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
  • 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

• the present invention contemplates austenitic stainless steels containing, by weight percent, at least about 12%, e.g., 13.5% to 24%, chromium; about 1.75%, e.g., 2% to 19%, manganese, the chromium and manganese being correlated such that when the chromium is from 18% to 24% the manganese does not exceed 10%; about 2.75% to about 3.5% or 4%, silicon; about 7% to less than 12% nickel; nitrogen up to about 0.25%; up to about 0.1% carbon; up to 3%, and advantageously, about 0.5% to 1.5%, copper; up to 1% titanium; and the balance essentially iron.
• the chromium content should be at least about 12%; otherwise, corrosion resistance suffers. Care must be exercised with regard to the upper limit lest a detrimental level of sigma and/or delta ferrite be introduced into the austenitic matrix. Delta ferrite and signa (an intermetallic precipitate) are notable for bringing about embrittlement and/ or hot working difiiculties. While the steels should be free of such phases, up to 2% or 5% thereof can be tolerated in hot workable steels. To that end and particularly for hot workable steels, it is beneficial that the chromium not exceed about 19%, a range of 14% to 16% or 17% being satisfactory.
• the chromium can be extended to 25% or 30% since hot workability is not of concern and this is the characteristic most aflFected by delta ferrite. In this instance a higher level of delta ferrite might be tolerated, e.g., up to 10%.
• Silicon is a potent ferrite and sigma former and preferably should not exceed about 3.5%. While the lower percentage might extend down to 2.5 particularly where service conditions are not overly severe, it is to considerable advantage that the percentage of silicon be at least 2.75% in seeking optimum stress corrosion resistance, a span of 2.75% to 3.25% giving excellent results.
• manganese With regard to manganese, it contributes to achieving and maintaining the desired austenitic face-centered-cubic structure and is also beneficial for deoxidation and other purposes. [It is at the same time, however, a constituent which, if to the excess, has been found to promote the sigma phase and for this reason it should be controlled so as not to exceed 19%. As noted above, it should not exceed 10% when the chromium is above about 18%.Although it may be as low as 1.5% or even 1%, it is preferred that it be at least 2%, a range of from 2% or 3% to 8% or 9% being satisfactory.
• Nickel in addition to promoting the formation of an austenitic structure also counteracts the sigma forming tendencies of chromium, silicon and manganese. For such reasons and for good mechanical characteristics and fabricability, at least 7% or 8% should be present, it being unnecessary that the upper level exceed 11% or 11.5%. The important point is high levels, e.g., 15% or more, are neither necessary nor indispensable in minimizing stresscorrosion attack.
• Carbon is also an austenite former and in combination with nickel tends to thwart the subversive roles of sigma and delta ferrite. While as indicated above herein, carbon should not exceed about 0.1%, it can be as high as 0.25% though in such instances an undue amount of carbides may form which will adversely affect other types of corrosion resistance, e.g., intergranular corrosion, in some corrosive media. Actually therefore, it is to advantage that carbon be held to levels below about 0.05% or 0.03%. Titanium is useful to tie up carbon. In this way intergranular corrosion, particularly in heat affected zones of welds, is inhibited.
• Copper With respect to copper, it is eflfecti ve in respect of corrosion resistance generally and also in offsetting the territe forming tendency of silicon in particular. Copper in percentages of about 0.5 to 1.5% or 2% is quite beneficial.
• a particular advantage of the instant invention is that relatively high nitrogen content can be utilized. This has not been the case historically. Even with high nickel austenitic stainless steels (e.g., 15 nickel, nickel being a known crack inhibitor at high levels), the art has been admonished that nitrogen not exceed about 0.05 For 8% nickel steels, lower levels have been imposed. This introduces recourse to high purity materials and/or special processing techniques and, consequently, higher cost. In accordance herewith, up to 0.25% can be used though lesser amounts, up to 0.15%, for example, may be desired. Nitrogen does stabilize austenite and this is of benefit with the higher chromium, e.g., 18%24% chromiurn, and lower manganese, e.g., less than 5% to 6%,
• Nickel and iron were [first charged and heated to approximately 2850 F. after which silicon-manganese and wash metal were added. The heats were killed with calciumsilicon whereupon the chromium, manganese, silicon and copper were added.
• Aluminum was used for deoxidation.
• Thirty pound ingots were produced using an air induction furnace and magnesia crucible, pouring being carried out at about 2650 F. Thereafter, the ingots were soaked for 3 hours at 2000 F. and rolled to /2" plate, a portion of which was further rolled to 4" plate. Prior to test the 4" plates were heat treated at 2000 F., air cooled and thereafter machined to /3 specimens.
• the specimens were stressed in the form of U-bends and immersed in boiling magnesium chloride (42% concentration, 154 C.).
• the U-bends were examined after the first, second and 24 hour periods and then put back into the same solution. They were then examined daily for approximately three days. Thereafter, they were examined approximately every seven days, a fresh magnesium chloride solution being used during each subsequent 7 day period. The full test period covered approximately thirty (30) days after which the test was discontinued.
• Table I includes data obtained with respect to the point in time at which cracking occurred as well as maximum depth of crack penetration. Data for 304 purchased from a commercial source is also given for purposes of comparison.
• Tests were also conducted in respect of various steels in accordance herewith using sodium chloride.
• specimens prepared as generally described above were exposed in an autoclave to both the aqueous and vapor phases containing chloride ions, the temperature being maintained at about 500 F.
• AISI 304 was also exposed for a comparison base. All steels within the invention survived for a period at least twice that of AISI 304.
• An advantageous alloy range is as follows: 14% to 17% chromium, about 1.9% to 5% manganese, about 2.75% to 3.5% silicon, about 7.5% to 10.5% nickel, up to about 0.03% or 0.05% carbon, about 0.5% to 1.5% copper, nitrogen in an amount, e.g., 0.02%, up to 0.15%, and the balance essentially iron, are especially suitable in the chemical processing industry where stress-corrosion cracking has heretofore been so prominent. It is to be further pointed out that no impairment in mechanical properties of the austenitic steels within the invention is encountered, the mechanical properties being quite comparable to properties of typical AISI grades of austenitic chromium-nickel stainless steels.
• balance or balance essentially when used in referring to the iron content does not exclude the presence of other elements commonly present as incidental ingredients, e.g., deoxidizing and cleansing constituents, and impurities ordinarily associated therewith in amounts that do not adversely affect the characteristics of the steels.
• Phosphorus and sulfur should preferably not exceed 0.02% and 0.015%, respectively, although up to 0.2% phosphorus and 0.03% sulfur might be tolerated.
• Aluminum can be used for oxidation purposes, an amount above 0.2% not being necessary.
• Molybdenum should be avoided. Carbide formers, e.g., columbium or vanadium can be used in lieu of or together with titanium. Conventional processing practices can be used. Pouring temperatures of 2650 F. to 2850 F. are recommended. Vacuum processing can be employed, though not necessary.
• An austenitic stainless steel which notwithstanding that it is of the 8% nickel type is characterized by good resistance to stress corrosion cracking when subjected to stress and in contact with halide solutions, said steel consisting essentially of at least about 12% and up to 30% chromium, the chromium not exceeding 24% in the wrought condition, about 1.75% to 19% manganese, the chromium and manganese being correlated such that when the chromium is from 18% to 24% the manganese does not exceed 10%, about 2.75% to 4% silicon, about 7% to less than 12% nickel, up to about 0.25% nitrogen, up to about 0.1% carbon, 0.5% to 3% copper, up to 1% titanium and the balance being essentially iron.
• An austenitic stainless steel in accordance with claim 1 containing 1.9% to 5% manganese, 2.75% to 3.5% silicon, 7.5% to 10.5% nickel, up to 0.05% carbon, 0.5% to 2% copper, nitrogen up to 0.25%, up to 1% of a carbide former from the group consisting of titanium, columbium and vanadium and the balance iron.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Heat Treatment Of Steel (AREA)

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Cited By (7)

• 1972
  • 1972-01-03 US US00215164A patent/US3806337A/en not_active Expired - Lifetime
  • 1972-12-29 JP JP48004513A patent/JPS4879117A/ja active Pending

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