GB2220674A

GB2220674A - Alloys useful at elevated temperatures

Info

Publication number: GB2220674A
Application number: GB8817510A
Authority: GB
Inventors: Chaur-Jeng Wang; Chi-Meen Wan; Chiu-Lung Yao
Original assignee: National Science Council
Current assignee: National Science Council
Priority date: 1988-06-29
Filing date: 1988-07-22
Publication date: 1990-01-17
Also published as: GB8817510D0

Abstract

The invention provides alloys consisting essentially of 20 to 35 wt% of manganese, 5 to 13 wt% of aluminum, 0 to 5 wt% of chromium, 0 to 2.5 wt% of silicon, 0.5 to 1.4 wt% of carbon and balance iron. These alloys may be in the form of casting pieces which may be used at temperatures as high as 1100 DEG C without sacrificing the excellent mechanical performance thereof. They may be further alloyed with a minute amount of other elements such as boron, tungsten, molybdenum, niobium, vanadium, nitrogen, copper, nickel, yttrium, scandium, hafnium and tantalum to meet special requirements for various specific applications, for example in incinerators and furnaces.

Description

NEW ALLOYS USEFUL AT ELEVATED TEMPERATURES Casting pieces which can resist elevated temperatures are desirable for applications such as incinerators, furnaces, reactors and steel refiners. The temperatures in such applications may be as high as 11000C. The design of casting pieces suitable for such applications are subject to the essential requirements of: (1) maintaining excellent mechanical performance; and (2) maintaining hot corrosion resistance under elevated temperatures.

Furthermore, the cost, the constancy of the supply or even the accessibility of the alloying components of the casting pieces are also of primary considerations.

The mechanical failures of casting pieces under elevated temperatures are caused primarily by softening and creep of the casting pieces under such temperatures.

Furthermore, the hot corrosion reaction initiated by hostile atmosphere will further accelerate the rate of mechanical failure.

The requirement for excellent mechanical performance of casting pieces under elevated temperatures varies greatly depending on the function and location of the desired application. For example, it is essential for casting pieces used as gas turbine blades to meet extremely high standard of mechanical properties such as stress, erosion and abrasion . On the other hand, the standards are not so high in "static" applications such as boiler shells or boiler linings. However, no matter how the mechanical performance requirements vary from applications to applications, the ability to resist the hot corrosion potential of the casting pieces in a hostile atmosphere is always absolutely essential.

As is well known in the art, aluminium, chromium and/or silicon are frequently used as alloying components with a view to improve the hot corrosion resistance of alloys for elevated temperature application. The efficacy of these elements is derived from their ability to form protective oxide films of aluminium oxide (A1203), chromium oxide (Cr203) and/or silicon oxide (SiO2), respectively. For some applications of lower temperatures, nickel may be used to provide a relatively weaker protection against hot corrosion by forming a nickel oxide film (NiO) on the surface of the protected material. Nickel oxide film does not provide the same level of hot corrosion resistance as those of the aluminium, chromium and silicon.However, nickel can provide the further benefit of promoting the mechanical performance of the protected materials and thus is also frequently considered as important alloying elements in the design of alloys for elevated temperature applications.

During 1920'-s and 1930's, Fe-Cr-Al alloys had been the major preference for elevated temperature applications.

However, suffering rom the inferior mechanical performance inherent from their ferrite or body-centered-cubic (hereinafter referred to as BCC) structure, these alloys are seldom used nowadays.

Recent commercialized alloys for elevated temperature applications are mainly characterized by containing 10 to 30 wt% of chromium. As previously mentioned, chromium forms a protective layer of Cr203 upon reaction with atmospheric oxygen. However, under temperatures higher than 1000 0C, Cr2O3 will be further oxidized into Cr03 which volatilizes during the course of the reaction and as a consequence the oxide layer will become loosened. For this reason, utilizations for this type of alloys are generally limited to be under 10000C. On the other hand, with such a high chromium content, sigma phase will precipitate in the alloy and make the alloy brittle. To overcome, this deficiency, some ingredients inhibiting the formation of sigma phase must be added.However, while the relatively cheap carbon is frequently used for such purpose in other types of alloys, carbon cannot be used in Fe-Cr alloys of this type because the alloy will be sensitized once the carbon content is higher than about 0.04 wt%. In an alternative, nickel is used instead because nickel can inhibit the formation of sigma phase while at the same time stabilize austenite phase to improve the toughness and other mechanical properties of the alloys. Based on the strategy, many species of Pe-Cr-Ni based alloys have been developed.For example, presently commercially available alloys for elevated temperature applications include wrought alloys such as Ni based super alloy Inconel-718 and Fe-Ni based stainless steel AISI 310, cast alloy such as Ni based super alloy Inconel- 738, Fe-Cr based alloys such as SAE 60410 (JIS SCH-1) and SAE 60446 (JIS SCH-2), and Fe-Ni based alloys such as SAE 60442 (JIS SCH-11), SAE 60316 (JIS SCH-12), SAE 60312 (JIS SCH-13), SAE 60309 (JIS SCH-14) and SAE 60310 (JIS SCH-15). Table I summarizes the compositions of the above mentioned alloys.

As is obvious from the Table, all of these alloys contain a relatively high amount of nickel or chromium or both. Since nickel and chromium are rare and expensive elements, these alloys are all very expensive. Over and above, since nickel and chromium are regarded as strategic material by most countries, they are not easily accessible to those countries which do not such resources. As a consequence, these alloys are extremely expensive to the extent tens or even hundreds times higher than carbon steel. This is why these alloys are priced on a kilogram basis rather than a tonnage basis. This obviously precludes these alloys from mass utilization. Furthermore, the high content of nickel or chromium also aggravate the difficulties in welding and thus further causes an increase in construction costs.

TABLE I Commercialized alloys for elevated temperature applications Designation Ni Cr C Al Mo JIS name Inconel-718 53.0 18.6 0.04 0.4 3.1 Inconel-738 61.0 16.0 0.17 3.4 1.7 AISI 310 19-22 24-26 < 0.08 SAE 60410 < 1.0 12-15 0.2-0.4 SCH-1 SAE 60446 - 25-28 < 0.5 SCH-2 SAE 60442 4-6 24-28 0.1-0.3 SCH-ll SAE 60316 8-12 18-23 0.2-0.4 SCH-12 SAE 60312 11-14 24-29 0.2-0.5 SCH-13 SAE 60309 18-22 24-28 0.3-0.6 SCH-14 SAE 60310 33-37 13-17 0.35-0.75 SCH-15 Facing said economic and availability problems, the art has attempted to find substitutes for chromium and nickel.

As a result, aluminium has been employed to replace chromium to provide the required hot corrosion resistance. Manganese and carbon together have been employed to replace nickel to stabilize the austenite phase so that high toughness can be realized. In particular, the ability of the combination of manganese and carbon to stabilize austenite phase has been proved to be more effective than that of the nickel.

However, these two strategies are conflicting. If alloys of this type contains too much carbon or manganese, the hot corrosion resistance of aluminium oxide will be greatly sacrificed. In many cases the hot corrosion resistance may be 50 times lower. No effective method to resolve this confliction between chemical performance and mechanical performance has been reported. Another problem with this type of alloys is that aluminium oxide is less effective in guarding against hot corrosion than chromium oxide. To overcome this deficiency, silicon has been added to enhance the formation of alpha-Al203 which contributes most to hot corrosion resistance. However, silicon functions only at temperatures around 1100 0C. At lower temperatures, the addition of silicon will not cause any substantial effect.

In conclusion, no alloy having satisfactory hot corrosion resistance and satisfactory mechanical performance simultaneously within the temperature range from ambient to as high as 11000C while being relatively cheap has been disclosed in the art.

In the past research on Fe-Mn-Al based. alloy, a portion of the three-element phase diagram of Fe-Mn-Al was constructed by Koster and Tonn in 1934 (W. Koster & W. Tonn, Arc Eisenhuettenw, 7, 365 (1933)) and then modified by Schmatz in 1959 (D. J. Schmatz, Trans. AIME, 215, 112114(1959)), Chakrabartic in 1977 (D. J. Chakrabartic, Metall. Trans., 8B, 121-123. (1977)) and V. G. Rivlin in l983 (V. G. Rivlin, International Metals Reviews, 28, No. 6, 309337 (1983)). These works, however, were restricted to few temperatures around 10000C and many areas of the phase diagram are still unclear. For example, the solidus line of the phase area around Mn 20 wt% an Al 6 t are still not resolve. The four-element phase diagram of Fe-Mn-Al-C are still lacking.

U.S. Patent No. 3,201,230 issued to Mitchell et al in 1965 discloses an alloy consisting essentially of 14 to 35 wt% of manganese, 3.5 to 8.75 wtE of aluminium, 4.5 to 8.75 wt% of chromium, 0.25 to 1.0 wt% of silicon, 0.25 to 1.0 wt% of carbon and balance iron. The alloy of this patent contains relatively high amount of chromium and thus would be relatively expensive if put into market. As a matter of fact, this patent has never been practiced, even after its expiration date. It is considered that the contents of the patent conflict with certain metallurgical theories and that the disclosed alloys cannot realize the desired efficacy, particularly when the alloys are designed for application under temperatures between 400-1000 C.The reasons for these are: (1) The manganese content of 14-35 wt of said patent is unreasonably wide since, as taught by Koster and Tonn in 1934 (W. Koster & ' W. Tonn, Arc Eisenhuettenw, 7, 365 (1933)), if the manganese content of the patented alloy is higher than 31 wtt, beta-Mn will precipitate around grain bounder of the alloy under 300 to 10000C. On the other hand, if the manganese content is lower than 20 wt%, larger Fe3AlCx carbides will emerge and thus make alloys brittle and consumes the aluminium content o -he alloy. Obviously, he hot corrosion resistance will be reduced rendering the alloys unpractical for elevated temperatures applications if the aluminium is consumed by the formation of Fe3AlCx Carbides.

(2) The high chromium content readily reacts with carbon to form chromium carbide (Cr23C6 or Cr7C3) under application temperatures. The alloy is thus sensitized and its mechanical properties reduced.

Furthermore, if the temperature is higher than 8000C and the chromium content falls between 5 and 8.7z wtss, the chromium content in the alloy will rapidly and massively react with atmospheric nitrogen to form needle-shaped aluminium nitride (Alml). This make the aluminium content insufficient to form a protective oxide layer.

Mitchell's patent, however, does not disclose how this problem could be resolved. In the past, aluminium nitrite had been mistakenly regarded as the internal oxidation of aluminium until the study of Wang et al of Material Science Research Institute of National Chin-hwa University, Taiwan with a scanning transmission eiectron microscope (STEM) overruled the erroneous recognition in 1987.

The report on the findings of Wang et al was published in Journal of Material Science of Great Britain in May, 1988.

The present inl-e-.~io. is superior over all the previously mentioned prior arts in: (1) The manganese content is restricted in a specific range that prove practical for the whole span of desired application temperatures. To suppress the formation or beta-manganese or Fe3AlCx around the marginal area of manganese content, other alloying elements are added to control the morphology and location of the precipitates.

(2) Silicon is added to enhance the hot corrosion resistance.

(3) Auxiliary elements are added to further enhance the mechanical performance of the casting pieces by the precipitation hardening mechanism under elevated temperatures.

(4) Through adequate control of the relative ratios of the alloying elements, the addition of chromium becomes an alternative or even totally unnecessary for certain applications. This greatly reduces the cost of the casting pieces of the present invention.

(5) The casting pieces of the present invention had lower oxidation weight gain under elevated temperatures than any of the previously mentioned prior arts.

The present invention provides new alloys which have excellent hot corrosion resistance and mechanical performance over the temperature range of from ambient to 11000C. The new alloys meet the above requirements with absolute necessity to incorporate expensive or rare elements such as chromium and nickel. Furthermore, the new alloys can be modified to meet special requirements for some specific applications by the addition of auxiliary elements.

The invention provides novel alloys and ready-for-application casting pieces based on Fe-Mn-Al-Cr-Si-C and a process for the preparation thereof.

More particularly, this invention provides novel alloys, especially in the form of ready-for-application casting pieces, consisting essentially of 20 to 30 wt% of manganese, 5 to 13 wt% of aluminium, 0 to 5 wt% of chromium, 0 to 2.5 wt% of silicon, 0.5 to 1.4 wt% of carbon and balance iron. These alloys may be used under temperatures up to as high as 1100 0C without sacrificing the excellent mechanical performance thereof. The alloys of the present invention may be further alloyed with a minute amount of other elements such as boron, tungsten, molybdenum, niobium, titanium, vanadium, nitrogen, copper, nickel, yttrium, scandium, hafnium and tantalum to meet special requirements for various specific applications.

The alloys of the present invention comprise mainly five alloying elements in addition to iron. The individual elements are described in detail below.

Manganese Manganese is used in the present invention as an austenite-stabilizer to improve the toughness of the alloy by stabilizing the austenite structure of the alloy.

Manganese is readily oxidizable under temperatures higher than 750 C. This problem is circumvented by addition of silicon as described below.

As previously mentioned, the corrosion resistant will be lowered in prior alloys if the manganese content is too high. This problem has been resolved by adjusting the relative ratios of manganese, carbon and aluminium in the present invention.

The ready-for-application casting pieces of the present invention comprise from 20 to 35 wt%, preferably from 20 to 32 t, and most preferably from 24 to 30 wt% of manganese.

Carbon Carbon is also used in the alloy of the present invention as an austenite-stabilizer to improve the mechanical performance of the alloy of the present invention. In case where any carbide-former such as niobium and molybdenum is added, the carbon content must be proportionally increased.

As previously mentioned, the corrosion resistant will be lowered in prior alloys if the carbon content is too high. This problem has been resolved by adjusting the relative ratios of manganese, carbon and aluminium.

The ready-for-application casting pieces of the present invention comprise from 0.5 to 1.4 wt%, preferably from 0.5 to 1.2 wt%, and most preferably from 0.5 to 11 wt% of carbon.

Aluminium Aluminium can be oxidized at elevated temperatures to form aluminium oxide layer to provide the major hot corrosion resistance. Aluminium is also used in the alloy of the present invention as a ferrite-former, and thus the relative ratios of aluminium with manganese and other alloying elements must be adequately balanced. Adequately arranged re' 2~ e ratios of other elements can provide a most effective aluminium oxide protection layer and develop ordered Fe3AlCx carbides within the matrix to further strengthen the alloy.

The ready-for-application casting pieces of the present invention comprise from 5 to 13 wt%, preferably from 8 to 12 wt%, and most preferably from 8 to 11 wt% of aluminium.

Chromium Chromium can be oxidized at elevated temperatures to form chromium oxide layer to provide the further hot corrosion resistance. The corrosion resistance provided by chromium oxide is more effective at room temperature.

Chromium is a ferrite-former and also an effective carbide former. To prevent the adverse effect of chromium on the mechanical performance of the alloy, care must be taken in adjusting the relative ratio of chromium.

The ready-for-application casting pieces of the present invention comprise from 0 to 5 wt%, preferably from 1 to 5 wt%, and most preferably from 2 to 4 wt% of chromium.

Silicon Silicon can form protective silicon oxide layer spontaneously. Because the oxidation rate and . the dissociation pressure of oxidation of silicon is lower than manganese oxide (MnO), silicon is used in the alloy of the present invention mainly as an inhibitor to the oxidation of manganese ad the nucleation and growth of manganese oxide.

As a result the chance of forming aluminium oxide is greatly enhanced. Silicon is, however, a strong ferrite-former and will greatly reduce the weldability of the casting pieces and thus its relative ratio must be carefully designed taking into consideration.

The ready-for-application casting pieces of the present invention comprise from 0 to 2.5 wt%, preferably from 0.5 to 2 wtt, and most preferably from 0.5 to 1.5 wtZ of silicon.

Auxiliary Elements In addition to the above essential alloying elements, the casting pieces of the present invention may be further alloyed with a minute amount of other elements such as boron, tungsten, molybdenum, niobium, titanium, vanadium, nitrogen, copper, nickel, yttrium, scandium, hafnium and tantalum to meet special requirements for various specific applications. The addition of these auxiliary element is well know in the art and is listed to show other -aspects contemplated by the present invention. The individual auxiliary elements are described below.

Boron The addition of boron will decrease the surface free energy of grain boundary which will further deduce the carbides precipitate in grain boundary. A better mechanical property will be achieved for the homogeneous precipitation in the matrix with the boron addition.

The casting pieces of the present invention may optionally comprise from 50 to 200 ppm of boron.

Tungsten, Molybdenum and Niobium These elements are used in the alloy of the present invention to form fine carbide strengthening alloy, to enhance the erosion resistance, to interfere the diffusion of carbon and to prevent the damage of protective oxide layer caused by the decarburation.

The casting pieces of the present invention may optionally comprise from 0.1 to 1.0 wt% of tungsten.

The casting pieces of the present invention may optionally comprise from 0.1-to 2.1 wt% of molybdenum.

The casting pieces of the present invention may optionally comprise from 0.1 to 1.0 wt% of niobium.

Titanium and Vanadium The function of titanium and vanadium is similar to those of tungsten, molybdenum and niobium. In addition, titanium and vanadium can form minute nitride to interfere with the damage caused by nitridation and divert the nitridation into an assistance to surface hardening.

The casting pieces of the present invention may.

optionally comprise from 0.1 to 2.0 wt% of titanium.

The casting pierces of the present invention max- optionally comprise from 0.1 to 1.0 wtt of vanadium.

Nitrogen Nitrogen is also a strong austenite-former. With the addition of nitrogen, the austenite phase will be stabilized with less amount of carbon by which a better hot corrosion resistance is achieved.

The casting pieces of the present invention may optionally comprise from 0.05 to 0.2 wtt of nitrogen.

Copper Copper may be used to enhance the corrosion resistance at lower temperatures. Copper can also used to form a compound with residual -phosphor in the casting pieces to prevent hot shortness caused by residual phosphor.

The casting pieces of the present invention may optionally comprise from 0.5 to 1.5 wt% of copper.

Nickel The compound of phosphor and copper as described above will still initiate hot shortness at temperatures. Nickel can be used to prevent such problem. Furthermore, nickel can be used to improve the luster of the alloy. Excessive nickel will form the metallic compound gamma' phase with aluminium which is harmful to alloy. The added amount of nickel and copper must be carefully adjusted in accordance with the residual phosphor content.

The casting pieces of the present invention may optionally comprise from 0.5 to 2.5 wt% of nickel.

Yttrium, Scandium, Hafnium and Tantalum Due to . their strong oxidation potential, these rare elements may be added to initiate internal oxidation within the alloy to absorb the vacancy formed during the formation of oxide film, to provide a key mechanism, and to enhance the adhesion of oxide film by providing an interlayer mechanism, such that the oxide film is not easily peeled off during the course of repeated rising and falling of temperature.

The casting pieces of the present invention may optionally comprise from 0.01 to 1.0 wt% of yttrium.

The casting pieces of the present invention may optionally comprise from 0.01 to 1.0 wt% of scandium.

The casting pieces of the present invention may optionally comprise from 0.01 to 0.5 wt% of hafnium.

The casting pieces of the present invention may optionally comprise from 0.01 to 0.8 wtZ of tantalum.

The following examples are offered to aid in understanding of the present invention and are not to be construed as limiting the scope thereof. Unless otherwise indicated, all parts and percentages are by weight.

EXAMPLE I This example exemplifies the method of preparing the casting pieces of the present invention.

A mixture of iron, manganese, aluminium, silicon and carbon was molten by a high frequency induction furnace to produce a melt of known composition. The melt was then cast with molds of different materials to produce casting pieces of different surface conditions.

Melting was conducted with a high frequency induction furnace with a capacity of 20 Kg. The composition, form and weight ot the alloying elements are shown in Table II.

TABLE II Element Composition Weiglit-Form wt% Kg Manganese 30 4.5 Commercial pure manganese Aluminium 10 1.5 Commercial pure aluminium Chromium 3 0.45 Commercial pure chromium Silicon 1 0.3 Ferrosilicon containing 50% Silicon Carbon 1 0.15 Active carbon powder Iron balance 8.1 1008 coil wire with 0.08% carbon Iron and ferrosilicon were fed into the furnace when cold . Power of the furnace was set as 20 kW and maintained for 10 minutes, after which the power was adjusted to 40 k.

7 minutes later, the iron and ferrosilicon melted completely at about 15500C. Power was then adjusted to 20 k, 2.25 Kg of manganese and all chromium were added, and then power was adjusted back to 40 kW. After the added manganese and chromium were melted, a further 2.25 Kg of manganese and 0.15 Kg of carbon were added. 10 minutes later the slag was removed. 1.5 Kg of aluminium was added. Slag forming and deslag operation were conducted after aluminium melted.The melt was decanted into a ladle with 30 Kg capacity after the temperature reached 1580 0C and then cast with four separate molds of different materials after the temperature lowered to 1480 OC . The fours molds are shell mold, sand mold, ceramic mold and steel mold respectively with capacities of 4 kilograms.

The surface conditions of the resulting casting pieces were observed. In regard of the shapes of the products, best results were observed with shell mold and then sand mold, ceramic mold and steel mold in the stated sequence.

In regard of the smoothness of the surface, best results were observed with steel mold and the shell mold, ceramic mold and sand mold in the stated sequence. However, all casting pieces are considered acceptable for application.

The casting piece cast with the ceramic mold was homogenized at 12000C for 4 hours and a sample was taken by drilling for chemical analysis. The result of the analysis is listed in Table III.

Table III The composition of the casting pieces in Table II as analyzed b chemical method Mn Al Cr Si C Fe 28.67 9.33 3.14 1.12 0.94 balance Example 2 A series of casting pieces within the context of the present invention were produced in a similar method as Example i and the mechanical properties (tensile strength, elongation and hardness) of the alloys were tested and compared with conventional casting pieces form elevated temperature applications.

17 casting pieces made of Fe-Mn-Al-C based alloys optionally added with other alloying elements such as niobium, molybdenum and nitrogen was prepared with the same procedure as described in Example 1.

The melt was them cast with ceramic mold to give cylindrical ingots of 80 mm in diameter, 250 mm in length and about 15 Kg weight. The ingots were cut into a cylinder of 4 cm height and then forged at 12000C until the height was reduced to 2 cm. The forged ingots were then cooled to room temperature and cut into testing samples of gage length of 1 inch for tensile property test. The tensile strength, elongation and hardness of the testing samples were tested and recorded as in Table IV. Uniform deformations were observed for all the testing samples during the course of the test. The necking rates are relatively low. Some typical mechanical properties of the conventional casting pieces for elevated temperature application as listed in Table I were listed in-Table V for comparison purposes.As shown by the comparison between Tables IV and V, The mechanical performance of the casting pieces of the present invention is superior to that of convention commercially available casting pieces for elevated temperature applications. In particular, the casting pieces of the present invention shows high elongation and toughness than that of the specification standard of AISI 310 which is working grade steel for elevated temperature application. TABLE IV Composition (wt%) tensile elon- hard No.Mn Al C Cr Nb Mo other Fe strength gation ness (Ksi) (%) &num;1 33.04 6.45 0.72 - - - - balance 117.2 45.6 65.9Rb &num;2 33.13 7.54 0.58 - - - - " 111.2 46.6 64.7Rb &num;3 28.78 7.83 0.6 - - - - " 117.8 48.2 59.5Rb &num;4 25.6 11.24 1.24 - - - - " 170.4 6.35 44.5Rb &num;5 25.3 6.72 0.46 5.52 - - - " 105.0 33.85 &num;6 27.2 7.22 0.79 5.92 - - 2.0Ni " 127.0 43.33 98.6Rb &num;7 27.8 7.24 0.41 5.89 - - - " 129.6 40.34 96.0Rb &num;8 32.7 7.36 0.88 5.73 - - - " 122.6 45.9 96.5Rb &num;9 25.1 8.1 0.74 5.54 - - - " 121.6 42.2 &num;10 30.51 10.99 0.78 - - - 1.40Si " 108.7 41.4 98.0Rb &num;11 30.3 7.6 1.08 - - 0.46 - " 144.85 29.85 30.4Rc &num;12 25.2 8.02 1.02 - - 0.66 - " 157.3 26.5 32.8Rc &num;13 29.4 10.9 1.14 - 0.05 - - " 177.5 13.3 36.4Rc &num;14 30.2 11.1 1.13 - 0.1 - - " 161,0 15 36.2Rc &num;15 33,9 11,0 1.19 - 0.3 0.76 - " 165.2 19.3 39.3Rc &num; ;16 30.7 10.9 1.09 - 0.1 - 0.004N " 158.0 12.9 36.5Rc &num;17 28.87 10.22 .72 - - - 0.02B " 123.2 23.7 2.0Ni Note: 1. Ksi x 0.70325 = kg/mm 2. Ksi x 6.8965 = MPa TABLE V Typical value of mechanical properties of commercially available high-tem?erature resistant alloys Designation Tensile strength elongation utility (Ksi) AISI 310 > 70 > 45 processing JIS SCH-ll > 85 Not specified casting JIS SCH-12 > 70 JIS SCH-13 > 70 " 'I JIS SCH-14 > 65 JIS SCH-15 > 57 Example 3 Six forged ingots of =10 alloy in the previous example prepared with the same procedure were cold-rolled with a 90% reduction in dimension to a thickness of 0.2 cm.The rolled pieces were then solution treated for 1 hour at 10500C and quenched into gear oil to room temperature. Five of the six rolled pieces were aged at 5500C for different time. The tensile strength, elongation and hardness of the six rolled pieces were tested with the result listed in Table VI obtained. As shown by Table VI, the tensile strengths remarkably increased, the elongation decreased by a limited level and the hardness increased. The rolled pieces were observed through an optical microscope. It is observed through microstructure analysis that the enhancement in mechanical performance was a result of the formation of minute Fe-Al-C carbides (Fe3AlCx).It was thus proved that the mechanical performance of the casting pieces of the subject invention is improved by controlling the morphology of the precipitates at elevated temperatures.

TABLE VI Mechanical properties of &num;10 alloy after aging Treating procedure Time tensile elongation hardness strength Ksi No aging - 127.7 51.4 54.8Ra With 550 0C aging 1 hr 138.5 39.6 64.5Ra With 5500C aging 4 hr 153.7 39.6 63.0Ra with 550 C aging 8 hr 158.3 37.4 64.9Ra with 550 C aging 16 hr 151.7 44.0 65.0Ra Example 4 This example illustrates the hot corrosion resistance of the alloys of the present invention.

A series of casting pieces with the composition listed in Table VII were prepared by melting and forging at 12000C and homogenization at 10500C for 8 hours as disclosed in Example 1, 2 and 3. The resulting pieces were then cut.into testing samples of 2x4x8 mm dimension. The surface of the testis samples were abraded with No. 1200 SiC abrasive paper. The abraded samples were then placed in a thermogravimetric analyzer (TGA) to test their oxidation weight gain. The dry air flow rate of the TGA was set at 100cc/min., the temperatures were set at 700, 900 and 1100 C and the experiment time was set at 24 hrs. The oxidation weight gains of these testing samples were listed in Table VIII.

TABLE VII Compositions of the alloys of Example 4 (wt%) No. Al C SFe 18 5.5 0.55 balance 19 8.5 0.65 balance i20 10.0 0.52 balance TABLE VIII Oxidation weight gain of the alloys of Example IV after 24 hours of oxidation at 700, 900 and 11000C (mg/cm2) No. 7000C 9000C 11000C &num;18 1.71 5.68 10.76 &num;19 1.19 0.46 0.89 &num;20 < 0.05 0.29 1.22 As shown by Tables VII and VIII, the addition of aluminium can inhibit the adverse effect of carbon to the corrosion resistance of the casting pieces of the present invention. This examples proved the disclosure of the invention that the corrosion resistance of carbon-containing alloys can be controlled by addition of aluminium.

Example 5 A series of casting pieces of composition listed in Table IX was prepared with the the procedure as disclosed in Example I and 2. These casting pieces were then processed as disclosed in Example 4 and places in a tube furnace set at 400 to 7000C together with a casting piece of AISI 304.

A dry air was introduced into the tube furnace at a flow rate of 200cm3/min to oxide the testing pieces. After 24 hours of oxidation, these testing pieces were air cooled to room temperature and the surface conditions of the testing pieces were observed. It was observed that the color of AISI 304 changed darker and darker as the temperature rose, and turned completely black when the temperature is higher than 6000C. On the contrary, the casting pieces of the present invention remained lustrous while being somewhat yellow at higher temperatures. At temperature rose higher, the color of the ca-sting pieces of the present invention changed from yellow to brown. No peeling was observed on either the casting pieces of the present invention or AISI 304.

The same procedure with the exception that the temperature was set at 11000C was repeated. No peeling-off and no apparent formation of oxide were observed on the surface of the casting pieces of the present invention. The color of the casting pieces of the present invention was lustrous brown. On the contrary, black homogeneous oxide layer was observed on the surface of AISI 304 and after being cooled to room temperature cracks were observed too.

This example proves the disclosure that excellent corrosion resistance can be realized by adjusting the composition of alloys even the carbon or manganese content is notably high.

TABLE IX Mn Al C Fe 28.3 11.3 0.5 balance Example 6 A series of chromium-containing casting pieces of composition listed in Table X were prepared as in Examples 1 and 2 and then cold-rolled with a 90% reduction in thickness to 2mm. The resulting pieces were then homogenized at 1150 0C for 50 hours to eliminate completely the working stress and promote complete growth of crystal grains. The rolled pieces were then cut, ground and polished up to 0.05 um aluminium oxide powder to give testing samples for oxidation test. The same procedure and apparatus were adopted to conduct the oxidation test with the exception that the oxidation temperature was set at 10500C and the rate of temperature rise from ambient temperature was set at 1000C per minute.The thermogravimetric analysis was conducted for 48 hours at 10500C. It was recorded that the total oxidation weight gains was only 0.42 mg/cm3. It was noted that the increase in oxidation weight gain was observed only in the leading 3 hours and no further increase in oxidation weight gain was observed during the later course. Analysis with X-ray analyzer and electron microprobe revealed that aluminium oxide crystal had grown completely during the starting 3 hours under above testing conditions. The testing samples were air cooled to room temperature and observed with the results that no peeling off of oxides were observed and the color of the tested samples changed to brownish silver.

It is evidenced by this Example that adequate addition of chromium to the casting pieces of the present invention and adequate control of the rate of temperature rise give a intact aluminium oxide protection layer which does not peel off after cooling.

TABLE X Composition of Fe-Mn-Al-Cr-C alloy in Example 6 No. Mn Al Cr C Fe &num;21 30.95 8.87 2.98 0.87 balance Example 7 A series of Fe-Mn-Al based alloy with compositions listed in Table XI was prepares as with the same procedure as described in Example 1. The resulting casting pieces were then homogenized at 12000C for 4 hours followed by hotrolling at 900-10000C. The rolled pieces were treated with solution treatment at 10500C for 1 hour and then oil quenched. The resulting pieces were cut to a dimension of 8x8x10 mm and abraded with X1200 SiC abrasive paper to give testing pieces for hot-corrosion test.The hot-corrosion test was conducted by dipping these casting samples of into commercially available Q-6 heat treatment salt solution containing essentially BaC12, KC1 and Licl for 268 hours.

The thickness of the corrosion product on 24 was only 30 um and it was observed that the corrosion proceeded along the ferrite phase.

It is evidence by this example that the casting pieces of the present invention show hot-corrosion resistance.

TABLE XI Compositions of Fe-Mn-Al-C based alloys in Example 7 No. Mn Al C Fe X22 32.3 9.7 0.03 balance &num;23 30.4 9.8 0.1 balance &num;24 30.8 9.8 1.0 balance Example 8 In areas where garbage classification is ' not prevailing, all kinds of garbage containing both acidic and basic wastes are sent into a garbage inzinerator. Analysis of samples taken from a garbage incinerator in Taiwan reveals that the garbage send to an incinerator contains on the average 56% of water, 15% of dust, 15% of carbon, 118 of oxygen, 0.1% of sulfur and other unidentified materials.

Due to its high water content,garbage in these areas cannot burn spontaneously and thus fuel is frequently sprayed to the garbage to facilitate incineration. It is considered that the environment within such incinerator presents a most hostile threat to alloys for elevated temperature applications.

A series of cast pieces of grates for incinerator made of Fe-Mn-Al-C-Cr-Si based alloys with compositions listed in Table XII were prepared by melting electrolytic pure raw materials in a high frequency induction. furnace with a capacity of 500 Kg and casting the resulting melt with a sand mold to give the desired base plates. Two types of incinerator grates were made, one fixed grates for supporting incinerator residue and the other movable grates for supporting burning waste and allowing hot air ventilation. Both two types of grates bear heavy weight load in corrosion atmosphere in burning temperature. In addition, these grates are required to resist prolonged erosion caused by transportation of trash. The movable grates which weigh about 12.5 Kg have a dimension of 398 mm in length and 245 mm in width.Square holes of 60 mm x 12 mm dimension distributes evenly over the whole grates to allow the falling of incinerator residue during inzineration and the ventilation of hot air. Fixed grates which are similar in shape to movable grates have a dimension of 398 mm in length and 289mm in width. The working temperature of these grates ranges between 700 and 95 OOC. These grates were tested in the trash incinerator for 60 days and 1 1/2 years during which terms the incinerator was fed with the constant load of 200 tons of trash per day. The thickness of reactant layer after the test was recorded as in Table XIII. It is evidence by Table XIII that the incinerator grates made of Fe-Mn-Al-C based alloys of the present invention meet the quality standard for incinerator grates which is made of high chromium cast steel.If chromium, silicon or titanium is further added, the performance of the grates will be further enhanced.

Table XII Compositions of Fe-Mn-Al-C based alloys in Example 8 No. Mn Al C Cr Si Ti Fe &num;24 As in Example 7 &num;25 29.53 9.5 1.31 3.0 0.8 - balance 826 30.12. 9.4 1.24 5.96 0.7 - " &num;27 29.45 10.2 1.28 2.98 1.02 0.3 TABLE XIII Reactant thickness of grates in incinerators No. First sampling Second sampling (2 months) (18 months) JIS SCH-ll 150 um 240 um &num;24 70 um 125 um =25 25 um 35 um i26 320 um damaged 27 18 um 26 um Example 9 A series of 15 Kg ingots made of Fe-Mn-Al-C based alloy of compositions listed in Table XIV were prepared by melting raw materials in a high frequency induction furnace and casting in ceramic molds.The melting raw materials are all in the form of electrolytic pure material with the exception that the manganese may be provide by low carbon ferromanganese (as noted by * mark in Table XIV). These ingots were cut and the hardness of the cut faces were measured. Part of the ingots were further cut into the testing samples for tensile test as in Example 2. The resulting hardness values and tensile strength values were listed in in Table XV.

It is evidenced by Table XV that the casting pieces of present invention present higher tensile strength and hardness than commercialized AISI series stainless steel even in the ingot form. In the working procedures, the casting pieces of the present invention also present excellent toughness. For example, an adequate combination of tungsten carbide drilling head, a high power driller and suitable drilling oil is required to drill the casting pieces of the present invention. In a test a saw of JIS SK series alloy was used to saw the &num;31 alloy of this example and the saw turned eroded after it sawed only 5 mm deep.

These phenomena reveals that the casting pieces of the present invention can hardly be further worked as casting.

TABLE XIV Compositions of Fe-Mn-Al-C based alloys in Example 9 No. Mn Al C Cr Mo Nb Ni B V Fe X28 31.92 10.44 1.10 - - - - - -bal.

&num;29 32.66 10.53 1.07 - 0.36 - - - &num;30 32.81 10.53 0.98 - 0.72 - - - &num;31 32.27 10.53 1.03 - 0.35 0.097 - - - " X32 32.99 10.67 1.03 - - 0.16 - - 0.298 &num;33 31.45 11.45 1.10 - - - - - - &num;34 31.64 11.14 1.11 - - - 1.9 - &num;35 26.78 9.34 1.11 - - - - 0.026 - " &num;36 33.79 10.39 1.07 - - - - 0.0284 - " &num;37 28.43 10.45 1.17 - - - - - &num;38 26.52 10.52 1.12 2.76 - 0.26 1.8 n - &num;39 27.64 10.31 1.05 3.15 - 0.3 1.8 0.0167 &num;40* 28.18 9.89 0.96 3.06 - 0.29 1.9 - * :manganese provided by ferromanganese TABLE XV Hardness and tensile strength of the alloys listed in Table XIV in ingot form No. Hardness (Rc) Tensile strength (Ksi) &num;28 32.8+0.9 &num;29 31.2#0.8 &num;30 30.5+0.7 &num;31 30.4+1.0 &num;32 31.4#0.4 &num;33 30.4+1.0 &num;34 30.1+2.3 &num;35 37.5#0.8 &num;36 35.7#0.9 &num;37 36.2+2.1 115+3 &num;38 38.0+1.4 80+5 &num;39 36.7+0.8 84+4 &num;40 31.1+1.2 117+6 While-the invention has been described with respect to certain preferred exemplifications and embodiments, it is not intended to limit the scope of the invention thereby, but solely by the claims appended hereto.

Claims

1. An alloy suitable for use at elevated temperatures consisting essentially of from 20 to 35 weight percent of manganese, from 5 to 13 weight percent of aluminium, from 0 to 5 weight percent of chromium, from 0 to 2.5 weight percent of silicon, from 0.5 to 1.4 weight percent of carbon and balance iron.

2. An alloy as claimed in claim 1 consisting essentially of from 20 to 32 weight percent of manganese, from 8 to 12 weight percent of aluminium, from 1 to 5 weight percent of chromium, from 0.5 to 2 weight percent of silicon, from 0.5 to 1.2 weight percent of carbon and balance iron.

3. An alloy as claimed in claim 1 consisting essentially of from 24 to 30 weight percent of manganese, from 8 to 11 weight percent of aluminium, from 2 to 4 weight percent of chromium, from 0.5 to 1.5 weight percent of silicon, from 0.5 to 1.1 weight percent of carbon and balance iron.

4. An alloy as claimed in any one of claims 1 to 3 further comprising one or more auxiliary elements selected from boron, tungsten, molybdenum, niobium, titanium, vanadium, nitrogen, copper, nickel, yttrium, scandium, hafnium and tantalum.

5. An alloy as claimed in claim 4 comprising from 50 to 200 ppm of boron.

6. An alloy as claimed in claim 4 comprising from 0.1 to 1.0 weight percent of tungsten.

7. An alloy as claimed in claim 4 comprising from 0.1 to 2.1 weight percent of molybdenum.

8. An alloy as claimed in claim 4 comprising from 0.1 to 1.0 weight percent of niobium.

9. An alloy as claimed in claim 4 comprising from 0.1 to 2.0 weight percent of titanium.

10. An alloy as claimed in claim 4 comprising from 0.1 to 1.0 weight percent of vanadium.

11. An alloy as claimed in claim 4 comprising from 0.05 to 0.1 weight percent of nitrogen.

12. An alloy as claimed in claim 4 comprising from 0.5 to 1.5 weight percent of copper.

13. An alloy as claimed in claim 4 comprising from 0.5 to 2.5 weight percent of nickel.

14. An alloy as claimed in claim 4 comprising from 0.01 to 1.0 weight percent of yttrium.

15. An alloy as claimed in claim 4 comprising from 0.01 to 1.0 weight percent of scandium.

16. An alloy as claimed in claim 4 comprising from 0.01 to 0.5 weight percent of hafnium.

17. An alloy as claimed in claim 4 comprising from 0.01 to 0.8 weight percent of tantalum.

18. An alloy as claimed in claim 1 substantially as described in any one of the foregoing Examples.

Amendments to the claims have been filed as follows

5. An alloy as claimed in claim 4 comprising from 50 to 200 ppm of boron.

11. An alloy as claimed in claim 4 comprising from 0.05 to 0.2 weight percent of nitrogen.