Corrosion Resistant, Magnetic Alloy Article

This invention relates to a corrosion resistant, ferritic alloy and more particularly to such an alloy having a novel combination of magnetic and electrical properties and corrosion resistance.

Heretofore, silicon-iron alloys and ferritic stainless steels have been used for the manufacture of magnetic cores for relays and solenoids. Silicon-iron alloys contain up to 4% silicon and the balance is essentially iron. Such alloys have excellent magnetic properties but leave much to be desired with respect to corrosion resistance. Ferritic stainless steels, on the other hand, such as AISI Type 430F, provide excellent corrosion resistance, but leave something to be desired with respect to magnetic properties, particularly the saturation induction property. Saturation induction, or saturation magnetization as it is sometimes referred to, is an important property in a magnetic material because it is a measure of the maximum magnetic flux that can be induced in an article, such as an induction coil core, made from the alloy. Alloys with a low saturation induction are less than desirable for making such cores because a larger cross-section core is required to provide a given amount of magnetic attraction force as compared to a material with a high saturation induction. In other words, low saturation induction in a core material limits the amount of size reduction which can be accomplished in the design of relays and solenoids.

The increasingly frequent use of such automotive technologies as fuel injection, anti-lock braking systems, and automatically adjusting suspension systems in late model automobiles has created a need for a magnetic material having good corrosion resistance but higher saturation induction than known ferritic stainless steels. The need for good corrosion resistance is of particular importance in automotive fuel injection systems in view of the introduction of more corrosive fuels such as those containing ethanol or methanol.

In an attempt to provide materials having a combination of corrosion resistance, good magnetic properties, and good machinability the following alloys were developed. The alloys, designated QMR1L, QMR3L, and QMR5L, have the following nominal compositions in weight percent.

Each of the alloys also includes lead for the reported purpose of improving machinability.

U.S. Pat. No. 3,925,063 issued to Kato et al. on Dec. 9, 1975 relates to a corrosion resistant, magnetic alloy which includes a small amount of lead, calcium and/or tellurium for the purpose of improving the machinability of the alloy. The alloy has the following broad range in weight percent:

at least one of the following are included: 0.03-0.40% lead, 0.002-0.02% calcium, or 0.01-0.20% tellurium; and the balance is essentially iron.

U.S. Pat. No. 4,705,581 issued to Honkura et al. on Nov. 10, 1987 relates to a silicon-chromium-iron, magnetic alloy having some corrosion resistance. The alloy has the following broad range in weight percent:

and the balance essentially iron wherein C+N.ltoreq.0.05%, and at least one of the following are included: 0.015-0.045% lead, 0.0010-0.0100% calcium, 0.010-0.050% tellurium or selenium.

U.S. Pat. No. 4,714,502 issued to Honkura et al. on Dec. 22, 1987 relates to a magnetic alloy having some corrosion resistance and which is reported to be suitable for cold forging. The alloy has the following broad range in weight percent:

and the balance essentially iron wherein C+N.ltoreq.0.040%, Si+Al.ltoreq.1.35%, and at least one of the following is included: 0.002-0.02% calcium, 0.01-0.20% tellurium, or 0.010-0.050% selenium.

The foregoing alloys include combined levels of chromium, silicon, and aluminum such that the alloys provide lower than desired saturation induction. The relatively high silicon and aluminum in some of those alloys also indicates that those alloys would have less than desirable malleability. Furthermore, all of the foregoing alloys contain lead which is known to present environmental and health risks in both alloy production and parts manufacturing.

It is a principal object of this invention to provide a corrosion resistant, magnetically soft alloy and an article made therefrom, which are characterized by an improved combination of magnetic properties and corrosion resistance.

More specifically, it is an object of this invention to provide such an alloy and article in which the elements are balanced to provide higher saturation induction than provided by known corrosion resistant, magnetic alloys.

The foregoing, as well as additional objects and advantages of the present invention, are achieved in a chromium-iron, ferritic alloy, and article made therefrom as summarized below, containing in weight percent, about:

The balance of the alloy is essentially iron except for additional elements which do not detract from the desired properties and the usual impurities found in commercial grades of such steels which may vary in amount from a few hundredths of a percent up to larger amounts that do not objectionably detract from the desired properties of the alloy.

The subject matter corresponding to the Broad, Preferred A, and Nominal A compositions is set forth and claimed in our copending application Ser. No. 07/379,486, filed on July 13, 1989. The present application is directed to the Preferred B and Nominal B compositions.

The alloy is preferably balanced within the preferred ranges to provide a saturation induction of at least about 17 kilogauss (hereafter kG) (1.7 teslas, hereafter T) and corrosion resistance in corrosive environments, such as fuel containing ethanol or methanol. Sulfur is preferably limited to about 0.05% max. when the alloy is to be cold formed rather than machined.

The foregoing tabulation is provided as a convenient summary and is not intended to restrict the lower and upper values of the ranges of the individual elements of the alloy of this invention for use solely in combination with each other, or to restrict the broad or preferred ranges of the elements for use solely in combination with each other. Thus, one or more of the broad and preferred element ranges can be used with one or more of the other ranges for the remaining elements. In addition, a broad or preferred minimum or maximum for an element can be used with the maximum or minimum for that element from one of the remaining ranges. Here and throughout this application percent (%) means percent by weight, unless otherwise indicated.

The alloy according to the present invention contains at least about 2% chromium. At least about 4% or better yet at least about 6% or 8% chromium increasingly benefits the corrosion resistance of the alloy. The best corrosion resistance is provided when the alloy contains at least about 10%, 10.5% or at least about 11% chromium. Up to about 13%. e.g., 12.75% max. or 12.5% max., chromium is advantageously used for its effect of increasing corrosion resistance, but above that amount the adverse effect of chromium on the saturation induction of this alloy outweighs its advantages. For a saturation induction of at least about 17 kG (1.7 T) chromium is limited to not more than about 12% and preferably to not more than about 10%. A chromium content of about 10% or 10.5% to about 12% provides the best combination of magnetic properties and corrosion resistance.

Up to about 1.5% molybdenum can be present in this alloy because it contributes to the corrosion resistance of the alloy in a variety of corrosive environments, for example, fuels containing methanol or ethanol, chloride-containing environments, environments containing pollutants, such as CO.sub.2 and H.sub.2 S, and acidic environments containing for example, acetic or dilute sulfuric acid. When present, molybdenum also benefits the electrical resistivity of this alloy. Molybdenum, however, adversely affects the saturation induction of the alloy and, preferably, no more than about 1.0%, better yet, no more than about 0.5% molybdenum is present.

From a small but effective amount up to about 0.5% sulfur can be present and preferably about 0.10-0.40% sulfur is present to benefit the machinability of the alloy. Selenium can be substituted for some or all of the sulfur on a 1:1 basis by weight percent.

Sulfur is not desired, however, when articles are to be cold formed from the alloy because sulfur adversely affects the malleability of the alloy. Accordingly, if the alloy is to be cold formed rather than machined or hot formed, preferably no more than about 0.05% sulfur is present.

Manganese can be present and preferably at least about 0.2% manganese is present in this alloy because it benefits the hot workability of the alloy. Manganese also combines with some of the sulfur to form manganese sulfides which benefit the machinability of the alloy. Too much manganese present in such sulfides adversely affects the corrosion resistance of this alloy and, therefore, no more than about 0.5%, preferably no more than about 0.4%, manganese is present.

Silicon can be present in this alloy as a residual from deoxidizing additions. When present silicon stabilizes ferrite in the alloy and contributes to the good electrical resistivity of the alloy. Excessive silicon adversely affects the cold workability of the alloy, however, and, accordingly, silicon is controlled such that no more than about 0.5%, better yet not more than about 0.4%, and preferably not more than about 0.3% silicon is present in the alloy.

The balance of this alloy is essentially iron except for the usual impurities found in commercial grades of alloys for the same or similar service or use and those additional elements which do not detract from the desired properties. The levels of such elements are controlled so as not to adversely affect the desired properties of the alloy. In this regard carbon and nitrogen are each limited to not more than about 0.05%, better yet not more than about 0.03%, e.g., 0.025% max., and preferably to not more than about 0.02%, e.g., 0.015% max. in order to provide a low coercive force of not more than about 4 Oe, preferably not more than about 3 Oe.

Phosphorus is limited to about 0.03% max., better yet to about 0.02% max., and preferably to about 0.015% max. Furthermore, titanium, aluminum, and zirconium are preferably limited to no more than about 0.01% each; copper is preferably limited to no more than about 0.3%; nickel is preferably limited to no more than about 0.5%, better yet to no more than about 0.2%; and lead and tellurium are preferably limited to not more than about twenty parts per million (20 ppm) each in this alloy.

The alloy according to this invention is preferably melted in an electric arc furnace and refined by the argon-oxygen decarburization (AOD) process. The alloy is preferably hot worked from a temperature in the range 2000.degree.-2200.degree. F. (1093.degree.-1204.degree. C.). The alloy is preferably normalized after hot working. For a billet having a thickness up to about 2 in (5.08 cm), the alloy is preferably normalized by heating at about 1830.degree. F. (999.degree. C.) for at least about 1 h and then cooled in air. A larger size billet is heated for a commensurately longer time.

The alloy is heat treated for optimum magnetic performance by annealing for at least about 2 hours at a temperature preferably below the ferrite-to-austenite transition temperature. Acceptable magnetic properties can be obtained, however, when the alloy has been cold worked, as by cold drawing, by annealing for at least about 1 hour. The annealing temperature and time are selected based on the actual composition and part size to provide an essentially ferritic structure preferably having a grain size of about ASTM 8 or coarser. For example, when the alloy contains less than about 4% or more than about 10% chromium the annealing temperature is preferably not higher than about 1475.degree. F. (800.degree. C.), whereas when the alloy contains about 4-10% chromium, the annealing temperature is preferably not higher than about 1380.degree. F. (750.degree. C.). Cooling from the annealing temperature is preferably carried out at a sufficiently slow rate e.g., about 150.degree.-200.degree. F./hr (83.degree.-111.degree. C./h), to avoid residual stress in an annealed article.

The alloy according to the present invention can be formed into various articles including billets, bars, and rod. In the annealed condition the alloy is suitable for use in automotive fuel injector components such as armatures, pole pieces, and injector housings and in magnetic cores for induction coils used in solenoids, relays and the like for service in such corrosive environments as alcohol containing fuels and high humidity atmospheres.

Examples of the alloy of the present invention having the compositions in weight percent shown in Table I were prepared. By way of comparison, Example alloys A and B outside the claimed range, having the compositions in weight percent also shown in Table I were obtained from previously prepared commercial heats. Example A is representative of ASTM A838-Type 2, a known ferritic stainless steel alloy and Example B is representative of ASTM A867-Type 2F, a known silicon-iron alloy.

Examples 1-4 and 6-9 were 17 lb (7.7 kg) heats induction melted under argon and cast into 2.75 in (6.99 cm) square ingots. Example 5 was a 400 lb (181.4 kg) heat induction melted under argon and cast into a single 7.5 in (19.05 cm) square ingot. Examples 10-15 were 30 lb (13.6 kg) heats induction melted under argon and cast into 2.75 in (6.99 cm) square ingots. Examples A and B were obtained from production-size mill heats that were electric arc melted and refined by AOD.

Examples 1-4 and 6-15 were each press forged from a temperature of 2100.degree. F. (1150.degree. C.) to 1.25 in (3.18 cm) square bar. Heat 5 was press forged from 2100.degree. F. (1150.degree. C.) to a 3.5 in (8.9 cm) round cornered square (RCS)

billet. A portion of the RCS billet was hot pressed to 1.25 in (3.18 cm) square bar.

Bar segments, each about 10 in (25.4 cm) long, were cut from the pressed bars of Examples 1-9, normalized at 1832.degree. F. (1000.degree. C.) for 1 h and then cooled in air. The normalized bars were milled to lin (2.54 cm) square. The bars from Examples 1-4 and 6-9 were annealed at 1472.degree. F. (800.degree. C.) for 4 h in a dry forming gas containing 85% nitrogen and 15% hydrogen, and then furnace cooled at about 200.degree. F./h (111.degree. C./h), to provide samples for magnetic and electric testing. The bar from Example 5 was annealed similarly but at 1380.degree. F. (750.degree. C.), the preferred annealing temperature for that composition.

A 12 in (30.5 cm) long bar segment was cut from each of the pressed bars of Examples 10-15, normalized at 1832.degree. F. (1000.degree. C.) for 2 h, and then cooled in air. The bars were spheroidized by heating for 24 h at 1380.degree. F. (750.degree. C.). From each bar a lin.times.lin.times.10 in (2.54 cm.times.2.54 cm.times.25.4 cm) bar and a 3/8 in (0.95 cm) diameter, lin (2.54 cm) long cylinder were machined. The 10 in (25.4 cm) bars and the cylinders of Examples 10-15 were annealed at 1472.degree. F. (800.degree. C.) for 4 h in dry forming gas and cooled at a rate of 180.degree. F./h (83.degree. C./h).

Direct current (dc) magnetic testing of Examples 1-15 was conducted per ASTM Method A341. The maximum permeability was determined using a Fahy permeameter. The residual induction, the maximum induction, and the coercive force were measured at a magnetizing force of 200 oersteds (Oe) (15.9 kA/m) on the Fahy permeameter. Testing to obtain the saturation induction of Examples 1-15 was performed using the isthmus magnet technique and was conducted per ASTM Method A773. The saturation induction was determined by extrapolation of induction data as a function of magnetizing force up to a maximum magnetizing force of 1500 Oe (119.4 kA/m).

The electrical resistivity was determined by measuring the voltage drop across a fixed length of bar at various dc currents up to 100 amperes and plotting a V-I characteristic curve from the measured test data.

The results of the magnetic and electric testing for Example 1-15 are shown in Table II including the maximum permeability (.mu.max), the residual induction (B.sub.r) in kG (T), the coercive force (H.sub.c) in Oe (A/m), the induction (B.sub.m) at 200 Oe (15.9 kA/m) and the saturation induction (B.sub.s) in kG (T), and the electrical resistivity (.rho.) in micro-ohm-centimeters (.mu..OMEGA.-cm). The percent chromium and percent molybdenum for each example are also given in Table II for easy reference.

Table II shows the improved saturation induction provided by this alloy in comparison with the known ferritic stainless steel. The data also show that the saturation induction provided by the present alloy approaches that of the silicon-iron alloy. It is also worthwhile to note the improvement in the coercive force between Examples 4 and 5: the former being annealed at an arbitrary temperature and the latter being annealed at the preferred temperature.

Additional samples of Examples 1-3, 5, 10-15, and the samples of Examples A and B were hot rolled from a temperature of 2100.degree. F. (1150.degree. C.) to 0.19 in (0.48 cm) thick strips and 2.25 in (5.72 cm) long segments were cut from 10 each strip. Strip segments of Examples 1-3, 5, and 6, and of Example A were annealed at 1380.degree. F. (750.degree. C.) for 4 h in dry forming gas and furnace cooled. The strip segments of Examples 10-15 were annealed at 1472.degree. F. (800.degree. C.) for 4 h in dry forming gas and cooled at a rate of 150.degree. F./h (83.degree. C./h). The strip segments of Example B were annealed at 1550.degree. F. (843.degree. C.) for 4 h in wet hydrogen and then furnace cooled at a rate of 150.degree. F./h (83.degree. C./h). Standard corrosion testing coupons 2in.times.lin.times.0.125 in (5.08 cm.times.2.54 cm.times.0.32 cm) were machined from the annealed segments and surface ground to a 32 micron (.mu.m) finish. All of the coupons were cleaned ultrasonically and then dried in alcohol.

Duplicate coupons of each example were tested in a salt spray of 5% NaCl at 95.degree. F. (35.degree. C.) in accordance with ASTM Standard Method B117. Additional, duplicate coupons of each material were tested for corrosion resistance in a 95% relative humidity environment at 95.degree. F. (35.degree. C.). The results of the salt spray and humidity tests for Examples 1-9, A, and B are shown in Table III. For the humidity test the data include the time to first appearance of rust (1st Rust) in hours (h), and a rating of the degree of corrosion after 200h (200h Rating). For the salt spray test, the data include the time to first appearance of rust (1st Rust) in hours (h), a rating of the degree of corrosion after 1 h (1 h Rating), and a rating of the degree of corrosion after 24 h (24 h Rating). The rating system used is as follows: 1=no rusting; 2=1 to 3 rust spots; 3 =approx. 5% of surface rusted; 4=5 to 10% of surface rusted; 5=10 to 20% of surface rusted; 6=20 to 40% of surface rusted; 7=40 to 60% of surface rusted; 8=60 to 80% of surface rusted; 9=more than 80% of surface rusted. Only the top face of each coupon was evaluated for rust.

Data for Examples 10-15 are not shown in Table III because those examples all had corrosion resistance similar to Example A, the 18% chromium heat, in both the 95% humidity and salt spray tests. Those results make clear that above about 12% chromium, there is no additional benefit to corrosion resistance. Regarding Examples 1-3, 5 and 6 of the invention, the data in Table III shows that the alloy according to this invention has corrosion resistance that is at least as good as to significantly better than the silicon-iron alloy, Example B, in high humidity. The salt spray 24 h test appears to be too severe for this alloy as it does not adequately discriminate between examples of the present alloy and the comparative examples.

Samples of Examples 1-4 and 6-15 were prepared similarly to the previous samples except that Examples 1-4 and 6 were annealed at 1475.degree. F. (800.degree. C.) this time. Duplicate coupons of each example were tested for resistance to corrosion in a simulated corrosive fuel mixture of 50% ethanol and 50% corrosive water at room temperature for 24 h, from which the rates of corrosion in mils per year (MPY) (g/m.sup.2 /h) were calculated. Additional duplicate coupons of each example were tested for corrosion resistance in boiling corrosive water for 24 h from which the corrosion rates in MPY (g/m.sup.2 /h) were determined. The results of the corrosive fuel testing are shown in Table IV. By way of comparison a sample of Example A measuring 0.450 in round.times.lin long (1.14 cm rd.times.2.54 cm lg) and a sample of Example B measuring 1.25 in square.times.0.19 in thick (3.175 cm sq.times.0.48 cm thk) were also tested and their results are shown in Table IV.

Table IV shows the improved corrosion resistance of this alloy compared to the silicon-iron alloy in the corrosive fuel mixture and in the boiling corrosive water. The corrosion resistance of Examples 10-15 approaches that of the 18% chromium stainless steel, Example A, in the corrosive fuel mixture test.

It is apparent from the foregoing description and the examples, as set forth in Tables II, III, and IV, that the alloy according to the present invention provides a unique and improved combination of magnetic properties and corrosion resistance. The alloy is well suited to applications where high saturation induction, low coercive force and good electrical resistivity are required and where the in-service environment is corrosive.

The terms and expressions which have been employed herein are used as terms of description and not of limitation. There is no intention in the use of such terms and expressions to exclude any equivalents of the features described or any portion thereof. It is recognized, however, that various modifications are possible within the scope of the invention claimed.