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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.