US3455681A - Stainless steel - Google Patents

Description

United States Patent 3,455,681 STAINLESS STEEL Arthur Moskowitz, Pittsburgh, Pa., and Gilbert Allen Saltzman, Plainfield, N.J., assignors to Crucible Steel Company of America, Pittsburgh, Pa., a corporation of New Jersey No Drawing. Continuation-impart of application Ser. No. 490,628, Sept. 27, 1965. This application Mar. 28, 1968, Ser. No. 716,981

Int. Cl. C22c 39/26; C21c N00 US. Cl. 75-126 6 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a ferritic stainless steel adapted for use in the manufacture of automobile mufflers and and other exhaust-system components of internal-combustion engines. Broadly, the steel contains chromium in an amount of 11 to 14% along with up to 0.08% carbon, up to 0.05% nitrogen, 0.4 to 0.7% titanium, up to 1.5% aluminum and with silicon present within the range of 0.80 to 2% and the balance iron.

This application is a continuation-in-part of pending application Ser. No. 490,628, now abandoned, filed on Sept. 27, 1965.

Our preferred steels have substantial advantage over the straight-chromium steel now most commonly used for automotive muffiers. There are several properties that are pertinent to the performance of a steel in a mufller or exhaust-system part, and the preferred steels of this invention substantially equal or surpass the known steel in respect to all of them.

The known steel with which the steels of our invention will be compared is designated E-2 and has the following composition: 0.08% max. car-bon, 0.60% max. manganese, 0.025% max. phosphorus, 0.025% max. sulfur, 0.50% max. silicon, 10.50 to 11.50% chromium, titanium in an amount of six times the carbon content to 0.70%, balance iron.

One pertinent property is resistance to dilute acids such as HBr when the steel is repeatedly exposed thereto, heated, and cooled. One way of evaluating this property is the Walker test. It is conducted by subjecting a specimen 2 inches wide, 4 inches long, 0.036 inch thick to a number of cycles as described below, e.g., 13 cycles, and determining the percent weight loss. Each cycle comprises: (1) dipping for eight seconds in an aqueous solution heated to 180- 190 F. and composed of 10 parts 5 N H 50 parts 1 N HBr and 980 parts deionized H O, all parts by volume, (2) suspending in vapor emanating from said solution for 59 minutes and 52 seconds, (3) repeating steps (1) and (2) additional times, and (4) baking the specimens at 480-490 F. for three hours. After 13 such one-day cycles, the above-mentioned E-2 steel exhibited a weight loss of 0.67%; our steels exhibit values as low as 0.2% together with the additional advantages with respect to other properties mentioned hereinbelow. This superiority in Walker test values is of particular importance because the Walker test is used as a measure of the expected service life of an automotive mufiler, and there is need for a mufiler steel with improved service life, particularly in view of the automobile manufacturers proposal to mitigate the contribution of auto exhaust gas to urban air 3,455,681 Patented July 15, 1969 pollution by admitting air to the exhaust line upstream of the muffler, thereby subjecting the mufller to higher service temperatures caused by combustion of the exhaust gases.

Another pertinent test is the ASTM salt-spray corrosion test. Sheet speciments are subjected to a fog of 5% squeous NaCl solution at F. in a closed cabinet for 96 hours, as described more fully in ASTM Specification B117-61. The test indicates resistance to chlorides, which mufilers might encounter in a seacoast atmosphere or in winter driving on salted roads. Both E-2 and our preferred steel exhibit less than 5% surface rust in this test. Both the silicon and the titanium contents of the steel affect the results in this test.

Another test is the CRL scaling-resistance test. Test coupons having a surface area of about 0.26 square decimeter are abraded with 120-grit paper, degreased, and then tested, using 20 cycles of 20 minutes at 1200 F. followed by 10 minutes of air cooling. The weight gain per surface area is determined. E2 steel exhibits a weight gain of 2.6 milligrams per square decimeter; our preferred steels are superior, with a weight gain of about 1.5 to 2.3 milligrams per square decimeter.

Another test, relating to formability before and after Welding, is the Olsen Cub Test. Samples about 0.036 in. thick by 3.5 in. wide by 4 in. long were tested in both the fully annealed condition and the as-welded condition (i.e., with a flat, full-penetration tungsten-inert-gas weld bead across their centers). The reported test values are the heights of the cup indentations required to initiate cracking of a sample; high values indicate better formability. The E-2 steel exhibited, as-annealed and as- Welded, values of 0.365 in. and 0.324 in., respectively, the latter figure being an average of two determinations. One of our preferred steels exhibited values of 0.332 in. and 0.318 in., respectively, which is substantially as good. In general, steels exhibiting a value over 0.250 in. in the as-welded condition have some useful formability after being welded, but steels exhibiting greater values, such as 0.290 in. or greater, are to be preferred, other things being equal.

The steels of our invention differ from the abovementioned E-2 steel principally in that they contain greater amounts of silicon. Although it has long been known that silicon additions promote the oxidation resistance of straight-chrome stainless steels (Armstrong [18. Patent No. 1,322,511, issued in 1919), it was soon thereafter discovered that silicon had a detrimental effect upon formability after a welding operation, and the art has looked away from making silicon additions as a way om improving the properties of the low-cost straightchrome ferritic stainlesssteels such as E-2 steel, chiefly because they are so frequently used in the form of strip, sheet, plate or other flat-rolled mill products subsequently intended to be welded without becoming too brittle to preclude further fabrication thereafter.

Accordingly, in accordance with the present invention we provide a steel that contains the following elements in weight percent:

Iron Balance.

-Withinthe above-mentioned. broad range, we prefer to use steels within the preferred ranges indicated below:

In each case, we consider it essential that the steel be fully ferritic. That is, it is not sufficient that the chemical composition of the steel satisfies one of the aboveindicated sets of ranges; rather, it is also essential that the contents of the austenite-promoting elements (carbon, nitrogen, nickel, manganese, and copper be so proportioned with respect to the contents of the ferritestabilizing elements (chromium, titanium, silicon, aluminum, and molybdenum that the steel exhibits a fully ferritic microstructure in both the as-annealed and the as-welded condition. Otherwise, difiiculties are encountered that make the steel unsuitable for a great number of its intended applications. When the contents of the austenite-promoting elements are slightly too high, the steel may be fully ferritic in the as-annealed condition, but when it is welded, some of the steel in the vicinity of the weld is transformed, first to austenite and then, upon cooling, to a low-carbon martensite that is substantially harder and less ductile than the surrounding ferrite. Thus, the steel loses its good formability in the welded condition. With higher contents of austenite-promoting elements, the steel may become partly martensitic, and consequently, less readily formable, even in the as-annealed condition, and subsequent welding may worsen the formability further. Moreover, steels with the two microstructural components, ferrite and martensite, tend in other respects, such as corrosion resistance in acids and oxidation resistance, to be inferior to fully ferritic steels. Accordingly, we limit our invention to the fully ferritic steels having compositions within the ranges stated above.

As an aid in determining whether or not a steel is fully ferritic, we have developed the following test equation:

A+B+C+D+E12.5O

where A=chromium content in weight percent B: times I, as defined below C=1.5 times the silicon content in weight percent D=3 times the aluminum content in weight percent E=the molybdenum content in weight percent F: the nickel content in weight percent 6:30 times K, as defined below H= 0.5 times the manganese content in weight percent I=the copper content in weight percent J=a factor related to the titanium, carbon, and nitrogen These elements are not mentioned in the above-indicated compositions of our steels, but in commercial steels they are usually present as residual and incidental impurities to some detectable extent; moreover, even in small quantities they InayI have influence on the microstructural makeup of the s cc contents of the steel, having the value 0 if the titanium content of the steel in weight percent is less than 6 times the sum of the carbon and nitrogen contents in weight percent, and otherwise having a value calculated by subtract'mg 6 times the sum of the carbon and nitrogen contents from the titanium content K=a factor related to the titanium, carbon, and nitrogen contents of the steel, having the value 0 if the titanium content of the steel in weight percent is greater than 6 times the sum of the carbon and nitrogen contents in Weight percent, and otherwise having a value calculated by subtracting one-sixth of the titanium content in weight percent from the sum of the'carbon and nitrogen contents.

The above-indicated formula is somewhat conservative. Some steels yielding values slightly below 3 may in fact be fully ferritic and have the desirable properties of the steels of our invention.

In our steels, the chromium content is necessarily held within the broad limits specified above. At low chromium contents, the steel loses its resistance to corrosion by chlorides. At high chromium contents, the steel becomes more costly to produce. Very small differences in cost per pound amount to considerable sums of money when a steel is used in large volume, e.g., for making the mufllers of seipral million automobiles per year. Moreover, steels of higher chromium content cannot be produced by modern oxygen-blowing steelmaking methods so efliciently as can steels of lower chromium content; more chromium is lost to the slag by oxidation.

1f the silicon content falls below 0.80%, the steel performs poorly in oxidation-resistance and corrosionresistance tests. High amounts of silicon tend to create other problems. The silicon may oxidize to form nonmetallic inclusions, rendering the steel dirty and unsaleable. In amounts over about 2%, or at the most 3%, silicon makes the steel more difiicult to work and may limit the formability in the as-welded condition.

Low titanium contents harm the general corrosion resistance (dilute acids, chlorides) and the atmospheric corrosion resistance. In certain cases, where the titanium present is insufficient to react with the carbon present, martensite can be formed, particularly after welding, and the steel will show reduced formability and lower corrosion resistance. On the other hand, high titanium contents harm the elevated, temperature oxidation resistance of the steel and. may introduce added difiiculties, such as increasing the tendency for the occurrence of inclusions.

Low carbon contents are desirable, but in general the carbon content of these steels cannot be lowered below about 0.04% by known steelmaking methods without using special practices that add undesirably to the cost of the steel. High carbon contents, such as 0.10%, tend to deprive the steel of its fully ferritic microstructure and its resistance to salts and acids.

Nitrogen in amounts greater than about 0.05% tends to deprive the steel of its fully ferritic microstructure.

Although satisfactory steels may be made without any addition of aluminum, we prefer to add a small amount to promote oxidation resistance. With an aluminum addition, it may be possible to obtain good properties at a lower titanium content. Amounts of aluminum greater than 1.5%. raise problems of cleanliness.

Manganese, nickel, sulfur and phosphorus may be present in our steels at the ordinary impurity levels for straight-chrome steels. The copper content should be limited to a maximum of 0.2% to preserve a fully ferritic structure.

Other elements such as molybdenum, columbium, tantalum, vanadium, and tungsten may be present in small amounts up to 1% for special purposes.

The value of the invention will be apparent from the following tables, wherein steels AB 17 and 148055 0011 stitute examples of our invention, and the other steels are presented for purposes of comparison. Steel AB 12 is the E-2 mufiier steel now on the market. The other steels will be discussed below.

TABLE I.COMPOSITIONS OF STEELS TESTED, WEIGHT PERCENT, BALANCE IRON C N Mn Cr Si Al T 0. 053 0. 001 0. 31 11. 76 0. 98 1 N.A. 0. 53 0. 063 0. 001 0. 33 11. 54 0. 79 1 N.A. 0. 42 0. 059 0. 002 0. 34 11. 62 0. 73 1 N.A. 0. 51 0. 066 0. 011 0. 53 12. 1. 34 0. 045 0. 51 0. 046 0. 002 0. 35 11. 76 0. 33 N.A. 0. 51 0. 068 0. 002 0. 35 ll. 54 0. 31 0. 98 0. 30 0. 080 0. 004 0. 32 11. 54 0. 50 0. 19 0. 34 0. 062 0. 004 0. 34 9. 57 1. 01 1 N.A. 0. 32 0. 054 0. 003 0. 34 11. 60 0. 35 1 N.A. 1 N-A. 0. 088 0. 002 0. 36 11. 54 1. 03 1 N.A. 0. 31 0. 060 0. 003 0. 33 11. 72 0. 31 0. 23 0. 36 0. 076 0. 015 0. 50 12. 1. 0. 03 0. 41

1 None added. 3 Also contained 0.26% nickel, 0.14% copper, and 0.17% molybdenum. 3 Also contained 0.25% nickel, 0.12% copper, and 0.10% molybdenum.

TABLE IL-PROPERTIES OF STEELS TESTED lit-cycle Salt- Walker spray test, surface Scaling Olsen cup ht., inches wt. loss, rust, test, percent percent mgJdm. Annealed Welded 0. 27 5 1. 7 0. 332 0. 318 0. 50 5 2. 1 0. 318 0. 272 0. 40 5 2. 3 0. 336 0. 291 0. 2 5 N-D. 363 0.313 0. 67 5 2. 6 0. 365 0. 324 0. 57 0 2. 4 0. 328 0. 280 0. 65 10 2. 5 0. 321 0. 265 4. 09 10 4. 0 0. 295 0. 247 1 09 77 4. 3 0. 339 0. 227 0. 42 0 1. 9 0. 320 0. 238 0. 45 0 4. 3 0. 338 0. 297 0. 35 5 1 N.D. O. 352 0. 100

1 Not determined.

The foregoing results demonstrate that the siliconmodified titanium-bearing straight-chrome steels of the invention surpass the properties of the known E-2 mufiler steel. With other steels of similar composition, however, disappointing results are obtained. Steels AB 48, AF. 51, AB 44 and AE 49 are too low in silicon, and no substantial improvement over the properties of Steel AB 12 is obtained. Steel AB 34 contains too little chromium, and it has poor performance in both the Walker test and the scaling-resistance test. Steel AB 50 shows that even at an appropriate chromium level, 11.5%, bad results in the salt-spray test and the Olsen Cup test are obtained if titanium is absent and no silicon is purposely added. Steel AB 12, which is essentially the same as AB 50 with titanium added, is the E-2 steel. It falls far short of equaling the properties of steels of our invention. Steel AB 45 is very close in composition to Steel AB 17, but it is a little too low in titanium and high in carbon, with the result that does not remain fully ferritic and fails in the as-welded Olsen Cup test. Steel AB is too low in silicon and gives poor values in the scaling-resistance test. Steel 148051 is not fully ferritic and fails in the Olsen Cup test.

The following Table 'III presents the results of tests of oxidation resistance conducted at higher temperatures, namely, 1500 F. and 1700 F. The tests were conducted by exposing samples to still air at the above-mentioned temperatures for five 20-hour cycles (100 hours in all) and determining the weight gain per unit surface area.

TABLE III.OXIDATION XT ELEVATED TEMPERA- As indicated above, the foremost use of our new steels is for automotive mufflers and for other parts of the exhaust system associated with an internal-combustion engine. Such mufliers and other parts exhibit desirably increased service life and/or ability to withstand more severe operating conditions and environments. The steel will find use in other applications requiring a weldable low-cost stainless steel with good resistance to acids and oxidation, such as in cement-kiln chains, chimney liners, liquid-fertilizer tanks, and the like. Other possible uses include industrial and farm roofing and siding, metal furniture, and tubing for hot and cold water systems.

While we have shown and described herein certain embodiments of our invention, we intend to cover as well any change or modification therein which may be made without departing from its spirit and scope.

We claim:

1. Fully ferritic stainless steel having good formability both before and after being welded, as well as good resistance to oxidation and good resistance to corrosion by acids and chlorides, said steel consisting essentially of about:

10 to 14% chromium,

up to 0.10% carbon,

up to 0.05% nitrogen,

0.80 to 3% silicon,

0.2 to 1.0% titanium,

up to 1.5% aluminum,

balance iron.

2. Steel as defined in claim 1, characterized in that it exhibits a 13-cycle Walker test value of 0.70% weight loss or lower, a weight gain of about 2.6 mg./dm. or lower in a CRL scaling-resistance test, a surface rusting of under 5% in an ASTM salt-spray test of 96-hour duration, and an Olsen cup test height in the welded condition of 0.250 in. or greater.

3. Fully ferritic stainless steel satisfying the equation A-I-B +C+D+E12.50

where the letters A through I have the meaning indicated in the foregoing specification, said steel having the composition and characteristics defined in claim 2.

4. Fully ferritic stainless steel satisfying the equation A+B+C+D+E12.50

where the letters A through I have the meaning indicated in the foregoing specification, said steel having good formability both before and after being welded, as well as good resistance to oxidation and good resistance to corrosion by acids and chlorides, said steel consisting essentially of about:

Percent Chromium 11 to 14 Carbon Up to 0.08 Nitrogen Up to 0.05 Silicon 0.80 to 2 Titanium 0.4 to 0.7 Aluminum Up to 1.5 Iron Balance 5. Steel consisting of about:

Percent Chromlum 11.5 to 12.5 Carbon Up to 0.08 N trogen Up to 0.05 SIlICOD. 0.80 to 2.00 Titanium 0.4 to 0.7

7 v A i 7 Percent Aluminum Up to 0.2 Iron Balance said steel .satisfying the equation A+B+C+D+E12.50 3

F+G+H+I where the letters A through I have the meaning indicated in the foregoing specification.

6. Steel as defined in claim 5, characterized in that it exhibits a 13-cycle Walker test value of 0.50% weight loss or lower, a Weight gain of about 2.0 mg./dm. or lower in a CRL scaling-resistance test, a surface rusting of under 5% in an ASTM salt-spray test of 96-hour duration, and

- 8 a 0155 P e ht in the welded condition of 0.290 in. or greater.

References Cited L. DEWAYNE RUTLEDGE, Primary Examiner TPIAUL WEINSTEIN, Assistant Examiner Y U.S. c1. X.R. 14837