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
  • C22C38/20—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
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • 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

Definitions

• chromium plus aluminum is at least about 13.0% and copper plus molybdenum is at least 0.5%.
• This invention relates generally to stainless steel, and more specifically, to a new, low cost, 6 to 12 percent chromium stainless steel having corrosion resistance equal to or better than the conventional 17 percent chromium stainless steels.
• the superior corrosion resistance of commercial stainless steels is primarily due to the addition of chromium to the alloy in amounts which vary from 4 to 30 percent. Since the corrosion resistance of stainless steel is more or less a direct function of the chromium content, it is well accepted that those stainless steels having large amounts of chromium are usually superior in corrosion resistance to those having lesser amounts.
• AISI Type 410, 430 and 304 which contain about 12, 17 and 18 percent chromium respectively.
• AISI Type 304 having the greatest amount of chromium plus about 8% nickel, is the best for conventional corrosion resistance applications.
• chromium In addition to being one of the major constituents for corrosion resistance, chromium is also the principal cost element in most commercial stainless steels. Therefore, AISI Type 304 stainless steel, in addition to being superior in corrosion resistance, is also more expensive than AISI Types 410 and 430 or most other lower chromium stainless steel.
• This invention is predicated upon my development of a new statinless steel wherein a portion of the chromium 3,690,870 Patented Sept. 12, 1972 content is replaced by controlled amounts of aluminum and copper and with optional additions of molybdenum. No columbium is added to this steel.
• This lower cost stainless steel contains chromium in amounts of from 6 to 12 percent, and yet has corrosion resistance characteristics equal to or greater than AISI Type 430 stainless steel.
• the steel of this invention is not quite as corrosion resistant as is the alloy containing columbium, i.e., it being comparable to AISI Type 430 rather than Type 304, it is substantially cheaper without additions of columbium, and further, molybdenum can be completely eliminated where economy is the major consideration.
• a stainless steel in accordance with this invention therefore may have corrosion resistance characteristics equal to or better than AISI Type 430 statinless steel while containing about half, or even less than half as much chromium and low cost quantities of aluminum and copper, and molybdenum if desired to optimize corrosion resistance.
• the statinless steel of this invention is ferritic and has the following general composition:
• the balance of the steel should of course be iron (i.e., iron with other usual steelmaking impurities).
• iron i.e., iron with other usual steelmaking impurities.
• the above composition range is further limited by the facts that the combination of chromium plus aluminum must be at least 13 percent or very near thereto and that the copper or combination of copper plus molybdenum must be at least 0.5 percent.
• molybdenum may be completely eliminated, copper, on the other hand, must be present in an amount 01 at least 0.5 percent, or at least 0.2 percent when the steel contains 0.3 percent molybdenum or more. Ideally, copper is maintained between 0.7 and 1.0 percent for opnmum effect on corrosion resistance, with or without the presence of molybdenum. Additions of molybdenum should be eliminated when economy is the major consideration, but for optimum corrosion resistance, molybdenum in concentrations of at least about 1.0 percent is preferred.
• a metal is passive if, on increasing the electrode potential toward more noble values, the rate of anodic dissolution in a given environment under steady-state conditions becomes less than the rate at same less noble potential.
• the tendency for passivation of stainless steels is a function of both a critical current density and a critical potential. Therefore, by measuring these parameters and comparing them with those values for commercially available stainless steels, the passivation tendency, and hence the corrosion resistance characteristics, for any experimental heat can be readily evaluated.
• a strong tendency to passivate is manifested by an active passivation potential and a low critical current density.
• Table I below contrasts the critical current density and chemistry of AISI Type 430 stainless steel with a representative sampling of the experimental heats, most of which had compositions in accordance with this invention.
• AISI Type 430 stainless steel was shown to have a critical current density of 7.2 ma./cm.
• the preferred composition of this stainless steel for optimum corrosion resistance at a minimum cost, would be about 8 percent chromium, 5 percent aluminum and 1 percent each of copper and molybdenum.
• Nickel, titanium and silicon additions were considered for this alloy, but neither showed any beneficial effect.
• Nickel and titanium are neither beneficial nor detrimental.
• Silicon on the other hand, not only has a detrimental effect on corrosion resistance, but further adversely affects the metallurgical quality of the steel. Therefore, silicon should be kept at residual impurity amounts of no more than 0.5 percent.
• Example 13 contained insufficieut aluminum at 1.94 percent
• Example 14 contained insufficient chromium plus aluminum at 12.05 percent
• Example 15 contained insuflicient copper at 0.31 percent.
• each of these examples displayed higher critical current densities than the standard, AISI 430.
• Examples 2, 4 and 10 contained very slightly less than 13 percent combined chromium plus aluminum, namely 12.93 percent, 12.98 percent and 12.93 percent respectively. These examples were, however, sufficiently close to the 13 percent limit that they did possess the desired lower critical current densities.
• Examples 13 and 14 containing combined aluminum and chromium levels of 12.24 percent and 12.05 percent respectively were insufficient, and hence, substantially inferior to the standard AISI 430 steel.
• a low chromium stainless steel according to claim 1 having about 8 percent chromium, 4 to 7 percent aluminum, and about 1 percent each of copper and molybdenum.
• Example 12 was indeed superior in corrosion resistance to AISI Type 430 steel.
• Example 16 which contained an insuflicient amount of copper and an insufliicient amount of chromium plus aluminum (12.02 percent) was inferior to AISI Type 430 steel.
• a low chromium ferritic stainless steel consisting essentially of up to 0.10 percent carbon, up to 2.0 percent manganese, 6 to 12 percent chromium, 2 to 7 percent aluminum, 0.2 to 3.0 percent copper, up to 2.0% molybdenum, and the balance substantially iron with the com- References Cited UNITED STATES PATENTS 'HYLAND BIZOT, Primary Examiner US. Cl. X.R. 75l25

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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)
• Heat Treatment Of Sheet Steel (AREA)

Applications Claiming Priority (1)

Publications (1)

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ID=22075026

Family Applications (1)

Country Status (8)

Cited By (5)

Families Citing this family (1)

• 1970
  • 1970-08-26 US US67293A patent/US3690870A/en not_active Expired - Lifetime
• 1971
  • 1971-08-18 CA CA120,853A patent/CA953949A/en not_active Expired
  • 1971-08-20 DE DE19712141899 patent/DE2141899A1/de active Pending
  • 1971-08-23 BE BE771676A patent/BE771676A/xx unknown
  • 1971-08-23 NL NL7111592A patent/NL7111592A/xx unknown
  • 1971-08-24 FR FR7130615A patent/FR2105944A5/fr not_active Expired
  • 1971-08-24 GB GB3971471A patent/GB1359850A/en not_active Expired
  • 1971-08-24 LU LU63768D patent/LU63768A1/xx unknown

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