US2862812A - Substantially nickel-free austenitic and corrosion resisting cr-mn-n steels - Google Patents

Description

Dec. 2, 1958 E. J. DULIS ETAL 2,862,812

SUBSTANTIALLY, NICKEL-FREE AUSTENITIC AND CORROSION masrs'r-mc Cr-Mn-N 'STEELS Filed m 16, 1958 3 Sheets-Sheet 2 COLD REDUCTION, PEI? cem- INVENTORS. E wA/eo dDuL/s. PETE/E R4 YSO/V. BY

ATTORNEYS.

United States Patent SUBSTANTIALLY NICKEL-FREE AUSTENITIC AND SCORRGSION RESISTING Cr-Mn-N- STEEL Edward J. Dulis and Peter Payson, Pittsburgh, Pa., as-

signors to Crucible Steel Company of America, Pittsburgh, Pan, a corporation of New Jersey Application May 16, 1958, Serial No. 735,900

5 Claims. (Cl. 7-5125) This invention pertains to corrosion resistant, austenitic stainless steels, and more particularly to a substantially nickel-free steel of this type, which contains as essential constituents in addition to iron, only carbon, manganese, chromium and nitrogen, and preferably also copper, within critically restricted ranges of each and in balanced proportions such as to impart an wholly austenitic structure together with excellent hot and cold forming properties as well as excellent strength and ductility and good corrosion resistance.

This application is a continuation-in-part of our copending application Serial No. 651,042, filed April 5, 1957.

The past, present, and anticipated future shortage of nickel has necessitated steel-industry-wide efforts to develop austenitic stainless steels in which the nickel of the Well-known 18Cr-8Ni (A181 300 series) steels is replaced by other austenite forming and stabilizing elements, such as carbon, nitrogen, manganese, and copper. At present, two new grades of such steels, in which nickel is partially replaced by manganese and nitrogen have successfully passed the development stage and are now accepted as the A181 Type 201 and 202 steels, having typical analyses of about 6.5Mn-4.5Ni17Cr-0.25N max. 0.15C max. and 9Mn5Ni-l8Cr0.25N max.-0.15C max., respectively. There has also been developed the 16-16-1 type of steel, i.. e., 16Cr-16Mn-1Ni which is competitive with the higher nickel-bearing grades aforesaid for certain applications. However, all such steels require substantial amounts of nickel and hence are objectionable on this ground for reasons above noted.

Recently, owing to the success of the CrNiMn steels, research directed to the development of completely nickel-free Cr-Mn--N austenitic, corrosion resisting steels has been intensified. In the development of a new steel of this type, the following goals are sought: The steel should retain a fully austenitic structure after solution annealing at about 2000" F. and rapid cooling to room temperature. Also, the retention of a fully austenitic structure at normal hot Working temperatures up to about 2300 F. is necessary because substantial amounts of delta ferrite will seriously impair the hottype of application.

workability of the steel by causing cracking and tearing during the initial conversion into bars, rods, and strip. Also such a steel should have suitable mechanical properties (strength, ductility, and Work-hardening characteristies) for successful fabrication and commercial. utility, and also adequate corrosion resistance for the intended In addition, the steel should have good drawability, weldability, and elevated-temperature mechanical properties.

Now we have developed a steel which meets these requirements, which requires no nickel as an essential constituent. The steel of our invention is characterized in being wholly or austenitic at all temperatures from the melting temperature down to room or atmospheric temperature. Our steel being thus free from high temperature ferrite at alltemperatures up to 2300 F., i. e., normal hot rolling temperatures, can :be easily reduced from the'ingot stage to hot bands without tearing or rupture.

Our steel can be annealed at about 2050 F. and as annealed has a yield strength of about 50,000 p. s. i. and a tensile strength of about 145,000 p. s. i. (a ratio of about 1:3), and an elongation of about 45%. When cold reduced 30%, the steel has a yield strength of about 140,000 p. s. i., a tensile strength of about 195,000 p. s. i. (a ratio of about 1:1.4) and an elongation of about 15%. The steel accordingly has excellent cold forming properties by reason of its low yield strength combined with high ductility, as annealed, and by reason of its high ductility after a 30% reduction. Likewise, as both annealed and cold reduced, it has adequate strength and ductility for use as a structural material in service.

Although the corrosion resistance of our steel to boiling nitric acid is not as high as in the relatively high nickelbearing grades, such as Types 201, 202 and 301, it is superior to that of the 16-16-1 nickel-bearing steel.

The steel of the invention is, however, primarily intended as a stainless type of steel for structural applications, subject primarily to atmospheric exposure, such as for exterior and interior paneling and trim on railway cars, automotive truck bodies, building structures and'the like. Our tests have shown that for such applications, our new steel has adequate corrosion resistance. In addition, it has the requisite hot and cold formability, and the requisite strength and ductility both as annealed and as cold reduced, for such applications.

The broad range of analysis of our steel is:

Composition, percent Manganese ll-under 14 Chromium 14-18 Copper 0-2 Silicon 0-3 Nitrogen 0.15-0.4 Carbon 0-0.15 Nitrogen plus carbon, minimum 0.075(Cr12.5)+0.1 Balance Iron Composition, Percent Broad Preferred 11-under 14- 11.5-13.5. 1446.5 14.5-15.5.

over 0.5-1.5.

The broad and preferred ranges for our typically 17Cr steel are:

Composition, Percent Broad Preferred 11-under14 11.5-13.5. 16.5-18 16.5-17.5.

over 0.5-1.5. 0.15-l.5.

. 0.05-0.15. 025-04 .t 0.075 (Cr-12.5) same.

+0.1. Iron Iron.

By balance iron in our steel is meant iron except for impurities within commercial tolerances, i. e., P and S about 0.04 max. each, and nickel about 0.5 max.

Copper is a preferred addition to our steel in the amounts stated, as it improves resistance of the steel to work hardening and also improves the corrosion resistance.

As stated, the ranges and proportioning of the alloying constituents in our steel are critical for providing the desirable combination of properties aforesaid. For assuring an wholly austenitic structure in our steel, we have found that the minimum carbon plus nitrogen contentshould be related to the chromium content as follows:'

(1) Percent (C-l-N) min.=

0.075 (percent Cr-12.5)+0.10

Although manganese over our broad range of about to under 14% helps to stabilize the austenite in the steel, beyond this range it tends to form ferrite. Likewise chromium above about l7% tends to introduce a ferritic structure.

Even with chromium and manganese thus limited, however, the steel will not have an wholly austenitic structure in the absence of appropriate additions of the potent austenite stabilizers carbon and nitrogen. As shown by the above formula, derived from test results hereinafter presented, the minimum carbon plus nitrogen, for assuring an wholly austenitic structure, increases with the chromium content. Thus for our typically Cr steel, the minimum carbon plus nitrogen is about 0.27%, and for our typically 17Cr steel the minimum carbon plus nitrogen is about 0.45%. On the high side there are definite upper limits for each of carbon and nitrogen that can be tolerated.

High carbon produces chromium carbide precipitation in weld zones of welded structures made of the steel, thus depleting the chromium and lowering the corrosion resistance of the steel thereat. And since the steel of the invention is primarily intended for applications aforesaid involving Welded structures, the carbon must be kept sufficiently low to assure good corrosion resistance throughout the entire structure including the weld zones. To this end, the carbon should not exceed about 0.15% and preferably should be held under 0.1%.

Nitrogen is limited on the high side by the extent of its solubility in the steel. Nitrogen present beyond the limit of solubility will evolve in gaseous form from the steel and thereby produce porous and unsound ingots. And while increasing amounts of the chromium and manganese in the steel, increase the solubility of nitrogen therein, and thus in this respect, function to increase austenite formation, there are definite upper limits beyond which an wholly austenitic structure cannot be achieved in this direction, owing to the aforesaid counteracting tendency for manganese above about 15% to produce ferrite, and the same tendency of chromium above about 17% to do so.

Our above formula for minimum carbon plus nitrogen shows, for example, that in a chromium-manganese steel containing about 18% chromium and 0.1% carbon, about 0.4% nitrogen is required to impart a completely austenitic structure at normal hot working temperatures. We have found, however, that in order to keep 0.4% nitrogen in solution, about 15% or more of manganese is required. But this amount of manganese in turn tendsto promote the formation of delta ferrite at the hot-working temperatures. The alternative of increasing the carbon thereby to permit of reducing the nitrogen and man-- ganese contents, does not solve this problem, since it reduces the corrosion resistance of the steel at weld zones as above explained.

We accordingly set the upper limit for nitrogen at 0.4% and preferably at 0.35%, in our steel in conjunction with the aforesaid upper broad and preferred limits for carbon, manganese and chromium and the minimum carbon plus nitrogen to assure the combination of good corrosion resistance and a completely austenitic structure at hot working temperatures and upon cooling to room temperature.

It will thus be seen that the ranges and relative proportions for each of the essential alloying constituents, namely, carbon, manganese, chromium and nitrogen, are quite critical in our steel, and insofar as we are aware, previously untaught in the art as to significance or as to the novel results achieved thereby.

In the accompanying drawings to be discussed more in detail, post:

Fig. l is a phase diagram showing the relation between the minimum of carbon plus nitrogen versus chromium content of the steels of the invention for maintaining an whollyaustenitic structure;

Fig. 2 illustrates graphically the variation in Rockwell C hardness versus percent of cold reduction of representative steels of the invention as compared to the Type 201 commercial grade;

Fig. 3 illustrates graphically the variation in tensile and yield strength with percent of cold reduction of representative steels of the invention as compared to commercial steels; while Fig. 4 is a graphical showing of the corre- The results as In the as-cast and hot rolled-con- After a solution anneal at 2300 F., howditions, as well as after a solution anneal at 2000 and 2100 ,F., all steels, with the exception of Steel 6971 (l5.4Cr-'0.08C0.'12N), were fully austenitic (non- F., magnetic). and rolled into strips 3" wide and thick. {Ihecompositions of these steels as determined by.chemical;analysis are given in the following Table I. 7

TABLE II Hardness, grain size, and austenite stability data presence of carbides or nitridesand delta ferrite phases in the microstructure.

ever somev steels developed delta ferrite. hot'rolled and as solution annealed at 2300" F. are compiled in Table 11.

thick.

is As dire-experimental basislfonouri findings abovestated, a seriesof l-i-p'ound' heats of varying :C Cr-fMn N analyses were induction melted and cast into ingots. The ingots were heated to about 2000 F., soaked one-half hour, and hammered into slabs 3%" wide and A The slabs were surface ground, heated to 1900-1950" As pointed out above, the preferred steels of the present invention are those in which the carbon does not exceed about 0.15% for reasons above explained. Accordingly, the steels in Table I.having carbon of not over 0.15% -were selected for further investigation as follows:

Samples (12 x 1 x in.) of these selected steels were solution-treated at 2100 F. for minutes and water quenched. The scaledsurfaces of the heat-treated samples were removed by sand blasting, and the samples of each steel were cold rolled to 10, and reductions in thickness. Standard 'ASTM tensile test specimens (2" in gage length and-0.5" ,in-width) were-pre- 75 pared from-samples-in -the-annealed"condition and-after 10, 20andj30% -cold reductions. 'The results of the de, and the Subsequently,

d M. J. Lavigne,-

#320 grit belts. Rockallographically, and de to establish the (l x 1 x in.) of all steels listed in Table 2100 and 2300 F. for 15 min- The Magne-Gage readings were t ferrite values ,on the basis of a 005413410100 0 0 .0 0.0305,... mawaammA mmammmm mmF W m warm m hw mm m m S m m .C md C na I S m mma mm mmm mmmna w a e s e mm 00% 3H 3000000000003 m 1 g m m wMAL n. m c an m m m mu o m m Wm 1 1 1 r on... hi mnc m S .BHmUn 1a @r$ .1 te do o s n F 76776989790860707115466 F m m H S e e .6 e m Hf 6b 0 m w u o 11111.11111211 121111111 e. 11 e r. .L t S u m 7 P 0 3 m m mmm mmwe eo m a f. m mmmswmammmm mo wwmm m f u r O 1 7 6 f. re1 a w w 7 mrMa 06146768801989910421789 e nh e 1 1 I 0. 2 C .C 0 MM 33333333344333344 333333 i. 1 w 1 t m m t m W HH S m S e .90. a 0 1 .0 h u e n .7 uF a ma mr b m maflmamw mfimmu h mm m f m .h 8% u 0 10111 n aiW C cme e 0 a0 e 1. h n I a u S f mp m t mIS tw n a. f .e.1. m. Wf u lIO a ma momm amm m wwm wwm .i h a I D w a Wot 6 g g u d m n S m a C 1 O a n 6 p u a. uano a ea fi m m m man m m w w ah m aa m a m. n O alaaaioizllllallltaiill n W. W t e 1% P m a o a t s 11111111111111111 11111 1m 3 m p e e w S O C m Mm F T D S f. S .n r e ho as cehm pt 1w .1dm 6 D. N 99061125 76152191472312 MW h d T m m Gayest-100 22 33 4 4 5455 562244 e 0 n n P V T 6 ..l.mt u m M 0000 0unmouuoaodoomouooa n Mccdd m l W H a a 3 t e h .1 n S .1 6 3 2 1 .1 S H yo ew su .e 1e r e o I a T n r a t X Y e r l d D. 22294465337280750808988 w 3 g a m H m W S m W n n m N w Pn a nb mt r a... nlli a qf ri a 0 00 000000000000000000000 m C w 0 5 0 5 0 y 4 A: 5 5 6 WW O WMWMNMMNNUHMM$MWMMMQM RM N wmmwhmmwmwn wmm o mfialnvlfiwmfilna uuouuaauuuuauuouuauouou .6 .2 3 .stAetteiaAAjt M C 0000.0000000000000000000 6 It S n m w 5 H N MEHHMM%%%%M&%%W%%H%%%H% mm P 0 0 0 0 0 0 0 fiw0 0 0 0 0 0 0 0 nw0 nw0 0 0 fiw D 0e r I. a W 0 MM% %%%%%M%W%fi% B m 0 0 0 0 0 0 0 flwO 0 0 0 0 flm0 0 0 01 .0 1 .0 L 1H S MW d M%%M%MM%%MWM%N%WHWQUM%% l 2 O A4 5 rw5 5 6 7 -L-L- 7 0 7 7 4-A 7 -L T. m a nmwnmnnwmnumumnwmnnnmww B m 0Q0 00 0 0 0Q00 0 00000000000 A. O .1 a a uauarmnn nnumwawmunncn 0 nmnw0 flm0 0 0M 0 fih0 0 0 0 flw0 0 0 0 0 0 0 0 W flwflnww% aw%mwm%mwwwwwfiw O LLLLLLZn MZZZZZLLZZLQMLLLL c 1111.1111111111111111111 M 244 mn mmwfiqudl O 222 0011 e n C r a B.

Specimens I were heated at 2000 utes and water quenched. To produce suitable surfaces for the determination of hardness and magnetic response, the scaled surfaces of the specimens were removed by magnetic response of the specimens was measured by use of a Magne-Gage.

converted into percen wet grinding and finishing on well C hardness determinations were ma correlation chart (T. V. Simpkinson an Metal Progress, February 1949 the specimens were prepared met microscopic examinations were ma room temperature tensile tests are shown in Table D1 below.

The data sho'wthat an increase in the carbon plus nitrogen content decreases the tendency of the steel to TABLE 111 I g g Room-temperature tensile properties l p v Gold 0.2% on ,-Tenslle nee. Steel N0. '0 N C+N Cr Cu Reducset Yield Strength, .E10ng., Area, tion, Strength, 1,000p.s.l.v1ercent, Percent Percent 1,000p.s.1.

a s as 2i 1 7 l 6962 0.14 0.33 0.47 17.6 20 170 20 46 30 141 191 12 37 a 22 a 10 6982 0.07 0.18 0.25 14.6 0.74 20 I 90 159 30 31 30 111 201 18 48 9% 2% s 0 6983 0.07 0.20 0.27 14.4 1.33 l 20 m0 138 32 30 121 157 16 45 0 2 s2 22 22 10 5 6979 0.14 0.28 0.42 17.2 0.67 20 114 153 22 52 30 141 173 14 44 O l8? s 22 10 8 4 6978 0.14 0.29 0.43 17.2 1.30 i 20 122 150 23 49 30 131 171 15 42 The yield strengths of the typically 15% chromium steels of the above table, in the annealed condition ranged from 47,000 to 50,000 p. s. i., the tensile strengths from 104,000 to 134,000 p. s. i., and the elongations from 46 to 58%. The yield strengths of the typically 17% chromium steels in the annealed condition ranged- H from 55,000 to 63,000 p. s. i., the tensile strengths fromj 109,000 to 116,000 p. s. i., and the elongations from' to 59%. The results show that an increase in the carbon plus nitrogen content tends to increase the yield I and tensile strengths slightly, whereas additions of copper .transform to martensite during cold deformation. Also demonstrated is the austenite-stabilizing effect of copper; that is, due to cold-working during the cold-rolling operation, the copper-bearing steels were less prone to Work harden as well as to transform to martensite than the steels that did not contain copper. For example, after 10% cold reduction Steel 6972 (no Cu) had 12% ferrite (martensite) in the microstructure; Steel 6982 (0.74Cu) had less than 1%; and Steel 6983 (1.33Cu) had none.

The average hardness increase of the 15% Cr and the 17% Cr steels upon cold rolling, together with a curve for Type 201 steel, are shown in Figure 2. On the basis of this graph, the work-hardening characteristics of the 17% Cr steel closely resemble the characteristics of Type 201 steel, whereas the 15% Cr steel shows a higher rate of work-hardening up to 10% reduction than the 17% Cr steel.

The tensile properties and work-hardening characteristics of our steels are summarized and compared with the respective properties of known types of steels in the following Table V and in Figs. 3 and 4 of the drawings.

TABLE IV Work-hardening and transformation characteristics Composition, Percent l 2,100 F., W. Q. 10% Cold-Reduced Steel N C N G+N Cr Cu Re Magne Percent Re Magne Percent Gage Ferrite Gage Ferrite 20% Cold-Reduced 30% Cold-Reduced All steels contained 11.5-12.5% manganese, balance iron.

In Table V the values for steels according to the invention are presented as the range of results given in the above Table IV, whereas the values for the comparison materials were obtained from the following sources: For U. S. Steels Cr-Mn-N steel Tenelon, from D. J. Carney, Steel, November 7, 1955, p. 138; for AISI Types 201 and 202, from R. A. Lula and W. G. Renshaw, Metal Progress, February 1956, p. 73, and E. V. Bennett, National Research Council, Materials Advisory Board Report No. MAB-4518M, June 10, 1935 (reprinted January 23, 1956); and for AISI Type 301, from the above cited Metal Progress article.

copper, up to 3% silicon, 0.15 to 0.4% nitrogen, up to 0.15% carbon, the minimum carbon plus nitrogen content being related to the chromium content in accordance with the following equation, percent (C+N) min.=0.075 (percent Cr-12.5 )+0.1, balance substantially iron, said alloy being characterized by an wholly austenitic struc- .ture as quenched from 2300 F. and by good corrosion resistance to atmospheric exposure.

2. An alloy steel consisting essentially of about: 11 to under 14% manganese, 16.5 to 18% chromium, up to 2% copper, up to 3% silicon, up to 0.2% carbon, 0.25 to 0.4% nitrogen, the minimum carbon plus nitrogen i TABLE V Comparison of tensile properties Yield Tensile Percent Percent Steel Condition Strength, Strength, Elonga- Reduction 1,000p. s. 1. 1,000p. s. 1. tion of Area This invention Cr {Annealed "5" 61:3 45-58 35458 This invention 17 Or igigii U. S. Steel's Or-Mn-N(Tenelon).. 192 lust Type 201 23 Annealed 40- 55 100-105 AISI Type 202 s ng Reduced.. 1% Annea e AISI Type 301 {30% Cold Reduced 140 170 An examination of values given in Table V and in 30 content being related to the chromium content in ac- Figs. 3 and 4 of the drawings shows that the yield and tensile strengths of our steels are slightly higher and the elongations slightly lower than those of the standard Type 301, 201 and 202 steels; however, the differences are not significant. Also, it is worthy of note that our 17% Cr steels, in general, exhibit more favorable tensile properties than the U. S. Steel experimental nickel-free, CrMn-N steel. This steel evidently corresponds to that of the U. S. Steel Corporations recently issued Patent 2,778,731, D. J. Carney, from Table II of which it is seen that the steel contains substantial amounts of delta ferrite as solution annealed at temperatures ranging from 1900 to 2300 F., this by reason of the high chromium content (17.520%) in conjunction with high manganese (15-20%) and low carbon plus nitrogen, as shown by the compositions given in Table I of the patent.

Reverting to Figs. 3 and 4 of the drawings, it will be seen, as previously discussed, that the effect of adding copper to the steel of the invention, is to decrease the martensite transformation and work-hardening tendencies during cold working. This behavior is manifested by a significant lowering of the yield strength and the tensile strength and accompanying increase in tensile elongation for the copper-bearing steels as compared to the same steel without copper.

To test the corrosion behavior of the steels of this invention in a relatively mild corrosive medium, such as is encountered in actual weathering conditions, specimens of the steels, one with copper and one without (Heats 6972 and 6983) were subjected to a water-vaporcolumn corrosion test in the solution-annealed (2100 F. for 15 min., water quench) condition. The test was conducted for 10 cycles, each cycle consisting of an 8- hour exposure to water vapor at 100 F. and of a 16- hour drying period. For comparison, specimens of the AISI Type 302 steel and of plain carbon steel were included in these tests. Visual examination of the surfaces of the specimens after the test exposure revealed that the steels of the invention tested showed some localized staining. The Type 302 specimen was apparently not affected, and the plain carbon steel was fully covered with heavy rust.

What is claimed is:

1. An alloy steel consisting essentially of about: 11 to under 14% manganese, 14 to 18% chromium, up to 2% cordance with the following equation, percent (C+N) min.=0.075 (percent Cr--12.5)+0.1, balance substantially iron, said alloy being characterized by an wholly austenitic structure as quenched from 2300 F. and by good corrosion resistance to atmospheric exposure.

3. An alloy steel consisting essentially of about: 11.5 to 13.5% manganese, 16.5 to 17.5% chromium, from more than 0.5 to 1.5% copper, from 0.15 to 1.5% silicon, from 0.05 to 0.15% carbon, from 0.25 to 0.4% nitrogen, the minimum carbon plus nitrogen content being related to the chromium content in accordance with the following equation, percent (C+N) min.=0.075 (percent Cr- 12.5)+0.1, balance substantially iron, said alloy being characterized by an wholly austenitic structure as quenched from 2300 F. and by good corrosion resistance to atmospheric exposure.

4. An alloy steel consisting essentially of about: 11 to under 14% manganese, 14 to 16.5% chromium, up to 2% copper, up to 3% silicon, up to 0.15% carbon, from 0.15 to 0.4% nitrogen, the minimum carbon plus nitrogen content being related to the chromium content in accordance with the following equation, percent (C+N) min.=0.75 (percent Crl2.5)+0.1, balance substantially iron, said alloy being characterized by an wholly austenitic structure as quenched from 2300 F. and by good corrosion resistance to atmospheric exposure.

5. An alloy steel consisting essentially of about: 11.5 to 13.5% manganese, 14.5 to 15.5% chromium, from more than 0.5 to 1.5 copper, from 0.15 to 1.5 silicon, carbon under 0.1%, from 0.25 to 0.35% nitrogen, the minimum carbon plus nitrogen content being related to the chromium content in accordance with the following equation, percent (C+N) min.=0.075 (percent Cr 12.5)+0.1, balance substantially iron, said alloy being characterized by an wholly austenitic structure as quenched from 2300 F. and by good corrosion resistance to atmospheric exposure.

References Cited in the file of this patent UNITED STATES PATENTS 2,198,598 Becket et a1. Apr. 30, 1940 2,698,785 Jennings Jan. 4, 1955 2,789,048 De Long et al Apr. 16, 1957 FOREIGN PATENTS 152,291 Austria Ian. 25, 1938 marsh sures. PATENT OFFICE CERTIHQATE GF CGRREQTION Patent No 2,862,812 December 2, 1958 Edward Jo Dulis et air,

It is hereby certified that error appears in the -printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 10,; line 53, for "min.=O.75" read minQ OQO'75 =0 Signed and sealed this 7th day of April 1959.

(SEAL) Attest:

KARL H, AXLINE BGBERT C. WATSON Attesting )fficcr {:ommissioner of Patents

Claims (1)

1. AN ALLOY STEEL CONSISTING ESSENTIALLY OF ABOUT: 11 TO UNDER 14% MANGANESE, 14 TO 18% CHROMIUM, UP TO 2% COPPER, UP TO 3% SILICON, 0.15 TO 0.4% NITROGEN, UP TO 0.15% CARBON, THE MINIMUM CARBON PLUS NITROGEN CONTENT BEING RELATED TO THE CHROMIUM CONTENT IN ACCORDANCE WITH THE FOLLOWING EQUATION, PERCENT (C+N) MIN=0.075 (PERCENT CR-12.5)+0.1, BALANCE SUBSTANTIALLY IRON, SAID ALLOY BEING CHARACTERIZED BY AN WHOLLY AUSTENITIC STRUCTURE AS QUENCHED FROM 2300*F. AND BY GOOD CORROSION RESISTANCE TO ATMOSPHERIC EXPOSURE.

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