US2624668A - Ferritic chromium steels - Google Patents

Description

Patented Jan. 6, 1953 FERR-ITIC CHROMIUM STEELS William Oakley Binder, Niagara Falls, N. Y., assignor to Union Carbide and Carbon Corporation, a corporation of New York No Drawing. Application January 19, 1951, Serial No. 206,905

2 Claims.

This invention relates to hot-workable ferritic chromium steels and articles m'ade therefrom, and has for its principal object the provision of steels containing 20% to 30% chromium which have good impact strength in the annealed condition at room temperature.

Ferritic chromium steels exhibit good resistance to many corrosive media and, hence, have attractive possibilities for industrial use. However, steels for corrosion service are frequently needed not only to exhibit good corrosion resist ance but also good toughness, and the latter needs are notfllled with the conventional ferritic chromium steels because they tend to be notch sensitive at room temperature. This is especially true when the chromium content exceeds 18%. On the other hand although the austenitic chrom'ium alloy steels have good impact properties, they are difficult to hot-work because of their great strength and low hot-ductility at elevated temperatures. Further, they contain larger quantities of alloying metals than ferritic steels and, hence, are more expensive to make. Tough ferritic chromium steel accordingly has long been a goal sought by metallurgists.

Although some expedients for providing tough ferritic chromium steels have been developed, they have not been adopted commercially to any great extent because of certain economic and inherent technical difliculties. Forinstance, one

expedient that was advanced in U. S. Patent No. 2,120,554 requires the introduction of substantial proportions of nitrogen to the steel, to-

gether with the addition of nickel or copper or both. Difiiculty encountered in the commercial production of sound high-nitrogen steels has hindered the widespread adoption of this expedient. Another attack on the problem has been directed toward the control of carbon and nitrogen. The carbon and nitrogen contents of the steels are lowered below certain values which are a function of the chromium content of the steels. For instance, the maximum tolerance for carbon plus nitrogen in 25% chromium steels is about 0.035% to achieve toughness in this way. Again, although the desired result can be obtained in the laboratory, industry is not yet ready to adopt, on a large scale, the vacuum-meltin or inert-atmosphere technique required to lower the nitrogen content of the steel below the critical level.

Stainless steels are generally made in electricarc furnaces, since they provide an economic means for refining the molten charge, and for the close control of the overall composition of the steel which is necessary to attain the properties desired. Since refining steps are possible, the arc-furnace gives the steelmaker an opportunity for utilizing scrap material and recovering eco nomically substantial amounts of alloying mate-.- rials from this scrap, as well as removing carbon. Thus, the furnace is charged with steel scrap, chromium-bearing steel scrap and a small amount of flux, and the initial charge is melted down and decarburized with an oxidizing agent such as iron ore or gaseous oxygen. The adoption of gaseous oxygen has given the steelmaker a means for attaining extremely low carbon contents since it permits the attainment of the high temperatures required to oxidize carbon preferentially to chromium. While the adoption of gaseous oxygen for oxidation permits the steelmaker to make very low-carbon steel from conventional low-carbon raw materials and steel scrap, the process will not eliminate the last traces of nitrogen from the steel. Nitrogen tends to be picked up from the atmosphere and the raw materials so that the steel contains about 0.04%

to 0.07% nitrogen. Since, as previously mentioned, the carbon-plus-nitrogen content must be kept below 0.035% in 25% chromium steel for high toughness ferritic chromium steels so pro duced have been notch sensitive.

In accordance with the invention, high-tough-' ness, ferritic 20% to 30% chromium steel, having in the annealed condition an Izod impact strength averaging at least 25 ft.-1b., is produced by the introduction of aluminum, nickel and copper and by proper control of thealloyingcon stituents and impurities to produce a substantially ferritic structure and satisfactory hotworkability. The steel of the invention exhibits enhanced toughness at levels of nitrogen and carbon, heretofore found detrimental to the toughness of ferritic 20% to 30% chromium steels.

More specifically, the invention is a ferritic chro-' m'ium steel containing 20% to 30% chromium, up to 0.035% maximum carbon, up to 0.08% maximum nitrogen, the sum of the carbon and nitro-, gen contents being greater than 0.06%; 0.25%

to 1.5% aluminum, 0.5% to 3.5% nickel, up to 3% copper, the sum of the nickel and copper,

contents not exceeding 4%; up to 1% silicon, up to 3% manganese, the remainder iron. A small proportion, say up to 3% molybdenum, may be added to the steel to improve its'corrosion resist-v ance, if desired, without detrimentally affecting its impact strength. Conventional impurities, such as phosphorus and sulphur, may be present but should be held quite low.

The composition limits of the steel are critical.

3 With a higher carbon content than 0.035%, that is, in the range of carbon content that can be produced practically in the arc furnace without the use of oxygen for refinement, the toughness of the steel is lowered even though the toughening elements aluminum, nickel and copper are present. Nitrogen is also critical and should not exceed 0.08%. Within the range of 0.03% and 0.08% nitrogen, aluminum, nickel and copper cfof tough ferritic steels containing to 30% chromium by conventional melting practice using conventional raw materials.

The toughness and corrosion resistance of these ferritic steels are enhanced by employing a suitable annealing heat-treatment. In the annealing operation, the steel is held at 900 C. for a sufficiently long time to put at least a part of the carbides and nitrides into solid solution. A

fectively neutralize the embrittling tendency of o preferred time for this purpose is 6 hours. Folnitrogen in chromium steels containing not more lowing the heating for solubility and homogenethan 0.035% carbon. Preferably, the steel of the ity, the steel should be cooled rapidly using air, invention contains 0.5% to 1% aluminum, as oil, or water as the quenching medium. In the higher percentages tend to lower toughness and annealed condition, the steel is substantially decrease ductility. It is preferred that nickel 15 ferritic and is formable, machineable, and suitconstitute 0.5 to 2% of the steel. Copper may able for manufacture into corrosion-resistant be present up to 3% in the steel, however, copper articles and numerous other products subjected is less efiective for the purpose of this invention to high stresses in service. The steels are not than nickel. As above stated, when copper is free from the tendency to embrittle when heated present, the sum of the nickel and copper metals go In the temperature range 01 are t ey f should not ex eed 4%, Wh n molybdenum i from susceptibility to intergranular attack when present, it is preferred that nickel constitute 1% heated above C; I to 2.5% of the steel, and when copper and nickel The S e s Of s Invention may be made in are both present in addition to molybdenum, it is flereult-type furnaces y n i l electric preferred that the same of the nickel and copper are furnace practices having al ppl i ty content should t exceed 4% The steel Should to the production of stainless steels, as nitrogen be deoxidized, suitably with manganese and silidoes 1101? a to be excluded from the furnace con, but t residual quantity f silicon in the atmosphere and conventional low-carbon raw steel should not xceed 1% as it imparts brittlemeierlels e employeble- They n e made ness Manganese may b present in th teel p by inductlon-furnace melting DIZCUCES 1f due t its action tending to supplement that f regard is given to the carbon content of the raw nickel nd copper materials, as the carbon. content of the steels In th following t bl several examples are must not exceed 0.035%. The steels in ingot given of steels containing about 26% chromium form are hot-workable at a temperature of about with varying proportions of carbon, nitrogen, 2100 F., and the comparative ease of working aluminum, nickel, and copper together with a at this temperature contributes to simplicity in number of Izod impact values obtained on testing achieving desired p ct by hilt-Working. each of the steels at room temperature after they These aluminum-containing ferritic stainless had been annealed by heating six hours at 900 steels have, by virtue of their chemical composi- C. and Water-quenching. tion, good mechanical properties and corrosion Composition: 26% Cr; 0.35% Si; Remainder Fe and- Izod Impact Values, percent 0 PercemN Pelfimt Pegcfnt Pecgnt Pelriclelnt Pergcnt Foot-pounds 0. 022 0. 059 0. 75 1.0 0. 7 35, 55, 40, 47 0. 025 0. 059 0. 75 2. 0 0. 7 s0, 74, 35, as 0.02s 0. 052 0. 75 a. 0 0. 7 47, 93, 54, 40 0. 020 0. 051 0. 05 3. 0 0. 7 44, 52, 50, 05 0. 025 0. 055 0. 75 2. 0 0. 7 44, 0s, 40, 51 0. 032 0. 052 0.75 1. 0 5. 0 54, 49, 44, 35 0. 032 o. 052 0. 75 2. 0 3. 0 45, 45, 57, 70 0.025 N. D. 0. 75 1.9 0. 7 41,41, 25, 42 0.025 0.074 0.7 4, 5, 5, 5 0. 02s 0. 055 0. 7 1s, 25, 35, 2s 0.025 0.055 0.7 7, 9,10,13 1

N. D.Not determined.

,For comparison, the third from last steel in the above table contains no aluminum, nickel. or copper, while the last two steels contain nickel but no aluminum. It will be evident that the addition of aluminum and nickel with or without copper raised the average impact strength from 5 ft.-lb. to well over 25 ft.-lb., and that the presence of nickel in the absence of aluminum does not produce the desired increase in toughness.

In the steel of the invention, aluminum presumably acts as a deoxidizer and a nitride former. Nickel, manganese, and copper probably increase the solubility of carbon in the steel by the formation of small uantities of austenite. Whether or not these efiects are the cause of the increased impact strength of the steel of the invention, the incorporation of these elements in the proportions specified makes possible the production resistance after a simple annealing treatment. They may be fabricated readily into products suitable for service requiring toughness and resistance to corrosion. The relative ease of melting and hot-working these steels and the fact that they can be made by conventional com mercial melting practices render them economical to produce. Further, they have the advantage of requiring much smaller quantities of strategic alloying metals than the standard austenitic grades of stainless steel.

Related subject matter is described and claimed in the application of Walter Crafts, Ser v ial No. 206,909, filed concurrently herewith and assigned to the assignee of this application.

I claim:

1. Ferritic chromium steel containing 20% to 30% chromium; up to 0.035% carbon; up to 0.08% nitrogen, the sumof carbon and nitrogen being in excess of 0.06%; 0.25% to 3% manganese; up to 1% silicon; 0.25% to 1.5% aluminum; 0.5% to 3.5% nickel; up to 3% copper, the sum of nickel and copper not exceeding 4%: up to 3% molybdenum; the remainder iron and incidental impurities, such steel having, in the annealed condition, an average Izod impact strength of at least 25 foot pounds at room temperature.

2. Ferritic chromium steel containing 20% to 30% chromium; up to 0.03% carbon; up to 0.08% nitrogen, the sum of carbon and nitrogen being in excess of 0.06%; 0.25% to 3% manganese; up to 1% silicon; 0.5% to 1% aluminum; 0.5% to 2.5% nickel; up to 3% copper, the sum of nickel and copper not exceeding 4%; up to 3% molybdenum, the nickel content not exceeding 2% in the absence of molybdenum; the remainder iron and incidental impurities, such steel having, in

the annealed condition, an average Izod impact strength of at least 25 foot pounds at room temperature.

WILLIAM OAKLEY BINDER.

REFERENCES CITED The following references are of record in the file of this patent:

FOREIGN PATENTS Nitrogen in Chromium Alloy Steels, page 16. Published in 1941 by the Electro-Metallurgical Corporation, New York.

Claims (1)

1. FERRITIC CHROMIUM STEEL CONTAINING 20% TO 30% CHROMIUM; UP TO 0.035% CARB ON; UP TO 0.08% NITROGEN, THE SUM OF CARBON AND NITROGEN BEING IN EXCESS OF 0.06%; 0.25% TO 3% MANGANESE; UP TO 1% SILICON; 0.25% TO 1.5% ALUMINUM; 0.5% TO3.5% NICKEL; UP TO 3% COPPER, THE SUM OF NICKEL AND COPPER NOT EXCEEDING 4%: UP TO 3% MOLYBDENUM; THE REMAINDER IRON AND INCIDENTAL IMPURITIES, SUCH STEEL HAVING , IN THE ANNEALDED CONDITION, A AVERAGE IZOD IMPACT STRENGTH OF AT LEAST 25 FOOT POUNDS AT ROOM TAMPERATURE.