The reduction of hardness of tubes No.C and No. B is the result of thermal energy that causes a change in internal energy including chemical bonding, strain energy and other energy sources. The graph also shows that the hardness of the tube No.B tends to be more constant than that of tube No.C because of the effect from the Cr23C6 with more area fraction and larger particle size, as shown in Figure 3-9 (a-b