Method for shot peening a gas carburised steel

Abstract

The present invention is to provide a method for shot peening for producing a high compressive residual stress in a gas carburized steel that has a soft layer. In this method, a depth where the maximum compressive residual stress is generated is estimated and the hardness on the surface or near the surface is not used. The depth where the maximum compressive residual stress is generated is estimated by multiplying the depth where the maximum stress is generated under contact stresses caused by the collision of shot media by the constant K. A processed steel that comprises a gas carburized steel and that has a hardness at that depth that exceeds 750 HV is used. Shot media that have a hardness that is greater than that of the processed steels at that depth by 50 HV or more are shot onto the processed steels to produce a high compressive residual stress in the processed steels.

1 . A method for shot peening,
 wherein a processed steel comprises a gas carburized steel that has hardness at 750 HV or more at a depth z, where a maximum compressive residual stress is generated, the depth z being estimated by using Equations (1) to (4), and   wherein shot media that have a hardness that is greater than the hardness of the processed steel by 50 HV or more are shot onto the processed steel to produce a compressive residual stress in the processed steel:   
       
         
           
             
               
                 
                   
                     α 
                     = 
                     
                       
                         1 
                         
                           8 
                            
                           
                             E 
                             
                               * 
                               
                                 3 
                                 5 
                               
                             
                           
                         
                       
                        
                       
                         
                           ( 
                           
                             
                               5 
                                
                               π 
                             
                             4 
                           
                           ) 
                         
                         
                           s 
                           5 
                         
                       
                        
                       
                         ρ 
                         
                           3 
                           5 
                         
                       
                        
                       
                         D 
                         3 
                       
                        
                       
                         V 
                         
                           6 
                           5 
                         
                       
                     
                   
                 
                 
                   
                     ( 
                     1 
                     ) 
                   
                 
               
               
                 
                   
                     
                       1 
                       
                         E 
                         * 
                       
                     
                     = 
                     
                       
                         
                           1 
                           - 
                           
                             v 
                             1 
                             2 
                           
                         
                         
                           E 
                           1 
                         
                       
                       + 
                       
                         
                           1 
                           - 
                           
                             v 
                             2 
                             2 
                           
                         
                         
                           E 
                           2 
                         
                       
                     
                   
                 
                 
                   
                     ( 
                     2 
                     ) 
                   
                 
               
               
                 
                   
                     z 
                     = 
                     
                       0.48 
                        
                       K 
                        
                       
                           
                       
                        
                       α 
                     
                   
                 
                 
                   
                     ( 
                     3 
                     ) 
                   
                 
               
               
                 
                   
                     K 
                     = 
                     1.25 
                   
                 
                 
                   
                     ( 
                     4 
                     ) 
                   
                 
               
             
           
         
         where 
         α: contact radius by shot media (m), 
         p: specific gravity of shot media (kg/m 3 ), 
         D: diameter of shot media (m), 
         V: shot speed(m/s), 
         E*: equivalent elastic modulus 
         E 1 : Young's modulus of the processed steel(Pa), 
         v 1 : Poisson's ratio of the processed steel, 
         E 2 : Young's modulus of shot media (Pa), 
         v 2 : Poisson's ratio of shot media, 
         K: constant, and 
         z: depth where the maximum compressive stress is generated (m). 
       
     
     
         2 . The method for shot peening of  claim 1 , wherein diameters of the shot media are in a range from 0.2 mm to 1.0 mm.