Table 1: Chemical composition of the drill bits.

Elements	C	Mn	Mo	Cr	Ni	P	S	Si	Fe
The unqualified	0.27	1.45	0.62	0.27	1.58	0.010	0.004	1.68	Bal
The qualified	0.28	1.48	0.62	0.32	1.41	0.013	0.006	1.67	Bal