US2890955A - Alloy compositions - Google Patents

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June 16, 19 59 J. K. STANLEY ETAL ALLOY COMPOSITIONS 2 Sheets-Sheet -1 PE ROE N 7' 04 R80 INVENTORS:

dam'es K. Stanley George F. Tlsmql fig/v5) June 16; 1959 S ANL Y E AL 2,890,955

ALLOY COMPOSITIONS 0.25 0.50 0.75 James A". Stanley BY George E Tisinai United States Patent 'ALLOY COMPOSITIONS Application July 29, 1955, Serial No. 525,275

sclaims. 01. 75-126 Our invention relates to the production of stable austenitic stainless steels without depending upon nickel as an essential alloyingelement.

The chrome alloy stainless steels are normally ferritic in structure. Although austenitic iron-chromium-nickel and iron-manganese-carbon alloys are well known, so far as we know, it has not been possible to produce completely austenitic alloys without the use of nickel or manganese. These alloys have useful, distinctive properties including a high degree of oxidationand corrosion resistance but their cost and the strategic significance of nickel and manganese have stimulated a great deal of research by investigators, both here and abroad, in an effort to produce austenitic alloys by other means.

The iron-chromium-carbon system, for example has been studied extensively. Ordinarily carbon-free ironchromium alloys containing more than 13% chromium are ferritic at all temperatures. Below 13% chromium the austenitic field eXists above 1550 F. Carbon additions will expand the austenite field up to about 21% chromium at temperatures above 1550 F. With increased chromium content above 2l%, the single phase austenite field is eliminated. Krivobok and Grossman, Trans. Am. Soc. Steel Treat, v. 18 (1930), pp. 760-807, have reported production of an essentially austenitic structure in a steel containing as high as 21% chromium by addition of 0.62% carbon. On the other hand, their work with 28% and 33% chromium alloys of variable carbon content (up to 0.70%) resulted in only minor amounts of austenite.

The effect of nitrogen on the structure of iron and chromium alloys also has been extensivelyinvestigated. The most significant work appears to be that of Colbeck and Garner, J. Iron and Steel Inst, v. 193 (1939), pp. 99-135, who have reported the production of 24% chromium alloys containing as much as 60% austenite and of 2-7%- chromium alloys containing as much as 50%-austenite. See also B. G. Bandel, Archiv f. d. .Eisenhuttenwesen, vol. 11, pp. 139-144, 1937, wherein the partial austenitization of a number of chromium al- .loys'by addition of nitrogen is reported. A completely austenitic structure, however, has not been realized for high chromium alloys insofar as we are aware, except forthe single non-reproducible example reported by Krivobok and Grossman, supra,without the use of upwards of about. 4%."nickel. In our research inthis field, we havefound that a critical relationship exists between carbon and nitrogen j aflectingaustenitization, a relationship which prior investigators appear to have overlooked, perhaps because of the unexpectedness of any cooperation between these minor constituents in this respect or perhaps because of the confusing fact that nearly all steels contain some concentration ofcarbon as a constituent impurity. We have found that the amount of nitrogen that can be retained by the higher chromium alloys is increased tremendously when thealloys contain up to about 1% ICC? 2 carbon. In investigating such alloys, we have discovered that completely austenitic alloys can be produced when the proportions of carbon and nitrogentareadjusted to a critical relationship for a particular chromium content, apparently through some synergistic action. We have found that a hyperbolic relationship exists between carbon and nitrogen in the chromium alloy steels which appears to be responsible [for stabilization of an austenitic structure. The higher the nitrogen content, the lower is the carbon content necessary to make the resulting alloy completely austenitic. Conversely, the lower the nitrogen content, the higher is the amount of carbonthat is required to make the alloy completely austenitic. As the amount of chromium in the alloy is increased, larger amounts of carbon and nitrogen appear to. be. required for a stable austenitic structure. At the'higher chromium contents, e.g. say 27 to 33% chromium, the stable austenite field becomes smaller although useful alloys of stable austenitic struc? ture containing co-eXisting chromium carbide may be formed over afield of broader range.

The new iron-chromium-carbon-nitrogen alloys have valuable corrosion-resistant properties, including oxidation resistance at elevated temperatures at least up to 900 F. and also above 2000 F., of a type associated with austenitic nickeland manganese-containing chrome alloys. In other respects, howeverthe new austenitic alloys appear to be quite distinctive in properties such as tensile and yield strength, work-hardening and the like.

The mechanism by which the carbon and nitrogen combination functions appears to-be fundamentally difierent from that of a metal such as nickel. Both the carbon and nitrogen dissolve interstitially between atoms, whereas the' larger nickel atoms take the place of other metal atoms.

Although the cooperative relationship between .carbon and nitrogen, which characterizes our invention, appears to have some general application to chrome alloy steels, alloys of less than about 21% chromium content appear to lack the distinctive corrosion resistance and high tensile strength properties of the new alloys. For example, a 21% chromium steel, even though of substantially austenitic structure, is borderline inoxida'tion resistance at 2000 F. Also, with chromium contents substantially above 33%, impracticably high nitrogen contents are required to avoid restriction of the austenite field to the vanishing point although, the two-phase field of austenite plus carbidemay have general usefulness.

In the broader aspects of the invention, we have found that completely austeniticalloys can be produced with chromium contents in therange of about 21 to 33% by using selected proportions of carbon. and nitrogen in the respective ranges of 0.1 to 1.0% and 0.2 to 1.0%. The resulting alloy may be pure, but as in conventional alloy production, it normally contains minor or nondetrimental amounts of constituent elements such as sulfur, silicon, manganese and phosphorus.

In the practice of the invention, we prefer to restrict the carbon content to a range of about 0.1 to 0.7 more desirably 0.1 to 0.3, and advantageously about 0.2%. The austenitic structure then is provided by using suflicient nitrogen in the range of about 0.2 to 1%, advantageously above about 0.3%, and with higher chromium contents, above about 0.5%. The resulting alloys appear to show greater corrosion resistance, particularly with chromium contents in the range of about 24 to 30%. Also, nitrogen appears to be more elfect-ive than carbon in imparting improvement in tensile and yield strength properties.

The accompanying drawings provide more graphic illustration of the alloys of the invention 'by reference to two quaternary phase diagrams which are based on the preparation and analysis of numerous samples of iron-chromium-carbon-nitrogen alloys. The alloys were produced-by several different techniques such as casting, powder metallurgy and nitrogenization.

Figure 1 ofthe drawings illustrates the approximate limits of the austenite field in the iron-chromium-carbonnitrogen system for four different chromium levels. In Figure 1, each panel represents a constant chromium content, the ordinates are percent nitrogen and the abs'cissas are percent carbon. The diagram applies to alloys cooled rapidly from above 2200 F. All of the alloys above the curved lines in Figure 1 are ferrite free, while the alloys below the curved lines contain ferrite. Thus, Figure l represents the boundaries between the combined austenite ('y) and austenite plus carbide ('y-l-c) fields, above the lines, and the combined austenite plus ferite ('y-l-a) and austenite plus ferrite plus carbide ('y|u-l-C) fields, below the lines. It will be noted that the boundaries resemble the hyperbolic form; i.e. a hyperbolic relation appears to exist :between the content of carbon and nitrogen which is responsible for the stable austenitic structure. The hyperbolic boundary shifts upward systematically as the chromium content is increased so that greater amounts of carbon and nitrogen are required to obtain the austenitic structure. Thus, from the 3-dimensional diagram of Figure 1, it may be seen that a curved surface exists which separates the upper austenitic field from austenite containing ferrite.

Figure 2 of the drawings presents a more detailed picture of the actual phases present in the iron-chrorniumcarbon-nitrogen systemat 2200 F. for five different chromium levels. As in the more simplified phase diagram of Figure 1, the approximate limits of the austenite field were determined from data obtained by preparation of alloys by casting, powder metallurgy and nitrogenization. The'incidental variation in composition due to minor elements which results from the use of the different preparation techniques did not appear to affect the location of the phaseboundaries. In Figure 2, the austenite and austenitev plus ferrite boundaries have been determined to rather close limits. The other boundaries are based on more limited information since less sampling was done on these alloys.

The alloys of greatest value according to the invention fall approximately within the area defined in Figure 2 by the austenite-austenite plus ferrite and the austenite-austenite plus carbide boundaries Alloys of austenite plus carbides, however, also have engineering value. Accordingly, in a broader aspect, the invention includes all of the austenitic alloys falling within the areas represented as above the curved lines of Figure 1 (or Figure 2) separating the combined austenite and austenite plus carbide fields from the combined austenite plus ferrite and austenite plus ferrite plus carbide fields. Although alloys in the austenite plus ferrite field having a predominantly austenitic structure may have engineering usefulness, the presence of more than about 15 to 20% ferrite will result in the loss of high strength and the loss of the non-magnetic nature, and possibly in the loss of general corrosion resistance commonly considered characteristic of the austenitic structure. The presence of small amounts of ferrite however may lower the susceptibility of the austenite toward intergranular brittleness, may favor the resistance of the alloy to corrosion under stress, and may be useful to prevent weld cracking in the event the alloys are Welded.

As noted above, the alloys of the invention may be .4 produced by a number of techniques. Essentially, preparation requires controlled addition of carbon and nitrogen to a high chromium steel, or to a mixture of the components thereof, and treatment of the resulting steel at a temperature above 20002200 F. followed by rapid cooling to a temperature below 800 F. For example, an iron-chromium-carbon alloy, containing the desired percent carbon, say 0.1% or higher, can be made in any conventional way. The resulting alloy then can be nitrogenized by the direct addition of nitrogen from a nitrogen atmopshere at a temperature above 2000 F., preferably at about 2250 F. The nitrogen is absorbed by the alloy and produces a case of austenite. The rate of penetration of the nitrogen in a concentration sufficient to form the austenitic case is a function of the carbon content and the nitrogen pressure, and is increased by each. By exposing the alloy to be austenitized to the nitrogen atmosphere for sufficient time, the cross sectional thickness of the alloy can be completely penetrated and converted to the completely austenitic state.

Another method of directly introducing nitrogen by means of solid-gas reaction requires the use of conventional nitriding, i.e. introduction of nitrogen from a partially dissociated ammonia gas atmosphere. In contrast to conventional nitriding, however, the iron-chromium-carbon alloys for austenitization are treated at about 1000 to 1200 F. and then are homogenized, or heat treated, at about 2.00 to 2300 F. in either vacuum or a protective atmosphere, for example, nitrogen gas.

The addition of nitrogen to conventional low-carbon, chromium-iron alloys has given considerable difficulty in conventional casting. If more nitrogen than about to of the chromium content is added, gassy ingots result during solidification due to the loss of a considerable amount of the nitrogen introduced. We have found, however, that when the carbon content of the melt is increased above 0.1% and up to about 1%, loss of nitrogen is minimized, or entirely prevented, .so thathigh nitrogen alloys containing up to as much as 1% nitrogen can be cast directly. Alloys can be produced in good quality with a completely austenitic structure at room temperatures. In casting, the retention of sufiicient nitrogen in alloys containing less than about 0.3% carbon may be dilficult when operating at atmospheric pressure but can be simplified, we believe, by use of pressure casting techniques. Cast ingots of completely austenitic structure can be satisfactorily reduced in size by forging.

In heat treatment, the alloy is cooled from about2200 F. to below about 800 to 900 F. rapidly enough to prevent transformationof the austenitic structure to the ferritic structure. Although the change in structure from austenitic to ferritic is sluggish, some decomposition of austenite may occur on slow cooling as by air cooling of large sections. Quenching or rapid cooling therefore is desirable. A partially decomposed austenite, however, can be converted to an all austenite structure by reheating to 2200 F. followed by rapid cooling. Also, although the austenitic grain size may be quite large in the cast alloys, it can be refined for control of mechanical properties by decomposing the alloys to ferrite, carbides and nitrides by heating to temperatures within the range of about 900 to 1300 F. followed by reaustenitizing for short periods of time at 2200' to 2300" F. p 7

Another means for production of the alloys of the invention is by the technique of powder metallurgy. The desiredportions of iron carbide (Fe C), chromium nitride (Cr N), chromium and iron powders are mixed thoroughly, compacted into the form desired, and sintered at 2200 to 2250 F. "under reduced pressure. Graphite may be used as a source of canbon instead of the carbides. Compacting pressures above 80,000 psi. and sintering times of 16 hours have been found satisfactory.

alloy compositions. powders were relatively pure so that the percentagesof elements other than the essential components which are ordinarily present as steelmaking impurities were very low. It was found. however that some of the alloys prepared by the powder metallurgy technique tended to become martensitic when quenched to room temperature. Since comparable compositions of the cast and forged materials did not undergo a martensitic transformation, it is believed that the steelmaking impurities suppressed the transformation. To illustrate, an alloy, prepared by melting had the following analysis: Cr, 20.81%; C, 0.46%; N, 0.23%; Si, 0.75%; Mn, 0.83%; S, 0.010%; P, 0.014%; Ni, 0.72%. This alloy did not transform martensitically even when cooled to liquid nitrogen temperature.

The invention will be further illustrated by reference to specific examples in whichalloys were prepared by melting and nitrogenization methods.

EXAMPLE I The steels used in this example were charged from either Armco Iron or billet slabs obtained from a 1030 carbon steel supplemented with granulated electrode carbon as required for carbon content. The melts were made in a high frequency induction furnace using an unlined magnesia crucible of either 200 pounds or pounds capacity.

The smaller heats were initially charged. Furnace heats larger than 10 pounds were slagged after the initial meltdown. For these heats, both the standard ferrochromium and nitrogen bearing ferrochromium were used, the nitrogen beanng'ferrochromium being added after the standard grade. I

Deoxidation was made with either an 80% Ni-20% Mg or MgFeSi (7% Mg-42% Si) alloy, or both, depending upon the quality of the alloy intended. Ferro-alloy additions were then made as desired. The melts were poured immediately into either cast iron or dry sand molds.

Analytical data on an alloy produced from a 10 pound heat, without slagging, and on a 100 pound heat, with slagging, are set outin Table I below. The alloys were produced as sound castings and, as-cast, were feebly magnetic. On reheating to 2200 F., followed by air cooling, the magnetism disappeared completely, indicating a completely austenitic structure. The material from the 100 pound ingot was hot swaged and appeared to have excellent hotworking properties.

In our use of the technique, the.

F. In certain instances, a nitrogenizing time of 72 hours also was employed. From previous experiments, it was known that 20 hours at 2200" F. was a sufilciently long time for the nitrogen to penetrate through a /a" x Va" cross section of a 27% Cr-0.3% C alloy and convert it to the completely austenitic state. Since it was believed that oxide formation on the surface interfered "with the dif-' fusion of nitrogen into such alloys, pains were taken to eliminate an oxidizing potential from the atmosphere. Tank nitrogen was passed through containers of Drierite and magnesium perchlorate to remove most. of the moisture, and then through calcium chips at 1100 F. to I'B-f move oxygen and residual moisture. The gas was passed a through an Inconel tube which contained the samples at 2200 F. All equipment joints were sealed with Glyptal paint. When the system was working properly, the dew point of the nitrogen gas was less than --80 F. and samples furnace-cooled in nitrogen after a 20 hr. treatment at 2200" F. did not contain a trace of oxide. The

alloys were furnace-cooled in the dry nitrogen, or, after modification of the apparatus, the samples: were pulled into the coldzone of the furnace while still being proteced by dry nitrogen.

To illustrate the results obtained by the above proce dure, a ferritic steel sample of Va x A" x. 3" Wassupported in the furnace at one end, with the longest dimension parallel to the length of the furnace tube. The sample was analyzed after one hour, 5 hours, and 20 hours. After 20 hours, the structure of the sample was completely austenitic throughout. The analysis follows: Si, 0.61%; Mn, 0.51%;S, 0.017%; P, 0.024%; C, 0.27%; Ni, 0.18%,Cr, 27.50%; N, 0.483%.

The rate of austenization of a ferritic steel can be increased by increasing the pressure of nitrogen. For example, a sample of 27% Cr-0.3 C was austenitized to a depth of about inch i n free flowing nitrogen after one hour at 2200 F., but to about inch in a nitrogen pressurized bomb (estimated as 2 to 3 atmospheres) after Analytical data on typical austenitic alloys of higher chromium content produced by the above procedure are set out in Table H below.

Table II COMPOSITIONS (PERCENT) OF WROUGHT ALLOYS AND PHASES PRESENT AFTER QUENGHING FROM 2200 F.

Si Mn s P 0 Ni Or N Phases 0.68 0.82 0.013 0.010 0.38 0.55 24.58 0.392 Austenitic.

0.82 0.86 0.017 0.014 0.33 0.71 27.57 0.538 Austenitic.

EXAMPLE II The addition of nitrogen to the starting ferritic alloys was accomplished by passing purified nitrogen (approx.

1 atm. pressure) over the alloys for 20 hours at 2200 75 ferrite, carbides, and nitrides above about 800 to 900 F readily with carbide tools. However, when the alloys are decomposed to ferrite, carbides and nitrides, they can be machined readily with ordinary cutting tools. The alloys then can be restored to the austenitic structure by reheating to greater than 2000 F. and cooling.

In the tensile test, very little localized necking occurs even though reduction of area values of 5 0 percent are attained. The tensile strength of a 27% chromium-0.5% carbon-0.7% nitrogen alloy is 138,000 p.s.i. The yield strength 90,000 p.s.i. The alloys, of course, are nonmagnetic when in the completely austenitic state. In general the new alloys have value in corrosive environments, particularly at the higher chromium contents, where strength and abrasion resistance are required. The new alloys as such may not be well adapted for use at elevated temperatures since they tend to decompose to The alloys can be beneficially modified in this respect by the introduction of additional alloying elements, for example, manganese, molybdenum, copper, cobalt, titanium, or aluminum. In slow bend tests, the austenitic alloys show adesirable degree of flexibility for mechanical shaping operations.

Generally it will be desirable to produce alloy structures that are completely austenitic throughout. In some applications, however, it may be desirable to apply only austenitic cases to ferritic. steel. Thus cast or wrought articles such as valves,.shears, knives and the like may be annealed. above 2000-2200 F. in nitrogen to obtain a corrosion resistant, high tensile strength outer case of austenitic structure.

We claim:

1. A stainless steel which is characterized by stable essentially austenitic structure consisting essentially of about 21 to 33% chromium and the presence of about 0.1 to 1.0% carbon and an amount of nitrogen in the range of about 0.2 to 1.0% which is suflicient to maintain the relative proportions of nitrogen to carbon for any particular chromium content above the line in the phase diagram of Figure 1 of the accompanying drawing and the. balance'being iron except for impurities ordinarily associated therewith.

2. The alloy steel. of claim 1 in which the chromium content is about 24 to 30%. t t 1 3. A stainless steel which is characterized by stable essentially austentic structure consisting essentially of about 21 to 33%' chromium and the presence of about. 0.1 to 0.7% carbon and an amount ofnitrogen in the range of about 0.3 to 1% which is sufficient to maintain the relative proportions of nitrogen to carbon for any particular chromium content above the line in the phasediagrarn Figure'l of. the accompanying drawing and the balance being iron. except for impurities ordinarily associated therewith.

4. The alloy steel of claim 3 in which the carbon content is about 0.1 to 0.3%.

5. A' stainless steel which is characterized by stable essentially austenitic structure consisting essentially of about 24 to 30% chromium and the presence of about 0.1 to 0.3% carbon and an amount of nitrogen in the range of about 0.4 to 1.0% which is sufficient to maintain the relative proportions of nitrogen to carbon for any particular chromium content above the line in the phase diagram Figure 1 of the accompanying drawing and the balance bjeing iron except forimpurities ordinarily associated therewith.

6. A stainless steel which is characterized by stable essentially austenitic structure consisting essentially of about 21 to 33% chromium and the presence of about 0.1 to 0.7% carbon and an amount of nitrogen in the range of about 0.2 to 1.0% wherein the relative proportions of nitrogen to carbon fora particular chromium content are such as to maintain the phase relationship substantially within the area defined by the 'Y 7 +7 and the 7+6 boundaries of the phase diagram of Figure 2 of the accompanying drawings and the balance being iron except for impurities ordinarily associated therewith.

7. The alloy steel of claim 6 in which the chromium content is about 24 to 30%.

8. A stainless steel which is characterized by stable essentially austenitic structure consisting essentially of about 21 to 30% chromium and the presence of about 0.1 to 0.3% carbon and an amount of nitrogen in the range of 0.4 to 1% wherein the relative proportions of nitrogen to carbon for a particular chromium content are such as to maintain the phase relationship within the area defined by the v 1 v +7 anu 7+ boundaries of the phase diagram of Figure 2 of the accompanying drawings and the balance being iron except for impurities ordinarily associated therewith.

References Cited in the file of this patent UNITED STATES PATENTS 1,990,589 Franks Feb. 12, 1935 2,657,130 Jennings Oct. 27, 1953 2,698,785 Jennings Jan. 4, 1955 2,764,481 Dyrkacz et al Sept. 25, 1956 OTHER REFERENCES Transactions, American Society for Metals, vol. 48,

Preprint No. 7. Edited by Tisinai, J. K. Stanley and C. H. Samans.

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Claims (1)

1. A STAINLESS STEEL WHICH IS CHARACTERIZED BY STABLE ESSENTIALLY AUSTENTIC STRUCTURE CONSISTING ESSENTIALLY OF ABOUT 21 TO 33% CHROMIUM AND THE PRESENCE OF ABOUT 0.1 TO 1.0% CARBON AND AN AMOUNT OF NITROGEN IN THE RANGE OF ABOUT 0.2 TO 1.0% WHICH IS SUFFICIENT TO MAINTAIN THE RELATIVE PROPORTIONS OF NITROGEN TO CARBON FOR ANY PARTICULAR CHROMIUM CONTENT ABOVE THE LINE IN THE PHASE DIAGRAM OF FIGURE 1 OF THE ACCOMPANYING DRAWING AND THE BALANCE BEING IRON EXCEPT FOR IMPURITIES ORDINARILY ASSOCIATED THEREWITH.

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