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
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
• • 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
  • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon

Definitions

• a heat-resistant ferritic steel contains from to chromium, the latters eifect in making the steel resistant to oxidation being helped by small amounts of aluminum, silicon, and to some extent by titanium, the latter, possibly assisted by other strong carbide-formers, also preventing chromium loss at the steels grain boundaries, the steel additionally containing small but effective amounts of calcium, cerium or other rare earth metals used singly or together and causing any scale that forms to be adherent and free from flaking.
• the steel is particularly adaped for elevated temperature use under oxidizing conditions, particularly when the service involves thermal shock conditions, exemplified by the operation of internal combustion exhaust after-burners or catalytic reactors required for pollution control.
• a particularly important example is provided by pollution-controlling automotive-vehicle exhaust after-burners or catalytic reactors which must be made in large numhers at the least possible cost and which, when in service, are subjected to thermal shock and corrosion in addition to temperatures of up to around 1000 C. under oxidizing conditions.
• a steel should be, in general, resistant to scaling, have thermal co-efiicients of expansion as low as possible so the products do not change in shape destructively at temperatures varying from room temperatures to the higher temperatures involved by their intended use, be resistant to corrosion, and have the ability to cause any scale that does form on its surface to be a tightly adherent layer having little or no tendency to flake off and cause trouble.
• the steel should be capable of production at the lowest cost possible, making the use of large amounts of the more expensive alloying metals undesirable.
• the ferritic steels normally contain a minimum of 18% chromium, possibly together with aluminum or silicon. In addition to 18% or more of the chromium, the austenitic stainless steels contain relatively large amounts of nickel such as up to 36% in some instances. These austenitic stainless steels are, of course, scale-resistant but their large amounts of high-cost alloying metals make them very expensive.
• the object of the present invention is to provide a heat resistant steel which may be used generally when condi tions such as described hereinabove are involved, but which is particularly adapted for use in the construction of internal combustion engine exhaust after-burners and the like, and which does not involve the use of large amounts of the more expensive alloying elements, while having a satisfactory combination of scale resistance while causing scale that does form to be tightly adherent, good structural stability when subjected to widely varying temperatures, and little or no tendency to be brittle when heated to the temperatures involved by the use of Welding during the manufacture of after-burners and encountered when they are in service.
• a ferritic stainless steel consisting essentially of only from 10% to 15% chromium and small amounts of aluminum, silicon, titanium and one of the rare earth metals, particularly calcium and/or cerium, all being used within limited percentage ranges specified hereinafter, the balance being iron and the usual impurities.
• the steel may be modified by additional alloying elements, used in relatively small amounts.
• the chromium provides the resistance to oxidation, assisted by the aluminum and silicon and to some extent by the titanium, the latter in addition, optionally aided by other strong carbide formers, bonding with the carbon contained by the steel and preventing chromium impoverishment at the steels grain boundaries; and recognizing that under the service conditions described, the formation of some scale is to be expected, the calcium, cerium and/or other rare-earth metal, causing such scale to very firmly adhere to the steel so that it does not crack or flake off during abrupt temperature changes to be expected, particularly in the case of the operation of after-burners.
• Chromium 10.0 to 15.0 Aluminum 1.0 to 3.5 Silicon 0.8 to 3.0 Titanium 0.3 to 1.5 Total calcium, cerium and/ or other rare earth metals 0.01 to 0.5 Total of niobium, tantalum and/ or zirconium to 1.0
• Nitrogen 0 to 0.10 Manganese 0 to 1.0 Nickel 0 to 1.0 Iron (with the usual impurities including sulfur) Balance and in which the sum (C+N) amounts to at least 0.05% and at most to 0.20%, the sum (Al+Si) amounts to at least 2% and at most to 5%, and the sum with the proviso of a free titanium content of at least 0.2% not bonded by the carbon, nitrogen and/ or sulfur.
• the carbon range is restricted to 0.04 to 0.08% and the chromium range to 11 to 13%. Less than chromium does not provide suflicient resistance to oxidation while more than 15% is not needed in the steel of the present invention.
• the influence of the chromium is assisted by the aluminum, the latter in this preferred embodiment having a range of from 1.8 to 2.5%.
• the silicon also assists the chromium in performing its intended function, and in this preferred form the silicon is restricted to a range of 0.8 to 1.5%; and the aluminum and silicon contents are adjusted relative to each other so that the aluminum content is about double the silicon content.
• the titanium which also assists the chromium, in this preferred form, may be within the full 0.3 to 1.5% range, because the titanium being a strong carbide former, also functions to tie up or combine with the carbon in the steel to prevent the formation of chromium carbides which would result in chromium impoverishment at the grain boundaries of the steel such as would make the steel subject to intergranular corrosion.
• An afterburner when in operation, is subjected to corrosion because of the condensate which for-ms on its interior.
• niobium columbium
• tantalum and zirconium used either singly or together to provide a content within a 0.2 to 1.0% range
• Other strong carbide formers such as niobium (columbium), tantalum and zirconium, used either singly or together to provide a content within a 0.2 to 1.0% range, may be added to the titanium to tie up the carbon and in addition, because of the low solubility of the carbides they form, to also prevent grain coarsening during the welding required for the fabrication of after-burners, a formation of a coarse grain structure near the welding area resulting in undesirable brittleness.
• niobium columbium
• tantalum and zirconium used either singly or together to provide a content within a 0.2 to 1.0% range
• the steel should contain at least this 0.2% uncombined titanium.
• the nitrogen from an effective amount up to an amount of 0.10%, because the special nitrides it forms keep the steels grain size relatively fine as compared to that to be expected in the absence of the nitrogen.
• the use of the calcium and/or cerium and/or other rare earth metals within the range of 0.1 to 0.5% is not only preferred but is of decisive importance. They may be used either singly or together. In this preferred embodiment if calcium is used, the range is from 0.05% to 0.2%; and if cerium or other rare earth metals are used either singly or together, their preferred range is from 0.03 to 0.1%. Within the range of 0.1 to 0.5% these rare earth metals may be used in any combination, singly or together.
• this new steel does not require the use of large amounts of the more expensive alloying metals. It may be produced as a wrought steel having good machining properties. It is a ferritic stainless steel but even with prolonged heating it is free from embrittlement due to the intermetallic compound, sigma phase, or in the 425 C. range where prior art ferritic chromium steels having a chromium content corresponding to that of this new steel, become brittle with prolonged heating. Its co-eflicient of thermal expansion is adequately low to permit it to be made into structural elements such as after-burners, of good stability regardless of rapid heating and cooling stressing. Its good resistance to oxidation is retained at temperatures up to about 1100" C. and scale that forms is tightly adherent to the steel and does not flake off with changing temperatures.
• this new steel can be used for many purposes, it is particularly adapted for use for making after-burners in view of the conditions they encounter when in service and the need for producing them in very large quantities at the minimum possible cost.
• Ferritic heat-resistant steel by weight consisting essentially of:
• Chromium s 10.0 to 15.0 Aluminum 1.0 to 3.5 Silicon 0.8 to 3.0 Titanium 0.3 to 1.5 Total calcium, cerium and/or other rare earth metals 0.01 to 0.5 Total of niobium, tantalum and/ or zirconium 0 to 1.0 Nitrogen 0 to 0.10 Manganese 0 to 1.0 Nickel 0 to 1.0 Iron (with the usual impurities including sulfur) Balance and in which the sum (0+N) amounts to at least 0.05 and at most to 0.20%, the sum (Al+Si) amounts to at least 2% and at most to 5%, and the sum with the proviso of a free titanium content of at least 0.2% not bonded by the carbon, nitrogen and/or sulfur.
• the steel of claim 1 excepting the carbon content is 0.04 to 0.08%, the chromium content is 11 to 13%, the aluminum content is 1.8 to 2.5% and the silicon content is 0.8 to 1.5%.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Catalysts (AREA)
• Exhaust Gas Treatment By Means Of Catalyst (AREA)

Applications Claiming Priority (1)

Publications (1)

Family

ID=5827926

Family Applications (1)

Country Status (5)

Cited By (10)

Families Citing this family (5)

• 1971
  • 1971-12-14 DE DE2161954A patent/DE2161954A1/de active Pending
• 1972
  • 1972-11-21 GB GB5373472A patent/GB1356897A/en not_active Expired
  • 1972-11-23 IT IT54238/72A patent/IT973699B/it active
  • 1972-12-07 FR FR7243506A patent/FR2165453A5/fr not_active Expired
  • 1972-12-13 US US00314559A patent/US3782925A/en not_active Expired - Lifetime

Also Published As

Similar Documents