US2624670A - Chromium steels - Google Patents

Description

Patented Jan. 6, 1953 CHROMIUM STEELS William 0. Binder and Roger A. Perkins, Niagara Falls, N. Y., assignors to Union Carbide and Carbon Corporation, a corporation of New York No Drawing. Application August 15, 1952, Serial No. 304,638

2 Claims.

This invention relates to hot-workable chromium steels and articles made therefrom, and has for its principal object the provision of steels containing 20% to 23 chromium which have resistance to corrosion in oxidizing or reducing acid conditions comparable to that of austenitic chromium steels, good impact strength at room temperature, and decreased sensitivity to elevated temperatures and conditions of heat-treatment.

High-chromium steels exhibit good resistance to many corrosive media and, hence, have attractive possibilities for industrial use. However, steels for corrosion service not only must be resistant to a wide variety of corrosive media, but also nust exhibit good toughness and must maintain these properties after exposure to broad, elevated temperature ranges. The latter needs are not filled with the conventional ferritic chromium steels because they tend to be notch sensitive at room temperature and exhibit reduced properties on exposure to certain elevated temperature ranges. This is especially true when the chromium content exceeds 18%. On the other hand, although the austenitic chromium alloy steels have good impact properties, they are difiicult to hot-Work because of their great strength and relatively low hot-ductility at elevated temperatures. Further, they contain larger quantities of strategic alloying metals and, hence, are more expensive to make. Tough, substantially ferritic,

high-chromium steel 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 diificulties. One 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.

Although the desired result can be obtained in proposed that a small portion of nickel be added to the steel in addition to aluminum. While the steels produced in accordance with these expedients have improved toughness, under some conditions they suffer from certain disadvantages, i. e., hot-cracking on Welding and segregated oxide inclusions which not only imparts poor surface qualities but also increases susceptibility to pitting. Further, the impact strength of the aluminum-containing steels is affected by section size and rolling practice, and the corrosion resistance and impact properties of these steels are undesirably sensitive to the conditions of heattreatment and to elevated temperature exposure. It is the main object of the present invention to provide a high-chromium steel having good impact strength at room temperature, useful resistance to both oxidizing and reducing acid corrosion, and decreased sensitivity to elevated temperatures and conditions of heat-treatment. In accordance with the invention, 20% to 23% chromium steel, having a room temperature Izod impact strength averaging at least 50 ft.-lb., having corrosion resistance comparable to that of austenitic chromium-nickel steel, and having decreased temperature sensitivity is produced by the introduction of nickel, manganese, copper, and molybdenum, and by maintaining a proper balance between the chromium content, alloying constituents, and impurities. The invention is a steel consisting of 20% to 23% chromium; 1% to 3% nickel; 2.5% to 4.5% manganese; the sum of said nickel and manganese being less than 6%; 0.2% to 1.0% copper; 0.2% to 0.8% molybdenum; up to 1% silicon; 0.02% to 0.05% carbon; 0.02% to 0.20% nitrogen; the sum of carbon and nitrogen contents being greater than 0.05%; the remainder iron and incidental impurities. Conventional impurities such as phosphorus and sulphur may be present but should be held quite low. The steel of the invention has an average Izod impact strength in the annealed condition of at least 50 ft.-1o. at room temperature, has a corrosion rate not exceeding 0.2 inch penetration per month in non-aerated 10% sulphuric acid at 70 C., and has the ability to retain these properties after exposure to temperatures from 700 C. to 1100 C. or on slow cooling from these temperatures. It possesses these properties despite the fact that it contains carbon and nitrogen in total amounts heretofore found detrimental to the toughness of .ferritic 20% to 30% chromium steels. The impact strength of several typical examples of the steel of the invention after annealing in the four different procedures indicated is set forth in Table I.

Table I d .th Composition percent I 1 Cr Ni Mn 11* Mo N i 1 2 3 4 21. 34 1. 75 2. 35 3. 5 5 0. 027 0. 040 9 50 72 21.75 1.75 2.80 .5 .5 .028 .048 113 00 85 17 21.91 1.75 2. 63 .5 .5 .034 .052 107 00 21 20. 10 l. 75 3. 5 5 0Z2 058 120+ 112 02 19 20.98 1. 3.40 .5 5 .028 .060 120+ 54. 52 21. 99 1.75 3. 56 .5 i .5 .033 .010 116 50 .90 32 22. 85 l. 75 3. 5 1 5 l 5 033 053 .104 45 93 '15 21.13 1.75 3.63 .5 .5 .038 .136 120+ 120+ 120+ 99 l i i l i l s 1 'The resistance of the steels of Table I to.cor- 2 rosion in non-aerated dilute sulphuric acid is recited in Table II.

Table 'II Inch penetration per month in nomaerated Composition percent lolzbsulphuric acid at i 4 Cr Ni 1 XVIII 011*{ C N 1 2 3 4 l 21.34 1. 75 2. 35 0. 5 L0. 5 0. 027 0. 040 O. 014 0. 31 0. 031 0. 33 .21175 1.75 2. 80 .5 .5 .028 .048 .020 .21 .025 .02

.21.91 1.75 2.63 5 .5 .034 .052 .015 .11 .023 .022 20.10 1.75 3.50 .5 .5 .032 .058 .118 .012 .738 .915 .20198 1.75 3.40 .5 .5 028 .000 .018 .009 .015 .310 21.99 1. 75 3. 56 5 5 033 049 011 O18 026 018 22.85 1. 75 3. 5i 5 5 033 053 014 038 02A 022 2113 1.75 3.03 5 .5 .038 I .130 .007 .011 .017 .024

Percent added.

.1. .Annealed by heating 3 hours at 900 C. and water-quenching.

2. Annealed by heating 1% hour to 1 hour at 1100 C. and wateriii iin aled by heating 3 hours at 000 C. and 1 hour. at 750 C.

and watenqnenching.

4. Annealed by heating 3 hours at 900 C. and furnace-cooled.

As can be seen from the dataof'Tables [and :II, the steel of the present invention not only shas unusually high impact 'strength'but also 'possesses useful resistance .to corrosion in dilute rion-aerated sulphuric acid. Ferritic 18% chromium'steels generally exhibit arcorrosion'rate-of 1:5 to 215 inch penetration per month in'nonaerated 10% sulphuricacid at'70 C., about .ten :to :one hundred 'times the rate of the steel of .the invention. Furthermorethe excellent propertiesof the steel of this'invention are attained lover a wide range of heat-treatment, whereas thenonventional ferriti'c chromium steels are far tmore sensitive "to the conditions of heat-treat- -ment.

Although the 'steel of the invention is, as -indicated, amenable to heat-treatment over a relatively 'wide temperature range, the optimum combination of impact strength and corrosion resistance is attained by heating the steel in the range 800 C. to 1100 C. for a sufficiently long time 'to form a substantial proportion of .austenite containing most of the carbides and nitridesin solid solution, the proportion of austenite generally being less than 50%. .A preferred time for this purpose is 3 to 6 hours. .Following the heating for solubility and homogeneity, the-steel should be cooled rapidly using air, 'oil, or water asthe quenching medium. In the annealed condition, the steel is partially austeni- -tic.-and-is formable, machineable, weldable, and *suitable zfor manufacture into corrosion-resistant articles and numerous other products subjected to high stresses in service.

The composition limits of the steel of the present invention are critical, and a precise chemical balance must be maintained to obtain the desired properties. For instance, to attain the desiredcorrosion resistance, the chromium content llybdenum; 0.5% copper; 0.4% silicon; 0.03% .carbon;.0.06%.nitrogen; the percentages of chromium shown in the table; remainder iron. The specimens were tested after having been annealed by heating three hours at 900 C. and cooled .in the furnace, furnace cooling generally being detrimental for high-chromium steels.

Table III 1 i Inch peneh gs i tration per in] month in hromiumpcrcent g i non-aerated in 25 d p one 501 foot pounds at 0 C.

l 49 0. 915 52 I .310 32 I 018 15 1 022 Similar tests demonstrate the criticality of the nickel content of the steel of the invention. In these tests'specimens of steels containingabout 21% chromium; 3.5% manganese; 0.5% molybdenum; 0.5% copper; 0.4% silicon; 0.03% car- 'bon; 0.06% "nitrogen; the different proportions of "nickel indicated in Table IV; .and the re- 'mainer iron, were annealed by heating at 900 C. for three hours (six hours in the case of theinickel-free steel) followed by water-quenching and subjected to the impact and corrosion tests reported in the .table. .It will be .seen that :nickel provides great improvement in impact strengthsand corrosion resistance. However, if -morethan -3,% nickel is added-to these steels they Lbecomemore 'austenitic and partake of the disadvantages of austenitic steels.

ric acid foot-pounds at c C.

The proportion-of manganese in the steel of the invention, too, is critical. Manganese within theranges indieatediinparts toughness and decreases the sensitivity of the steel to exposure to e'rlevated temperatures. Demonstrating this .are the data of Table V below which were obtainedby testing specimens of steels containing about 21% chromium; 1.8% nickel; 0.5% molyb- -denum; 0.5% copper; 0.4% silicon; 0.03% carbon: 0.05% nitrogen; 0.7% or 3.5% manganese as indicated in tnetable; remainder iron. The speci- -mens were tested after having been annealed by heating for three hours at 900 C. and water- .quenched and then subjected to the indicated heat-treatments.

Table V Inch penetra- Average Izod tion per month impact values in non-aerated sulphuric Condition of metal foot-pounds acid at 70 C.

Alloy Alloy Alloy Alloy Heated 1 hr. at 600 C. and Water-quenched .1 20 112 l. 64 0.59 Heated 1 hr. at 700 0. and water-quenched 23 106 1. 33 .026 Heated 1 hr. at 800 C. and Water-quenched 120+ 07 .016 Heated 1 hr. at 900 C. and Water-quenched 46 120+ 029 .018 Heated 1 hr. at 1,000 C. and Water-quenched. 95 120+ .017 .008 Heated 1 hr. at 1,100 O. and water-quenched 40 106 .57 .013

Heated 1 hr. at 1,200 C. and watcr-quenched.. .90 .43

Decreased temperature sensitivity of the steel of this invention is illustrated by the retention of good impact strength and excellent corrosion resistance after exposure to this broad range of high temperatures. Similar tests have shown that a minimum of 2.5% manganese is required for best performance under a Wide variety of conditions of heat-treatment. Also, it has been found that if the sum of the manganese and nickel contents is greater than 6.0%, the alloy becomes more susceptible to intergranular corrosion, and it is for this reason that the manganese content should not exceed 4.5% and that the sum of the manganese and nickel must be maintained below 6.0% in the steel of the present invention.

ihe combination of copper and molybdenum in the range 0.2% to 0.8% each exerts a stron influence on sensitivity to conditions of heat-treatment and is similar to manganese in this respect; in addition, these elements improve the resistance of the steel to corrosion by reducing acids and to pitting. If the molybdenum content is greater than about 0.8%, however, the steel becomes more susceptible to embrittlement on exposure to temperatures below 800 C.

Carbon and nitrogen also have a marked effect on both the corrosion resistance and impact strength. With carbon contents greater than 0.05%, the steel exhibits reduced properties after exposure to temperatures below 800 C., and is more susceptible to intergranular corrosion after exposure to temperatures of 1100 C. or above. To insure high impact strength and excellent corrosion resistance under a wide variety of conditions of heat-treatment, the steel of this invention should not contain over 0.05% carbon and the nitrogen content should be in excess of 0.08%.

The residual silicon content should not exceed 1.0% as it imparts brittleness.

The steel of this invention may be made in Heroult-type furnaces by conventional electric arc furnace practices having general applicability to the production of stainless steels, as nitrogen does not have to be excluded from the furnace atmosphere and conventional low-carbon raw materials are employable. It can be made by induction-furnace melting practices if due regard is given to the carbon content of the raw materials, as the carbon content of the steel must not exceed 0.05%. The steel ingot form is hotworkable at a metal temperature of about 1100- simple annealing treatment. It may be heated in the range of 700 C. to 1100" C. without detriment to impact strength and corrosion resistance, but is subject to loss of toughness on prolonged exposure below these temperatures and becomes susceptible to intergranular corrosion after exposure to temperatures above this range. The loss of toughness and corrosion resistance can be restored by annealing in the range of 800 C. to 1100 C. It may be fabricated readily into products suitable for service requiring toughness and resistance to corrosion. The steel has a high yield strength, tensile ductility, and reduction of area indicatin its suitability for structural service where high stresses are involved. The relative ease of melting and hot-working the steel and the fact that it can be made by conventional commercial melting practices render it economical to produce. Further, it has the advantage of requiring much smaller quantities of strategic alloying metals than the standard austenitic grades of stainless steel.

What is claimed is:

1. Steel consisting of 20% to 23% chromium; 1% to 3% nickel; 2.5% to 4.5% manganese; the sum of nickel and manganese being less than 6%; 0.2% to 1.0% copper; 0.2% to 0.8% molybdenum; up to 1% silicon; 0.02% to 0.05% carbon; 0.02% to 0.20% nitrogen; the sum of the carbon and nitrogen contents being greater than 0.05%; the remainder iron and incidental impurities; said steel having, in the annealed condition, an average Izod impact strength of at least 50 footpounds at room temperature.

2. Steel consisting of 20% to 23 0 chromium; 1.5% to 2.0% nickel; 2.5% to 4.5% manganese; the sum of nickel and manganese being less than 6%; 0.4% to 0.6% molybdenum; 0.4% to 0.8% copper; 0.3% to 0.6% silicon; 0.02% to 0.04% carbon; 0.08% to 0.15% nitrogen; the remainder, iron and incidental impurities; said steel having in the annealed condition an average Izod impact strength of at least 50 foot-pounds at room temperature.

WILLIAM O. BINDER. ROGER A. PERKINS.

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

UNITED STATES PATENTS 1175 C., and the comparative ease of working at Number Name Date this temperature contributes to simplicity in 2 554 Franks achieving desired products by hot-working. June 1938 The steel of this invention, by virtue of its chem- FOREIGN PATENTS ical composition, possesses good mechanical prop- Number Country Dat erties and. corrosion resistance after a relatively 803,361 France Sept, 29, 193

Claims (1)

1. STEEL CONSISTING OF 20% TO 23% CHROMIUM; 1% TO 3% NICKEL; I.5% TO 4.5% MANGAMESE; THE SUM OF NICKEL AND MANGANESE BEING LESS THAN 6%; 0.2% TO 1.0% COPPER; 0.2% MOLYBDENUM; UP TO 1% SILICON; 0.02% TO 0.5% CARBON; 0.02% TO 0.20% NITROGEN; THE SUM OF THE CARBON AND NITROGEN CONTENTS BEING GREATER THAN 0.05%; THE REMAINDER IRON AND INCIDENTAL IMPURITIES; SAID STEEL HAVING, IN THE ANNEALED CONDITION, AN AVERAGE IZOD IMPACT STRENGTH OF AT LEAST 50 FOOTPOUNDS AT ROOM TEMPERATURE.