Patent Application
20070166183
This invention relates generally to stainless steels, and in particular, to a magnetic stainless steel that provides good corrosion-resistance and machinability.
There are many known magnetic stainless steels that provide good machinability. Such steels are often described or identified as being “free-machining”. It is desirable for free-machining magnetic stainless steels to be able to resist corrosion when used in aggressively corrosive environments. However, chromium, the element most used to improve corrosion resistance in steel alloys, adversely affects the magnetic properties of magnetic stainless steel. More specifically, increasing the amount of chromium in magnetic stainless steel typically results in lower magnetic saturation induction and higher coercivity. In magnetic stainless steel, the magnetic saturation induction should be as high as possible to minimize the size and weight of parts made from the steel. On the other hand, the coercivity should be as low as possible to minimize “sticking” of a component when it is subjected to a magnetic field for the inducement of movement, such as in a solenoid magnet.
To provide a balance between corrosion resistance and the desired magnetic properties, chromium has been limited to a level sufficient to provide effective “stainless” properties. This level is typically in the range of about 8-14% chromium. There are known magnetic stainless steels that provide a good combination of corrosion-resistance, magnetic properties, and free machinability. However, a need has arisen for a free-machining magnetic stainless steel that provides a better combination of corrosion resistance and magnetic properties than the known magnetic stainless steels. In particular, it would be desirable to have a free-machining stainless steel that is capable of providing a better combination of corrosion resistance and magnetic properties than the nominal 13%-chromium magnetic stainless steels and corrosion resistance that is at least as good as the nominal 18%-chromium magnetic stainless steel.
The above-described needs are fulfilled to a significant degree by the alloy according to the present invention which has the following Broad and Preferred weight percent ranges.
The balance in each case is iron and usual impurities.
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 for use in combination with each other, or to restrict the ranges of the elements for use solely in combination with each other. Thus, one or more of the ranges can be used with one or more of the other ranges for the remaining elements. In addition, a minimum or maximum for an element of a broad, intermediate, or preferred composition can be used with the minimum or maximum for the same element in another preferred or intermediate composition. Here and throughout this specification the term “percent” or the symbol “%” means percent by weight unless otherwise specified.
The alloy according to the present invention contains at least about 1.0% and preferably at least about 1.2% silicon to benefit the electrical resistivity of the alloy and to promote a ferritic structure in the alloy. Too much silicon adversely affects the cold workability of the alloy. Good cold workability is desirable so that the alloy can be cold worked easily during manufacture to achieve optimum magnetic properties when annealed. Therefore, the alloy contains not more than about 2.0%, and preferably not more than about 1.8%, silicon.
At least about 12.0% and preferably at least about 12.5% chromium is present in this alloy to provide good corrosion resistance and to promote the ferritic structure. Too much chromium adversely affects magnetic properties such as magnetic saturation induction and coercivity. Accordingly, chromium is restricted to not more than about 14.0% and preferably to not more than about 13.5% in this alloy.
Molybdenum and vanadium are present together in this alloy to contribute to the good corrosion resistance provided by the alloy. Vanadium also benefits the magnetic properties of this alloy because it contributes to the low coercivity provided by the alloy. For those reasons this alloy contains at least about 0.5% molybdenum and at least about 0.5% vanadium. Preferably, the alloy contains at least about 0.8% molybdenum and at least about 0.8% vanadium. For best results, the alloy contains at least about 0.9% molybdenum and at least about 0.9% vanadium. No significant additional benefit is provided when the alloy contains more than about 1.3% molybdenum and 1.3% vanadium. Preferably, the alloy contains not more than about 1.2% molybdenum and not more than about 1.2% vanadium.
This alloy includes at least about 0.15% and preferably at least about 0.20% sulfur to benefit the machinability property. Sulfur is limited to not more than about 0.40% and preferably to not more than about 0.35% because it adversely affects the corrosion resistance, the hot workability, and the magnetic properties of the alloy.
The elements nickel and copper are inevitably present in commercial grades of magnetic stainless steels. However, the amounts of nickel and copper are limited in the alloy according to the present invention because nickel and copper promote the formation of austenite which adversely affects the magnetic properties of the alloy. Accordingly, this alloy contains not more than about 0.5% nickel and not more than about 0.5% copper. Preferably, the alloy contains not more than about 0.4% nickel and not more than about 0.4% copper.
Manganese is also typically present in commercially available magnetic stainless steels. When present in this alloy, manganese combines with available sulfur to form manganese sulfides which benefit the machinability property. However, manganese sulfides also adversely affect the corrosion resistance of this alloy. Therefore, manganese is restricted to not more than about 0.60% and preferably to not more than about 0.50% in this alloy.
Carbon and nitrogen also are inevitably present in the known commercial grades of magnetic stainless steels. Carbon and nitrogen form carbides and nitrides that adversely affect the magnetic properties of this alloy. Carbon and nitrogen also promote the formation of austenite which is undesirable in the alloy. Therefore, carbon is restricted to not more than about 0.025% and preferably to not more than about 0.020% in this alloy. Similarly, nitrogen is restricted to not more than about 0.025% and preferably to not more than about 0.020% in the alloy.
The balance of the alloy is iron and usual impurities. Such impurities include phosphorus which is undesirable in this alloy. Therefore, phosphorus is restricted to not more than about 0.035% and preferably to not more than about 0.025% in this alloy. Aluminum contributes to the electrical resistivity of this alloy. However, it also adversely affects the machinability of the alloy by combining with available oxygen to form aluminum oxides. Accordingly, aluminum is limited to not more than about 0.020% and preferably to not more than about 0.010%.
The alloy according to this invention can be prepared by any convenient melting technique. However, the alloy is preferably melted in an electric arc furnace and refined by the argon-oxygen decarburization process (AOD). The alloy is usually cast into an ingot form. However, the molten alloy can be cast in a continuous caster to directly provide an elongated billet form. The ingot or the continuously cast billet is hot worked, as by pressing, cogging, or rolling, from a temperature in the range of about 1100-1200° C. to a first intermediate size billet. The billet is then hot and/or cold worked to reduce its cross-sectional area. When the alloy is cold worked, intermediate annealing steps are conducted between successive cold reductions as necessary in keeping with good commercial practice. Where the appropriate equipment is available, the foregoing steps can be avoided by casting the molten alloy directly into the form of strip or wire. An intermediate form of the alloy can also be made using powder metallurgy techniques.
Regardless of the method used to make the intermediate form of the alloy, the alloy is mechanically worked to provide an elongated form having a penultimate cross-sectional dimension that permits the final cross-sectional size of the finished form to be obtained in a single cold reduction step of about 10-25%, preferably about 10-20%, reduction in cross-sectional area (RCSA). This final cold reduction step may be accomplished in one or more passes, but when multiple passes are employed, there is no annealing between consecutive passes. After the intermediate form of the alloy has been reduced to the penultimate cross-sectional dimension, and before it is cold worked to final cross-sectional dimension, it is annealed at a temperature in the range of about 700-900° C. for at least about 2 hours and then cooled to room temperature. Preferably, this penultimate anneal is conducted at a temperature in the range of about 750-850° C.
Cold working of the intermediate form to final cross-sectional dimension is carried out with any known technique including rolling, drawing, swaging, stretching, or bending. As indicated above, the cold-working step is performed so as to provide no more than a 10-25% RCSA of the intermediate form. In some instances it may be advantageous to further reduce the outside dimension(s) of the intermediate form by machining or by such surface finishing techniques as grinding or shaving in order to ensure that the final cold reduction is within the specified range.
After the final cold reduction, the elongated form, or a part machined therefrom, is heat treated for optimum magnetic performance by annealing for at least 4 hours at a temperature in the range of about 700-1050° C., preferably about 800-900° C. Cooling from the annealing temperature is carried out at a slow rate to avoid residual stress in the annealed alloy or part. Good results are obtained with a cooling rate of about 80-110 C°/hour.
In order to demonstrate the novel combination of properties provided by the alloy according to the present invention, examples of the alloy and several comparative alloys were prepared and tested. The weight percent compositions of the various heats are set forth in Table 1 below.
The balance of each composition is iron and usual impurities. Heats 874, 907, 908, 909, and 910 are examples of the alloy according to the broad range of the present invention.
All of the heats were vacuum induction melted under a partial pressure of argon gas. The molten heats were cast as 2¾ inch square ingots. The ingots were heated at 2050° F. for 2 hours and then forged to 1¾ inch square bars. The bars were reheated at 2050° F. and then forged to 1¼ inch square bars. Heats 868-873 and Heats 906-910 were annealed at 1508° F. for 4 hours and then air cooled in accordance with the usual processing of those alloys. Heats 867 and 914 were annealed at 1526° F. for 4 hours and then cooled in air in accordance with the usual processing for those alloys.
Two test specimens in the form of cones were prepared from each heat for corrosion testing. Both of the cones of each example were tested in a salt spray of 5% NaCl at 95° F. in accordance with ASTM Standard Method B117 after being passivated. The results of the salt spray tests are shown in Table 2. The test data include the time to first appearance of rust (First Rust) in hours (h) and a rating of the degree of of corrosion after 200 h (Rating) for each specimen. The rating system used is as follows: 1=no rust; 2=1 to 3 rust spots; 3=less than 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
To evaluate the magnetic properties of the alloy according to the present invention relative to the comparative alloys, material from heat Nos. 906, 907, 908, 909, and 910 was processed into strip from using the following procedure. Segments of the 1¼ inch bar material were heated to 2000° F. and then hot rolled to band having a thickness of 0.213 to 0.221 inches. The band material was grit blasted and then cleaned in a 1:1 solution of hydrochloric acid. The band material was annealed in dry hydrogen at 1508° F. for 4 hours and cooled at 180 F° per hour. After annealing, the band was grit blasted again and then ground to a thickness of 0.208 to 0.213 inches. The surfaces of the band material were smoothed. The band material was then cold rolled to strip having a thickness of 0.181 to 0.182 inches. After cold rolling, the strip was ultrasonically cleaned and then annealed in dry hydrogen at 1562° F. for 4 hours followed by cooling at 180 F° per hour.
The coercivities of the various materials were measured on a Förster-Koerzimat machine. The results of the coercivity testing in oersteds (Oe) are presented in Table 3 for each of two test specimens. The data for each specimen is the average value of four readings.
The data in Tables 2 and 3 show that Examples 874 and Examples 907-910 provide a better combination of corrosion resistance and magnetic properties than the comparative heats of the nominal 123%-chromium alloys (Heats 868-873 and 906). The improvement is significant and would not have been expected from the know free-machining, magnetic stainless steel alloys. The data in Table 2 show that Examples 908-910 provide corrosion resistance that is about as good as, and Examples 874 and 907 provide corrosion resistance that is better than, the nominal 18%-chromium alloy (Heats 867 and 914). Those results are also unexpected because those examples have significantly less chromium than the known alloy.
As a further demonstration of the combination of properties provided the alloy according to this invention, two 400 lb. heats were prepared and tested. The weight percent compositions of the two heats are shown in Table 4 below. Also shown in Table 4 is the weight percent composition of a nominal 18%-chromium magnetic stainless steel that was used for comparison of corrosion resistance.
The balance of each composition is iron and the usual impurities. Heats 850 and 851 contain less than 0.01% Co and less than 0.01% Al. Heat 851 is an example of the alloy according to this invention.
Heats 850 and 851 were vacuum melted under a partial pressure of argon gas and cast as 7½ inch square ingots. The ingots were forged to 4-inch square billets from a temperature of 2000° F. and then processed to 0.4717 inch round bars. Heat 191 was obtained from continuously cast material that had been previously prepared and was processed to 0.500 inch round bar.
Samples of the bars of Heats 850 and 851 cold drawn 16.8% RCSA were tested to evaluate the effect of different annealing temperatures on magnetic properties. Shown in Table 5 are the results of magnetic testing on samples of each heat after three different annealing heat treatments. The annealing treatments were performed in dry hydrogen for 4 hours followed by cooling at the rate of 180 F° per hour. The data presented in Table 5 include the annealing temperature (Temp.), the applied magnetic field strength (H) in oersteds (Oe), the measured magnetic flux density (B) in kilogauss (kG), the permeability (Perm.), the maximum permeability (Max. Perm.) and the coercivity (Hc) in oersteds (Oe).
Salt spray testing was performed for 200 hours on passivated cone specimens machined from the experimental heats and the known alloy. The cone specimens were prepared as described above. The results of the 200 hour salt spray tests are shown in Table 6 below including the time to first appearance of rust (First Rust) in hours (h) and a rating of the degree of corrosion after 200 h (Rating) for each specimen. The corrosion rating system is the same as that described previously above.
The data in Tables 5 and 6 show that the example of the alloy according to the present invention, provides a better combination of corrosion resistance and magnetic properties than the comparative heat of the nominal 13%-chromium alloys. The improvement is significant and would not have been expected from the known free-machining, magnetic stainless steel alloys. The data in Table 6 show that the example of the alloy of this invention provides corrosion resistance that is better than the nominal 18%-chromium alloy. That result is also unexpected.
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.
