Corrosion resistant austenitic stainless steel containing 0.1 to 0.3 percent manganese
This is a continuation of application Ser. No. 903,258 filed May 5, 1978, now abandoned. 
    
    
     The present invention relates to an austenitic stainless steel, which is completely or practically completely austenitic in a hot worked, solution heat-treated state, with properties characteristic for such steel and with a corrosion resistance clearly superior to the AISI 304 type of 18/8-steel, at least on a par with AISI 316 type of acid resistant steel 17/12-2Mo. 
     It is well known that the acid-resistant steels, e.g. AISI 316, are superior to the stainless steels, e.g. AISI 304, not only with regard to general corrosion resistance in reducing and weakly oxidizing acids such as hydrochloric acid, sulphuric acid and various organic acids, but also against atmospheric corrosion and the especially dangerous type of local corrosion such as pitting and crevice corrosion caused by chlorides. 
     More particularly, the invention relates to an austenitic stainless steel of the AISI 304 type which, by alloying with copper in combination with a minor manganese content as well as alloying with boron, gives a corrosion resistance clearly superior to that applying for the AISI 304 steel, and at least on a par with that for AISI 316, while at the same time retaining a completely austenitic structure and the good manufacturing and utilization properties of the austenitic steels. 
     The object of the invention is to obtain an acid-resistant steel, novel from the point of view of alloying techniques, with corrosion properties which are as good as, or better than those applying for the AISI 316 type of steel. 
     In order to attain the object of the invention regarding resistance to corrosion, it is necessary to lower the manganese content as well as to alloy copper. Boron is added primarily to compensate for the somewhat negative effect a lowered manganese content and the alloying of copper has on hot ductility, and therefore to fulfil the object of the invention that manufacturing properties shall also be satisfactory. 
     With regard to chemical composition, the invention can be exemplified by the following typical analysis: 18% Cr, 9% Ni, 3% Cu, 0.15% Mn, 15 ppm B. 
     The novel and characterizing composition of a steel according to the invention is apparent from the patent claims. 
     The effects of the individual alloying substances Mn, Cu and B in austenitic stainless steel of the AISI 304 type, with respect to corrosion resistance are as follows: 
     Manganese: Lowering the Mn-content from the normal ratio of 1 to 2% down to 0.1 to 0.2% results in an increase in the initiating resistance to pitting and crevice corrosion to at least that applicable for the AISI 316 type of steel. The reason for this positive effect is that the manganese sulphide normal in austenitic stainless steel is transformed into a chemical resistant chromium sulphide when the manganese content of the steel is lowered. The normal manganese sulphide has poor chemical durability and serves as initiation location for pitting and crevice corrosion. The amount of manganese may not be reduced completely arbitrarily however, since initiation resistance begins to drop at contents below 0.05 to 0.10%. With regard to the resistance against propagation of pitting and crevice corrosion, general corrosion in weakly oxidizing acids and stress corrosion, lowering the manganese content only results in a marginally positive effect. 
     Copper: 2 to 3% copper does not notably affect initiation resistance to pitting and crevice corrosion, but has a strongly positive effect on the resistance to propagation, and alloying copper similarly drastically improves the general corrosion resistance of the austenitic stainless steels in hydrochloric acid, sulphuric acid and remaining reducing and weakly oxidizing acids, and as a consequence, resistance to atmospheric corrosion and corrosion fatigue increases. 
     Boron: In amounts laid down by the invention, boron has a positive effect on the resistance to intercrystalline corrosion caused by heat treatment in the temperature range 600°-800° C. For the remainder, boron has no notable effect on corrosion resistance. 
     With regard to the effect of manganese, copper and boron on remaining properties, the following may be stated: 
     Lowered manganese content causes a certain depreciation of hot ductility and somewhat less austenitic stability. 
     Alloying copper increases austenitic stability and decreases cold hardening. 
     Alloying boron improves hot ductility. 
    
    
     The invention will now be more closely described in conjunction with a series of steels having compositions falling in the vicinity of, or within the purview of the invention. 
     Corrosion resistance, strength and hot ductility have been investigated for steels according to the invention, their chemical composition being accounted for in table A, such as the steels 1-3 and 5-6. The compositions of the AISI steels 304 and 316 are given for comparison. 
     The initiation resistance to pitting (and crevice corrosion) is denoted by the &#34;pitting potential&#34; in table B. The higher the potential, the better its initiation resistance. 
     A total effect of the corrosion initiation and propagation resistance is accounted for in table C in the form of loss in weight obtained in 10% FeCl 3 . 
     Resistance to strongly oxidizing acids is exemplified by the general corrosion speed in 65% boiling nitric acid (the Huey-test). See table D. 
     In table E are shown the results from potentio-dynamic anodic polarisation in sulphuric acid in the form of corrosion potential, passifying potential and critical passifying current density, I crit . The most important parameter I crit  indicates how easily a steel is passified. The lower I crit  is, the better the condition. 
     Corrosion resistance in sulphuric acid is accounted for in table F. 
     Corrosion resistance in hydrochloric acid is accounted for in table G. 
     Tensional strength is accounted for in table H. 
     Hot ductility, i.e. the effect of boron, is shown in table J. 
     It will be seen from tables B to J that steel according to the invention has corrosion resistance in environments and for types of corrosion typical for steel of the AISI 316 type, which are as good as or better than those applying for this steel, that the strength is normal for austenitic stainless steel and that boron clearly improves the hot ductility of the steel. 
     
                                           TABLE A                                 
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Chemical composition                                                      
Steel Nb Ti C  Si                                                         
                 Mn P  S  Cr Ni Mo N  Cu B ppm                            
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1     -- 0.48                                                             
            .050                                                          
               .55                                                        
                 .11                                                      
                    .021                                                  
                       .011                                               
                          19.7                                            
                             9.0                                          
                                .20                                       
                                   .032                                   
                                      3.16                                
                                         15                               
2     0.70                                                                
         -- .055                                                          
               .55                                                        
                 .12                                                      
                    .021                                                  
                       .019                                               
                          19.1                                            
                             7.5                                          
                                .50                                       
                                   .031                                   
                                      3.65                                
                                          5                               
3     -- -- .048                                                          
               .55                                                        
                 .13                                                      
                    .015                                                  
                       .011                                               
                          18.7                                            
                             9.1                                          
                                .11                                       
                                   .031                                   
                                      2.12                                
                                         20                               
5     -- 0.45                                                             
            .045                                                          
               .46                                                        
                 .30                                                      
                    .011                                                  
                       .014                                               
                          18.4                                            
                             9.1                                          
                                .24                                       
                                   .023                                   
                                      2.65                                
                                         --                               
6     0.45                                                                
         -- .032                                                          
               .49                                                        
                 .26                                                      
                    .011                                                  
                       .012                                               
                          18.5                                            
                             8.9                                          
                                .22                                       
                                   .029                                   
                                      2.63                                
                                         13                               
7 AISI 304  .034                                                          
               .47                                                        
                 1.08                                                     
                    .024                                                  
                       .011                                               
                          18.3                                            
                             8.6                                          
                                .17                                       
                                   .020                                   
                                      -- --                               
8 AISI 316  .029                                                          
               .38                                                        
                 1.48                                                     
                    .029                                                  
                       .005                                               
                          16.7                                            
                             11.7                                         
                                2.53                                      
                                   .021                                   
                                      -- --                               
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                       TABLE B                                                     
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Pitting potential, Ep, in 0.1 M NaCl at 25° C.                     
Steel           Ep in mV E.sub.sce (average)                              
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1-3             450                                                       
AISI 304        300                                                       
AISI 316        420                                                       
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                       TABLE C                                                     
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 Pitting in 10% FeCl.sub.3 at room temperature, for a period of           
24 hours                                                                  
Steel             Corrosion, g/m.sup.2 . h                                
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3                 3.1                                                     
7 AISI 304        9.8                                                     
8 AISO 316        3.8                                                     
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                       TABLE D                                                     
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Huey test.                                                                
Corrosion in boiling 65% HNO.sub.3 for 5 periods at                       
48 h/period                                                               
Corrosion, g/m.sup.2 . h                                                  
period                                                                    
Steel   1        2        3      4      5                                 
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2       0.25     0.17     0.17   0.18   0.19                              
3       0.22     0.15     0.15   0.17   0.15                              
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                       TABLE E                                                     
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Potentio-dynamic anodic polarization in 1 M H.sub.2 SO.sub.4 at           
25° C.                                                             
mV E.sub.sce           μA/cm.sup.2                                     
        Corrosion  Passification                                          
                               Critical passification                     
Steel   potential  potential   current density                            
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1-3     -320 a     -230 a      50 a 30                                    
        -330       -260                                                   
AISI 304                                                                  
        -380 a     -310 a      310 a 560                                  
        -410       -330                                                   
AISI 316                                                                  
        -320 a     -260 a      ˜100                                 
        -380       -310                                                   
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     TABLE F 
     Corrosion in sulphuric acid, weight loss 
     The experiments were carried out at room temperature for 1+3+3 periods of 24 hours, the solution being changed after each period. Before each of the three test periods, the samples were activated by means of a Zn rod. 
     
         ______________________________________                                    
           Corrosion, grams/m.sup.2 · h (mm/year)                
Steel                                                                     
     % Cu    H.sub.2 SO.sub.4                                             
                     24 hours                                             
                            3 × 24 h                                
                                   3 × 24 h                         
                                          Average                         
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3    2.12            0      0.08   0                                      
1    3.16    10      0      0.01   0                                      
2    3.65            0.01   0.01   0                                      
&#34;    &#34;               0.11   0.22   0.19   0.19                            
&#34;    &#34;       15      0.06   0.19   0.15   0.15                            
&#34;    &#34;               0.08   0.04   0      0.03                            
&#34;    &#34;               0.31   0.22   0.21   0.23                            
&#34;    &#34;       20      0.25   0.23   0.19   0.21                            
&#34;    &#34;               0.21   0.20   0.08   0.15                            
&#34;    &#34;               0.46   0.22   0.11   0.21                            
&#34;    &#34;       25      0.24   0.15   0.08   0.13                            
&#34;    &#34;               0.20   0.15   0.11   0.14                            
&#34;    &#34;               0.62   0.19   0.18   0.25                            
&#34;    &#34;       30      0.26   0.14   0.10   0.14                            
&#34;    &#34;               0.22   0.15   0.11   0.14                            
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     As is apparent from the tables, all three steels according to the invention are in a stably passive condition at room temperature in 10% H 2  SO 4  and steel No. 2 (with 3.65% Cu) in 15% H 2  SO 4  as well. For a further increased H 2  SO 4  concentration (15→30%), the corrosion rate only increases very marginally. Excepting period 1 (which is as it should be), the corrosion rate is 0.14 g/m 2  ·h (approved result) for the steels with the highest Cu-content, and 0.19 g/m 2  ·h for the steel having the lowest Cu-content. 
     The steel type AISI 304 is only good for 3% H 2  SO 4 , while the steel type AISI 316 is good for about 20% H 2  SO 4 . 
     
                       TABLE G                                                     
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Corrosion in HCl. Room temperature. Time 24 hours                         
                        Corrosion g/m.sup.2 . h                           
Steel        HCl %      (mm/year)                                         
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3            1          0.05                                              
1            1          0.00                                              
2            1          0.01                                              
7 AISI 304   1          0.58                                              
8 AISI 316   1          0.00                                              
&#34;            2          0.12                                              
&#34;            2          0.12                                              
&#34;            2          0.14                                              
&#34;            2          1.42                                              
&#34;            2          0.38                                              
&#34;            3          0.15                                              
&#34;            3          0.16                                              
&#34;            3          0.18                                              
&#34;            3          1.63                                              
&#34;            3          0.57                                              
&#34;            5          0.15                                              
&#34;            5          0.14                                              
&#34;            5          0.18                                              
&#34;            5          0.82                                              
&#34;            5          0.78                                              
&#34;            10         0.21                                              
&#34;            10         0.18                                              
&#34;            10         0.44                                              
&#34;            10         1.43                                              
&#34;            10         1.58                                              
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     According to this table and available corrosion tables, the AISI 304 and 316 types of steel are only good for 0.1% and 1.0% HCl, respectively, at room temperature and for stationary solution. Furthermore, the AISI 304 and 316 steels are attacked at a considerably greater rate e.g. for 10% HCl than the steel according to the invention, when all three steels are in an active state. 
     
                       TABLE H                                                     
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Tensional strength                                                        
                    Rp 0.2    Rm      A5                                  
Steel     % Cu      N/mm.sup.2                                            
                              N/mm.sup.2                                  
                                      %                                   
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1         3.16      254       560     52                                  
2         3.65      256       557     52                                  
3         2.12      247       547     59                                  
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     TABLE J 
     Hot ductility according to hot tensile strength test with the strain rate of 10·s -1  for temperature 950°-1250° C. 
     The samples are taken perpendicular to the fusion direction from the ingot halves. 
     
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                    Area contraction in %                                 
                    Average value 950,                                    
Steel               1000 and 1950° C.                              
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5      According to the invention                                         
                        22                                                
6      According to the invention                                         
                        38                                                
4      Steel type AISI 306 with                                           
       2.7% Cu          40                                                
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