WO1995023776A1

WO1995023776A1 - 'iron aluminide alloy' reinforced composite materials

Info

Publication number: WO1995023776A1
Application number: PCT/AU1995/000103
Authority: WO
Inventors: William Sinclair; Neil Blake
Original assignee: Bhp Steel (Rp) Pty. Ltd.
Priority date: 1994-03-02
Filing date: 1995-03-01
Publication date: 1995-09-08
Also published as: ZA951690B; AUPM417294A0

Abstract

A composite material suitable for use in high temperature applications is disclosed. The complete material comprises a refractory material reinforced with an iron aluminide alloy. The term 'iron aluminide alloy' means alloy base on Fe3Al.

Description

"IRON ALUMINIDE ALLOY" REINFORCED COMPOSITE MATERIALS

The present invention relates to a composite material suitable for use in high temperature applications.

It is known to use refractory materials in high temperature applications, such as the linings of furnaces, coke oven doors, injection lances, soaking pit components, and ladle lip segments.

It is also known to reinforce refractory materials with stainless steel fibres to improve the mechanical strength and thermal shock properties of the refractory materials so that the refractory materials have increased resistance to cracking and thermal spalling under severe service conditions. The improvement in these properties which is obtained with stainless steel fibres is particularly important in order to extend as long as possible the useful life of the refractory materials before replacement becomes necessary. The grades 310 and 446 are the stainless steel grades that are most widely used as reinforcing fibres for refractory materials. However, the upper operating temperature of these grades is limited to 1150°C, which is relatively low in the context of high temperature applications, such as the linings of furnaces, since above this temperature there is rapid oxidation of the stainless steel.

An object of the present invention is to provide a composite material which has properties, including oxidation resistance, mechanical strength and thermal shock properties, which are at least comparable to known stainless steel fibre reinforced refractory materials.

According to the present invention there is provided a composite material comprising a refractory material reinforced with an iron aluminide alloy.

The term "iron aluminide alloy" as used herein is understood to mean alloys based on Fe₃Al type compositions. t is noted that the alloys may be ordered intermetallic compounds with a D0₃ structure type or may be a disordered alloy.

It has been found by the applicant that the composite material of the present invention has oxidation resistance, mechanical strength and thermal shock properties which are at least comparable to conventional stainless steel fibre reinforced refractory materials at high temperatures. This was an unexpected outcome because, whilst iron aluminide alloys are known to have good oxidation resistance at elevated temperatures up to 1000°C, they are generally regarded to have poor ductility at room temperature which has been attributed to hydrogen e brittlement. This poor ductility has significantly limited the use of iron aluminide alloys. The applicant has found in experimental work that in the application of iron aluminide alloys to reinforcing refractory materials the brittle failure problem appears to have been substantially reduced after firing the composite material. This was unexpected as the mix with refractory materials before firing contained substantial amounts of water which was expected to make worse the hydrogen embrittlement problem.

The refractory material may be any suitable material.

It is preferred that the iron aluminide alloy comprise iron and 8 to 20 wt% aluminium.

It is preferred that the composite material comprise up to 16 vol% (approximately 29 wt%) of the iron aluminide alloy.

The iron aluminide alloy may comprise other elements, such as chromium, niobium, and silicon, in relatively small quantities.

Typically the iron aluminide alloy comprises up to 12 wt% chromium. It is preferred that the iron aluminide alloy comprises at least 5 wt% chromium.

Typically, the iron aluminide alloy comprises between 1 and 3 wt% niobium.

Typically, the iron aluminide alloy comprises between 1 and 3 wt% silicon.

The iron aluminide alloy may also comprise other elements, such as cerium and yttrium, in very small quantities, typically up to 1 wt%. It is preferred that the iron aluminide alloy be in the form of fibres.

The fibres may be manufactured by any suitable means.

For example, the fibres may be manufactured by direct casting or by chopping fibres from a strip which has been direct cast or manufactured by hot or cold rolling.

According to the present invention there is also provided a process for forming a composite material comprising a refractory material reinforced with an iron aluminide alloy, the process comprising:

(a) casting a mixture of iron aluminide fibres and a refractory material;

(b) curing the cast composite material;

(c) drying the cured composite material; and

(d) firing the dried/cured composite material.

It is preferred that the curing step (b) be carried out at ambient temperature.

It is preferred that the drying step (c) be carried out at a temperature in the range of 100 to 150°C.

It is preferred that the firing step (d) be carried out at a temperature of at least 900°C, more preferably 1000°C, typically 1100°C. This firing can be carried out prior to or during use.

The present invention is described further by reference to the following Examples. EXAMPLE 1 .

An experimental program to evaluate the composite material of the present invention was carried out on a standard refractory material (BHP Refractories Pty Ltd product designated 1600AR) reinforced with fibres of (i) iron aluminide alloy (hereinafter referred to as "iron aluminide") and (ii) stainless steel (grades 310 and 446).

The 160OAR refractory material has a maximum service temperature of 1600°C and has the following composition:

A1₂0₃ 54-57 wt%

Si0₂ 34 -37 wt%

Fe₂0₃ 0 . 5-1 . 0wt%

Alkalis 0 . 5-1 . 5wt%

The iron aluminide fibres were produced from

60 kg ingots of iron aluminide having low chromium and high chromium compositions as set out below in Table I.

Table 1

Grade Al (wt%) Cr (wt%) Fe (wt%)

Low Cr 10 .8 5 .1 84 .1

High Cr 9 . 8 10 .1 80 .1

The ingots were cast from a melt made up from electrolytic iron and pure aluminium and chromium in a vacuum induction furnace.

Slices of iron aluminide were cut at 12 mm thickness from the ingots and after heating to 1080°C were hot rolled in stages to 2.3 mm thickness.

The hot rolled 2.3 mm strips were annealed at 850°C for 4 minutes to recrystallise the structure. The sheets were then warm rolled at 750 to 650°C to a thickness of 1.3 mm.

The warm rolled strips were annealed at 850°C for 4 minutes and the edges were then trimmed using a file.

The annealed strips were then cold rolled (using an oil based lubricant) in accordance with the following procedure:

(a) cold rolled to 1 mm and annealed at 850°C for 4 minutes;

(b) cold rolled to 0.6 mm and annealed at 850°C for 4 minutes; and

(c) cold rolled to 0.4 mm.

The cold rolled strips were then trimmed to a width of approximately 25 mm and the strips were again annealed at 850°C for 4 minutes.

The strips were then chopped into fibres 19 mm long, 0.7 mm wide and 0.4 mm thick by a process of

Australian patent 478169 in the name of Aquila Steel Company Pty Ltd which enlarges the end of the fibres to produce a characteristic dogbone shape.

The stainless steel fibres were produced in lengths of 25 mm:

(a) from grade 310 (i) in a chopped form by the process described in the preceding paragraph, and (ii) in a melt extract form; and

(b) from grade 446 in a melt extract form.

The refractory material reinforced with the iron aluminide and stainless steel fibres was prepared in bars of dimensions of 50 x 50 x 180 mm. The bars were prepared by casting the refractory material containing a 2 wt% loading of fibres into wooden moulds and thereafter curing the bars for 24 hours at room temperature and drying for a further period of at least 4 hours at 110°C.

The properties determined in the experimental program included:

(a) oxidation resistance of the fibres at high temperatures; and

(b) (i) fracture toughness, (ii) permanent linear change, (iii) thermal shock resistance, and

(iv) modulus of rupture (MOR) , of the fibre reinforced refractory materials at high temperatures.

The oxidation resistance of the fibres at high temperatures was evaluated by heating a small quantity of chopped fibres in high purity alumina crucibles in a resistance heated furnace. Two series of tests were performed. In the first series, the temperature was varied between 1100 and 1300°C throughout a constant soak time of 24 hours and, in the second series, the soak time was varied while the temperature was held constant at 1200°C or at 1300°C.

The behaviour of the two types of iron aluminide fibres was compared with chopped grade 310 stainless steel fibres. The weight gain of the fibres from oxidation was recorded as a percentage of the original weight. in comparing the weight changes on this basis it was assumed that the scale densities were the same. It is noted that this is correct for chromia and alumina but is an oversimplification for the iron, nickel, chromium oxide spinels which also form on the stainless steel fibre surface.

The results from the oxidation resistance tests are collected in the graphs of Figure 1.

The weight gain of fibres held at temperatures between 1100 and 1300°C over a period of 24 hours in the first series of tests is plotted in Figure 1(a). The figure shows that the high chromium iron aluminide fibres gained only 6 wt% at 1300°C compared to a 40 wt% gain for the grade 310 stainless steel fibres. The fibres of the latter sample had completely converted to oxide at the completion of the test while only a thin white alumina scale had developed on the high chromium iron aluminide fibres.

The weight gain of the fibres held at a constant temperature of 1200°C and at a constant temperature of 1300°C in the second series of tests is plotted in Figures 1(b) and 1(c), respectively. The figures show that complete conversion of the grade 310 stainless steel fibres to oxide occurred after 24 hours at 1300°C and greater than 60 hours at 1200°C and that the corresponding weight gains for the high chromium iron aluminide fibres were only 4 and 3.5%, respectively.

The fracture toughness of the fibre reinforced refractory materials was determined for bars fired to 1100, 1200 and 1300°C for 24 hours and broken in a 4-point bending rig. A toughness index was calculated from the resultant load/deflection curve for each bar by dividing the area under the curve until "failure" by the area up to the first-crack deflection. "Failure" is understood to mean in this context the point where a load of 5% of the maximum load was reached whereas the first crack was taken as the point on the load/deflection curve where the curve deviates from the linear. An upper span of 50 mm, a lower span of 150 mm, and a cross head speed of 0.2 mm/min was used for the mechanical bend tests.

The calculated values of toughness index for bars fired at 1100, 1200 and 1300°C for 24 hours is shown in Figure 2.

With reference to the figure, the significant result is that the fibre reinforced refractory materials containing high chromium iron aluminide fibres maintained a reasonable toughness after firing at 1300°C whilst those containing the other types of fibres showed a severely diminished toughness. Peak loads (k ) of the refractory materials and the fibres were identified on the load deflection curve and are collected in Table 2 below (with fibre values in parentheses) .

Table 2

Identity/Firing 1100°C 1200°C 1300°C Conditions

High Cr FeAl 3.5 (2.7) 3.9 (2.1) 4.7 (2.1)

446 (M) 3.7 (2.7) 4.8 (1.3) 4.7 (1.3)

310 (M) 3.7 (3) 2.8 (1.0) 6.2 (0.0)

310 (C) 3.5 (3.6) 4.7 (3.8) 5.0 (0.8)^'

The values in Table 2 show that at 1300°C the high chromium iron aluminide fibres provide reinforcement support at a load around half that of the refractory material while the other fibre types had minor load bearing capacity. The fracture surfaces of the broken bars of iron aluminide fibre reinforced refractory materials showed that many of the fibres exhibited pull-out from the matrix while others showed ductile fracture at the fracture surface. The other fibre types exhibited brittle fracture with no evidence of fibre pull-out on bars fired to 1300°C.

In relation to oxidation resistance, the appearance of the bars in cross-section after firing at 1300°C revealed the extent of oxidation of the fibres. The grade 310 stainless steel fibres were severely oxidised with many fibres converting entirely to the oxide while the high chromium iron aluminide fibres showed only minor alteration. Stainless steel grade 446 fibres showed less oxidation than grade 310 stainless steel fibres, however, these fibres no longer had the ability to act as reinforcement.

The thermal shock resistance of the fibre reinforced refractory materials was assessed by measuring the flexural strength and toughness following 10 cycles of rapid heating and quenching. The procedure involved heating the bars at 1100°C for 20 minutes followed by immersion into a water bath for 10 minutes.

The results of the thermal shock resistance tests indicated that the thermal shock resistance of fibre reinforced refractory materials containing iron aluminide fibres was comparable to that of the stainless steel fibre reinforced refractory material.

The hot MOR of the fibre reinforced refractory materials was measured using bars of 25 x 25 x 250 mm due to size limitations of the testing apparatus. The bars were sawn from cast bricks, pre-heated at either 1200°C or 1300°C for 24 hours and then broken in a 3-point bending rig at 1000°C and 1200°C. The permanent linear change was measured by recording the dimensions before and after firing to 1300°C.

The results of the MOR tests indicated that none of the fibres appeared to provide any advantage in terms of hot MOR. After pre-firing at 1300°C, all the bars tested showed brittle failure at 1000°C. While ductility was measured in the bars pre-fired at 1200°C and broken at 1000°C, no fibre type obviously outperformed the others. The results from the hot MOR tests are summarised in Table 3 below.

Table 3

Pre-fired at 1200°C Pre-fired at 1300°C

MOR at MOR at MOR at MOR at

1000°C 1200°C 1000°C 1200°C

(MPa) (MPa) (MPa) (MPa)

FeAl - High Cr 5.6 3.0 12.1 8.5

FeAl - Low Cr 6.8 3.4 10.8 7.7

310 (M) 5.5 3.8 11.9 7.7

446 (M) 4.9 3.6 9.4 5.8

An examination of the iron aluminide fibre reinforced refractory materials revealed mostly ductile fracture at the fractured surfaces and, in many cases, the fibres had actually pulled out of the matrix to some degree before fracturing.

In summary, the results of the experimental program showed that:

(a) the properties of oxidation resistance and fracture toughness of the iron aluminide fibre reinforced refractory materials, particularly high chromium iron aluminide fibres, at high temperatures up to 1300°C were significantly better than the same properties for stainless steel fibre reinforced refractory materials; and

(b) in terms of the other properties tested, namely, thermal shock resistance and modulus of rupture, the performance of the iron aluminide fibre reinforced refractory materials was comparable to that of the stainless steel fibre reinforced refractory materials.

EXAMPLE 2.

The experimental program described in Example 1 was based on samples of composite material having a 2 wt% loading of fibres. In order to further investigate the effect of fibre reinforcement on the properties of the composite material of the present invention an experimental program was also carried out on samples having a fibre loading of 6 wt%.

Iron aluminide fibres having the high chromium composition set out in Table 1 of 19mm length were produced in accordance with the procedure outlined in Example 1.

For comparative purposes, stainless steel fibres of grades 310 and 446 in melt extract form of 25mm length were also produced.

Two refractory materials were used in the experimental program. In addition to the 1600AR refractory material referred to in Example 1, a refractory material designated OLA was used.

The 9OLA refractory material is based on a BHP Refractories Pty Ltd low cement, high alumina product marketed under the trade mark QDICKCAST 90L. The andalusite fraction of QUICKCAST 90L was removed and replaced with alumina in the 9OLA refractory material.

As in Example 1, bars of the refractory materials reinforced with the iron aluminide and stainless steel fibres of dimensions of 50 x 50 x 180 mm were prepared.

The bars were prepared by casting the refractory materials containing a 6 wt% loading of fibres into wooden moulds. The water content used for casting 1600AR refractory material was 11%.

The cast, fibre reinforced, refractory materials were cured for 24 hours at room temperature and dried for a further period of 4 hours at 110°C. All samples were prepared in triplicate.

The properties determined in the experimental program were fracture toughness and thermal shock resistance.

The toughness index was obtained using the same procedure as that outlined in Example 1.

Firing conditions of 1300°C for 24 hours were used for most of the samples. Two sets of samples

(reinforced with stainless steel grade 446 fibres) were fired at 1200°C for 3 hours.

The calculated toughness index values for the fibre reinforced 160OAR refractory material samples that were fired at 1300°C for 24 hours are shown in Figure 3. With reference to the figure, the fibre reinforced refractory material containing high chromium iron aluminide fibres gave the highest toughness index of these samples.

Also included in Figure 3 is the toughness index of the samples of 1600AR refractory material reinforced with stainless steel grade 446 fibres that were fired at 1200°C for 3 hours. With reference to the figure, the toughness index values for these samples exceeded the values for all others and illustrates the superior performance of reinforcement with fibres that have undergone minimal oxidation.

Variation of toughness index values within each group of samples was recorded although the general trends were internally consistent.

A comparison of the peak loads of the refractory materials and the fibres for samples fired at 1300°C for 24 hours and at 1200°C for 3 hours is shown in Table 4 (with fibre values in parentheses) .

Table 4

Identity/Firing 1200°C/3h 1300°C/24h Conditions (kN) (kN)

High Cr FeAl - 6.3 (5.0)

446 (M) 4.5 (3.5) 5.8 (2.5)

310 (M) - 4.6 (1.6)

Standard ( 0% fibre) - 5.1

The data in Table 4 show that the iron aluminide fibres provide reinforcement support to a higher load than stainless steel grade 446 (heated to 1200°C for 3 hours). However, the stainless steel grade 446 fibres provide support to a much larger deflection as shown by the high toughness index value. The fracture surfaces of the broken bars of stainless steel 446 fibre reinforced refractory showed that the fibres are generally aligned with the long direction of the bars and exhibited a significant degree of pull-out from the matrix.

In contrast, the shorter iron aluminide fibres displayed more random alignment within the bars and exhibited a mixture of fibre fracture and pull-out from the matrix as previously observed. It is important to note that these fibres had enlarged ends which restrict the degree of pull-out.

The fracture toughness properties of 160OAR refractory material reinforced with a 6 wt% loading of high chromium iron aluminide fibres after firing to 1300°C for 24 hours was comparable to that of 160OAR refractory material with a 2 wt% loading reported in Example 1.

The fracture surface of the iron aluminide fibres showed ductile failure and a simple bending test suggested that the ductility of these fibres was increased after high temperature heat treatment. This is an unexpected observation as it was thought that the presence of water within the refractory composite would promote hydrogen embrittlement and reduce the ductility of the fibres.

The main differences between the two reinforcement loadings are the peak loads of the refractory materials and fibres. These values are higher for the 6% fibre content which reflects the contribution of the metallic reinforcement to the properties of the composite material. The values are compared in Table 5.

Table 5

Identification/Loading 2% (kN) 6% (kN)

Hi Cr FeAl 4.7 (2.1) 6.3 (5.0)

446 (M) 4.7 (1.3) 5.8 (2.5)

310 (M) 6.2 (0.0) 4.6 (1.6)

As in Example 1, thermal shock resistance was assessed by measuring the flexural strength and fracture toughness following 10 cycles of rapid heating and cooling. The procedure involved heating the bars at 1100°C for 20 minutes followed by fan cooling for 10 minutes.

The calculated toughness index values for the fibre reinforced 1600AR and 90LA refractory material samples that were tested for thermal shock resistance are shown in Figure 4.

The figure indicates that superior performance of iron aluminide fibre reinforced refractory materials over stainless steel fibre reinforced refractory materials was obtained for both types of refractory material.

The lower toughness index values for the standard

( 0% fibre) 9OLA refractory material compared to standard (0% fibre) 1600AR refractory material showed that the former had a relatively poor thermal shock resistance.

An average peak load of 2 kN was recorded for the standard ( 0% fibre) 9OLA refractory material samples but increased to 6.3 kN for 9OLA refractory material reinforced with iron aluminide fibres and 6.6 kN for the fibre support load. The average peak load values for the standard ( 0% fibre) and iron aluminide fibre reinforced 1600AR refractory material samples were 0.1 and 2.0 kN, respectively. The fibre support load was also 2 kN for the latter samples. The 1600AR refractory material is a significantly weaker material than the low cement 90LA product.

Figure 4 also includes the toughness index value for stainless steel grade 446 fibre reinforced 9OLA refractory material which had a short firing time of 3 hours at 1200°C. The toughness index is considerably greater than all other samples. Although there was major variation in toughness index values of the stainless steel grade 446 fibre samples, the lowest value was almost twice that of iron aluminide fibre reinforced 9OLA refractory material .

The results in Figure 4 demonstrate the benefits of higher loadings of fibres in improving the thermal shock resistance. By way of comparison, in the experimental program at 2 wt.% fibre loadings reported in Example 1, the toughness index of thermally shocked iron aluminide fibre reinforced 1600AR refractory material was only slightly better than that of the highly oxidised stainless steel fibre reinforced refractory materials.

The improvement in thermal shock resistance is also demonstrated by the higher average peak load of the fibre reinforced refractory materials compared to the unreinforced refractory materials. The higher strength indicates that the former product sustained less damage to the matrix microstructure during the thermal shock cycling procedure. The fibres have obviously acted to maintain matrix bonding and absorb crack extension.

The composite material of the present invention, as described above in Examples 1 and 2 has the following advantages over known refractory materials reinforced with stainless steel fibres.

(a) The cost of iron aluminide fibres is particularly lower compared with stainless steel fibres.

(b) As a consequence of the properties of oxidation resistance and fracture toughness at high temperatures, it is expected that the composite material of the present invention can be used at higher temperatures than known refractory materials. By way of example, typically, the maximum operating temperature for refractories reinforced with stainless steel is in the order of 1150°C, and it is expected that the maximum operating temperature for the composite material of the present invention will be 100 to 150°C higher. This is considered by the applicant to be a considerable advantage in relation to high temperature furnaces.

(c) The ductility of the fibres has been maintained and enhanced by high temperature heat treatment despite the water vapour from the refractory which should have been a source of hydrogen for fibre embrittlement.

(d) The weight of the iron aluminide fibres of the composite material of the present invention is of the order of 13% lower than that of stainless steel fibres of refractory materials reinforced with stainless steel. Hence, composite material of the present invention with high loadings of iron aluminide fibres will be significantly lighter than the conventional stainless steel fibre reinforced refractory materials.

Many modifications may be made to the composite material of the present invention, as described above, without departing from the spirit and scope of the present invention.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A composite material comprising a refractory material reinforced with an iron aluminide alloy.

2. The composite material defined in claim 1 wherein the iron aluminide alloy comprises iron and 8 to 20 wt% aluminium.

3. The composite material defined in claim 1 or claim 2 which comprises up to 16 vol% (typically 29 wt%) of the iron aluminide alloy.

4. The composite material defined in any one of the preceding claims wherein the iron aluminide alloy comprises any one or more of chromium, niobium, and silicon, in relatively small quantities.

5. The composite material defined in claim 4 wherein the iron aluminide alloy comprises up to 12 wt% chromium.

6. The composite material defined in claim 4 or claim 5 wherein the iron aluminide alloy comprise at least 5 wt% chromium.

7. The composite material defined in any one of claims 4 to 6 wherein the iron aluminide alloy comprises between 1 and 3 wt% niobium.

8. The composite material defined in any one of claims 4 to 7 wherein the iron aluminide alloy comprises between 1 and 3 wt% silicon.

9. The composite material defined in any one of the preceding claims wherein the iron aluminide alloy is in the form of fibres.

10. A process for forming a composite material comprising a refractory material reinforced with an iron aluminide alloy, the process comprising:

(a) casting a mixture of iron aluminide fibres and a refractory material;

(b) curing the cast composite material;

(c) drying the cured composite material; and

(d) firing the dried/cured composite material.

11. The process defined in claim 10 wherein the curing step (b) is carried out at room temperature.

12. The process defined in claim 10 or claim 11 wherein the drying step (c) is carried out at a temperature in the range of 100 to 150°C.

13. The process defined in any one of claims 10 to 12 wherein the firing step (d) is carried out at a temperature of at least 900°C and this firing can be carried out prior to or during use.