US 3893849 A
A ferritic stainless steel having the composition:
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United States Patent [191 Brickner 1 July 8,1975
[ OXIDATION-RESISTANT FERRITIC STAINLESS STEEL  Inventor: Kenneth G. Brickner, Pittsburgh,
 Assignee: United States Steel Corporation, Pittsburgh, Pa.
 Filed: Oct. 30, 1970  App]. No.: 85,738
FOREIGN PATENTS OR APPLICATIONS 476,115 12/1937 United Kingdom 75/124 Primary Examiner-L. Dewayne Rutledge Assistant Examiner-Arthur J. Steiner Attorney, Agent, or FirmForest C. Sexton  ABSTRACT A ferritic stainless steel having the composition:
carbon 0.03% max. manganese 050% max. silicon 0.10% max. phosphorus 0.025% max. sulfur 0.025% max. chromium 2.75 to 6.25% aluminum 5.0 to 7.0% molybdenum 2% max. iron balance plus incidental impurities,
wherein the percent chromium plus 2 times the percent molybdenum is at least equal to the integer 5. The steel has good high-temperature oxidation resistance at all temperatures up to about 2,200F and has sufficient ductility to be hot and cold worked into sheet and strip.
1 Claim, No Drawings 1 OXIDATION-RESISTANT FERRITIC STAINLESS STEEL BACKGROUND OF THE INVENTION This is a need in industry for better economical hightemperature oxidation-resistant steels for use in applications where high-strength at elevated temperatures is not required. In these applications, such as furnace parts, heat exchanger parts, automobile anti-smog devices and the like, ferritic stainless steels, particularly AlSl Types 430 and 446, are frequently used. Although these steels are lower in cost than comparable chromium-nickel austenitic stainless steels, their cost is still relatively high because of the high chromium content and their service temperatures are limited. Specifically, Type 430 steel contains about 17 percent chromium and has a maximum service temperature of about l,500F. Type 446 steel contains about 25 percent chromium and has a maximum service temperature of about 2,000F.
In addition to the ferritic stainless steels, high aluminum steels have been developed for comparable hightemperature applications. These high aluminum steels (i.e., above 4 percent aluminum) have excellent oxidation resistance up to about 2,200F. These aluminum steels do, however, have two serious disadvantages in that they are very difficult to produce in wrought form, and they exhibit relatively high and erratic oxidation rates between about l,0OO and l,400F. These high aluminum steels, therefore, cannot be used in applications where service temperatures will be in the l,lOl,400F range or where cyclic temperatures through this range may be encountered.
More recently, a combination chromium-aluminum steel has been developed which does not experience the unusual oxidation characteristics within the troublesome l,lOOl,400F range. (See for example U.S. Pat. No. 3,068,094, Alloy of Iron, Aluminum and Chromium, Zackay et al.) In spite of its good oxidation resistance, this alloy is not commercially attractive because to impart any useful degree of ductility, the alloy must be rigidly deoxidized and degassed by costly and time-consuming vacuum melting procedures or complex chemical procedures.
SUMMARY OF THE INVENTION In accordance with the present invention, there is provided a new chromium-aluminum alloy which is not only oxidation-resistant at all temperatures up to about 2,200F but which possesses a high degree of ductility without complex deoxidation or degassing. The ductility of this inventive alloy is sufficient to permit cold rolling of the alloy to sheet and strip products in accordance with conventional cold rolling practices. In addition, this inventive steel utilizes lesser amounts of alloy additives, notably chromium, and is therefore more economical than the comparable prior art alloy discussed above.
It is therefore an object of this invention to provide a new, low-cost, high-temperature oxidation-resistance stainless steel for use in applications where high elevated temperature strength is not required.
Another object of this invention is to provide a new chromium-aluminum steel alloy having good oxidation resistance at all temperatures up to about 2,200F, and further having sufficient ductility when produced by conventional steelmaking practices to be cold rolled into sheet and strip products.
A further object of this invention is to provide a new, high-temperature oxidation-resistant, chromiumaluminum stainless steel alloy which utilizes less chromium and is therefore more economical than similar prior art alloys.
These and other objects and advantages will become apparent from the following detailed description.
DESCRIPTION OF THE PREFERRED EMBODIMENTS The prior art chromium-aluminum high-temperature steel alloy, mentioned above, contains 7.0 to 8.0 percent chromium and 6.5 to 8.0 percent aluminum. The balance thereof is of course iron with incidental impurities. Because of the high alloy content, this steel cannot be cold rolled without excessive edge cracking when produced in accordance with conventional steelmaking practices. In order to have any useful degree of ductility, this alloy must have exceptional low oxygen and gas contents, i.e., such low gas levels as can only be achieved by vacuum melting practices or by specialized chemical degassing procedures.
As noted above, the alloy of this invention is a similar chromium-aluminum steel alloy, but further having exceptional ductility even when produced in accordance with conventional steelmaking practices. The crux of this invention resides in my discovery that slight reductions in the alloy content to levels below certain critical limits will yield such a ductile alloy without any marked sacrifice in the desired high-temperature oxidation resistance. Stated simply, the essential critical feature of this inventive alloy is the restriction of chromium contents to not more than 6.25 percent, and the restriction of aluminum contents to not more than 7.0 percent. In its broadest aspect, the composition limits of this alloy are as follows:
carbon 0.03% max. manganese 0.50% max. silicon 0.10% max. phosphorus 0.025% max. sulfur 0.025% max. chromium 2.75 to 6.25% aluminum 5.0 to 7.0% molybdenum 2% max.
iron balance with incidental impurities.
In addition to the above ranges, the alloy of this invention is further limited to the relationship in which the percent chromium plus two times the percent molybedenum is at least equal to the integer 5. In the absence of molybdenum, therefore, the inventive alloy must contain from 5 to 6.25 percent chromium. With given additions of molybdenum up to 2 percent, however, chromium reductions equal to twice the molybdenum content can be tolerated, down to a minimum of about 2.75 percent chromium.
Considering the embodiment of this invention in detail, the more basic embodiment contains only chromium and aluminum without molybdenum, as has been the prior art practice. Unlike the prior art practice, however, this embodiment of the invention contains not more than a critical 6.25 percent chromium, for a chromium content of from 5.0 to 6.25 percent. In a like manner, the aluminum content of this alloy must be restricted to the range 5.0 to 7.0 percent. As has been noted, this contrasts with prior art limits of 7.0 to 8.0 percent chromium and 6.5 to 8.0 percent aluminum. 1n limiting the alloy additives to such lower levels, I have found that there is not a marked sacrifice in the alloys high-temperature oxidation resistance. Because of these possible lower alloy additions while maintaining useful high-temperature oxidation resistance, the economic advantages of this invention become readily apparent.
Another advantage more significant than economy is that of ductility. That is to say, I have discovered that if the chromium, aluminum and normal residual elements are all maintained below given critical maximum limits, the alloy will have a rather high degree of ductility. In fact, the alloy will possess sufficient ductility to be readily cold rolled into sheet and strip products using conventional commercial equipment and procedures. Of most significance is the fact that this ductility is achieved without any highly special processing to degas or purify the alloy. To achieve this ductility the chromium content in the alloy must not exceed a critical 6.25 percent, the aluminum content must not exceed a critical 7.0 percent and the residual elements must be limited as follows: carbon, 0.03 percent maximum; manganese, 0.5 percent maximum; silicon 0.10 percent maximum; phosphorus, 0.025 percent maximum; and sulfur, 0.025 percent maximum. In order to retain a useful degree of high-temperature corrosion resistance, however, at least 5.0 percent chromium and 5.0 percent aluminum must be provided. In some embodiments of this alloy, molybdenum may be present in amounts not exceeding 2 percent.
Although the above discussed composition limits will provide an alloy having good high-temperature oxidation resistance and exceptional ductility as stated, there are certain more preferred embodiments of this inventive alloy which will provide even better properties for specific given applications. Another embodiment of this invention, therefore, is an alloy substantially as described above, but further containing molybdenum in amounts not exceeding about 2 percent, and preferably about 1 percent molybdenum. When molybdenum is in the alloy in quantities exceeding about 2 percent, even with substantially reduced chromium contents, an alloy is produced that is difficult to hot work and that has a marked tendency towards edge cracking when cold rolled to sheet thicknesses. To assure ductility, therefore, molybdenum contents in excess of about 2 percent should be avoided, and molybdenum contents of about 1 percent or less are preferred.
In discussing the basic embodiment, it was stated that chromium contents of at least about 5.0 percent were essential in order to achieve any useful degree of hightemperature oxidation resistance. In these latter embodiments, however, where molybdenum is added to the alloy, as little as 2.75 percent chromium can be used without sacrificing the alloys high-temperature oxidation resistance. More specifically, l have found that such small quantities of molybdenum are twice as effective as chromium for imparting high-temperature oxidation resistance in the presence of at least 2.75 percent chromium. Hence, chromium reductions to values between 2.75 and 5.0 percent can be realized without sacrifice in oxidation resistance if half that chromium reduction is replaced with molybdenum. Broadly stated, therefore, the alloy of this invention is limited to compositions within the above discussed ranges wherein the percent chromium plus two times the percent molybdenum is at least equal to the integer 5. For example, a molybdenum-free alloy having 6.25 percent chromium is comparable, for the objectives of this invention, to an alloy having 4.25 percent chromium and 1 percent molybdenum, or to an alloy having 5.25 percent chromium and 0.5 percent molybdenum and so on. It is not essential, however, that the chromium content be reduced proportionally, or at all, when molybdenum is used. Insofar as ductility and hightemperature oxidation resistance are concerned, there is no beneficial or detrimental effect in adding the molybdenum to the basic alloy containing 5.0 to 6.25 percent chromium. In order to optimize economy, however, chromium contents of from 2.75 to 5.0 percent should be used when adding molybdenum.
Concerning the molybdenum-containing alloy discussed above, it was stated that chromium contents above about 5 percent would not be beneficial or detrimental to the alloys high-temperature oxidation resistance or ductility. I have discovered, however, the above molybdenum-containing alloy will possess superior resistance to certain other corrosive environments if from 5.0 to 6.25 percent chromium is provided in addition to about 1 percent molybdenum. Therefore, a third preferred embodiment of this alloy would be one containing 5.0 to 6.25 percent chromium, about 1 percent molybdenum, 5.0 to 7.0 percent aluminum and of course having residual elements controlled as described above. Compared to the other preferred embodiments, this alloy will have superior resistance to corrosion in certain corrosive environments such as automobile engine condensate that is encountered in automobile mufflers or anti-smog devices.
The following examples are presented to more graphically illustrate the advantages of this invention, particularly in contrast with comparable prior art alloys. In this series of tests, 12 alloys were prepared having compositions as shown in Table I below.
TABLE I Compositions of Steels Investigated Percent Steel Grade C Mn P S 51 Cr A1 Mo 1 Type 430 0.064 0.46 0.021 0.022 0.57 16.9 X X 2 Type 446 0.093 0.06 0.022 0.012 0.42 23.9 X X 3 6A1 0.014 0.10 0.005 0.018 0.05 0.05 5.54 0.02 4 3Cr-6A1 0.012 0.14 0.009 0.009 0.08 3.02 5.96 0.005 5 3Cr6Al-1 Mo 0.013 0.065 0.007 0.010 0.06 2.99 6.06 1.04 6 3Cr-6A1-3Mo 0.012 0.084 0.008 0.008 0.07 3.03 6.06 2.92 7 6Cr-6Al 0.012 0.10 0.008 0.012 0.09 6.04 6.62 0.006 8 6Cr-6Al-1Mo 0.013 0.062 0.004 0.013 0.07 6.01 6.54 0.97 9 6Cr-6Al-3Mo 0.015 (0.049 0.012 0.06 6.03 6.55 2.97
.008 10 6.5Cr-7.3Al 0.022 0.22 0.023 0.021 0.026 6.48 7.32 X 11 7Cr-7A1 0.018 0.35 0.023 0.022 0.05 7.10 6.97 X 12 6Cr-6A1 0.010 0.35 0.010 0.010 0.07 6.03 5.70 X
x Not determined; residual amounts only.
As noted in the above table, Steel 1 was a conventional AISI Type 430 stainless steel, Steel 2 was a conventional AISl Type 446 stainless steel, Steel 3 was a high-temperature aluminum steel as known in the prior art, and Steels and 11 were chromium-aluminum steels having compositions in accordance with prior art teaching. but not specially degassed. Steels 4 through 9 and 12 were steels having compositions in accordance with this invention, except that Steels 6 and 9 had molybdenum contents in excess of that taught and Steel 4 contained insufficient chromium without molybdenum. Hence Steels 5, 7, 8 and 12 had compositions completely within the scope of this invention.
Steels l through 9 shown in TAble I above were identically tested for high-temperature oxidation resistance. For this test, identical sheet samples were exposed for eight days in a furnace through which air was circulated by natural convection. The samples were tested at each of the following temperatures: 1,200, 1.500,l .800,2,000 and 2,200F. After exposure, the samples were carefully descaled and the amount of weight lost by each sample was determined. The weight-loss data so obtained established quantitatively the oxidation resistance of each steel. A small weight loss by a steel indicates good oxidation resistance, whereas a large weight loss indicates poor oxidation resistance. The results of this oxidation test are shown in Table 11.
TABLE II Results of Laboratory High-Temperature Air Oxidation Tests Weight Loss, mg/sq in., after 8 days exposure at Notes: The results are the average of two tests. except where noted. NT not tested.
'- average of 4 tests.
range obtained in 8 tests.
As shown in Table 11 above, Steel 1, the Type 430 steel. had small weight losses at 1,200 and 1,500F, but
weight loss increased markedly at 1,800F. Similarly,
Steel 2, the Type 446 steel, had a small weight loss at 1,500F but, as expected, the weight loss increased substantially at 2,000 and 2,200F. Steel 1 was not tested at 2,000 and 2.200F because the sample would have been completely oxidized at these temperatures. Steel 3, the 6 percent aluminum steel, exhibited relatively high and erratic weight losses at 1,200F and substantially lower weight losses between l,500 and 2,200F. Steel 4, containing 3 percent chromium and 6 percent aluminum, did not exhibit weight losses appreciably different from those of Steel 3. However, the addition of 1 percent molybdenum to such a steel as Steel 3, i.e., Steel 5, resulted in a substantial reduction in weight loss at 1,200F. The weight losses of this steel at 1,500 and 1,800F were also slightly lower than those of Steel 3. At 2,000F, however, Steel 5 exhibited a weight loss somewhat higher than Steel 3 but markedly less than Steel 2 at this temperature. In Table II, it can be seen that Steel 7, with 6 percent each of chromium and aluminum, was the best sample for oxidation resistance over the total range of temperatures.
Table II further shows that the addition of 1 percent molybdenum to a 6 percent chromium-6percent aluminum alloy (Steel 8) had no appreciable detrimental or beneficial effect on oxidation resistance. however, 3 percent molybdenum in such a steel (Steel 9) resulted in a relatively high weight loss at 1,800F. In addition, Steel 9, as well as Steel 6, was difficult to hot work, and cold rolled sheets thereof had marked tendencies toward edge cracking.
As already noted, Steels 6 and 9, having 3 percent molybdenum, were quite difficult to hot roll, and had marked tendencies towards edge cracking when cold rolled. In addition, Steels 10 and l l, which are outside the scope of this invention, exhibited deep transverse cracks in 1 inch plates that were hot rolled at 2,200F from slab ingots. In contrast, Steel 12, which is within the scope of this invention, was hot rolled in the same manner without difficulty. Portions of the 1 inch plates of Steels 10 and 11 that did not contain cracks were subsequently hot rolled at 2,200F to 0.225-inch plates, and all the resulting plates exhibited a moderately slivered surface. Steel 12, which was hot rolled identically, was essentially free of slivers. The 0.225 inch hot rolled plates were then annealed at 1,500F and descaled by grit blasting and then cold rolled. Steel 10 cracked on cold rolling to a thickness of 0.204 inch, and Steel 11 cracked on cold rolling to a thickness of 0.098 inch. In contrast, Steel 12 was cold rolled to 0.096 inch completely without difficulty. Similarly, Steels 5, 7 and 8, which are also within the scope of this invention, were hot rolled and double cold rolled with an intermediate anneal to 0.065 inch thick completely without difficulty.
I claim: 1. A ferritic stainless steel consisting essentially of carbon 0.03% max. manganese 0.50% max. silicon 0.10% max. phosphorus 0.025% max. sulfur 0.025% max. chromium 2.75 to 5.0% aluminum 5.0 to 7.0% molybdenum about 1% iron balance plus incidental impurities.
wherein the percent chromium plus two times the percent molybdenum is at least equal to the integer 5, said steel characterized by high-temperature oxidation resistance at all temperatures up to about 2,200F, and sufficient ductility to be hot and cold worked into sheet and strip products.
UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 5, 95, 49 Dated y 1975 O Inventofls) Kenneth G. Briokner' It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:
In Table 1, Steel 9, P should be 0.008
S should be 0.012
' Si should be 0.06 Cr should be 6.03
A1 should be 6.55
Mo should be 2.97 O
Between lines 9 and 10 of Table 1, under Mn, delete Signed and Scaled this seventh Day of 0mm 1975 [SEAL] Attest:
RUTH C. MASON C. MARSHALL DANN' Arresting Officer Commissioner nj'Parents and Trademarks FORM PO-1050 (10-69) USCOMM DC 603764369 U.S. GOVERNMENT PRINTING OFFICE: 8 69. 93 o