Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese

Definitions

• ABSTRACT FREE MACHINING STAINLESS STEEL ALLOY This invention relates to stainless steel alloys and more particularly to ferritic chromium-bearing steel alloys and martensitic chromium-bearing steel alloys having improved free machinability or improved combined properties of free machinability and corrosion resistance in selected media.
• Chromium-bearing stainless steel containing little or no nickel may contain from about to 30 percent chromium.
• the microstructure of the alloy may be ferritic or martensitic, or both phases can be present in varying proportions. Because of their mechanical and chemical properties, and in some instances their magnetic properties, such alloys are highly desired for a wide variety of uses. Because of the difficulty encoimtered in machining such alloys, standard grades have been modified by means of freemachining additives for use where the accompanying reduction in corrosion resistance could be tolerated. For example, both A.l.S.I. type 416 and type 430F are illustrative of grades which contain substantial amounts of sulfur or selenium, that is more than about 0.15 percent and usually 0.3 percent or more, for improved machinability.
• manganese is used because of its beneficial effect on the free-machining properties of the alloy in spite of its detrimental effect on corrosion resistance.
• manganese probably because of the relative solubility of manganese sulfides, has an adverse effect on corrosion resistance, but we found that it could be offset to some extent by carefully passivating the surface of the part. As would be expected, as the passivated surface from which the manganese sulfides has been removed wears away, the underlying material becomes exposed and is attacked.
• Our present invention stems from our discovery that when a relatively small amount of tellurium is present together with the elements copper and aluminum, they work together with sulfur or selenium or both of them to impart a unique degree of free machinability to chromium-bearing stainless steel alloys without reducing the corrosion resistance to the extent expected from previous experience with the sulfur additions required to attain a like degree of machinability.
• varying amounts of other elements as for example: up to about 3% silicon but preferably no more than about 1%, up to about 0.5% preferably no more than 0.035% phosphorus, up to about 2.5% nickel, up to about 5% preferably no more than l.5% molybdenum, or tungsten can replace all or part of the molybdenum in the ratio of about 2 to 1, up to about 0.01% boron, and up to about 1% preferably no more than 0.5% each of columbium or titanium. Except for incidental impurities, the balance of the alloy is iron.
• the elements sulfur and/or selenium together with controlled amounts of copper, aluminum and tellurium work together in our composition to provide an unexpected degree of free machinability and corrosion resistance. While sulfur in amounts ranging from about 0.015 to 0.75 percent can be present, above about 0.5 percent increasing difficulty may be encountered in both hot and cold working the composition. We therefore preferably limit sulfur to no more than about 0.4 percent to minimize working difficulties.
• Selenium on a l-for- 1 basis can be substituted for all or part of the sulfur in our composition as is indicated in the foregoing tabulation where the ranges stated are to be read as the broad and preferred amounts of the combined content of both sulfur and selenium.
• the elements sulfur and selenium, individually or together, are not equivalent to and cannot be substituted for the element tellurium in our composition.
• Copper can be present in solid solution in our composition or out of solution as a copper-rich phase or both forms can be present. Its most beneficial effect on free machinability seems to occur when at least some of the copper is present out of solution and forms a copper-rich phase which acts as a chip breaker. Below about 0.5 percent, there is not enough copper present to provide a significant effect while above about 7 percent, increasingly severe segregation problems appear which cause hot shortness.
• Aluminum contributes to the formation of smaller and thereby better dispersed sulfides. Because aluminum is a powerful ferrite former, its use must be carefully controlled in compositions which are intended to be entirely or primarily martensitic. Broadly, from about 0.25 to 4 percent aluminum can be present in our composition. Larger amounts of aluminum cause embrittlement and excessive grain size. In order to facilitate maintaining the martensitic balance in some compositions, less than 2 percent aluminum should be used. We prefer to use aluminum in an amount ranging from about 0.50 to 1.25 percent.
• tellurium appears to form tellurium-rich compounds, which may be tellurides, attached to the sulfides. The more tellurium present, the more the sulfides are surrounded by the tellurium-rich compounds.
• the sulfur content of a given alloy can be reduced so as to obtain better corrosion resistance with little or no loss in free machinability or even in some instances with some improvement in machinability.
• the addition of the elements copper, aluminum and tellurium makes possible significantly improved free machinability.
• manganese is preferably not added to our composition. If added, manganese is kept below about 5%. When its beneficial effect on free machinability is wanted, manganese can be added where the resultant impairment of corrosion resistance can be tolerated, and for this purpose is included in the range of about 0.4 to 2.5 percent.
• the effect of manganese on corrobility, as measured by the drill penetration test was 415 as compared to the value of 45 1' obtained with specimens of Example l0. The small differences noted in the hardnesses of Example l-l 1 are not considered significant.
• sion resistance seems to be mainly that of increasing the solu- 5 Modification of A.l.S.I. type 416 in accordance with the bility of the sulfides in dilute acids.
• free present invention is defined by the following intermediate machining stainless steel grades are not intended for use range: where, as in chemical-processing equipment, they would be exposed to very rigorous media, the smaller amounts of man- Weight Percent ganese, from about 0.4 percent to about 0.6 percent to 0.7 gr M015 2 M I v romlum percent, provide better corrosion resistance than when man- Sum" pmsselcnium 0154's ganese is present in the larger amounts. Copper 0,754
• A.l.S.l. type 430F and Examples 12-14 differ therefrom by the addition of copper, aluminum and tellurium in accordance with the present invention.
• ingots were cast of each of the foregoing examples, hot worked and shaped to form test pieces which were annealed before testing.
• Examples l] l were annealed in the temperature range of 1,300-l,450 F. for 1 hour per inch of material followed by air cooling.
• Examples l2-l5 were annealed in the temperature range of 1,400l,500 F. for 1 hour per inch of material and were also air cooled.
• the hardness of the examples in their annealed condition was measured on the Rockwell B Scale, and the results are recorded in the righthand column.
• the machinability of the specimens of each of the examples was determined as the average depth of penetration in thousandths of an inch into the specimens under carefully controlled conditions. While there is Test, accepted standard for measuring machinability, the free machining values were obtained by measuring the depth of penetration into the specimens by a quarter-inch drill in a time intrval of 15 seconds with the drill rotating at or very close to 670 r.p.m. under constant torque. Before the start of each drilling operation, the drill mounted in a conventional drill press was brought against the surface of the specimen where it was maintained by a constant weight of 100 pounds. The results of the, tests are recorded in Table l under Drill Test", and each is an average of three tests.
• Examples l-9 clearly demonstrate the improvement in machinability obtained in accordance with the present invention when, with the sulfur content held at about the same level or somewhat reduced, the elements copper, aluminum and tellurium are added.
• the improvement in free machinability is attained without any significant effect on the corrosion resistance of the composition as compared to that of A.l.S.l. type 4l6.
• the elements copper, aluminum and tellurium acting alone or only two at a time cannot provide this effect.
• f 'r e e machi na and the balance iron except for incidental impurities or other additions which do not adversely affect the desired properties of the composition.
• Examples 12-15 demonstrate the improved free machining properties obtained by modifying A.l.S.l. type 430F alloy in accordance with the present invention, which properties are accompanied by an improvement in the corrosion resistance properties of .the composition. These improved properties are provided in accordance with the present invention by the addition of copper, aluminum and tellurium in the amounts indicated for better free machinability and by a reduction in the sulfur content by more than about 50 percent to obtain better corrosion resistance. It is to be noted that the hardness of Example 15 is significantly lower than that of Examples 12-14, and, having in mind that machinability as measured by the drill test decreases as the hardness increases from R 81, the improvement in free machinability is greater than appears from a direct comparison of the drill test results of Examples l2-l5.
• Example 12 with a value of 0.358 in. average from the drill test at a hardness of R 88 actually demonstrates improved free machinability as compared to Example 15 when the difference in hardness is taken into account.
• the following intermediate range defines the modification 'of A.I.S.l. type 430F in accordance with the present invention:
• Chromium l4-l8 Sulfur plus Selenium (HS-0.5 pp 1 1.. Aluminum 0.5-! .25
• Nickel up to about 0.5
• specimens of Example 15 rusted after an average of about 2 hours, two specimens of Example 12 rusted after an average of about 4 hours, four specimens of Example. 13 gave an average of about 38 hours with one of the specimens showing no rust after 50 hours exposure when it was removed from the test medium, and of the two specimens of Example 14 tested, one rusted after 48 hours, and the other was removed from the test medium after 50 hours exposure without rusting.
• passivated specimens of each of the examples were first subjected to a by weight NaOl-l dip at 120 F.
• Example 15 yielded an average of about 21 hours
• Example 12 yielded an average of about 35 hours with one specimen being removed from the test medium after 50 hours without rusting
• Example 13 yielded an average of about 37 hours with two of the four specimens tested being removed from the test medium after 50 hours without rusting
• both specimens tested were removed from the test medium after 50 hours without rusting.
• sulfur in our composition to less than 0.25 percent and preferably to no more than about 0.15 percent to 0.20 percent.
• our compositions which are wholly ornormal hot working temperature, the microstructure is less than about 50 percent ferrite and more than 50 percent austenite.
• the forging temperature for the martensitic chromium steels usually ranges from about 2,l00 F. to about 2,250 F. and, in some instances, closer to 2,300 F. depending upon the alloy content.
• the martensitic compositions of the present composition which contain less than about 50 percent ferrite at the normal hot working temperature tend to develop body and corner tears (similar to those found in an alloy that is hot short) presumably due to the presence of a low melting point constituent and require special treatment.
• hot working does not get worse as the forging temperature is raised as in the case of the usual hot short compositions.
• those hot working difficulties disappear, and forging can be successfully carried out at temperatures above about 2,300 F.
• the hot working range should be above the delta ferrite transformation point (the start of the austenite to delta ferrite transformation).
• the preferred temperature is at about 2,400 F.
• a stainless steel alloy as set forth in claim 1 containing about 0.02-0.4% sulfur plus selenium, about 0.75-4% copper, about 0.50-1 25% aluminum, and about 0.010.l% tellurium.
• a stainless steel alloy as set forth in claim 2 containing about:
• a stainless steel alloy as set forth in claim 2 containing less than 0.25 percent sulfur plus selenium.
• a stainless steel alloy having good free machinability in its annealed condition consisting essentially in approximate weight percent of:
• Nickel up to 2.5 Boron up to 0.0l Columbium up to 0.5 Titanium up to 0.5 Molybdenum up to L5 and the balance essentially iron.
• a stainless steel alloy as set forth in claim 6 containing about 0.4-0.6 to 0.7 percent manganese.
• a stainless steel alloy as set forth in claim 7 containing less than 0.25 percent sulfur plus selenium.
• a stainless steel alloy having good free machinability in its annealed condition consisting essentially in approximate weight percent of:
• Titanium up to about 0.25 Molybdenum up to about 0.0 I
• a stainless steel alloy as set forth in claim 12 containing about 0.02-0.4% sulfur plus selenium, about 0.754% copper, and the balance essentially iron. and about 0.5-1 25% aluminum 11.
• a stainless steel alloy as set forth in claim l containing a r (SEAL) Attest: v v
• Patent No. 3 45 322 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

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• 1969
  • 1969-09-04 US US855416A patent/US3645722A/en not_active Expired - Lifetime
• 1970
  • 1970-08-11 CA CA090435A patent/CA925725A/en not_active Expired
  • 1970-08-19 FR FR7030450A patent/FR2058984A5/fr not_active Expired
  • 1970-08-19 JP JP45072725A patent/JPS4933243B1/ja active Pending
  • 1970-08-20 SE SE11381/70A patent/SE349062B/xx unknown
  • 1970-09-01 DE DE19702043229 patent/DE2043229A1/de active Pending

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