US3235417A - High temperature alloys and process of making the same - Google Patents

Description

Feb. 15, 1966 A. ROY

ETAL

HIGH TEMPERATURE ALLOYS AND PROCESS OF MAKING THE SAME Filed Jan. ll, 1965 2 Sheets-Sheet 1 Feb. 15, 1966 A. ROY ETAL 3,235,417

HIGH TEMPERATURE ALLOYS AND PROCESS 0F MAKING THE SAME f/Qawe INVENTORLS.

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United States Patent O 3,235,417 HlGH TEMPERATURE ALLOYS AND PROCESS F MAKING THE SAME Amedee Roy, Birmingham, Frederick A. Hagen, Detroit, and John M. Corwin, Royal Oak, Mich., assignors to Chrysler Corporation, Highland Park, Mich., a corporation of Delaware Filed Jan. 11, 1965, Ser. No. 424,538 16 Claims. (Cl. 148-136) This invention relates to austenitic alloys of the csharacter disclosed and claimed in the copending application of Amedee Roy, `one of the present applicants and Walter E. Jominy, Serial No. 119,902 led June Z7, 1961, :now Patent No. 3,165,400 issued January l2, 1965, which is by this cross reference made a part hereof. Said application describes the preparation of alloys containing low amounts of strategic elements such as nickel and cobalt and containing carbon in amounts higher than those normally used in conventional stainless steels and relatively small amounts of a plurality of carbide forming additions such as tungsten, columbium, molybdenum for high temperature applications. It further emphasizes the preparation of alloys with good stress-rupture properties characterized by ne secondary precipitates, heavily concentrated toward a semi-continuous interdendritic network and randomly dispersed within the rest of the austenitic matrix and where the best stress-rupture alloys are those containing the finer secondary precipitates.

The present invention is directed to procedures involving heat treatment of alloys of the character described in said copending application to increase the mechanical properties of such compositions and reduce the scatter in stress-rupture results of as cast structures. It is based on the discovery that although austenitic alloys are not generally heat treatable it is possible whereas here -a proper balance exists between carbon content and the carbide forming elements of the composition to `effect such a treatment with good results.

Thus, we have discovered that when alloys of the aforementioned character are solution heat treated and aged by heat treatments at optimum temperature or are aged at particular temperatures but not solution treated the random distribution of the carbide precipitates is less random Vand there is a substantial concentration and increase of the iine precipitates about the eutectic carbides comprising the network of lederburite phases.

Where the solution heat treatment and aging procedural combination was followed it was found that a high temperature (greater than 2100 F.) solution treatment followed by aging at a temperature below 1500 F. could increase the stress value for a 100 hour rupture life at l500 F. for an alloy such as 6D (hereinafter described) about 3000 p.s.i. whereas simply aging the as cast alloy at l200 F. to 1400 F. and eliminating the solution treating operation did not produce a greater increase in stress-rupture life over the as cast alloy as compared to ICC the combination treatment but improved room temperature ductility and substantially increased short time tensile properties. It was also discovered that the upper limit of ythe solution heat treatment temperature was quite critical. Thus, it was found that as the solution treating temperature was raised from 2150 F. to 2300 F. the dot-type matrix precipitate observed at 2150 and 2200 F. which are of an undesirable character and which it is desired to get into solution by this treatment started to go into solution above 2200 F. and was practically absent -at 2250 F. and that the massive carbide network started to show a substantial change with temperature at about 2300 F. such being the remelting of the grain boundary eutectic structure of the carbide phases which is undesirable ltending among other things to promote a pearlitic structure. It was also `found that the higher the solution heat treatment temperature of the alloy the ner was the desirable matrix precipitate upon subsequent aging by heat and the better the mechanical properties. Hence, it is preferred that the solution heat treat temperature prior to aging be below 2300 F., preferably 22.50 F. and as near `these limits as possible.

The carbon content of the alloy also has a material effect on the results obtained by the solution heat treat procedure, it being found that a carbon content below about 0.6% by weight where the austenitic forming elements of the alloy are not in proper balance with the chromium in the alloy would promote instability in -the alloy structure.

In our substantial experimentation it was further noted that articial aging of the as cast alloys such as described and claimed in said copending application; either after solution treatment or in the absence thereof, causes the formation of relatively ne precipitate carbides in the austenitic matrix of the alloy, that the size, shape, location and concentration of these precipitate carbides influence the strength properties of the as cast alloy Iand can generally enhance it, and that aging temperature and time control the precipitate formation to effect best results.

Thus, it was discovered that properties of the as cast alloy could be improved by aging above l000 F., preferably at 17.00 F. to 1250 F. for a suitable time, for example, 20 to 100 hours and that aging of the as cast alloy should not be carried out at a temperature exceeding about 1400 F. since the size and shape of the precipitate formed above 1400 F. are undesirable for maximum properties. Moreover, it was unexpectedly found that a reduction in total aging time and best optimum mechanical properties could be realized by a step aging procedure involving a combining of time and temperature at various levels. For example, these novel results were obtainable by initiating aging at a temperature of 1200 F. to 1250" F. for 10 to 20 hours and finishing aging treatment by an additional l0 to 20 hours `at 1300 F. to 1400 F. It was also observed that where solution heat treatment preceded aging heat treatment that the best aging temperature for optimum stress-rupture results for a given alloy following solution treatment at the optimum temperature described above is determined by its temperature of use. The higher the latter, the 'higher the aging temperature to be employed.

Set forth below are various tables, and graphs giving the chemical composition of many alloys tested and the effects thereon of solution heat treatment followed by aging and of aging only by a constant temperature or step temperature treatment.

Thus, Table I gives the chemical composition of the as cast alloys for which test data are given and upon which the above conclusions are based and identifies the alloys by number, many being substantially the same or substantially same as alloys reported in the said copending application. At the end of this table is also included the composition of as forged alloys for which test data appear in certain tables.

Table II gives the stress-rupture values and properties for alloys 6D and 15D obtained at 1500 F. where these alloys were solution heat treated and then aged before testing, the results for four different solution heat treatments being recorded. In treatment A the alloy test bar was solution heat treated at 2250 F. for 1/2 hour, then air cooled and then aged at 1400 F. for 20 hours. In treatment B, the soluiton treatment was at 2300" F. for 1/2 hour and the aging treatment was 1400 F. for 20 hours after air cooling. In treatment D the solution treatment was at 2200 F. for 1 hour and the aging treatment after air cooling was at 1400 F. for 20 hours. In treatment E the solution heat treatment was at 2150 F, for 1 hour and the aging treatment after air cooling was 1400 F. for 20 hours.

Table No. III records test data comparing stress-rupture values for 6D alloys and 6D alloys in which deviations were made in the specific amounts of certain elements especially carbon which alloys in as cast condition were solution treated at 2250 F. for 1/2 hour then air cooled. and aged at 1400 F. for 20 hours before testing. The last part of the table includes data showing the results of varying carbon in forged test bars similarly solution treated and aged.

These data show that the stress-rupture values for solution treated and aged alloys such as reported increased with increases of carbon.

Table No. IV records test data comparing stress-rupture values at 1500 F. of nickel variations in 15D base alloys without manganese which have been solution heat treated at 2250 F. for 1/2 hour, air cooled and then aged at 1400 F. for 20 hours. The test indicate that the base alloys 15D which contains 5% nickel and 5% manganese gives the best results and that when the magnese is omitted (see the third alloy in the table) there is a decrease in stress rupture properties. Also that reductions in the amount of nickel in the absence of manganese has a similar effect.

Table No. V records Vtest data comparing stress-rupture values at 1500o F. of chromium variations in 6D base alloys which have been solution heat treated at 2250 F. for 1/2 hour, air cooled, and then aged at 1400 F. for 20 hours. The tests indicate that there is a material decrease in stress-rupture properties with increases in the chromium content much above 20% believed to be about 23% in this base alloy.

Table No. VI records test data comparing stress-rupture values at 1500 F. of Boron variations in 6D base alloys which have been solution heat treated at 2250 F. for 1/2 hour, air cooled, and then aged at 1400 F. for 20 hours. The tests indicate that the desirable range of Boron for solution treated and aged 6D alloys is from about 0.003 to 0.010% by weight.

Table No. VII records test data comparing stress-rupture values at 1500 F. of titanium variations in 6D and 1|5D alloys which have been solution heat treated at 2250 F. for 1/2 hour, air cooled, and then aged at 1400 F. for 20 hours. The tests indicate that no improvement in stress-rupture values resulted from the addition of titanium to these base alloys which are solution treated and aged.

Table No. VIII records test data comparing stressrupture values at 1500 F. of phosphorous additions in 6D base alloys which have been solution heat treated at 2250 F, for 1/2 hour, air cooled and then aged at 1400 F. for 20 hours. The tests indicate that for solution treated and aged 6D alloys no improvement in stressrupture values is obtained by phosphorous additions and a material decrease in such properties when the level of phosphorous exceeds about 0.1% by weight.

Table No. IX records test data comparing stress-rupture values at 1500 F. of sulfur additions in 6D base alloys which have been solution heat treated at 2250 F. for 1/2 hour, air cooled and then aged at 1400 F. for 20 hours. The -tests indicate that sulfur does not improve the stress-rupture values of 6D alloys.

Table No. X records test data at 1500 F. showing the effect on solution treated and aged alloys of the use of a single carbide forming element therein. The heats were solution treated at 2250 F. for 1/2 hours, air cooled, and then aged at 1400 F. for 20 hours. These test data when compared with the results obtained for the 6D and 15D base alloys, for example, that employ multiple carbide forming elements. (See also heat 6DH in Table XI) indicate that the use of multiple carbide forming element additions gives optimum results.

Table XI records test data at 1500 F. showing the effect on some solution treated and aged alloys of the use of multiple carbide forming elements therein. The heats recorded were solution treated at 2250 F. for 1/2 hour, air cooled, and then aged at 1400 F. for 20 hours. The data show, for example, as in heat 6DH that advantages may accrue from the presence of multiple carbide forming elements.

Table XII records test data at 1500a F. showing the effect on solution treated and aged 6D and 15D base alloys of miscellaneous variations in alloy composition. The heats recorded were solution treated at 2250 F. for j/2 hour, air cooled, and then aged at 1400 F. for 20 hours.

Table XIII records test data showing the effect of step aging heat treatment on room temperature and 1200 F. short time tensile properties of as cast 6D base alloys. The properties appear improved by this treatment over those obtained with similar alloys subjected to a single step aging treatment for the same total aging time. In this rtable the heat code is indicative of the aging treatment. Thus he first digit represents the temperature of the first aging step, the numeral 2 representing 1200 F., the numeral 3 representing 1300 F. and so on. The second digit represents the time at the first aging temperature, the numeral 1 representing 10 hours, the numeral 2 representing 20 hours and so on. The third and fourth digits give similar representations to those respectively of the first and second digit for the aging step.

Table XIV records test data showing the effect of step aging heat treatment on stress-rupture and creep properties at 1 200 F. and 1500 F. of as cast 6D base alloys. The properties appear improved by this treatment over those obtained with similar alloys subjected to a single step aging treatment for the same `total aging time. The heat treat code in this table has the same meaning as in. Table Xl/II.

Other 0000000000000000000000 oaii 0000000000000050000002000 0 2.LL.L2.2.2.A42.2.ZZZQLLLLLQRWL Rw 00000000000000500000020 00 m50 0000000000 .3. 0M LLLLLLLLLL frs-SR at 1,500" F.

TABLE I 1 Table XVI records test data showing the eifect of sin- Forged test ba gle step agingY on short time tensile strength and stressrupture properties of as cast 15D alloys.

Alloy Table XV records test data showing the effect of single step aging 0n short time tensile strength and stress-rup ture properties of as cast M`6D alloys.

Heat

Other ent of the alloys reported. It essenti-v ounts are in percent by weight of the y. All am Alloy 1 Iron (Ije) is not set forth in this table as a constitu ally constitutes the remainder of each allo entire composition.

TABLE XI Eect on stress rupture at 1500 F. of using multiple carbide former elements n solution heat treated and aged alloys TABLE V III Effect on stress rupture at 1500 F. of variations in phosphorous on 6D base, solution treated and aged alloys l l TABLE Xlv Eect of step aging heat treatment n stress rupture and creep properties of as cast 6D alloy tested at 1200 F. and 1 5 00 F.

FIGURE l is a graph showing the strain percent against time in hours for 2 gage length test bars of alloy 6D given two different aging treatments designated M and Z and subjected to a load of 20,000 p.s.i. at 1500 F. and 40,000 p.s.i. at l200 F. for times up to 300 hours.

It will be noted that the test data are recorded in the Elongation and Time to Rupture Creep Rate as percent OIITI Of bands between pairs 0f CuI'VeS. The Width Of the at Load Stfallnlfrlllgaours band in each case represents the normal range of test results obtained with a plurality of tests with bars sim- Heat Treat GZKDSLM Kwijt ilarly treated and tested under the same conditions. Code 1,200F. 1,500F. l The band between the dotted curves are for test bars gss'' 53112503511.' of the 6D alloy given an M aging treatment (20 hours Hrs. Percent Hrs Percent at 12007 F.) and that between the full line curves is for Elong- Elon@ test bars of the 6D alloy given a Z aging treatment (100 hours at l200 E). 53h21: Eig ggz? gjg :(11% The two upper bands show the results obtained where 2131 52.6 6.0 111.4 10.0 .177 .643 the test bars were loaded at 20,000 p.s.i. at a temperature 122g 1% 133:3 jg :QE of 1500 F. and the two lower bands show the results obtained where the test bars were loaded at 40,000 p.s.i. 215421:: 217 1310 119 150 i293 :150 20 atatemperatufe 120Go F' 3231 1% These test data show that the bars given the Z aging 41:22: 6:5 9:0 58:0 13:0 NT :087 treatment have a lower creep rate (strain+time) than ggg gg 13(5) 12.8 1.537 .ggg those given the M treatment. Also that the band for the 2254:: ,1:7 12:0 11:0 21:0 1104 :237 test at. the higher loading temperature for the same load gg 3.8 .ggg was wider for the bars aged by the M treatment than those 2441:: 11:3 11g 12:3 10:0 0j 11100 aged by the Z teatlrjient thus indicating that the Z aging 2444 .2 9. T treatment is pre era e.

41 4.4 10.0 1.3 11.0 .2 42.; 13 0 18,0 59 9 5 Nif? FIGUREZ 1s a baigraph showing the room tempera- 515g 2.8 100-g Jg .ggg ture short time tensile properties for 6D, M6D and 15D 31411:: 5:0 10:0 118:1 010 :140 'NT 30 alloys aged respectively by the M and Z treatments. The 3.3 lg-g ggg 13g ggg lower block 1n each instance represents the average yield 3154:: 5j 5 1010 03; 0 7j 0 :093 :100 strength obtained from a plurality of test bars as measured g 110g g-g Pfg gi by the 0.2% offset test methods. The upper block in 35512:: 0:0 0:0 87:1 8:0 :20 :087 each case represents the average ultimate tensile strength 151.3 12?(1) 11kg 35 for the same alloys. The width of the blocks represents 214 1010 2115 13.0 1 74 2003 the extent of variation in test results i.e. within the stand- Q- 1113-2 ggg ard 1 sigma (e) range for a plurality of tests of the saine 113 11o 17.2 12 0 133 'NT alloy under the same conditions. The line in the brackets 3-1 10-5 23-5 11-5 307 NT at the bottom of FIGURE 2 is a scale line representing quantitatively the amount of scatter in the test results in TABLE XV Effect of single step aging on short time tensile strength and stress rupture properties of as cast M6D alloy Time to Rupture at 1,200 F. Room Temperature 1,200 F. Under Load Alloy Age Temp., Age Time,

F. hrs.

UTS. 0.2YS Percent UTS 0.2YS Percent 62 64 (K p.s.i.) (K p.s.i.) Elong. (K p.s.i.) (K p.s.i.) Elong. (K p.s 1 (K p.s.i.) (K p.s.i.)

M6D 1,100 100 121.5 76.4 3.5 76.3 43.4 3.5 1,100 200 121. 3 30. 6 3. 0 77. 6 43.1 8.0 1,200 40 113. 2 76. 3 4. 5 73. 3 52. 0 9. 0 1,200 60 115. 7 77.4 3. 5 79. o 50. 3 s. 0 1,200 30 120. 3 79. 6 3. 0 77.4 50. 0 6. 0 1,200 121. 0 79. 1 3. 0 73. 2 51. 0 5. 4

TABLE XVI Eect of single step aging on short time tensile strength and stress rupture properties of as cast 15D alloy Room Temperature 1,200 F. Time to Rupture nt 1,200 F.

Under Load Alloy Age Temp., Age Time,

F. hrs. UTS. 0.2YS Percent UTS 0.2YS Percent 60 62 64 (K p.s.i (K p.s.i.) Elong. (K p.s.i.) (K p.s.i.) Eloiig. (K p.s.i.) (K p.s.i.) (K p.s.i.)

13 thousands of pounds per square inch; the line as shown representing 10,000 p.s.i. By measuring the Width of the blocks in the ligure and comparing with the scale line it is Ipossible to note the amount of scatter in p.s.i.

FIGURE 3 represents a similar type of block showing as in FIGURE 2 but directed to the room temperature short time tensile test elongation in percent obtained with a one inch gage length on tests for the same test bars used in the tests of the alloys reported in FIGURE 2. The scale at the bottom of FIGURE 3 is quantitatively in percent as distinguished from p.s.i. in FIGURE 2.

The signicance of the showing in FIGURE 2 is the strength properties for alloys receiving the Z aging treatment are superior to the same alloys receiving the M aging treatment. With respect to FIGURE 3 the recorded data show that the percent elongation is less for the alloys having the Z aging treatment than the same alloys which received the M againg treatment.

We claim:

1. A process for producing iron base alloy articles having substantial oxidation resistance and high strength at elevated temperatures comprising preparing an iron base austenitic alloy consisting essentially of about 0.6 to 1.6% carbon; about 12 to 35% chromium; up to 2.5% silicon; between 0.2 to 12% of a plurality of carbide forming elements from the group consisting of tungsten in the range 0.1 to 10%, molybdenum in the range 0.1 to 9%, columbium and tantalum, said columbium and tantalum combined being in the range 0.1 to 5%; up to 15% of metal from the group consisting of, nickel in amount up to 15 manganese in amount up to 15% and cobalt in amount up to 8% and up to 0.6% nitrogen; the balance essentially iron, forming an article from said alloy, solution treating the alloy of said article by heating the same for about one-half to one hour at a temperature between about 2200 F. to 2300 F. and then cooling said article and aging said alloy thereof by heating the same for 5 to 100 hours at a temperature between about 1000 F. to 1400 F.

2. A process for producing iron base alloy articles having substantial oxidation resistance and high strength at elevated temperatures comprising preparing an iron base austenitic alloy consisting essentially of about 0.6 to 1.6% carbon; about 12 to 35% chromium; up to 2.5 silicon; between 0.2 to 12% of a plurality of carbide forming elements from the group consisting of tungsten in the range 0.1 to 10%, molybdenum in the range 0.1 to 9%, columbium and tantalum, said columbium and tantalum combined being in the range 0.1 to 5%; up to 15% of metal from the group consisting of, nickel in amount up to manganese in amount up to 15 and cobalt in amount up to 8% and up to 0.6% nitrogen; the balance essentially iron, forming an article from said alloy, aging the alloy of said article by subjecting the same to a plurality of heating steps each at a different temperature between about 1000 F. to 1400 F. and each for a total time not exceeding 60 hours.

3. The process as claimed in claim 2 wherein the article is air cooled between the aging steps.

4. A process for producing iron base alloy articles having substantial oxidation resistance and high strength at elevated temperatures comprising preparing an iron base austenitic alloy consisting essentially of about 0.6 to 1.6% carbon; about 12 to 35% chromium; up to 2.5% silicon; between 0.2 to 12% of a plurality of carbide forming elements from the group consisting of tungsten in the range 0.1 to 10%, molybdenum in the range 0.1 to 9%, columbium and tantalum, said columbium and tantalum combined being in the range 0.1 to 5%; up to 15 of metal from the group consisting of, nickel in amount up to 15 manganese in amount up to 15 and cobalt in amount up to 8% and up to 0.6% nitrogen; the balance essentially iron, forming an article from said alloy, aging the alloy of said article by subjecting the same to a pair of heating steps totaling a time not exceeding 100 hours,

14 each step being at a temperature between about 1000 F. to 1400 F. and -the second step being at a higher temperature than the rst.

5. The process as claimed in claim 4 wherein the article is air cooled between the aging steps.

6. A process for producing iron base alloy articles having substantial oxidation resistance and high strength at elevated temperatures comprising preparing an iron base austenitic alloy consisting essentially of about 0.6 to 1.6% carbon; about 12 to 35% chromium; up to 2.5% silicon; between 0.2 to 12% of a plurality of carbide forming elements from the group consisting of tungsten in the range 0.1 to 10%, molybdenum in the range'0.1 to 9%, columbium and tantalum, said columbium and tantalum combined being in the range 0.1 to 5%; up to 15% of metal from the group consisting of, nickel in amount up to 15 manganese in amount up to 15% and cobalt in amount up to 8% and up to 0.6% nitrogen; the balance essentially iron, forming an article from said alloy, aging the alloy of said article by subjecting the same to a pair of heating steps, the rst at a temperature of about 1200 F. to 1250 F. for 10 to 40 hours and the second being at a temperature between about 1300 F. to 1350 F. for 10 to 60 hours.

7. The process as claimed in claim 6 wherein the article is air cooled between the aging steps.

8. A process for producing iron base alloy articles having substantial oxidation resistance and high strength at elevated temperatures comprising preparing an iron base austenitic alloy consisting essentially of about 0.6 to 1.6% carbon; about 12 to 35 chromium; up to 2.5% silicon; between 0.2 to 12% of a plurality of carbide forming elements from the group consisting of tungsten in the range 0.1 to 10%, molybdenum in the range 0.1 to 9%, columbium and tantalum, said columbium and tantalum combined being in the range 0.1 to 5%; up to 15% of metal from the group consisting of, nickel in amount up to 15%, manganese in amount up to 15 and cobalt in amount up to 8% and up to 0.6% nitrogen; the balance essentially iron, forming an article from said alloy, and aging the alloy of said article by subjecting the same to heating at a temperature of about 1200 F. to 1250 F. for about 20 to 100 hours.

9. A process as claimed in claim 1 where the alloy prior to treatment is in the as cast condition.

10. A process as claimed in claim 1 where the alloy prior to treatment is in a forged condition.

11. A process as claimed in claim 2 where the alloy prior to treatment is in the as cast condition.

12. A process as claimed in claim 2 where the alloy prior to treatment is in a forged condition.

13. The process as in claim 8 wherein the temperature is between 1000 F. to 1400" F. and the time is 10 to hours.

14. A process as claimed in claim 1 wherein the nickel content of the alloy is at least about 2% when the manganese content is less than about 1%, wherein the manganese content is at least between 2 to 10%, when the nickel is less than 2% and nitrogen is present, at least between about 5 to 10% when substantially no nitrogen is present and the nickel is less than about 2% and at least about 10% when the nickel is zero.

15. A process as claimed in claim 2 wherein the nickel content of the alloy is at least about 2% when the manganese content is less than about 1%, wherein the manganese content is at least between 2 to 10% when the nickel is less than 2% and nitrogen is present, at least between about 5 to 10% when substantially no nitrogen is present and the nickel is less than about 2% and at least about 10% when the nickel is zero.

16. A cast article comprising an iron base alloy having substantial oxidation resistance and high strength at elevated temperatures, said alloy consisting essentially of about 0.6 to 1.6% carbon; about 12 to 35% chromium; up to about 2.5% silicon; between 0.2 to 12% of a plu- 15 rality of carbide forming elements from the group consisting of tungsten in the range 0.1 to 10%, molybdenum in the range 0.1 to 9%, columbium and tantalum, said columbium and tantalum combined being in the range 0.1 to 5%; up to 15% of metal from the group consisting of nickel in amount up to 15%, manganese in amount up to 15% and cobalt in amount up to 8% and up to 0.6% nitrogen; the balance being essentially iron; the alloy of said cast product being characterized by a hundred hour rupture strength at 1500 F. of at least about 27,000 p.s.i. and a test bar thereof 1A" in diameter being generally characterized by a structure having an interdendritic network of substantially lederburite phases outlining an austenitic phase embedding relatively line dotlike precipitates randomly distributed therein and substantially concentrated about the interdendritic network of substantially lederburite phases.

References Cited by the Examiner UNITED STATES PATENTS 2,879,194 3/1959 Eichelberger 148-136 2,892,702 6/1959 Walton et al 75-124 10 2,984,563 5/1961 Tanczyn 148-136 X DAVID L. RECK, Primary Examiner.

HYLAND BIZOT, Examiner.

Claims (1)

1. A PROCESS FOR PRODUCING IRON BASE ALLOY ARTICLES HAVING SUBSTANTIAL OXIDATION RESISTANCE AND HIGH STRENGTH AT ELEVATED TEMPERATURES COMPRISING PREPARING AN IRON BASE AUSTENITIC ALLOY CONSISTING ESSENTIALLY OF ABOUT 0.6 TO 1.6% CARBON; ABOUT 12 TO 35% CHROMIUM; UP TO2.5% SILICON; BETWEEN 0.2 TO 12% OF A PLURALITY OF CARBIDE FORMING ELEMENTS FROM THE GROUP CONSISTING OF TUNGSTEN IN THE RANGE 0.1 TO 10%, MOLYBDENUM IN THE RANGE 0.1 TO 9%, COLUMBIUM AND TANTALUM, SAID COLUMBIUM AND TANTALUM COMBINED BEING IN THE RANGE 0.1 TO 5%; UP TO 15% OF METAL FROM THE GROUP CONSISTING OF, NICKEL IN AMOUNT UP TO 15%, MANGANESE IN AMOUNT UP TO 15% AND COBALT IN AMOUNT UP TO 8% AND UP TO 0.6% NITROGEN; THE BALANCE ESSENTIALLY IRON, FORMING AN ARTICLE FROM SAID ALLOY, SOLUTION TREATING THE ALLOY OF SAID ARTICLE BY HEATING THE SAME FOR ABOUT ONE-HALF TO ONE HOUR AT A TEMPERATURE BETWEEN ABOUT 2200*F. TO 2300*F. AND THEN COOLING SAID ARTICLE AND AGING SAID ALLOY THEREOF BY HEATING THE SAME FOR 5 TO 100 HOURS AT A TEMPERATURE BETWEEN ABOUT 1000*F. TO 1400*F.

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