TESTS FOR HARDNESS. 



277 



material tested. This value is taken as the measure of 

 the indentation hardness of the metal tested. Writing 

 this equation in the form 



it will be seen that p and i can be measured during the 

 test, the index 1*2 being practically constant for all ductile 

 metals. 



The actual value of this index was found by Professor 

 Unwin by plotting curves showing the relation between 

 the logarithms of the indentations and the logarithms of 

 the loads per inch of width of the indenting tool, and 

 finding the slope of the straight line, which form the curve 

 takes. By using this test Unwin has made it possible 

 to make actual quantitative tests of the hardness of 



f- 



FIG. 139. 



ADMIRALTY HARDENING AND 

 COLD BENDING TEST. 



FIG. 140. 



ADMIRALTY HOT BENDING TEST FOR 

 WROUGHT IRON. 



various metals, in the place of simply establishing scales 

 of relative hardness. The following is a table of some of 

 the values obtained : 



HARDNESS. 

 Metal. Values of C. 



Cast steel, normal 554-0 



Mild steel, normal 143-5 



Copper, annealed 62-0 



,, unannealed 105-2 



Brass, No. 1 221-0 



No. 2 246-0 



Aluminium, squirted 41-8 



,, alloy, cast 103-5 



Lead, cast 4-2 



Zinc, cast 40-8 



