US2624671A - Ferritic chromium steels - Google Patents

Description

11953 w. o. BINDER ET AL 2,624,657l

FERRITIC CHROMIUM STEELS Filed Jan. 19, 1951 3 C -BRITTLE FRACTURE' U DUCTILE FRACTURE llllllllllllllll 2o 25 30 as '40 C HROMIUM INVENTORS 5 fTTORNEY Patented Jan. 6, 1 953 FERRITIC CHROMIUM STEELS William O. Binder, Niagara Falls, N. Y., and Bussell Franks, Pittsburgh, Pa., assignors to Union Carbide and Carbon Corporation, a corporation of New York Application January 19, 1951, Serial No. 206,906

6 Claims.

This invention relates to ferritic chromium steels suitable for the fabrication of equipment required to possess good toughness and ductility in addition to resistance to corrosion and oxidation.

The so-called "straight ferritic chromium steels have found limited utility in the past because as chromium is increased to provide neces sary resistance to oxidation and other corrosion, the toughness of the steels at ordinary room temperatures decreases sharply. Thus, although steels containing more than 20% chromium are excellent from th standpoint of corrosion and oxidation-resistance, they are notch sensitive, and for applications where toughness is required, of the ferritic chromium steels only those containing not more than 20% chromium could be used, and generally the chromium content has not exceeded 16%. The notch sensitivity developed by ferritic steels containing more than 20% chromium has engaged the attention of metallurgists for many years, but no satisfactory solution to the problem, whether heat treatment or modification of composition, has been achieved.

It is the principal object of this invention to provide ferritic chromium steels containing 20% to 40% chromium which steels are tough and ductile at ordinary room temperatures and lower temperatures. More specifically, it is an object of the invention to provide ferritic steels containing 20% to 40% chromium which have an Izod impact strength of at least 18 foot pounds at room temperature. Another object is the provision of articles fabricated from such steels,

which articles in normal use are required to possess good resistance to corrosion and oxidation and to retain toughness and ductility.

The invention by means of which these objects are attained is based upon the discovery that there is a critical relationship in'ferritic chromium steels containing 20 to 40% chromiumbetween the toughness of such steels and the sum of their carbon and nitrogen contents. Further, it has been found that as chromium is increased from 20% to 40% the tolerance for carbon and nitrogen to retain toughness decreases and that the relationship between chrominum and the sum of carbon and nitrogen may be represented by a curve which separates steels which retain' toughness at room temperature and below from steels which are brittle at room temperature.

The single figure of the drawing is a pair of curves, AB and CD, representing the critical relationship between chromium and the sum of car- Mn and nitrogen in ferritic chromium steels containing 20% to 40% chromium.

The invention comprises ferritic chromium steels containing 20% to 40% chromium, carbon and nitrogen in such proportions that their sum for any selected chromium content when plotted against the chromium content as in the drawing does not fall above the curve, AB, and is preferably below the curve CD, the remainder essentially iron and the customary additions and impurities ordinarily present in steels of this type. As is illustrated by the drawing, a steel containing 20% chromium may contain carbon and nitrogen in a sum not above about 0.06% andbe ductile, but a steel containing chromium must contain not more than 0.035% carbon and nitrogen to be free of brittleness at room temperature, while at chromium the sum of carbon and nitrogen must be leSS than about 0.02%. For applications where it is important to have greatest impact strength, it is preferred that the composition of a steel to be used fall below the curve, CD. Representative upper limits for the sum of carbon and nitrogen under these conditions are 0.04% at 20% chromium; 0.025% at 25% chromium and 0.015% at 40% chromium.

The explanation for the rapid decrease in toughness of commercial ferritic chromium steels is found in these data which show that the critical sum of carbon and nitrogen for optimum toughness decreases rapidly with increasing chromium content and in carbonand nitrogen contents. In

such tests hot-workedsamples of steels were subjected to Izod impact tests at room temperature and at temperatures above and below room temperature. The samples were heated for six hours at 900 C. and quenched in water before testing.

Representative examples'of results of these tests are given in the'following tables.

In; Table I Izod impact values obtained at room temperature, C. and C. are set forth. In Table II are presented tensile test data taken at room temperature, tensile strength being measured in pounds per square inch, elongation (per cent E1.) in percentage of a two-inch gage length, and

percent R... A. is percentage reduction of area at fracture;

Table I Composition-Remainder Fe Izod impactft.-lb.

Pep Percent Room l00 6 cecnt 3 other temp. C. C.

20. 0. 013 0. 003 Nil 89 98 1 15/47 21. 79 .014 .002 Nil 92 90 100 23. 56 .012 .016 Nil 97 102 9 7/33 25. 44 010 001 Nil 96 100 51 25. 13 014 003 Nil 100 100 100 28.17 .018 .002 Nil 90 N.T N. '1 31. 24 012 007 Nil 91 N. T N. T 34. .010 .002 Nil 00 91 0 37. 37 .016 004 Nil N. T N. T. 25.13 0.01 .014 .003 Nil 100 100 100 25. 70 02 018 003 Nil 98 106 39 83 l4 014 002 O. 04 n 100 106 116 26. 41 .23 .018 .012 0. 03 Mn 110 114 66 2G .24 .016 .003 Nil 100 110 25.18 53 .005 .003 Nil 102 N.T. 5 25. 55 78 .005 003 Nil 112 116 110 26.05 1.80 .008 .010 Nil 4 N.T N. T. 25 004 002 2 M0 102 N. T N. '1. 25 003 004 1 Ni 93 100 108 25 .006 005 2 Ni 87 97 95 25 .004 002 3 Ni 97 86 101 i No Mn or Si added, but traces of both probably present.

2 Two specimens with diflcrent values.

N. T. means not tested.

Table IL Composition-Remainder Fe 1 Tensile Percent Percent Percent Percent Percent Percent Strength Cr C N other 25. 44 0. 010 0. 001 Nil 01, 600' 43 74. 25. 13 014 003 Nil. 53, 600 37 71. 31. 39 016 002 Nil 65, 000 29 31 34175 .010 .002 Nil 40, 000 20 44 37. 77 .016 .004 Nil 71,000 33 57 25 003 004 1 Ni 64, 300 27 43 25 006 .005 2 Ni 66, 100 31 60 25 004 002 3 Ni 70, 500 I 24 41 1 N0 Mn 01' Si added, but traces of each probably present.

their carbon and nitrogen contents is kept below the curves shown in the drawing. The data show further that by lowering the sum of carbon and nitrogen in such steels below the curve in the drawing the transition from toughness to brittleness is retarded and that in general,

the lower the sum of carbon and nitrogen, the lower the temperature of transition from toughness to brittleness.

Reduction and control of carbon and nitrogen can be carried out by utilizing materials of very low carbon and nitrogen contents and by melting and casting the steel at a reduced pressure in contact with an oxygen-bearing material. Under the latter conditions, carbon can be oxidized in preference to chromium and the carbon monoxidev gas which forms can be readily removed from the steel as. it is not very soluble. The oxidation of carbon, however, only proceeds. if the partial pressure of the. carbon monoxide in the atmosphere over the steel is less than the equilibrium value at a given temperature. This means, that to remove carbon from the steel the carbon monoxide gas produced in the carbonoxygen reaction must be continuously pumped from the system. As the process of oxidizing carbon proceeds relatively slowly, suflicient time must be allowed after the charge is melted for the reaction to approach equilibrium.

The mechanism of nitrogen removal is best explained by Henrys law which states that the.

partial vapor pressure of the solute is proportional to the moi fraction of the solute, which means that the solubility of a gas in a liquid decreases as the pressure is reduced. The reduction in nitrogen content of the steel is due to the lowering of the partial pressure of nitrogen over the bath.

To obtain the desired carbon and nitrogen levels it has been found necessary to keep the pressure less than about 1000 microns of mercury if excessive bath temperatures are to be avoided. The use of low pressures, therefore, has the added advantage of prolonging furnace refractory life. Excessive sublimation of chromium occurs with long holding at pressures of less than about 50 microns of mercury, and pressures in the range of 200 to 500 microns of mercury give best operating results.

Instead of melting the steel at a reduced pressure, the desired control or reduction of carbon and nitrogen may be attained by maintaining an inert atmosphere free from traces of nitrogen over the steel while melting. Argon or helium are ideal for this purpose as they are inert gases, but in using a gas atmosphere it is necessary to circulate purified gas continuously over the surface of the metal in order to prevent the partial pressure of nitrogen and carbon monoxide from building up in the inert atmosphere.

Small amounts of nickel, copper and cobalt, say up to 3% of nickel or cobalt and 2% copper, may be present in the 20% to 40% ferritic chromium steels of the invention without seriously altering their characteristics provided that the total carbon and nitrogen content is such that it falls below th curvesv in the drawing. Likewise, small amounts of other carbide-forming elements than chromium may be resent, for instance up to 3% of molybdenum or tungsten, for improved corrosion resistance or high-temperature strength. The steels should be fully deoxidized, manganese and silicon being suitable deoxidizing agents, but for optimum toughness at low temperatures silicon should fall well below 1%, and the carbon and nitrogen contents as far below the critical level as possible. The amount of residual silicon should not greatly exceed that needed for deoxidation purposes. Silicon apparently tends to shift the temperature at which embrittlement occurs upwards, particularly if carbon and nitrogen are near the critical total and chromium is in excess of 25%. Manganese is difficult to retain if the steel is melted at pressures below about 4 mm. of mercury. However, if the steel is made under conditions favorable to the retention of manganese, it is suggested that manganese-not exceed 1 maximum. Sulfur and phosphorus, usual impurities in steel, may be present, but phosphorus tends to shift the embrittlement temperature upward, and hence should be kept as low as possible. Other impurities in the raw materials may have a similar influence, and for optimum toughness at low temperatures, the purity of the. steel should be kept high.

Thev very low carbon and nitrogen steels covered by'thls invention are capable of being welded. The steels should be welded under conditions such that nitrogen and carbon pick-up is avoided, or with austenitic electrodes such as 18% chromium- 8% nickel steels or 25% chromium-20% nickel steels as the austenitic steels are not seriously susceptible to loss of toughness due to carbon and nitrogen pick-up during welding, and they team will produce welds matching the good ductility and toughness of the base metal.

This application is in part a continuation of our application Serial No. 36,558, filed July 2, 1948, now abandoned.

What is claimed is:

l. A ferritic chromium steel containing 20% to 40% chromium, up to 3% nickel, up to 3% cobalt, up to 2% copper, up to 3% molybdenum, up to 3% tungsten, the remainder, except for incidental impurities, being iron, carbon and nitrogen, the sum of the carbon and nitrogen contents for a selected chromium content being no greater than the value determined by the intersection of the plot of the selected chromium content with the curve AB of the drawing, said steel having in the annealed condition an Izod impact strength of at least 18 foot pounds at room temperature.

2. A ferritic chromium steel containing 20% to 40% chromium, up to 3% nickel, up to 3% cobalt, up to 2% copper, up to 3% molybdenum, up to 3% tungsten, the remainder, except for incidental impurities, being iron, carbon and nitrogen, the sum of the carbon and nitrogen contents for a selected chromium content being no greater than the value determined by the intersection of the plot of the selected chromium content with the curve CD of the drawing, said steel having in the annealed condition an Izod impact strength of at least 18 foot pounds at room temperature.

3. An article which in its normal use is required to withstand corrosion and to have high toughness at ordinary room temperatures, said article being composed of a ferritic chromium steel containing 20% to 40% chromium, up to 3% nickel, up to 3% cobalt, up to 2% copper, up to 3% molybdenum, up to 3% tungsten, the remainder, except for incidental impurities, being iron, carbon and nitrogen, the sum of the carbon and nitrogen contents for a selected chromium content being no greater than the value determined by the intersection of the plot of the selected chromium content with the curve AB of the drawing, said steel having in the annealed condition an Izod impact strength of at least 18 foot pounds at room temperature.

4. An article which in its normal use is required to withstand corrosion and to have high toughness at ordinary room temperatures, said article being composed of a ferritic chromium steel containing 20% to 40% chromium, up to 3% nickel, up to 3% cobalt, up to 2% copper, up to 3% 3 molybdenum, up to 3% tungsten, the remainder, except for incidental impurities, being iron, carbon and nitrogen, the sum of the carbon and nitrogen contents for a selected chromium content being no greater than the value determined by the intersection of the plot of the selected chromium content with the curve CD of the drawing, said steel having in the annealed condition an Izod impact strength or at least 13 foot pounds at room temperature.

5. A welded article which in its normal use i required to withstand corrosion and to have high toughness at ordinary room temperatures, said article being composed of a ferritic chromium steel containing 20% to 40% chromium, up to 3% nickel, up to 3% cobalt, up to 2% copper, up to 3% molybdenum, up to 3% tungsten, the remainder, except for incidental impurities being iron, carbon and nitrogen, the sum of the carbon and nitrogen content for a selected chromium content being less than the value determined by the intersection of the plot of the selected chromium content with the curve AB of the drawing, said steel having in the annealed condition an Izod impact strength of at least 18 foot pounds at room temperature.

6. A welded article which in its normal use is required to withstand corrosion and to have high toughness at ordinary room temperatures, said article being composed of a ferritic chromium steel containing 20% to 40% chromium, up to 3% nickel, up to 3% cobalt, up to 2% copper, up to 3% molybdenum, up to 3% tungsten, the remainder, except for incidental impurities, being iron, carbon and nitrogen, the sum of the carbon and nitrogen contents for a selected chromium content being less than the value determined by the intersection of the plot of the selected chromium content with th curve CD of the drawing, said steel having in the annealed condition an Izod impact strength of at least 18 foot pounds at room temperature.

WILLIAM O BINDER. RUSSELL FRANKS.

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

Transactions, American Society for Metals, vol. 23, page 33, published by the American Society for Metals, Cleveland, Ohio.

Claims (1)

1. A FERRITIC CHROMIUM STEEL CONTAINING 20% TO 40% CHROMIUM, UP TO 3% NICKEL UP TO 3% COBALT UP TO 2% COPPER, UP TO 3% MOLYBDENUM, UP TO 3% TUNGSTEN, THE REMAINDER, EXCEPT FOR INCIDENTAL IMPURITIES, BEING IRON, CARBON AND NITROGEN, THE SUM OF THE CARBON AND NITROGEN CONTENTS FOR A SELECTED CHROMIUM CONTENT BEING NO GREATER THAN THE VALUE DETERMINED BY THE INTERSECTION OF THE PLOT OF THE SELECTED CHROMIUM CONTENT WITH THE CURVE AB OF THE DRAWING, SAID STEEL HAVING IN THE ANNEALED CONDITION AN IZOD IMPACT STRENGTH OF AT LEAST 18 FOOT POUNDS AT ROOM TEMPERATURE.

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