US2225440A - Austenitic alloy steel - Google Patents

Description

Patented Dec. 17, 1940 UNITED STATES I PATENT OFFICE 2,225,440 AUSTENITIC ALLOY SlEEL Frederick M. Becket, New York, and ltussell Franks, Niagara Falls, N. Y., assignors to Electro Metallurgical Company, a corporation of West 3 Claims.

The invention relates to corrosion resistant austenitic chromium-manganese-nickel alloy steels and to articles made therefrom, and has for its primary object the improvement of the characteristics of such steels. This application is a division of our application Serial No. 238,560, filed November 3, 1938, issued April 30, 1940, as U. S. Pat. No. 2,198,598, and a continuation of our application filed May 13, 1939, Serial No. 273,424.

Steels containing about 12% to 25% chromium, 6% to 14% manganese, a minor part of the manganese being replaced by nickel, and carbon in a proportion not exceeding about 0.2%, are corrosion resistant, tough and ductile. Such steels normally consist of austenite or of a mixture of austenite and a ferritic constituent, the proportion of austenite being largely dependent on the ratio of chromium to austenite-promoting elements in the steel and also on the rate at which the steel has been cooled from elevated temperatures above its upper critical temperature (about 950 C.). The corrosion resistance, toughness, and ductility of these steels are usually greatest when their structure is most nearly completely austenitic and the proportions of chromium and austenite-promoting elements should therefore be carefully selected in order to ensure a steel having the maximum proportion of austenite. The austenite so produced is relatively, unstable and the resultant steels are dimcult to hot work and to heat treat successfully.

During hot working, these steels have a tendency to tear, check or crack, apparently because of the partial decomposition of the austenite under the conditions of high temperature and mechanical work, forming a small amount of an undesirable territic constituent throughout the austenite matrix. This small proportion of ferritic constituent formed during hot working appearsto have a serious detrimental effect on the hot working characteristics of steels oi the class in question.

Measures relying for their beneficial effect on the production of a stable austenite have been proposed to improve the hot workability and heat treating characteristics of the foregoing steels, but none has been entirely satisfactory. One proposal has been to increase the carbon content and thus, by promoting a more stable austenite, to restrict the decomposition of that constituent. This expedient, while improving the hot work ability and heat treating characteristics of the steel, has a deleterious effect on the corrosion resisting properties of the metal and is of little or no value. It has also been proposed to promote a more stable austenite by increasing the manganese content or combined nickel and manganese content of the steel, but this expedient is only partially effective and is, moreover, relatively expensive. The use of a very high proportion of nickel has the added disadvantage of increasing the hot-stillness of the metal, thereby increasing the dimculty of hot working it.

We have found that the addition of relatively small amounts of nitrogen to steels containing about 12% to 25% chromium, 6% to 14% of a mixture of manganese and nickel wherein the percentage of manganese is greater than that of the nickel; and carbon in an amount less than 0.20%, and preferably less than 0.12%, considerably improves the hot working, heat treating, and other characteristics of such steels without impairing their excellent corrosion resisting and physical properties or without substantially increasing their resistance to deformation at hot working temperatures. The nitrogen content is uniformly distributed throughout the steel and should be in an amount at least 0.05% but not exceeding 0.5%. As is customary in steels of this class, copper maybe added in amounts up to about 2.5%, and one or more elements of the group consisting of aluminum and silicon may be present in an amount not exceeding 3% and preferably not exceeding 1%. The percentage otnitrogen that can be held in stable combination in these alloys depends on their chromium andmanganese contents. If the chromium content is about 25% and the manganese content is high, the nitrogen content may rise to 0.5%, but when the chromium and manganese contents are lower, it is advisable to keep the nitrogen content below 0.2%, and usually below 0.15%.

We have observed that the nitrogen imparts stability to the austenitic constituent of the steel, strengthens and otherwise improves the properties of the ierritic constituents, particularlyat elevated temperatures, and promotes a fine- .grained structure throughout the steel.- It is toughness to articles which have been fabricated 'by welding, and to those articles either cast or wrought, which, owing to their shape, thickness or other factors, could not be very rapidly cooled by known Practicalmethods. I

Other valuable characteristics resulting from the addition of nitrogen to the steels herein described are that the yield point, maximum strength, and impact strength of the hot-worked steels tested at room temperature are at least as good as those properties of nitrogen-free steels of otherwise similar analysis, while their ductility is substantially increased.

Improvements in the physical properties of the hot-worked steels are indicated in Table A which gives the results of tests made on representative samples of steels of the class in question, with and without additions of nitrogen.

these steels is that they may be cold worked to give a material of high yield strength without deleteriously affecting their ductility and toughness. It is believed that this benefit is due largely to the stabilizing effect of nitrogen on the austenite of the steel inasmuch as magnetic tests show that after a given degree of cold work has been imparted to steels of the invention and to I nitrogen-free steels of otherwise similar analysis. the nitrogen-free steels are considerably more magnetic'than those of the invention.

While we have disclosed several specific embodiments of our invention, it is evident that suchembodiments are by way of example, and may be modified within the scope of the invention as defined in the appended claims.

We claim:

1. Austenitic alloy steel containing between Table .4

Composition (remainder Fe) Tensile test results Perce t Percent Percent Percent Percent Percent Percent Mn N1 0 N Y. P. M. 5. EL RA Imd B. H.

STEELS HOT WORKED ANflWATER QUENCHED FROM ABOUT 1050 0. BEFORE TESTING STEELE HOT WORKED AND AIR COOLED FROM ABOUT 1050 0. BEFORE TESTING In the above table, the following symbols are used: Y. P. for yield point in thousands of pounds per square inch; M. S. for maximum stress in thousands of pounds per square inch; %EL for percentage elongation; %RA for percentage reduction in area of cross-section accompanying the elongation; Izod to designate the Izod impact resistance in foot pounds; B. H. for the hardness values on the Brinell scale. The tensile tests were made on standard 0.505 in. diameter tensile test samples as specified by the American Society for Testing Materials.

The nitrogen addition is also beneficial in improving the physical characteristics of the coldrolled chromium-manganese steels. The steels of the invention can be more severely cold worked than nitrogen-free steels of similar analysis without inducing an undesirable degree of brittleness in the material. This characteristic is of particular importance in articles which have been fabricated by deep drawing inasmuch as the residual stresses ordinarily present in such articles are thus appreciably reduced and a greater degree of permanency in the finished article is ensured. Another valuable characteristic of 12% and 25% chromium; between 6% and 14% of manganese and nickel, the percentage of manganese being greater than that of the nickel; carbon in a percentage not over 0.2%; 0.05% to 0.5% nitrogen; remainder iron; the nitrogen serving to materially improve the hot working characteristics of the steel.

2. Austenitic alloy steel containing between 12% and 25% chromium; manganese and nickel in total percentage between 6% and 14%, the percentage of manganese being greater than that of the nickel; carbon in a percentage not over 0.12%; 0.05% to 0.2% nitrogen; remainder iron; the proportions of chromium, carbon, manganese,

and nickel being inefiective to prevent the formation of an undesirable ferritic constituent if the nitrogen were not present.

3. A steel article hot worked under conditions that promote the formation of a ferritic constituentand subsequently quenched from above the upper critical temperature (about 950 C.) and composed of a steel as defined in claim 2.

FREDERICK M. BECKET. aussnu. mums.