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
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F28—HEAT EXCHANGE IN GENERAL
  • F28F—DETAILS OF HEAT-EXCHANGE AND HEAT-TRANSFER APPARATUS, OF GENERAL APPLICATION
  • F28F21/00—Constructions of heat-exchange apparatus characterised by the selection of particular materials
  • F28F21/08—Constructions of heat-exchange apparatus characterised by the selection of particular materials of metal
  • F28F21/081—Heat exchange elements made from metals or metal alloys
  • F28F21/082—Heat exchange elements made from metals or metal alloys from steel or ferrous alloys
  • F28F21/083—Heat exchange elements made from metals or metal alloys from steel or ferrous alloys from stainless steel

Definitions

• ABSTRACT A ferritic stainless steel having the composition:
• the steel has good high-temperature oxidation resistance at all temperatures up to about 2,200F and has sufficient ductility to be hot and cold worked into sheet and strip.
• high aluminum steels have been developed for comparable hightemperature applications. These high aluminum steels (i.e., above 4 percent aluminum) have excellent oxidation resistance up to about 2,200F. These aluminum steels do, however, have two serious disadvantages in that they are very difficult to produce in wrought form, and they exhibit relatively high and erratic oxidation rates between about l,0OO and l,400F. These high aluminum steels, therefore, cannot be used in applications where service temperatures will be in the l,lOl,400F range or where cyclic temperatures through this range may be encountered.
• a new chromium-aluminum alloy which is not only oxidation-resistant at all temperatures up to about 2,200F but which possesses a high degree of ductility without complex deoxidation or degassing.
• the ductility of this inventive alloy is sufficient to permit cold rolling of the alloy to sheet and strip products in accordance with conventional cold rolling practices.
• this inventive steel utilizes lesser amounts of alloy additives, notably chromium, and is therefore more economical than the comparable prior art alloy discussed above.
• Another object of this invention is to provide a new chromium-aluminum steel alloy having good oxidation resistance at all temperatures up to about 2,200F, and further having sufficient ductility when produced by conventional steelmaking practices to be cold rolled into sheet and strip products.
• a further object of this invention is to provide a new, high-temperature oxidation-resistant, chromiumaluminum stainless steel alloy which utilizes less chromium and is therefore more economical than similar prior art alloys.
• the prior art chromium-aluminum high-temperature steel alloy contains 7.0 to 8.0 percent chromium and 6.5 to 8.0 percent aluminum. The balance thereof is of course iron with incidental impurities. Because of the high alloy content, this steel cannot be cold rolled without excessive edge cracking when produced in accordance with conventional steelmaking practices. In order to have any useful degree of ductility, this alloy must have exceptional low oxygen and gas contents, i.e., such low gas levels as can only be achieved by vacuum melting practices or by specialized chemical degassing procedures.
• the alloy of this invention is a similar chromium-aluminum steel alloy, but further having exceptional ductility even when produced in accordance with conventional steelmaking practices.
• the crux of this invention resides in my discovery that slight reductions in the alloy content to levels below certain critical limits will yield such a ductile alloy without any marked sacrifice in the desired high-temperature oxidation resistance.
• the essential critical feature of this inventive alloy is the restriction of chromium contents to not more than 6.25 percent, and the restriction of aluminum contents to not more than 7.0 percent.
• the composition limits of this alloy are as follows:
• the alloy of this invention is further limited to the relationship in which the percent chromium plus two times the percent molybedenum is at least equal to the integer 5. In the absence of molybdenum, therefore, the inventive alloy must contain from 5 to 6.25 percent chromium. With given additions of molybdenum up to 2 percent, however, chromium reductions equal to twice the molybdenum content can be tolerated, down to a minimum of about 2.75 percent chromium.
• the more basic embodiment contains only chromium and aluminum without molybdenum, as has been the prior art practice. Unlike the prior art practice, however, this embodiment of the invention contains not more than a critical 6.25 percent chromium, for a chromium content of from 5.0 to 6.25 percent.
• the aluminum content of this alloy must be restricted to the range 5.0 to 7.0 percent. As has been noted, this contrasts with prior art limits of 7.0 to 8.0 percent chromium and 6.5 to 8.0 percent aluminum. 1n limiting the alloy additives to such lower levels, I have found that there is not a marked sacrifice in the alloys high-temperature oxidation resistance. Because of these possible lower alloy additions while maintaining useful high-temperature oxidation resistance, the economic advantages of this invention become readily apparent.
• ductility Another advantage more significant than economy is that of ductility. That is to say, I have discovered that if the chromium, aluminum and normal residual elements are all maintained below given critical maximum limits, the alloy will have a rather high degree of ductility. In fact, the alloy will possess sufficient ductility to be readily cold rolled into sheet and strip products using conventional commercial equipment and procedures. Of most significance is the fact that this ductility is achieved without any highly special processing to degas or purify the alloy.
• the chromium content in the alloy must not exceed a critical 6.25 percent, the aluminum content must not exceed a critical 7.0 percent and the residual elements must be limited as follows: carbon, 0.03 percent maximum; manganese, 0.5 percent maximum; silicon 0.10 percent maximum; phosphorus, 0.025 percent maximum; and sulfur, 0.025 percent maximum.
• at least 5.0 percent chromium and 5.0 percent aluminum must be provided.
• molybdenum may be present in amounts not exceeding 2 percent.
• composition limits will provide an alloy having good high-temperature oxidation resistance and exceptional ductility as stated, there are certain more preferred embodiments of this inventive alloy which will provide even better properties for specific given applications.
• Another embodiment of this invention is an alloy substantially as described above, but further containing molybdenum in amounts not exceeding about 2 percent, and preferably about 1 percent molybdenum.
• molybdenum is in the alloy in quantities exceeding about 2 percent, even with substantially reduced chromium contents, an alloy is produced that is difficult to hot work and that has a marked tendency towards edge cracking when cold rolled to sheet thicknesses.
• molybdenum contents in excess of about 2 percent should be avoided, and molybdenum contents of about 1 percent or less are preferred.
• the alloy of this invention is limited to compositions within the above discussed ranges wherein the percent chromium plus two times the percent molybdenum is at least equal to the integer 5.
• a molybdenum-free alloy having 6.25 percent chromium is comparable, for the objectives of this invention, to an alloy having 4.25 percent chromium and 1 percent molybdenum, or to an alloy having 5.25 percent chromium and 0.5 percent molybdenum and so on. It is not essential, however, that the chromium content be reduced proportionally, or at all, when molybdenum is used.
• Steel 1 was a conventional AISI Type 430 stainless steel
• Steel 2 was a conventional AISl Type 446 stainless steel
• Steel 3 was a high-temperature aluminum steel as known in the prior art
• Steels and 11 were chromium-aluminum steels having compositions in accordance with prior art teaching. but not specially degassed.
• Steels 4 through 9 and 12 were steels having compositions in accordance with this invention, except that Steels 6 and 9 had molybdenum contents in excess of that taught and Steel 4 contained insufficient chromium without molybdenum.
• Steels 5, 7, 8 and 12 had compositions completely within the scope of this invention.
• Steel 2 the Type 446 steel, had a small weight loss at 1,500F but, as expected, the weight loss increased substantially at 2,000 and 2,200F.
• Steel 1 was not tested at 2,000 and 2.200F because the sample would have been completely oxidized at these temperatures.
• Steel 3 the 6 percent aluminum steel, exhibited relatively high and erratic weight losses at 1,200F and substantially lower weight losses between l,500 and 2,200F.
• the addition of 1 percent molybdenum to such a steel as Steel 3, i.e., Steel 5 resulted in a substantial reduction in weight loss at 1,200F.
• Table II further shows that the addition of 1 percent molybdenum to a 6 percent chromium-6percent aluminum alloy (Steel 8) had no appreciable detrimental or beneficial effect on oxidation resistance. however, 3 percent molybdenum in such a steel (Steel 9) resulted in a relatively high weight loss at 1,800F. In addition, Steel 9, as well as Steel 6, was difficult to hot work, and cold rolled sheets thereof had marked tendencies toward edge cracking.
• a ferritic stainless steel consisting essentially of carbon 0.03% max. manganese 0.50% max. silicon 0.10% max. phosphorus 0.025% max. sulfur 0.025% max. chromium 2.75 to 5.0% aluminum 5.0 to 7.0% molybdenum about 1% iron balance plus incidental impurities.
• the percent chromium plus two times the percent molybdenum is at least equal to the integer 5, said steel characterized by high-temperature oxidation resistance at all temperatures up to about 2,200F, and sufficient ductility to be hot and cold worked into sheet and strip products.
• A1 should be 6.55

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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 Sheet Steel (AREA)
• Heat Treatment Of Steel (AREA)

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Citations (4)

• 1970
  • 1970-10-30 US US085738A patent/US3893849A/en not_active Expired - Lifetime
• 1971
  • 1971-10-05 CA CA124,491,A patent/CA951148A/en not_active Expired
  • 1971-10-11 GB GB4727371A patent/GB1373231A/en not_active Expired
  • 1971-10-22 BE BE774362A patent/BE774362A/xx unknown
  • 1971-10-25 IT IT70499/71A patent/IT945777B/it active
  • 1971-10-25 NL NL7114656A patent/NL7114656A/xx unknown
  • 1971-10-26 FR FR7138379A patent/FR2110058A5/fr not_active Expired
  • 1971-10-29 DE DE19712154148 patent/DE2154148A1/de active Pending

Patent Citations (4)

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