US3458366A - Wrought chromium base alloy - Google Patents

Description

July 429, 1969" w. H. cHANG 3,458,366

i WROUGHT CHROMIUM BASE ALLOY I Filed on. 1,- 1965 s sheets-sheet 1 @si *e NQS. Iv l INVENTOR /4//4/.57//1/ //l// July 29, 1969 w. H. CHAN@ 3,458,366

WROUGHT CHROMIUM BASE ALLOY Filed Oct. 1l 1965 3 Sheets-Sheet 2 1N VENTOR. Afin/ July 29, 1969 Filed 001. l, 1955 W. H. CHANG WROUGHT CHROMIUM BASE ALLOY 3.Sheets-Sheet 5 VENTOR.

fang@ United States Patent O 3,458,366 WROUGHT CHROMIUM BASE ALLOY Winston H. Chang, Cincinnati, Ohio, assignor to General Electric "Company, a corporation of New York Filed Oct. 1, 1965, Ser. No. 491,978 Int. Cl. C22c 31/00; C22f 1 /11 U.S. Cl.. 148-32 3 Claims ABSTRACT OF THE DISCLOSURE This invention relates to chromium base alloys and, more particularly, to a wrought chromium base alloy of improved oxidation resistance and ductility.

The need for improved material for use at elevated temperatures such as 1800 F. and above, particularly above 2000 F., has resulted in the Irecognition of certain alloys -based on chromium. Emphasis on such an alloy is based in part on the inherently better oxidation resistance of chromium as compared with other refractory metals such as molybdenum and columbium. Nevertheless, at temperatures particularly in excess of 1800 F., strengthened chromium base alloys used in an oxidizing atmosphere are subject to oxidation or nitrication problems. Oxidation behavior affects the load-bearing capability through metal loss as well as through embrittling reactions. Other problems relate to the temperature at which a chromium changes from a ductile to a brittle condition as temmrature is lowered.

One improved chromium base alloy of the type to which the present invention relates is described in U.S. Patent 3,174,853-Sims et al., issued Mar. 23, 1965 and assigned to the assignee of the present invention. That patent describes a chromium base alloy dispersion strengthened through the formation of carbides of such elements as titanium, zirconium and hafnium or mixtures thereof. The oxidation resistance of such alloys as that described in the Sims et al.. patent, are improved by the retention of small amounts of yttrium in the alloy. Other additions such as thorium improve the oxidation resistance of these kinds of alloys as described in U.S. Patent 3,011,889- Baranow, issued Dec. 5, 1961 and assigned to the assignee of the present invention. However, further improvement in oxidation resistance as well as ductility are desired so that designers can apply wrought chromium base alloys as articles, such as in the high temperature sections of propulsion apparatus.

A principal object of the present invention is to provide a wrought chromium base alloy of improved resistance to oxidation, nitriiication and scaling and of improved ductility.

Another object is to provide an improved 'wrought chromium base alloy moderately strengthened with a dispersion of carbides which at the same time enhance oxidation and nitrication resistance by inhibiting the formation of heavy scale and the penetration of oxygen and nitrogen into the structure of the alloy, the alloy having additional oxidation resistance as a result of the inclusion of yttrium and thorium `while maintaining good ductility.

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Still another object of the invention is to provide such an improved chromium base alloy further strengthened with a solution strengthening element.

These and other objects and advantages will be more fully understood from the following detailed description and examples which are meant to be representative of but not to be any limitation on the scope of the present invention.

In the drawings:

FIG. 1 is a graphical comparison of oxidation characteristics between cast and wrought alloy forms;

FIG. 2 is a graphical comparison of the effect oxidation has on depth hardening between cast and wrought alloy forms; and

FIG. 3 is a graphical `comparison of air oxidation characteristics of one form of the alloy of this invention with a reported sheet alloy.

The alloy provided by the present invention is a wrought chromium base alloy strengthened by a zirconium carbide dispersion which has, in addition, a scavenging effect. Such carbide dispersion which exists as a grain boundary network in the as-cast condition is controlled and made to be discontinuous in the wrought condition in order to inhibit the migration of oxygen and nitrogen along a carbide path or network normally susceptible to oxygen and nitrogen. Thus the discontinuous carbide phase in the wrought condition performs a triple function of acting both as a strengthening mechanism, as a scavenging agent by absorbing interstitials from the matrix and as an oxidation and nitrication inhibitor. The alloy is improved further by the addition of a combination of yttrium and thorium retained in the alloy for additional oxidation resistance improvement. In another form of this invention, because Zr does not form a tine carbide dispersion which would provide the alloy with high strength, a solution strengthening element such as molybdenum can be added to provide the alloy with the combination of dispersion and solution strengthening without significant effect on oxidation-nitrification resistance.

More particularly, the wrought chromium base alloy of the present invention consists essentially of, by weight, 0.05-0.15% C; O.1-0.2% Y; 0.05-0.2% Th; an amount of zirconium from a minimum of 0.1% -|-7.6 wt. percent C up to about 3% Zr; with the balance chromium. For solution strengthening, up to about 10% Mo can be added.

A review was made of the oxidation behavior of and structural changes in alloys to which elements of Group IV A of the Periodic Table of Elements (Ti, Zr and Hf) were added, particularly Zr. It was recognized that titanium while strengthening the alloy through its carbide, can be detrimental to oxidation resistance. Titanium, like columbium, will, even in the as-cast condition, form a ne dispersion of carbides both in the grain matrix as well as a network along the grain boundary. By -way of contrast, zirconium will not form a fine carbide dispersion but in the as-cast condition forms a net-work of larger carbides in the grain boundary. The relatively concentrated dispersion of ne carbides of titanium, -while strengthening the alloy perhaps to a greater extent than does zirconium carbide, at the same time provides a more readily traversed path for oxygen and nitrogen to penetrate into the alloy. Working of titanium carbide-bearing chromium base alloys from cast to wrought form does not improve this condition. Working of zirconium carbide-bearing alloys of this invention breaks up the grain boundary zirconium carbide network and distributes large, isolated masses of zirconium carbide throughout the alloy. This strengthens the alloy but requires any oxygen or nitrogen attempting to penetrate the alloy structure to pass through the more difticult-to-traverse chromium matrix.

Zirconium will rst enter into a solid solution with chromium up to the solubility of zirconium, which is less than about 0.1 weight percent Zr. Additional zirconium added then will combine with available carbon to form relatively large portions of zirconium carbide. Thereafter excess amounts of zirconium will form an intermetallic ZrCr2. In the absence of carbon, a precipitate of the ZrCrZ phase would form Iwith zirconium in excess of that in solid solution.

When carbon is present with zirconium, zirconium carbide Will form preferentially in the grain boundaries, which are the last to solidify after casting of an alloy of this type, rather than in the grain matrix. Because the amounts of carbon required to dispersion strengthen the alloy are suiciently high, enough zirconium carbide precipitates on casting to from a continuous network along the grain boundaries. In addition to strengthening the al loy, zirconium carbide has the characteristic of accepting considerable amounts of oxygen and nitrogen. This scavenging characteristic is beneticial with regard to removing interstitials from the grain matrix. However, the continuous carbide grain boundary network in the cast condition provides a ready oxidation path through the alloy in which the zirconium carbide first forms a Zr (C, O, N) compound which changes to ZrO2. The alloy of the present invention is characterized by a discontinuous, isolated carbide str-ucture to inhibit oxygen and nitrogen penetrating actions while retaining the scavenging and strengthening characteristics.

In the evaluation of alloys of the type to which the present invention relates, from a strengthening viewpoint itis necessary to control the relative amounts of zirconium and carbon within a range optimum for providing the strongest possible alloy yet retain ductility. superimposed upon the problem of controlling the zirconium and carbon as it relates to the formation of zirconium carbide and the intermetallic ZrCrZ is the problem that carbon in excess of that which would preferentially join with zirconium to form the carbide, would form a chromium carbide such as Cr23Cs. Whereas ZrC tends to enhance oxidation resistance, Cr23C6 will embrittle the alloy, as will be discussed in connection with the alloy of Example 1 and is not known to improve oxidation resistance.

It has been recognized from the evaluation of the present invention that at least a particular amount of zirconium is required to provide with carbon the combination of carbide dispersion strengthening and the formation of a discontinuous, isolated carbide structure to improve oxidation resistance while at the same time retaining ductility. The minimum Zr required is the sum of Athe -amount of zirconium which -is soluble in chromium (up to abuot 0.1 weight percent) and the amount of zirconium required to combine with al-l of the carbon present preferentially to form zirconium carbide rather than chromium carbide.

The amount of carbon required to dispersion strengthen the alloy of the present invention is at least about `0.05 weight percent. Less than 0.05% C does not form enough zirconium carbide to getter the interstitials. Thus there would be insuicient scavenging both during melting and during exposure in an oxidizing environment. However, more than about 0.15 weight percent carbon with sufficient zirconium forms excessive carbides and will not allow the creation of a discontinuous carbide structure. With insufficient zirconium, it forms the embrittling chromium carbide. Therefore the zirconium required in the alloy of the present invention at a minimum is about 0.1 weight percent +7.6 wt. percent carbon based on an atomic ratio consideration of 1 zirconium atom to 1 carbon atom. In addition to this minimum for zirconium, Vit was recognized that although a small excess of zirconium can be tolerated allowing the formation of the intermetallic ZrCr2, the inclusion of more than about 3 weight percent zirconium allows the formation of amounts of ZrCr2 which will embrittle the alloy. Furthermore, long time oxidation resistance increases with increasing zirconium carbide up to the point where an oxidation path is created.

This control, according to the present invention, of the elements zirconium and carbon in a chromium base to provide an alloy of an improved combination of streng-th, oxidation resistance and ductility, particularly at 2200o F. and above, can be assisted further from an oxidation viewpoint by the inclusion of a combination of retained yttrium and thorium in the range of about 0.05-0.2 weight percent yttrium and about 0.05-0.2 weight percent thorium. Inclusion of amounts of each of these elements much below their respective lower limits provides insutiicient improvement in oxidation resistance. The addition of amounts greater than those speciiied cause the alloy to be brittle during reduction. Sometimes this condition is referred to as hot short.

Melting and casting of an alloy having the composition described above is not suicient to provide the discontinuous, isolated or interrupted carbide structure which is an important characteristic of the alloy of the present invention. Therefore, it has been recognized that such a cast composition must be reduced to wrought form such as by extrusion or forging -a-t a temperature between about 2000-2500 F. A reduction below about 2000 F. results in cracking of the material whereas above 2500 F. incipient melting tends to occur. Thus the alloy of the presen-t invention, in one fonm, can be formed by rst casting an alloy having a composition consisting essentially of, by weight, 0.050.l5% C; about ODS-0.2% Y; about ODS-0.2% Th, the above described minimum zirconium up to about 3%, up to about 10% Mo with the balance esentially chromium, and then reducing the cast structure to wrought form at a temperature between 2000-2500 F.

Typical of the composition of alloys included in the study of, and which were melted in the evaluation of the presen-t invention are those shown in the following Table I.

tion melted under argon to insure homogeneity and to minimize ingot cracking. The alloy was cast into a 3" diameter Y2O3-stabilized zirconia crucible. The Y and Th additions in the charges were about 6 and 2 times, respectively, larger than the nominal contents to compensate for their losses lthrough scavenging effects. Therefore, the percentages for those elements listed in this specific-ation refers to the retained amount rather than the amount added.

Micrographic studies of these alloys in the as-cast condition show an almost continuous grain-boundary network. X-ray diffraction and emission evaluations showed the network to consist predominantly of ZrC with minor amounts of ZrO2 and in some cases some of the facecentered-cubic, high temperature form of the intermetal lic compound ZrCrz. The alloy of Example 1, which lies outside the scope of the present invention, included the embrittling Cr23C6 in addition to the predominant ZrC. As will be shown in detail later in connection with Table III, the effect of CrzaCs in reducing the ductility of the alloy of Example 1 is signicant.

-In order to prvoide the alloy form of the present invention by creating a discontinuous or interrupted carbide structure of ZrC, the ingots were subsequently extruded to sheet bar at a temperature of 2400 F. without any diiculty a-t an extrusion ratio of Iabout 9 to 1. As was mentioned before, this temperature was selected to avoid cracking of the ingot during reduction and to avoid incipient melting. The extrusion temperature of -about 2400 provided ease of processing while at the same :time avoiding excessive gr-ain growth.

For subsequent testing, the extruded bar was rolled to 0.05 sheets. The first reduction Was 50% in thickness at 2000 F. followed by finish rolling half of the material at 1800 F. and the other half at 1500 F. Both procedures yielded excellent sheets of 0.05" thick material. A photomicrographic analysis of the grain structure after extrusion and rolling showed discontinuous isolated portions of ZrC and ZrCr2. In alloy Example 2 of Table I, as a typical example, in the as-cast condition the grain boundary included about 92% ZrC and about 8% lZrCr2. After rolling, about 75% of the particles were ZrC and about 25% ZrCrz with the ZrC fragmented and discontinuous. The preferential grain boundary precipitation in the as-cast condition is probably based on zirconiums low solubility in chromium and the fact that zirconium was segregated in the last solidifying massthe grainboundary-which formed the ZrC network upon complete solidiication. In the absence of carbon, the network would have been composed of Cr-ZrCr2.

Oxidation tests of 100 hour duration were conducted at temperatures of between 1600 and 22.00 F. For these tests, bar specimens of 0.22 x 0.35 X 0.5 for the rolled conditions were used. Specimen preparation consisted of grinding and polishing through 400 grit paper followed by water and alcohol rinsing. Specimens were placed in zirconia crucibles and oxidized continuously in a tubular furnace with natural air convection. The following Table II gives data for specimens in the cast condition in which the grain boundary carbide precipitate was continuous and in the wrought condition in which the zirconium car bide was discontinuous.

A continuous scale on the surface of the alloys of Examples 1, 2 and 3 after oxidation testing was tightly adherent and not suiiiciently thick to cause scale cracking. If the scale remains free from mechanical defects, anion diffusion of oxygen and nitrogen has been found to be negligible. Hence there is little or no nitriding. However, if the surface scale is too thick, mechanical failure of the scale occurs. Oxygen and nitrogen, to the detriment of ductility, then diffuses into the alloy structure. The alloy of the present invention is tightly adherent and resists heavy oxidation scaling.

The effect of oxidation temperature on 100 hour weight gain of the alloy of Example 2 within the scope of the present invention is shown in FIG. l. The upper curve represents the weight gain of a cast bar specimen whereas the lower band represents the range for extruded bar, extruded sheet and rolled sheet. It is interesting to note in FIG. 2 the significant effect of oxidation temperature on the hour depth of hardening of the alloy of Example 2. The upper curve represents that for the as-cast condition whereas the lower curve is that for both the extruded bar and rolled sheet condition.

Further evidence that the alloy of the present invention has significantly improved resistance to oxidation and nitrication is condensed in FIG. 3. That ligure compares air oxidation test data for a known sheet alloy, reported to be one of the best available based on chromium, with the sheet alloy form of Example 2. The known alloy has a composition, by Weight, of 93.5% chromium, 0.5% titanium and 6% magnesium oxide. Both sheet alloys were at the same thickness of about 50 mils. The solid lines in FIG. 3 show weight gain data and the broken line shows nitride thickness. The significant difference between the two alloys is easily recognized. No nitride line appears for the alloy of Example 2 because no nitride was found up to 2400 F. for 100 hours.

Although the alloy of Example l has better oxidation resistance at lower temperatures than does the alloys of Examples 2 and 3, which are within the scope of the present invention the reverse becomes true at temperatures above about 2000 F. Furthermore, metallographic microstudies of the structure of the alloys of Examples l, 2 and 3, showed that because of the excess of carbon as compared with zirconium, there existed the carbide CrzgC which appears to have a significant embrittling effect on the alloy of Example 1. This is shown more particularly from the bend test data on sheet material shown in the following Table III.

TABLE IIL- ROOM TEMP. BEND TESTS SHEET FORM The low-temperature ductility of the sheet materials evaluated in connection with the present invention was rst assessed by room temperature bend tests on electropolished specimens about `1A wide by 2" in length. These tests were conducted on an Instron testing machine using a. 4T ram radius and a 110 v.block with a 0.75 span. The deflection rate was 0.05 in./min. In the above Table III full bend is meant to be The ductility of the alloy of Example 2 is particularly outstanding in that a full bend was obtained at room temperature'in all` conditions. It was found that the temperature at which the alloy of Example 2 changes from a brittle to ductile form is -between 0 F. and room temperature whereas that transition temperature for the alloy of Example 1 is at about F.

The microstructure studies of the alloys of Examples 1, 2 and 3 show that after exposure for 100 hours at temperatures from 1600-2200 F., nitrilication of the alloys of Examples 1, 2 and 3 is effectively blocked with the presence of fragmented, discontinuous ZrC. The weight gain data of Table II reflect nitrication inhibition which affects the alloy ductility. However, in the case of Exam ple 1 alloy, the presence of the embrittle Cr23C6 has a significant eifect on ductility as shown by Table III. That alloy was so brittle as a result of the inclusion of the Cr23C6 carbide that it fractured without any deflection in the as-rolled condition and was significantly less ductile in the other conditions. Thus, although the alloy of Example 1 had somewhat better oxidation resistance than does alloys 2 and 3 at lower temperatures, the existence of the chromium carbide resulting from the lack of further control of the elements zirconium and carbon results in an alloy having a significantly poorer combination of oxidation resistance and ductility.

7 8 Typical of the data resulting from mechanical testing Which are intended to be covered by the appended claims. in evaluating the present invention is shown in the fol- What is claimed is: lowing Table 1V. 1. A wrought chromium base alloy of improved oxidation resistance and ductility consisting essentially of, by TABLE IV.-TENSILE DATA, ROLLED SHEET, RECRYSTAL- 5 Weight:

LIZED CONDITIONl 0.05-0.l5% C; (LUS-0.2% Y; UGS-0.2% Th; Zirconi- Ultma 02% yield um from a minimum of 0.1% +7.6 wt. percent C up Telll; Stfgh' Stfelggh: Elogftoht, to about 3% Zr; up to about 10% Mo with the bal- 'sm 'SM peice ance chromium and incidental impurities; Example: the alloy having as a carbide precipitate discontinuous 1 7s 50 30 14 10 1,000 30 13 42 portlons of zirconlum carbide 1n the gram boundaries 1,500 25 11 46 and further characterized b y the absence of carbides 1,800 15 s 54 2, 000 10 5 71 of chromium and titamum. 2,2%) (lg 2. The alloy of claim 1 in which the Zr is O.5-3'%. 1,000 43 i0 50 15 3. The alloy of claim 1 in which C is about 0.1%; ggg Y is about 0.1-0.2%; Zr is about l-2%; and M0 is up 2,' 000 12 0 5g to about 5%. 2,200 s 4 5* 3 1 0.6% ig 0g References Cited 38 g3 20 UNITED STATES PATENTS 2000 24 15 33 3,011,889 12/1961 Baranow 75-176 21200 15 11 43 3,174,853 3/1965 Sims et al 75--176 1 Examples 1 and 21ecrystallized at 1,900" F. for 1 hour. Example 3 3,227,548 1/1966 Clark '7S-176 fcl'ysmulzed at 21000 F-f0f1 1101111 3,347,667 10/1967 Wukusick et a1. 75-176 Although the present invention has been described in 5 RICHARD 0, DEAN, Primary Examiner connection with specic examples, it will be recognized by those skilled in the metallurgical art the modifications U.S. Cl. XR. and variations of which the invention is capable and 75-176; 148-2

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