AU600009B2 - Dispersion strengthened alloy - Google Patents

Abstract

A dispersion strengthened powder metallurgical iron-base alloy combines good stress rupture strength and high resistance to oxidation attack at temperatures as high as 1300 DEG C and contains special amounts of chromium, aluminum, a refractory metal dispersoid and preferably titanium in addition to iron. Advantageously, the alloy is prepared by mechanical alloying.

Description

600009 S F Ref: 33592 FORM COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952 COMPLETE SPECIFICATION

(ORIGINAL)

FOR OFFICE USE: Class Int Class Complete Specification Lodged: Accepted: Published: Priority: Related Name and Address of Applicant: Inco Alloys International, Inc.

Huntington West Virginia 25720 UNITED STATES OF AMERICA Address for Service: Spruson Ferguson, Patent Attorneys Level 33 St Martins Tower, 31 Market Street Sydney, New South Males, 2000, Australia Complete Specification for the invention entitled: Dispersion Strengthened Alloy The following statement is a full description of this invention, Including the y best method of performing it known to me/us 5845/3 -1 1 PC-2200/FOR.

Abstract A dispersion strengthened powder metallurgical iron-base alloy combines good stress rupture strengch and high resistance to oxidation attack at temperatures as high as 1300°C and contains special amounts of chromium, aluminum, a refractory metal dispersoid and preferably titanium in addition to iron. Advantageously, the alloy is pref 'ed by mechanical alloying.

-I L ICIIIP-- sllCI~- i l~r_ F_ -IA- PC-2200 FIELD OF INVENTION The present invention is directed to dispersionstrengthened (DS) alloys, and more particularly to oxide-dispersion strengthened (ODS) iron-base alloys which manifest an exceptional degree of resistance to oxidation at temperatures as high as 1300C (approx. 2400*F) whereby the alloys are useful in the production of advanced aircraft gas turbine engine components and in demanding industrial applications.

BACKGROUND OF INVENTION In U.S. Patent 3,992,161 ('161) ODS iron-chromium alloys are described as having very good oxidation resistance coupled with high-strength at elevated temperatures. The results set forth therein reflect a decided improvement over iron-chromium alloys produced by the more conventional melt/ingot processing practices.

More specifically, it was disclosed that the ODS alloys could be produced by the now well known Mechanical Alloying process, a technology developed nearly twenty years ago and described in such U.S. Paterts as 3,591,362 and 3,837,930.

PC-2200 Notwithstanding the virtues of the '161 alloys such materials have be found wanting in certain aerospace and industrial environments. By way of explanation, though the '161 ODS material (commercially contains about 20% chromium, aluminum) exhibits good corrosion and oxidation resistance at, say, up to 1200°C, it is prone to undergo premature slagging attack (formation of low melting point phases/compounds through a chemical reaction with corrosive deposits from and/or the environment per se) and/or accelerated attack upon exposure at higher temperatures after short intervals of time, the failure being of the catastrophic type. In this connection, accelerated oxidation may be considered as the rapid mass change of an alloy by oxidation. The mass change is virtually always dramatically positive if all the oxide is collected and weighed. In undergoing the ravages occasioned by such attack the alloy surface converts to friable iron oxide and iron-chromium spinels.

For example, burner cans in aircraft gas turbine engines of advanced design are currently intended for use at increasingly higher operating temperatures, about 1250 0 C (2308 0 and above, e.g., 1300 0 C (2372 0 Similarly, industrial applications involving intimate contact with such aggressive corrosives as flue dust, fly ash, molten glass, etc. require more oxidation and/or corrosionresistant materials, Apart from the above, what is also required for such applications is a material which offers in addition to high strength at operating temperatures, including stress'rupture and tensile characteristics, sufficient fabricability that it can be formed into flat rolled products such as sheet, strip, etc, which product forms can be formed into tubing, rings, canisters and other shapes. WithuI out fabricability the utility of an ODS material is significantly diminished.

Apart from '161 reference also might be made to the work of Kornilov, "Aluminum in Iron and Steel" by S.C. Case and KR. Van Horn, John Wiley and Sons (1953). Kornilov studied the effect of up to 10% aluminum and up to 5% chromium on scaling losses in both cast and wrought Fe-Cr-Al alloys. Aluminum benefited scaling resistance but seemingly there was little benefit conferred by chromium beyond i i 72' 'U B^ 3the 25% level at 1100-1400 0 C. Nothing in the Kornilkv investigation involved fabricability of an ODS product or manufacture of sheet.

R. Allen and R. Perkins (in a contract report for the Naval Air Systems Command, May 1973) investigated ODS iron-chromium-aluminum-yttrium alloys with 16-25wt% chromium at an aluminum level of 5.7-6.0wt% versus conventional wrought and cast 25wt% Cr/4wt% Al and 15wt% Cr/4wt% Al alloys.

It wa? indicated that such alloys could be extruded but nothing was given in terms of fabricability and the production of, say, the important sheet product form, O10 SUMMARY OF INVENTION ,o It has now been found that certain ODS iron-base compositions having S special and correlated percentagcs of chromium and aluminum and a o. refractory dispersoid afford an outstanding degree of resistance to o oxidation/corrosion such that the alloys can be used in the hot sections of 15 gas turbine enginas, e,g, burner cans, and in industrial applications where aggressive corrosives are encountered, molten glass, flue dust, fly ash, etc.

According to a broad form of the present invention there is provided a powder metallurgi al iron-chromium-aluminium dispersion-strengthened alloy in the form of a flat rolled product characterized by good fabricability and resistance to oxidation at temperatures as high as 1300°C, said product being formed from an alloy consisting of 20 to chromium, 5 to 8wt% aluminium, 0,4 to 0.7wt% Y 2 0 3 0 to 5wt% titanium, O to 2wt% each of zirconium, hafnium, tantalum and vanadium, 0 to 6wt% each 23 of molybdenum and tungsten, 0 to 0,5wt% each of silicon and niobium, 0 to 0,05wt% each of calcium, yttrium and rare earth metals, 0 to 0,2wt% boron, the balance being iron, In the specification and the claims all the percentages relating to composition are by weight unless otherwise stated, NVELTBG .N E AL LY.,A-D.EAMg E Generally speaking, the present invention contemplates dispersion strengthened powder metallurgically produced iron-chromium-aluminum alloys containing 22,5 to 30wt% chromium and 5 to 8wt% aluminum. Where flat rolled products are required, eg., sheet, for Intended use and thus a significant degree of fabricability is necessary, the aluminum content should not exceed 6.25wt% the aluminum should be from 5wt% to 6.25wt%, Advantageously, in this regard, the chromium should be from 23 to 27wt% and j t the aluminum from 5 to 6wt%. The alloys may also contain up to titanium, up to 2wt7 each of zirconium, hafnium, tantalum and vanadium, up 'KWK/599y L~ 1 -4 to 6wt% each of molybdenum and tungsten, up to 0.5wt% each of silicon and niobium, up to 0.05wt% each of calcium, yttrium and rare earth metals, up to 0.2wt% boron and the balance iron.

In carrying the invention into practice, the chromium should not exceed 30wt% to minimize the formation of deleterious levels of topologically close packed (TCP) phases such as sigma, phases which adversely impact mechanical properties. Given cost, there is no significant benefit derived with chromium percentages above about 27wt%, The percentage of chromium can be extended downward to 20wt% where less 6 o10 demanding operational parameters are contemplated but at the risk oxidation resistance will be decreased at a given aluminum level, Aluminum should be from about 5wt% to 8wt% for oxidation and .oo corrosion resistance but as indicated, supra, preferably should not exceed 6wt% when seeking the optimim in terms of fabrication into sheet, strip, 1 etc. Such elements as nickel and cobalt are not required and confer no particular advantage. Carbon need not exceed 0.lwt% though higher percentages can be tolerated. Our investigation has not shown silicon or boron to be particularly beneficial. Boron is thought to be causative of o'" 1 (or a contributor to) distortion when the sheet product form is heat 29 treated at elevated temperatures, It preferably should not exceed O0lwt%.

Such constituents as titanium, zirconium, tantalum, niobium, hafnium, O zirconium and vanadium need not exceed lwt%. Tantalum, for example, at the lwt% level has resulted in a loss of fabricability, It tends to stiffen the alloys of the invention and possibly raises the ductile-brittle trans- 2r formation temperature too much. A range of titanium from 0.2 to O.75wt% is preferred.

The alloys of the invention are most preferably produced by Mechanical Alloying as described in U.S. 3,992,161, incorporated herein by reference, although other dispersoid strengthening powder metallurgy processes may be employed.

To give those skilled In the art a better understanding of the invention the following information and data are presented.

KHK/599y P C- 2200 A series of alloy compositions were prepared using raw material powders namely, elemental. Fe, Cr, Al), master alloy Fe-Cr-Al-Ti) and yttrium bearing oxide (Y 2 0 3 which powders were thereafter blended to produce the chemistries given in Table I.

The powder blends were mechanically alloyed (MA) in high energy ball mills under an argon atmosphere for about 24 hours at a ball-topowder ratio of about 20:1 using steel balls as the impacting! grinding media. The MA powders were screened to remove the coarser particles (above about 600 microns), placed in mild steel cans, sealed and hot compacted by extrusion. The extrusions were decanned and then hot and cold rolled to 1.25 mm (0.05in) thick sheet, the sheet thereafter being subjected to a final anneal which was typically 1315'C (2400'F) for I hour to achieve recrystallization.

TABLE I Composition (Weight Percent) Y 0 Alloy C Si Mn Al Cr Ti P S N 0 Fe 2 3 A 0.016 0.10 0.13 4.36 16.04 0.27 0.011 0.006 0.052 0.21 Bal. 0.27 B 0.020 0.14 0.14 4.36 20.07 0.36 0.007 0.001 0.040 0.18 Bal. 0.36 C 0.023 0.08 0.10 4.27 19.50 0.36 0.006 0.004 0.028 0.20 Bal. r 20 D 0.019 0.09 0.13 4.41 23.50 0.34 0.007 0.007 0.038 0.19 Bal. 0.34 E nd nd nd 4.3 24.0 nd nd nd 0.023 0.28 Bal. F d45 2. d d 00104 G nd nd nd 4.5 25.0 0.5 nd nd 0.051 0.42 Bal. H 0,021 0.16 0.16 6.58 24.73 0.42 0.010 0.005 0.075 0.21 Bal. 0.42 S 25 1 0.030 0.02 5.50 20.93 0.47 0.100 0.62 0.66 NOTE: nd =Not Determined Ba]. balance iron Nominal Standard size specimens were cut from the sheets produced and the ground to approximately 600 grit for use in accelerated oxidation tests. Cyclic oxidation testing was used and this consisted of exposing samples at temperatures of 1200'C, 1250'C and 1300'C in air 5%H 0 for 24 hour cycles then cooled to room temperature and weighed. Results are reported in Tables Il and III.

PC-2200 TABLE II Time (hours) Before Initiation of Accelerated Oxidation At Alloy

A

B

C

D

E

F

G

H

I

1200°C 3216* 4704 4800 4224 3384 3384 4498 8208 4656 1250 0

C

1152 1838** n.d.

1992 1656 1320 1656 3216 3624 1300°C 168 348 n.d.

168 528 148 480 600 576 Average of 2 results Average of 5 results nd not determined In Table III below oxidation to completion the times from initiation of are reported: accelerated TABLE III Time from Initiation of Compositional Variation, Accelerated Oxidation to Wt. Cr/Al Completion* at 1300 C Alloy

B,C

D

20/4.3 23.5/4.4 24.7/6.5 24.7/6.5 less than 2 days 5 days 10 days 21 days at 1200°C ssr r osep n 00411ir iS *Completion defined as Attack over 100% of surface area of specimen.

An examination of the data in Table II and III reflects Wt t-ft that increasing the chromium level from 164i to 2% resulted in some improvement in oxidation resistance at a constant aluminum lovel, Alloy A vs. Alloys B and C, the results being quite poor at the 1300°C test temperature. However, raising the chromium level to 23.5%, Alloy D, did not manifest any significant improvement, particularly at the 1300°C test condition.

K;.ip' 7 Alloys B, and C are representative of a typical '161 composition, 20wt% Cr/4.5wt% Al. At 1300 0 C, the initiation of accelerated oxidation to the point of completion spanned but 2 days. See Table III.

Increasing the chromium content to 24wt% reduced in half the rate of accelerated oxidation (Alloy D, Table III) and increasing the aluminum level from 4.5 to 6,5wt% again markedly reduced the rate of attack (Alloy H, Table III). This pattern of behaviour is of practical importance because a significant reduction in the rate of attack may extend service life to allow a repair operation and, thus, avoid the consequences of a catastrophic failure.

Figures 1-3 illustrate more graphically what happens by increasing the chromium level of a typical commercial '161 alloy which contained, apart from the different chromium levels, 0.02wt%C, 4.5wtAl, 0.3wt%Ti, 0.5wt%Y 2 0 3 incidental impurities, with iron being essentially the balance. At each test temperature of 1200 0 C, 1250°C and 1300°C, the spallation rate (mass change) was greater in respect of the higher percentage of chromium. In accordance with the subject invention, the aluminum content should also be increased, preferably proportionately, to reduce the rate of spallation and ensure better integrity of the alloy composition. This is reflected by Figures 4 and 5 where at a 25wt% Cr level the spallation rate is markedly reduced through the co-presence of an additional 2wt% of aluminum above the '161 alloy, A further practical advantage of the alloys of our invention is that 2* they are deemed to afford improved high temperature oxidation and corrosion resistance in thin gauges in comparison with prior art material. Sheet thickness, for example, of 1.25 mm (0.05 in.) are typical for the ZO Al '161 alloy as commercially produced. In such gauge section there is a propensity to uaiergo accelerated oxidation attack early on for lack of, comparatively speaking, bulk concentraticn of aluminum and chromium atoms available for surface (oxide) protection, Put another way, such accelerated attack can cause pitting, pitting which will penetrate through, for example, sheet, Alloys of the invention offer a higher concentration of reserve aluminum and/or chromium atoms, 'Ath regard to fabricability Figure 6 depicts a general correlation between chromium and aluminum in respect of their combinative effect on bendability, a criterion used to assess fabricabllity. In this connection, sheet specimens approximately 0.05 in, (1 t> thick, 1/2 inch in width and about 2 to 4 inches In lingth were bent over a rod of approximately 0.1 L /5 9 9 y Lp9y 1 inch thick (2 Tests were made in both the longitudinal and transverse directions. The black shaded area is indicative that some cracking was evident from the tests. As can be seen, the standard '161 alloy of 20 Cr/ 4.5 Al is quite fabricable. But at a 30 Cr/4.5 Al level cracking was experienced. Some cracking was noted In the transverse direction with an alloy of approxymately 19wt% chromium and 5.2wt% aluminum. The alloy containing 6.6wt% aluminum and about 25wt% chromium cracked excessively in the transverse direction, the bend angle being less than 50° versus a desired 105" or more. For purposes of fabricability the aluminum content, as noted above herein, advantageously should not exceed 6wr% B and more preferably is not above 5.75wt%.

Apart from flat rolled products, the alloys contemplated herein can be used in hot worked and/or machined bar and other mill product shaped forms including fQrgings and tubing. It may be cost effective, for example, to machine compositions from bar for, say, flame guides or glass extrusion dies.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand.

Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

JLH/599y

Claims (1)

1. 9- The claims delining the invention are as follows: 1. A powder metallurgical iron-chromium-aluminium dispersion-strengthened alloy in the form of a flat rolled product characterized by good fabricability and resistance to oxldation at temperatures as high as 1300 0 C, said product being formed from an alloy consisting of 20 to 30wt% chromium, 5 to 8wt% aluminium, 0,4 to 0,7wt% Y 2 0 3 0 to 5wt% titanium, 0 to 2wt% each of zirconium, hafnium, tantalum and vanadium, 0 to 6wt% each of molybdenum and tungsten, 0 to each of silicon and niobium, 0 to 0.05wt% each of calcium, yttrium and rare earth metals, 0 to 0.2wt% boron, the balance being iron, 2. The alloy of claim 1 where the form of the flat rolled product is a sheet or strip, 3. The alloy of claim 1 or claim 2 containing from 0.2 to 0.75wt% S titanium. 4. The alIloy of any one of claims 1 to 3 in which the aluminum does not exceed 6wt%, The alloy of any one of claims 1 to 4 in which chromium is from 23 to 27wt%, 6. As a new article of manufacture, a metal component in the hot stage section of an aircraft gas turbine engine and formed from the alloy product of any one of claims 1 to 7, The article of manufacture of claim 6 in the form of a burner can, 8, The alloy cf any one of claims 1 to 7 wherein the powder is produced by mechanical alloying, 9, A powder metallurgical Iron-chromium-aluminum dispersion strengthened alloy in the form of a flat rolled product characterized by good fabricability and resistance to oxidation at temperatures as high as 1300 0 C substantially as hereinbefore described with reference to alloy H or I, DATED this EIGHTEENTH day of MAY 19W0 Inco Alloys International, Inc. Patent Attorneys for the Applicant S SPRUSON FERGUSON KWK/599y I