Duplex stainless steel alloy for use in seawater applications

Abstract

The present invention relates to a stainless steel alloy, more precisely a duplex stainless steel alloy having ferritic-austenitic matrix and having high corrosion resistance in combination with good structural stability and hot-workability, in particular a duplex stainless steel having a ferritic content of 40-65% and a well-balanced composition, which gives the material corrosion properties making it more suitable for use in chloride-containing environments than what has been found possible previously. The material according to the present invention has, in view of the high alloy content thereof, extraordinarily good workability, in particular hot-workability, and should thereby be very suitable to be used for, for instance, the manufacture of bars: pipes, such as welded and weld less pipes, weld material, construction parts, such as, for instance, flanges and couplings. These objects are met according to the present invention with duplex stainless alloy, which contain (in % by weight): C more than 0 and up to max 0.03 Si up to max 0.5 Mo 0-3.0 20 Cr 24.0-30.0 Ni 4.9-10.0 Mo 3.0-5.0 N 0.28-0.5 B 0-0.0030 25 S up to max 0.010 Co 0-3.5 W 0-3.0% Cu 0-2.0 Ru 0-0.3 3 0 Al 0-0.03 Ca 0-0.010%, balance Fe together with inevitable contaminations.

1 . Ferrite-austenitic duplex stainless steel alloy having the following composition (in % by weight):  
     
       
         
               
               
               
             
                   
                   
               
                   
                   
               
                   
                 C 
                 more than 0 and up to max 0.03% 
               
                   
                 Si 
                 up to max 0.5% 
               
                   
                 Mn 
                 0-3.0% 
               
                   
                 Cr 
                 24.0-30.0%   
               
                   
                 Ni 
                 4.9-10.0%  
               
                   
                 Mo 
                 3.0-5.0%   
               
                   
                 N 
                 0.28-0.5%   
               
                   
                 B 
                   0-0.0030% 
               
                   
                 S 
                 up to max 0.010% 
               
                   
                 Co 
                 0-3.5% 
               
                   
                 W 
                 0-3.0% 
               
                   
                 Cu 
                 0-2.0% 
               
                   
                 Ru 
                 0-0.3% 
               
                   
                 Al 
                  0-0.03% 
               
                   
                 Ca 
                  0-0.010% 
               
                   
                   
               
                   
                   
               
           
              
              
             
             
              
              
              
              
              
              
              
              
              
              
              
              
              
              
              
              
              
             
          
         
       
     
     as well as balance Fe together with normally occurring contaminations and additives, the ferrite content being 40-65% by volume and the relation PRE=% Cr+3.3% Mo+16% N exceeding 46 for the total composition of the alloy, as well as that PRE in austenite and ferrite phase exceeds 45 as well as that the yield point in tension Rp 0.2  of the alloy exceeds 720 N/mm 2 , as well as that CPT>90° C. as well as CCT≧60° C.  
   
   
       2 . Alloy according to  claim 1 , wherein the chromium content is between 26.5 and 29.0% by weight.  
   
   
       3 . Alloy according to  claim 1 , wherein the manganese content is between 0.5 and 1.2% by weight.  
   
   
       4 . Alloy according to  claim 1 , wherein the nickel content is between 5.0 and 8.0% by weight.  
   
   
       5 . Alloy according to  claim 1 , wherein the molybdenum content is between 3.6% and 4.9% by weight.  
   
   
       6 . Alloy according to  claim 1 , wherein the nitrogen content is between 0.35 and 0.45% by weight.  
   
   
       7 . Alloy according to  claim 1 , wherein the ruthenium content is between 0 and 0.3% by weight.  
   
   
       8 . Alloy according to  claim 1 , wherein the cobalt content is between 0.5 and 3.5% by weight.  
   
   
       9 . Alloy according to  claim 1 , wherein the copper content is between b  0 . 5  and 2.0% by weight.  
   
   
       10 . Alloy according to  claim 1 , wherein the ferrite content is between 42 and 60% by volume.  
   
   
       11 . Alloy according to  claim 1 , wherein the total PRE or PREW value of the alloy exceeds 46, wherein PRE=% Cr+3.3% Mo+16 N and PREW=% Cr+3.3 (% Mo+0.5% W)+16 N, wherein % relates to % by weight.  
   
   
       12 . Alloy according to  claim 11 , wherein the PRE or PREW value of both the ferrite and austenite phase is greater than 45 and the PRE or PREW value of the total alloy composition is greater than 46.  
   
   
       13 . Use of an alloy according to  claim 1  as umbilical cord pipe in chloride-containing environments.  
   
   
       14 . Use of an alloy according to  claim 1  for the manufacture of bars, pipes, weld material, and construction parts.  
   
   
       15 . Alloy according to  claim 7 , wherein the ruthenium content is greater than 0 and up to 0.1% by weight.  
   
   
       16 . Alloy according to  claim 8 , wherein the cobalt content is between 1.0 and 3.0% by weight.  
   
   
       17 . Alloy according to  claim 9 , wherein the copper content is between 1.0 and 1.5% by weight.  
   
   
       18 . Alloy according to  claim 10 , wherein the ferrite content is between 45 and 55% by volume.  
   
   
       19 . Use of an alloy according to  claim 13 , wherein said chloride-containing environments comprise sea-water environments.  
   
   
       20 . Use of an alloy according to  claim 14 , wherein said pipes comprise welded and weldless pipes, and said construction parts comprise flanges and couplings.