US2229065A - Austenitic alloy steel and article made therefrom - Google Patents

Description

Patented Jan. 21, 1941 UNITED STATES PATENT orrlce Russell :Franks, Niagara Falll,'N. Y., assignor to. Electro Metallur icai Company, a corporation of West Virginia fio Drawing. Application December 14, 1938,

Serial No. 245,593

scum.- (01.75-128) The invention relates to austenitic chromium alloy steels and is particularly concerned with means for improving the hot working characteristics of such steels.

Chromium steels containing about 12% to 25% chromium, 6% to 20% of austenite-promoting metal oi! the group consisting oi nickel, manganese, and mixtures theerof, and carbon in an amount not exceeding about 0.12%, are corrosion resistant, tough, and ductile. To improve certain of the characteristics of these steels, for example, their resistance to intergranular corroslon, it has been proposed to add a substantial amount of one or more of the ferrite-promoting, carbide-forming elements of the group consistmg or titanium, zirconium, vanadium, columbiurn, tantalum, molybdenum and tungsten.

hile such ,elements effectively inhibit intergranular corrosion, their presence appears to have a deleterious effect onthe hot working characteristics of the steels. It is believed that the difficulties in hot working, usually manifest by tears, checks, or cracks in the steels, are caused by the partial decomposition of the austenite oi. the steel under the conditions of high temperature and mechanical work, forming a small amount of an undesirable ferritic constituent throughout the austenite matrix. This small proportion of ierritic constituent appears to detrimentally afiect the hot workability of steels oi the class in question.

Measures have been proposed to improve the hot working characteristics of such 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 both the tendency of the carbideforming elements to form the undesirable ferritic constituent, and the tendency of the austenite to decompose and form such a constituent during hot working. This expedient, while improving hot workability, has a deleterious effect, not only on the general corrosion resistance of the metal, but also on its resistance to inter-granular attack. It has also been proposed to promote a more stable austenite by increasing the nickel or manganese content, or combined nickel and manganese content of the steel above 20%, 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-stiffness of the metal, thereby increasing the difilculty of working it.

i have found that the addition of relatively small amounts of nitrogen to steels containing about 12% to 25% chromium, 6% to 20% of an austenite-promoting element of the group consisting of nickel, manganese, and mixtures thereof, carbon in an amount less than 0.12%, and a 0 total amount, not over of one or more of the ferrite-promoting, carbide-forming elements 0! the group consisting of titanium, zirconium, vanadium, columbium, tantalum, tungsten, molybdenum, and mixtures thereof considerably improves the hot working and other properties of such steels without impairing their excellent corrosion-resistance or without substantially increasing their resistance to deformation at hot working temperatures. The nitrogen content is 15 uniformly distributed throughout the steel and should be between 0.05% and 05%, preferably between 0.05% and 0.2%. As is customary in steels of this class, copper may be added in amounts up to about 2.5%. The maximum permy centage of nitrogen that can be held in stable combination in these alloys depends on their chromium, nickel, and manganese contents. If the chromium content is or over, andthe nickel, manganese, or nickel-plus manganese content is low, the nitrogen content may rise to 0.5%, but when the chromium content/is below 20%, and the nickel, manganese, or nickel-plus manganese content is high, it is advisable to keep the nitrogen content below 0.2%.

l have observed that the nitrogen imparts stability to the austenitic constituent of the steel, strengthens and otherwise improves the properties of the ierritic constituents particularly at elevated temperatures, and promotes a finegrained structure throughout the steel. It is probable that these effects are responsible, at least in large part, for the improved hot workability of the steels and for the fact that during the heat treating of such steels the cooling rate necessary for a given section, to retain a high proportion of austenite, can be slower than that necessary to retain the same proportion of austenite in .1 steel containing little or no nitrogen but otherwise of similar analysis. The reduced cooling as rate is particularly valuable in imparting good ductility and toughness to those articles, either cast or wrought, which, owing to their shape, thickness or other factors, could not be very rapidly cooled by known practical methods.

Another valuable characteristic resulting from the addition of nitrogen to the steels herein described is that the yield point and maximum strength of the hot-worked steels are increased substantially without the proportionate decrease til in impact strength and ductility which usually accompanies such improved properties.

This and other improvements in the physical properties of the hot-worked steels are indicated nitrogen, which counteracts the said adverse effect of the columbium: remainder iron and incidental impurities including not over 0.12% carbon.

in the following table: 2. Hot worked austenitic alloy steel containing Composition (remainder Fe) Tensile test results Percent Percent Percent Percent Percent Percent Percent Percent M 0 ob Mo N Y. P. M. 8. EL RA Izod B. H.

1 17. 5 9 0. 07 None None 0. 04 84 87 59 64 107 128 2.. l8. 5 0 0.09 l. 1 None 0. 04 86 90 64 70 101 3 18.0 9 0. 08 l. 1 None 0. ll 48 1(1) 50 66 107 4. l8. 3 l0 0. 06 None 2. d 0. 04 37 90 57 73 98 146 6.- 18. 3 10 0. 08 None 2. 6 0. 10 42 92 58 74 102 156 6.- l8. 2 9. 6 0.06 None 3. 2 0. 04 40 92 54 73 97 163 7.-- 18. 2 9. l 0. 08 None 3. 2 0. 13 49 99 66 74 113 170 N own-Steels No. l to 3 hot worked and water quenched from l,lfi0 C. before testing.

and air cooled from l,i00 C. 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; per cent EL for percentage elongation in two inches in a standard 0.505 inch tensile sample; per cent RA for percentage reduction in area of cross section accompanying the elongation; Izod to designate the Izod impact resistance in foot pounds; and B. H. for the hardness on the Brineil scale.

The steels of the invention may be satisfactorily hot worked following the procedure ordinarily employed in working plain austenitic chromium nickel steels. The resulting articles are free from the surface imperfections usually produced in hot worked austenitic chromium nickel steels containing ferrite-promoting, carbide-forming elements. Further, the improved hot working and other characteristics of the steels herein described are obtained without deleteriously afl'ecting the strength, toughness, ductility, or corrosion-resistance of the metal.

I claim:

1. Hot worked austenitic alloy steel containing between 12% and 25% chromium; between 6% and 20% nickel, a substantial proportion of the ferrite-promoting ingredient columbium, having a favorable eflect on the corrosion resistance of the steel but an adverse effect on its hot working properties, but not exceeding 5%; 0.05% to 0.2%

Steels No. 4 to 7 hot worked between 12% and 25% chromium; between 6% and 20% nickel; substantial proportions of the ferrite promoting ingredients columbium and molybdenum, having a favorable effect on the corrosion resistance of the steel but an adverse effect on its hot working properties, but not exceeding 5% in total sum; 0.05% to 0.2% nitrogen which counteracts the said adverse effect of the columbium and molybdenum; remainder iron and incidental impurities including not over 0.12% carbon.

3. Hot worked corrosion resistant austenitic alloy steel article containing between 12% and 25% chromium; between 6% and 20% of at least one material, promoting an austenitic structure, selected from the group consisting of nickel and manganese; a substantial proportion of at least one ferrite-promoting ingredient having a favorable efl'ect on the corrosion resistance of the steel, but an adverse eflect on its hot working properties, such ingredient or ingredients being in the aggregate not over 5% of the steel and being selected from the group consisting of titanium, zirconium, vanadium, columbium, tantalum, molybdenum, and tungsten; between 0.05% and 0.5% nitrogen which counteracts the said adverse effect of the ferrite-promoting ingredients; the remainder iron and incidental impurities including not over about 0.12% carbon.

RUSSELL FRANKS.