Metal treatment – Stock – Ferrous
Reexamination Certificate
2002-01-09
2004-07-13
Oltmans, Andrew L. (Department: 1742)
Metal treatment
Stock
Ferrous
C148S324000, C148S327000, C420S011000, C420S012000, C420S056000, C420S059000, C420S064000, C420S065000, C420S121000, C420S128000
Reexamination Certificate
active
06761777
ABSTRACT:
This application was originally deposited on Aug. 6, 2001, in the United States Patent and Trademark Office under the Disclosure Document Deposit Program and was assigned Disclosure Document No. 497,934.
FIELD OF INVENTION
This invention relates generally to the art of alloys and more particularly to a high chromium, nitrogen bearing alloy having high corrosion resistance. The instant invention also relates to a high chromium-nitrogen bearing castable alloy, a high chromium-nitrogen content alloy, and a process for producing the high chromium-nitrogen bearing alloy, and articles prepared from the same. This invention further relates to a corrosion resistant high chromium, nitrogen bearing austenitic alloy which is also excellent in strength at high temperatures and suitable for materials of boilers, chemical plant reactors and other apparatus which are exposed to severely high temperature and corrosion environments at work. The instant invention is also directed to a heat resistant high Chromium, nitrogen bearing austenitic alloy having high strength and excellent corrosion resistance in high temperature corrosive environments. The present also addresses the problem of creating a metal casting material, the wear resistance of which will correspond approximately to common commercial types of white iron, but which additionally will be characterized by high corrosion resistance in aggressive media. In addition to high corrosion and wear resistance, the alloy material according to the invention has good casting characteristics. Consequently it can be produced in conventional high-grade steel foundries. Moreover, the casting material has good working characteristics. Furthermore, the aforementioned positive quantities are primarily a chromium content of 28 to 48 wt. %, a carbon content of 0.3 to 2.5 wt. %, and a nitrogen content of 0.01 to 0.7% which result in a sufficiently high volume proportion of carbides and nitrides. The large increase of the chromium content decreases the chromium depletion of the matrix. With regard to the combination of corrosion resistance and wear resistance, the material according to the invention is decidedly superior compared to the known types of castings previously utilized in applications subjected to hydroabrasive wear. The present invention is also directed to an air-meltable, castable, workable, alloy resistant to corrosion and acids such as sulfuric acid and phosphoric acid over a wide range of acid strengths.
BACKGROUND OF INVENTION
Equipment used in highly corrosive environments typically is constructed of metal alloys such as stainless steel or other high alloys. These alloys are necessary to withstand the extremely corrosive effects of environments in which the equipment encounters chemicals such as concentrated sulfuric acid or concentrated phosphoric acid. A particularly difficult environment is encountered in making phosphate fertilizer. In the digestion of phosphate rock with hot, concentrated sulfuric acid, equipment must resist the environment at temperatures up to about 100° C. The impure phosphoric acid which is produced can be extremely corrosive and contains some residual sulfuric acid. The corrosive effect is often increased by other impurities in the phosphoric acid, particularly by halogen ions such as chloride and fluoride, which are normally present in the phosphate rock feedstock used in the process. An extremely corrosive environment is encountered in the concentration of the crude phosphoric acid.
Phosphate rock deposits at various locations in the world vary greatly in chemical composition. The most severe corrosion environments are typically encountered in processing deposits of phosphate rock which contain a high content of halogens, such as chloride or fluoride.
It is also generally known that increasing the Cr content is effective to improve corrosion resistance of steel. Hi-Chrome alloys containing 23-40% Cr, 0.8-2% C, 2.5% Si, and up to 5% Mo, have been known since the 1930's. See for Example German Patent No 7,001,807. U.S. Pat. No. 5,252,149 represents a modernization of this alloy, followed by the German Patent No. 8,612,044 or No. 4,417,261. It is noted that in both patents the alloys exhibit a high resistance to abrasion and good resistance to corrosion. However, both exhibit poor mechanical properties, especially low toughness, brittleness, sensitivity to heat, sensitivity to notch all of which limit their usefulness. It is evident that their structure contains ferrite (Fe &agr;).
The ferritic structure in these alloys is inherently very brittle, and the carbide phase embedded in such a brittle phase, results in a very low toughness, high notch sensitivity, as well as sensitivity to heat. Besides, the ferritic structure supersaturated with Chrome, causes the creation of the sigma phase, which drastically lowers toughness and corrosion resistance.
U.S. Pat. No.5,320,801 is directed to alloys having the following composition: Cr—27 to 34% by weight, Ni+Co—13 to 31%, Si—3.2 to 4.5%, Cu—2.5 to 4%, C—0.7 to 1.6%, Mn—0.5 to 1.5%, Mo—1 to 4%, and Fe—essentially the balance. The alloy of the '801 patent possesses good toughness, but has very poor hardness and very poor wire resistance and low tensile strength. The hardness of 208 to 354 HB, is similar to that of CD4MCU stainless steel (260-350 HB), which has excellent corrosion resistance, but poor wear resistance. The alloy disclosed and claimed in U.S. Pat. No. 5,320,801 is similar to austenitic, high Nickel stainless steels in that is has good toughness, but very low tensile strength and hardness, as well as poor wear resistance. The Nickel present in corrosion resistant alloys, serves mainly for structural stabilization and adds very little to their corrosion resistance. Good examples of this are the stainless austenitic steels containing 12-35% Ni, which have corrosion resistance approaching that of duplex stainless steels which have a low percentage of Nickel (4-8%), or High-Chrome stainless steels with Ni only up to 4%. The primary elements of stainless alloys are Chromium, Molybdenum and Nitrogen as illustrated in the models used to show how various alloying elements influence the corrosion resistance of stainless steel. For example: Pitting Resistance Equivalent Number, PREN=% Cr+3.3*Mo+16*% N illustrates that Nitrogen is an important, very powerful alloying element of corrosion resistant alloys.
The main flaw of the High-Chrome alloys of the prior art is the difficulty in dissolving of Chrome, Molybdenum and Nitrogen in the matrix, without a negative effect on the mechanical properties of the alloy, such as toughness, tensile strength, brittleness, heat sensitivity and weld ability. This is the result of the precipitation of the sigma phase from alloys saturated with Chrome and Molybdenum. Premature wearing out of pump parts made from the above-mentioned High-Chrome alloys is a common occurrence. The main contributing factors are: very low toughness, brittleness and low endurance. Most often a failure happens with a casting worn thin in an isolated area where, due to the poor mechanical properties of the alloy, a crack develops leading to the eventual disintegration of the otherwise still viable component.
The mechanism for corrosion and erosion in acidic environments of the alloys of the prior art are accelerated corrosion due to the continuous removal of the passive corrosion resistant layer by particles in solids containing corrosive fluid. This is especially evident in alloys containing a higher volume of Chrome and Molybdenum, where significant amount of sigma phase is unavoidable and the metal matrix possesses very poor toughness. In order to restore the passive layer, it is necessary to have the Chrome and the Molybdenum concentration at as high a level as possible.
Increasing the Chrome/Carbon, or Cr+Mo/C ratio, increases corrosion resistance up to the critical point, after which begins the formation of the sigma phase, which drastically reduces the toughness and lowers the corrosion resistance of the alloy by de
Greenblum & Bernstein P.L.C.
Oltmans Andrew L.
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