ASTM Hardness Minimum Thickness Requirements

Minimum allowable thickness for a corresponding hardness in the respective scales

Minimum Minimum - Rockwell - - - Superficial Rockwell - -
thickness thickness C A B 15N 30N 45N 15T 30T 45T
in. mm.                  
                     
0.006 0.15       92          
0.008 0.20       90          
0.010 0.25       88     91    
0.012 0.30       83 82 77 86    
0.014 0.36       76 78.5 74 81 80  
0.016 0.41   86   68 74 72 75 72 71
0.018 0.46   84     66 68 68 64 62
0.020 0.51   82     57 63   55 53
0.022 0.56 69 79     47 58   45 43
0.024 0.61 67 76 94     51   34 31
0.026 0.66 65 71 87     37     18
0.028 0.71 62 67 80     20     4
0.030 0.76 57 60 71            
0.032 0.81 52   62            
0.034 0.86 45   52            
0.036 0.91 37   40            
0.038 0.96 28   28            
0.040 1.02 20                

The above tables are from ASTM E 18.
Reprinted from the Annual Book of ASTM Standards, copyright American Society for Testing and Materials.