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
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
• • 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
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
• • 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

Definitions

• the present invention relates to a low interstitial, corrosion resistant, weldable ferritic stainless steel with improved toughness and to a process for the manufacture thereof. More specifically, the present invention is directed to a low interstitial/corrosion resistant, Weldable ferritic stainless steel with a small addition of either columbium or titanium to improve the toughness when the maximum achievable cooling rate is limited.
• a ferritic stainless steel must have superior pitting and crevice corrosion resistance in order to be used in certain chemical environments as for example in power plants exposed to sea water, and pulp and paper process equipment.
• Stainless steel containing 29% chromium and 4% molybdenum are highly resistant to crevice corrosion. These steels require a low level of interstitials, for example a total carbon plus nitrogen content of less than 0.025% by weight, to have good post-welding ductility and intergranular corrosion resistance.
• Toughness is the ability of a metal to absorb energy by deforming plastically before fracturing.
• the transition temperature is the temperature at which the fracture which occurs from the impact is 50 percent shear (ductile) and 50 percent cleavage (brittle).
• the toughness of the 29% chromium - 4% molybdenum alloy is low compared to substantially lower chromium alloys of an equivalent carbon and nitrogen content because of the high alloy content.
• the toughness of the 29% chromium - 4% molybdenum alloy is improved by water-quenching to speed up the cooling process instead of the slower air-cooling.
• the water-quenching process is not possible or practical to use, so a method is needed to improve the toughness of the steel in those situations where the maximum cooling rate is limited.
• United States Patent No. 3,807,991 teaches the addition of between 13 and 29 times the amount of nitrogen or between 0.065% and 0.363% columbium to a steel of 1 % molybdenum for improved toughness and intergranular corrosion resistance in the air-cooled condition.
• An object of the present invention is to provide a low interstitial, corrosion resistant, weldable ferritic stainless steel with a high molybdenum content which exhibits improves toughness in heavier section thickness when the maximum achievable cooling rate is limited.
• a further object of the present invention is to provide a low interstitial, corrosion resistant, weldable ferritic stainless steel with a high molybdenum content which exhibits improved toughness due to a small addition of either columbium or titanium.
• this invention provides a low interstitial ferritic stainless steel which is corrosion resistant and weldable at room temperature.
• the improvement of the present invention being an addition of a small critical amount of columbium and/or titanium to improve the toughness of the steel in situations where water quenching is impossible or impractical.
• the present invention provides a low interstitial, corrosion resistant, weldable ferritic stainless steel with improved toughness characterized in that it consists of, by weight percent: 25.0%-35.G% chromium, 3.6%-5.6% molybdenum, less than 3% nickel, less than 2% manganese, less than 2.0% silicon, less than 0.5% aluminum, less than 0.50% copper, less than 0.050% phosphorus, less than 0.05% sulfur, less than 0.01 % carbon, less than 0.02% nitrogen, the sum of the carbon and nitrogen being less than 0.025%, 0.05-0.50% of columbium and/or titanium, and the balance of iron.
• the present invention further provides a process for the manufacture of ferritic stainless steel of improved toughness wherein said steel is hot rolled, annealed and cold-rolled to strip thickness, characterized in that the process comprises air-cooling from the annealing temperature before cold rolling a ferritic stainless steel consisting essentially of, by weight per cent: 25.0%-35.0% chromium, 3.6%-5.6% molybdenum, less than 3.0% nickel, less than 2.0% manganese, less than 2.0% silicon, less than 0.5% aluminum, less than 0.5% copper, less than 0.5% phosphorus, less than 0.05% sulfur, less than 0.01% carbon, less than 0.02% nitrogen, the sum of the carbon and nitrogen being less than 0.025%, 0.05%-0.50% of columbium and/or titanium, and the balance iron, so as to produce a steel with a Charpy impact transition temperature of below -17.8°C (0°F) as cold-rolled to a thickness of 1.575 mm (0.062 inches).
• Chromium and molybdenum are preferably present in respective amounts of 28.5% to -30.5% and 3.75% to 4.75%.
• Columbium and/or titanium is present preferably in the amount of 0.05% to 0.20%.
• Manganese and silicon are each usually present in amounts of less than 1%.
• Aluminum, copper, phosphorous, and sulfur are present usually in amounts of less than 0.1 %.
• Carbon and nitrogen are present preferably in amounts of less than 0.008% and 0.016% respectively.
• This invention relates to a low interstitial ferritic stainless steel having a chromium content of between 25.0% and 35.0% and a molybdenum content of between 3.6% and 5.6%.
• a small amount of columbium or titanium of between 0.05% and 0.20% is added to the steel composition to improve its toughness when the maximum achievable cooling rate is limited.
• the ingots were conditioned, heated to a temperature of 1121°C (2050°F) and hot rolled to a strip about 3.556 mm (0.140 in.) thick.
• the hot rolled band was annealed at a temperature of 1010°C (1850°F), and water-quenched and cold rolled to a thickness of 1.575 mm (0.062 in.).
• the strip was then annealed at a temperature of 1010°C (1850°F), water-quenched and TIG welded.
• the impact transition temperatures of the air-cooled specimens are higher than those of the water-quenched specimens it can be seen from Table II that the difference in impact transition temperatures of an air-cooled and a water-quenched 1.575 mm (0.062 in.) thick steel strip is less than 55°C (100°F). Whereas the difference in impact transition temperature for prior art compositions is 133°C (240°F).
• the impact transition temperature of the 1.575 mm (0.062 in.) steel strip of a composition according to the present invention is below -17.8°C (0°F) whereas that of prior art composition A is 54°C (130°F). It is essential that the impact transition temperature of such a steel strip be below room temperature so that the steel strip will not crack upon welding. Steels of prior art compositions had to be water-quenched before cold rolling in order to achieve the necessary toughness characteristics.
• the composition of this invention enables us to achieve good toughness characteristics by air-cooling the steel before cold-rolling instead.
• Corrosion tests were also performed on the 1.575 mm (0.062 in.) thick strip in the as welded condition and on the base metal specimens which were heat treated at 1232°C (2250°F) and air-cooled to simulate the heat affected zone upon welding a heavier thickness.
• the specimens 25.4 mmx50.8 mm (1 in.x2 in.) were exposed to a boiling solution of ferritic sulfate-50% sulfuric acid for 120 hours according to ASTM A262 Practice B for intergranular corrosion testing. The corrosion rates are given in Table Ill.

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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)
• Treatment Of Steel In Its Molten State (AREA)
• Heat Treatment Of Sheet Steel (AREA)
• Pressure Welding/Diffusion-Bonding (AREA)
• Laminated Bodies (AREA)
• Soft Magnetic Materials (AREA)
• Heat Treatment Of Articles (AREA)

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• 1981
  • 1981-12-02 EP EP81305667A patent/EP0057316B1/en not_active Expired
  • 1981-12-02 AT AT81305667T patent/ATE12527T1/de not_active IP Right Cessation
  • 1981-12-02 DE DE8181305667T patent/DE3169748D1/de not_active Expired
  • 1981-12-04 AU AU78294/81A patent/AU7829481A/en not_active Abandoned
  • 1981-12-08 CA CA000391706A patent/CA1184403A/en not_active Expired
  • 1981-12-09 KR KR1019810004803A patent/KR880001356B1/ko active Pre-grant Review Request
  • 1981-12-28 ES ES508364A patent/ES8305049A1/es not_active Expired
• 1982
  • 1982-01-13 BR BR8200150A patent/BR8200150A/pt unknown
  • 1982-01-14 MX MX190981A patent/MX156238A/es unknown
  • 1982-01-16 JP JP57005324A patent/JPS57137455A/ja active Pending

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