Stainless alloys for enhanced corrosion resistance

Abstract

An improved stainless alloy has application in handling concentrated sulfuric acid. The alloy of the invention preferably comprises 23 to 33% chromium, more than 20% nickel, 6% or more of silicon, 0.2% to 0.4% nitrogen and minor amounts of molybdenum, copper, and tungsten with the balance iron. The alloy offers superior corrosion resistance by comparison with the standard stainless steels over the range from 90% sulfuric acid into the oleum range. The alloy is readily weldable and may be used in either cast or wrought form. After heat treating the alloy acquires a duplex structure which has a mixture of austenite and ferrite phases. The heat-treated alloy is ductile.

We claim:  
     
         1 . A weldable alloy comprising: 
 24% to 36% chromium;    6% to 10% silicon;    at least 18% nickel;    0.5% to 6% of an alloying element selected from the group consisting of copper, molybdenum, and a mixture of copper and molybdenum, the copper not exceeding 4%;    0.05 to 0.45% nitrogen; and    the balance comprising iron and incidental impurities,    the alloy being heat treated at a temperature in the range of 1100° C. to 1200° C. to produce a duplex microstructure consisting of a ferrite phase and a austenite phase, the ferrite phase being about 24% to 30% by volume, and the austenite phase being the balance, the alloy further having a tensile elongation in excess of 40%.    
     
     
         2 . The alloy of  claim 1  wherein the ratio of silicon to chromium is in the range of 0.25 to 0.36.  
     
     
         3 . The alloy of  claim 1  wherein the relative amounts of silicon and chromium are expressed by the formula: 
         W   Si =(0.33±0.06)× W   Cr   where W Si  is the percentage of silicon and W Cr  is the percentage of chromium.    
     
     
         4 . The alloy of  claim 1  wherein the ratio of the chromium content of the austenite phase and the chromium content of the ferrite phase is 1±0.2, and the ratio of the silicon content of the austenite phase and the silicon content of the ferrite phase is 1±0.07.  
     
     
         5 . The alloy of  claim 1  further comprising 2% to 4% tungsten.  
     
     
         6 . The alloy of  claim 1  further comprising 2% to 4% vanadium.  
     
     
         7 . An acid contacting component in a plant for manufacturing concentrated sulfuric acid by the contact process, the component having an acid-contacting surface made from the alloy of any one of the above claims.  
     
     
         8 . The use of the alloy of  claim 1 , as an acid-contacting component in a heat exchanger in a process for recovering heat from acid having a concentration in excess of 95% and temperature in excess of 130° C.  
     
     
         9 . The use of the alloy of  claim 1 , as an acid-contacting component in a heat exchanger wherein one surface of the component is in contact with concentrated sulfuric acid and another face of the component is in contact with concentrated phosphoric acid.  
     
     
         10 . A stainless steel alloy consisting essentially of: 
 24% to 36% chromium;    6% to 8% silicon;    18% to 32% nickel;    0.2% to 0.45% nitrogen;    2% to 4% copper;    and, optionally,    0% to 2% molybdenum, 2% to 4% tungsten, 2% to 4% vanadium and 0% to 0.05% carbon;    the balance comprising iron and incidental impurities.    
     
     
         11 . An alloy comprising 
 22% to 36% chromium,    6% to 10% silicon,    0.5 to 6% of an alloying element selected from the group consisting of up to 4% copper, molybdenum, and a mixture of up to 4% copper and molybdenum;    0.05 to 0.45% nitrogen; and,    the balance iron and nickel.    
     
     
         12 . The alloy of  claim 11  comprising molybdenum in an amount of less than 5%.  
     
     
         13 . The alloy of  claim 11  comprising chromium in an amount greater than 23%.  
     
     
         14 . The alloy of  claim 13  wherein the relative amounts of silicon and chromium are expressed by the formula: 
         W   Si =(0.33±0.06)× W   Cr   where W Si  is the percentage of silicon and W Cr  is the percentage of chromium.    
     
     
         15 . The alloy of  claim 13  wherein the ratio of silicon to chromium is in the range of 0.25 to 0.35.  
     
     
         16 . The alloy of  claim 10  comprising approximately 6.6% silicon and approximately 23% chromium.  
     
     
         17 . The alloy of  claim 11  comprising 0.2% to 0.4% nitrogen.  
     
     
         18 . The alloy of  claim 11  comprising 2% to 4% tungsten.  
     
     
         19 . The alloy of  claim 11  comprising 2% to 4% vanadium.  
     
     
         20 . The alloy of  claim 11  comprising copper in an amount of less than 4%.  
     
     
         21 . A stainless steel alloy consisting essentially of: 
 22% to 36% chromium;    6% to 8% silicon;    18% to 32% nickel;    0% to 4% copper; and,    the balance iron.    
     
     
         22 . A method for making a workable metallic alloy which is resistant to corrosion by sulfuric acid, the method comprising: 
 a) alloying at least 22% chromium, at least 6% silicon, at least 20% nickel, up to 4% copper, and iron; and,    b) heat treating the resulting alloy at a temperature sufficient to cause the alloy to become ductile and to have a duplex structure consisting of a mixture of ferrite and austenite.    
     
     
         23 . The method of  claim 22  wherein the heat treating is performed at a temperature in the range of 1100° C. to 1200° C.  
     
     
         24 . The method of  claim 22  wherein, after heat treatment the alloy has a duplex microstructure characterized by 24% to 30% by volume ferrite and the balance austenite.  
     
     
         25 . The alloy of  claim 2  further comprising 2% to 4% tungsten.  
     
     
         26 . The alloy of  claim 3  further comprising 2% to 4% tungsten.  
     
     
         27 . The alloy of  claim 4  further comprising 2% to 4% tungsten.  
     
     
         28 . The alloy of  claim 2  further comprising 2% to 4% vanadium.  
     
     
         29 . The alloy of  claim 3  further comprising 2% to 4% vanadium.  
     
     
         30 . The alloy of  claim 4  further comprising 2% to 4% vanadium.  
     
     
         31 . The alloy of  claim 5  further comprising 2% to 4% vanadium.  
     
     
         32 . The use of the alloy of  claim 2  as an acid-contacting component in a heat exchanger in a process for recovering heat from acid having a concentration in excess of 95% and temperature in excess of 130° C.  
     
     
         33 . The use of the alloy of  claim 3  as an acid-contacting component in a heat exchanger in a process for recovering heat from acid having a concentration in excess of 95% and temperature in excess of 130° C.  
     
     
         34 . The use of the alloy of  claim 4  as an acid-contacting component in a heat exchanger in a process for recovering heat from acid having a concentration in excess of 95% and temperature in excess of 130° C.  
     
     
         35 . The use of the alloy of  claim 5  as an acid-contacting component in a heat exchanger in a process for recovering heat from acid having a concentration in excess of 95% and temperature in excess of 130° C.  
     
     
         36 . The use of the alloy of  claim 6  as an acid-contacting component in a heat exchanger in a process for recovering heat from acid having a concentration in excess of 95% and temperature in excess of 130° C.  
     
     
         37 . The use of the alloy of  claim 2  as an acid-contacting component in a heat exchanger wherein one surface of the component is in contact with concentrated sulfuric acid and another face of the component is in contact with concentrated phosphoric acid.  
     
     
         38 . The use of the alloy of  claim 3  as an acid-contacting component in a heat exchanger wherein one surface of the component is in contact with concentrated sulfuric acid and another face of the component is in contact with concentrated phosphoric acid.  
     
     
         39 . The use of the alloy of  claim 4  as an acid-contacting component in a heat exchanger wherein one surface of the component is in contact with concentrated sulfuric acid and another face of the component is in contact with concentrated phosphoric acid.  
     
     
         40 . The use of the alloy of  claim 5  as an acid-contacting component in a heat exchanger wherein one surface of the component is in contact with concentrated sulfuric acid and another face of the component is in contact with concentrated phosphoric acid.  
     
     
         41 . The use of the alloy of  claim 6  as an acid-contacting component in a heat exchanger wherein one surface of the component is in contact with concentrated sulfuric acid and another face of the component is in contact with concentrated phosphoric acid.  
     
     
         42 . The alloy of  claim 12  comprising chromium in an amount greater than 23%.  
     
     
         43 . The alloy of  claim 11  comprising approximately 6.6% silicon and approximately 23% chromium.  
     
     
         44 . The method of  claim 23  wherein, after heat treatment the alloy has a duplex microstructure characterized by 24% to 30% by volume ferrite and the balance austenite.