Method of producing a tube of a duplex stainless steel

Abstract

The present disclosure relates to a method of producing a tube of a duplex stainless steel, in particular a duplex stainless steel suitable for use in fuel injection systems for injection of fuel into the combustion chamber of a combustion engine.

1 . A method of producing a tube of duplex stainless steel comprising the following composition, in weight %, 
       
         
           
                 
                 
                 
               
                     
                     
                 
                     
                   C 
                   max 0.06; 
                 
                     
                   Cr 
                     21-24.5; 
                 
                     
                   Ni 
                    2.0-5.5; 
                 
                     
                   Si 
                   max 1.5; 
                 
                     
                   Mo 
                   0.01-1.0  
                 
                     
                   Cu 
                   0.01-1.0; 
                 
                     
                   Mn 
                   max 2.0; 
                 
                     
                   N 
                   0.05-0.3; 
                 
                     
                   P 
                   max 0.04; 
                 
                     
                   S 
                   max 0.03; 
                 
                     
                     
                 
             
                
               
               
                
                
                
                
                
                
                
                
                
                
                
               
            
           
         
       
       and
 a balance of Fe and unavoidable impurities, and with a PRE-value of at least 23.0, wherein the method comprises the steps of:
 a) providing a melt of the duplex stainless steel; 
 b) casting a body of the duplex stainless steel from the melt; 
 c) forming a bar of the body; 
 d) forming a tube of the bar by generating a hole therein; 
 e) reducing the diameter and/or wall thickness of the tube by hot extrusion ; 
 f) further reducing the diameter and/or wall thickness of the tube by cold deformation; and 
 g) annealing the cold deformed tube, wherein after step g), the duplex stainless steel of the tube includes 40-60% austenite and 40-60% ferrite and wherein step g) comprises subjecting said tube to a temperature in the range of from 950° C.-1060° C. for a time period of from 0.3-10 minutes and to an atmosphere consisting of a gas mixture comprising 1-6 vol % nitrogen gas and the remainder is H 2  or an inert gas. 
 
 
     
     
         2 . The method according to  claim 1 , wherein said temperature range is of from 970° C.-1040° C. 
     
     
         3 . The method according to  claim 1 , wherein said temperature range is of from 1000° C.-1040° C. 
     
     
         4 . The method according to  claim 1 , wherein said annealing step comprises subjected said tube to said temperature for a time period of from 0.5 to 5 minutes. 
     
     
         5 . The method according to  claim 1 , wherein said inert gas is argon or helium or a mixture thereof. 
     
     
         6 . The method according to  claim 1 , wherein the content of nitrogen gas in said gas mixture is equal to or less than 4 vol %. 
     
     
         7 . The method according to  claim 1 , wherein the content of nitrogen gas in said gas mixture is equal to or above 1.5 vol %. 
     
     
         8 . The method according to  claim 1 , wherein step e) comprises subjecting the tube to said hot extrusion at a temperature in the range of from 1100° C. to 1200° C. and a cross-sectional area reduction in the range of from 92-98%. 
     
     
         9 . The method according to  claim 1 , wherein step f) comprises subjecting, without pre-heating, the tube to cold deformation. 
     
     
         10 . The method according to  claim 1 , wherein step f) comprises subjecting the tube to a cross sectional area reduction in the range of from 50-95%. 
     
     
         11 . The method according to  claim 1 , wherein the cold deformation is pilgering. 
     
     
         12 . The method according to  claim 11 , wherein, in said pilgering step, the relationship between the wall thickness reduction and the outer diameter reduction of the tube is expressed as a Q-value, wherein the
     Q -value=( Wallh−Wallt )*( Odh−Wallh )/ Wallh  (( Odh−Wallh )−( Odt−Wallt )),
   
       wherein
 Wallh=hollow wall=the thickness of the wall before pilgering 
 Wallt=tube wall=the thickness of the wall after pilgering 
 Odh=hollow OD=the diameter of the tube before pilgering 
 Odt=tube OD=the diameter of the tube after pilgering, 
 
       and wherein Q is in the range of from 0.5-2.5. 
     
     
         13 . The method according to  claim 12 , wherein Q is in the range of from 0.9-1.1. 
     
     
         14 . The method according to  claim 1 , wherein said duplex stainless steel comprises, in weight %: 
       
         
           
                 
                 
                 
               
                     
                     
                 
                     
                   C 
                    0.01-0.025; 
                 
                     
                   Si 
                   0.35-0.6;  
                 
                     
                   Mn 
                   0.8-1.5; 
                 
                     
                   Cr 
                     21-23.5; 
                 
                     
                   Ni 
                   3.0-5.5; 
                 
                     
                   Mo 
                   0.10-1.0;  
                 
                     
                   Cu 
                   0.15-0.70; 
                 
                     
                   N 
                   0.090-0.25;  
                 
                     
                   P 
                   less than or equal to 0.035; 
                 
                     
                   S 
                   less than or equal to 0.003; 
                 
                     
                     
                 
             
                
               
               
                
                
                
                
                
                
                
                
                
                
                
               
            
           
         
       
       and
 a balance of Fe and unavoidable impurities. 
 
     
     
         15 . The method according to  claim 1 , wherein the tube is a tube arranged for conduction of a fuel in a fuel injection system for injecting fuel into the combustion chamber of a combustion engine.