevated at one end to an angle, so that there was a strong draft through the tube a t all times. Under these conditions an iron-const.antan thermocouple lasted only 2 or 3 days. At the eud of tlie 3 weeks there was an adherent scale on ltezistal 4 which was removed with difficulty by hammering with a final loss in weight of 1.1 per cent. The commcrcial steel, under identical conditions showed a loss of 67.4 per cent..
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condition a t the end of the ruu. The same pieces were then heated for 95 hours at 980" C. In this run only No. 175 failed. It should be explbined that No. 175, although it failed in the first ticat,ing, was further tested a t 980" and also a t 1090" C. to study the effect of time at various temperatures. This steel withstood C@ hours at 870" C., failed in 24 hours a t 980" C. and in 3 hours a t 1090" C . This indicates that miscellaneous data in regard to what temperatures various products will stand must be accepted very cautiously unless conditions of tile experimeut and tlie time and t,emperature are all given. The third run was at 1090' C. Here failures
Table 111--Steel Used in Scaling Tesfs Maw=*-
No.
Ca~os
C**B"N
NIS*
93 12 176
Silchrocne Strinler3 12
0.41
0.51
0.22
0.46
0.19
%perimental inless
1s
perimentai 26 679 2 15
26 4 7
perilnentr~ pertmental Stainless 24
Reririal 2-A Rerisial 2-C Experilnenlal Rezirtai 2600 Reristal 4 Reiistal 7
n.10 0.28 n,ii 0.19 0.40
n.18
0.20
n.11
0.18
0.37 0.37 0.24 0.22
Srr.rcoN 2.92 0.21 0.22 0.29
n
49 0.43
0.77 2.90
0.48
?,a6 2.97
nau
0.37 0.47 0.64 0.85 0.79 0.63 0.71
0.40
0.67 2.14
2.45 1.42 3.08 1.64
NLCIOI.
0.10
0.19 n.18
n,18 o,18 0.28 9.01 9.24
15.87 22.91 24.99
21.10
CawoariuM
9.30 12.81 17.33 18.~0 17.90 22.04 ZS.M
25.57
17.30 18.86 16.76 7.9s 17.47 26.33
The purpose of these tests was to discover the length of time a t virions temperatures at which there was an appearance nf free scale or scale which fell o f f freely or could be readily detached. Short bars (long enough to use later in the manufacture of Cliarpy impact test pieces) were used and they were smooth-machined before startirig the test. The test itself was run as an elimination contest to see which steel lasted the longest. All steels which failed a t one tem-
A5 RO'
Figure 4-Flactures of Charpr Test Pieces of t h e Chrome-Nickel and Chrome-Nickel-Sillcon N i o y s
L ED
Figure 3-Fraefures of Charpy Teef Pieces of Chrome and Chrome-Silicon Alloy8
perature were eliminated and the rest were taken to a higher temperature. In order to facilitate the liberation of male, the test pieces were cooled from time to time, so that the scale might be loosened by contraction; they were also lightly hammered and put back in the test. These tests were conducted in a gas-fired semi-muffle furnace in which the steels were subjected directly to products of combustion. The first heating period was for 140 hours at 870" C. This eliminated Nos. 26 and 175 in 68 hours, and No. 12 in 74 hours. All the rest were in good
were frequent. No. 93 failed in Z1/, hours, Nos. 18 and 15 in 22*/? hours, No. 2 in 24 hours, and No. 174 in 47 hours. The remaining st,ecls, Nos. 4, 7, 25, 172, and 173, stood the full 56 hours of the experiment. The next nzn was for 54 hours a t 1200' C. No. 4 failed in 2 hours and No. 172 in 6 hours, while Nos. 7, 25, aird 173 stood the full time. The two steels which were carried through for the fifth run at 1260" C. were Rezistal 7 and Stainless 24, which failed in 2 hours and 3'/2 hours, respectively. It will be noted from Table 111 that two steels contained more than 25 per cent chromium and one of them, No. 7, also contained nickel and silicon. The difference in the effect of temperature upon the physical properties will be discussed later. Figpro 1, a photograph of actual specimens after full time at 1090' C., gives a visual impression of how the steels behave. Nos. 2 aiid G71 differ only slightly except in the silicon content. The one that is scaled or blistered is the steel low in silicon. Rezistal 4 and 7 preserve a somewhat steely appearance and No. 25 is discolored even at 870" C., but shows little tendency to develop free scale. Figure 2 shows the specimens which remzined for the 1090" C. test. Only two of them showed no free scale aud these were the ones which failed soon a t 1260" C. Figure 3 shows impact fractures of all the chromium aiid chromiumsilicon steels as they appeared after subjection to the temperatures noted at tho left. It will bc noted that the fractures are all flat breaks without any evidence of plastin deformation due to impact, with the possible excep tiou of No, 175 as rolled. The effect of high silicon on Nos. 172 and 173 in coarsening the grain is ab0 noticeable, when compared with Nos. 174 and 175, which were similar steels without the high silicon content. However, i t should also be noticed that the steels containing silicon stood up better, as far as scaling is concerned, since they appear opposite the temperature of 1200" C. (2200" F.), while the other two were eliminated after the 1090' C. (2000' F.) test. Figure 4 shows the austenitic chromium-nickel and chromium-nickel-silicon steels. With the exception of No. 7 in the as-rolled and at 1600" F. (870" C.) condition all the
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1164
pieces show marked evidence of plastic deformation and a tough break which seems to go through the crystals rather than between them, as was the case in Figure 3. There is some, but much less marked, evidence of coarsening of grain in these steels. A microscopic examination of all these specimens in both Figures 3 and 4 shows that marked grain growth does occur in both types of steel, but it is very much more marked in the steels in Figure 3 than in Figure 4. Rezistal 7 and Stainless 24, as was observed, were above 25 per cent in chromium and, so far as impact resistance is concerned, the former shows some of the characteristics of the latter in the as-rolled condition and after 870" C. (1600" F.). In these two conditions Rezistal 7 shows, respectively, 4.10 and 6.0 foot-pounds (0.566 and 0.828 kilogram-meter) in the Charpy tests. Even this, however, is considerably higher than Stainless 24, which shows but 1.8 foot-pounds (0.248 kilogram-meter) after 870" C. (1600" F.), and at the higher heats is slightly diminished, while the austenitic steel, No. 7, a t the higher temperatures, showed a very great improvement and the Charpy test gave, respectively, 40.7 foot-pounds (5.62 kilogram-meters) after 980" C. (1800' F.), 52.6 foot-pounds (7.26 kilogram-meters) after 1090" C. (2000" F.), and 54.3 foot-pounds (7.49 kilogram-meter) after 1200' C. (2200" F.). Rezistals 2 and 4 were still higher in impact resistance after being subjected to high temperatures, while for the chromium and chromium-silicon steels a t 870" C. (1600" F.) or higher the maximum Charpy impact test showed 5.8 foot-pounds (0.80 kilogram-meter) and the minimum showed 1.4 foot-pounds (0.19 kilogrammeter). This very marked difference, particularly in the effect of temperature on the chromium and chromium-silicon steels versus the austenitic steels, is very remarkable and demonstrates why the latter are superior for applications involving high temperatures, particularly if there are any shock or impact conditions to be met, either intentionally or accidentally. Where only resistance t o scaling is concerned and high physical requirements are not necessary, Stainless 24 is very good. Under other conditions, however, the tougher steels, such as the austenitic types, are to be preferred as a matter of precaution. In case of exposure to products of combustion from high-sulfur fuels, such as lowgrade coal, those steels which are very high in nickel are liable to fail rapidly, but if the chromium is much higher than the nickel content there is still the possibility of pre-
Vol. 21, No. 12
serving the austenitic characteristics by the addition of a small amount of nickel to a high-chromium alloy and a t the same time producing a satisfactory steel for sulfurous conditions. Future of Alloy Steels
As t o the future of alloy steels for both ordinary corrosion and high temperature conditions, little imagination is required to foresee endless applications for products possessing such unusual properties. The austenitic heat-resistant steels have been used very successfully in furnace and retort work involving high temperatures for long periods of time. Welded carburizing boxes made from sheets or plates are gaining rapidly in popularity as compared with the much heavier cast boxes, which are not only more difficult to handle in the shop but also require a definite increase in the amount of fuel and time in order to carry out these carburizing operations. Boxes made of welded plates offer an economy in weight, time, and fuel. The same has been found true of annealing boxes for bright-annealing steel sheets and strips. This material has also been used in the form of endless belts, particularly in furnaces for continuous heat-treating operations. Endless belts built up of sheets and traveling over large pulleys sarry the work from the low-temperature end of the furnace up to the higher-temperature end. In other heat-treating furnaces the work is carried directly through the furnace on the surface of parallel driven rollers which constitute the hearth of the furnace. For this application strength, under load, and resistance to scaling are of great importance; while in the oil-cracking industry and other operations in chemical manufacturing, the same materials have been very successfully employed to withstand, not only chemical corrosion, but resistance to heat and scaling. While such applications have been very numerous, the writer is convinced that industry has only commenced to use these materials and in the very near future their employment will become much more general. Literature Cited (1) Evans, Trans. A m . Znst. Mtning M e t . Ens., Institute of Metals Div., 1929, p. 7. (2) Mathews, Zbid., 71, 568 (1935). (3) Pfeil, J . Zron Steel Znst. (London), 119,501 (1929). (4) Vernon, "Bibliography of Metallic Corrosion," E. Arnold & Co.,London, 192s.
Effect of Atmospheres on the Heat Treatment of Metals' E. G. de Coriolis and R. J. Cowan SURFACE COMBUSTION COMPANY, 2375 DORRST., TOLEDO, OHIO
M
ETALS are heat-treated for the purpose of effecting B change in their structure, their shape, or their composition. Some heat-treating operations may involve two of these changes simultaneously. It is not the purpose of this paper to deal with these changes per se, for they lie in the realm of the metallurgist. Incidental to them, however, are questions which deeply concern the chemist. As the name implies, heat treatment involves subjecting the metal to the effect of a thermal gradient, which in turn implies the derivation of heat in some form. As usually en1 Received August 10, 1929. Presented before the Division of Gas and Fuel Chemistry at the 78th Meeting of the American Chemical Society, Minneapolis, Minn., September 9 to 13, 1929, on behalf of the American Gas Association.
countered in the arts, the source may be the combustion of a fuel, in which case heat is imparted to the metal by convection and radiation, or radiation alone may be applied, as when a muffle is intercepted between the metal and the burning fuel or an electric current is utilized through a resistor, resulting in the conversion of electric to radiant heat energy. I n every instance some of the heat is imparted to the work by conduction, but this is not important in this analysis. Again the metal to be treated may itself be used as the resistor, resulting in the most direct possible application of heat. Finally, the metal may be placed within the field of a high-frequency current wherein by inductance energy is imparted t o the metal with a resultant thermal rise. The last two cases-via., resistance and inductance-make
