Bolt Head Markings And Grades

Head Marking	Grade and Material	Nominal Size Range (inches)	Mechanical Properties
Head Marking	Grade and Material	Nominal Size Range (inches)	Proof Load (psi)	Min. Yield Strength (psi)	Min. Tensile Strength (psi)
US Bolts
No Markings	Grade 2 Low or medium carbon steel	1/4 thru 3/4	55,000	57,000	74,000
No Markings	Grade 2 Low or medium carbon steel	Over 3/4 thru 1-1/2	33,000	36,000	60,000
3 Radial Lines	Grade 5 Medium Carbon Steel, Quenched and Tempered	1/4 thru 1	85,000	92,000	120,000
3 Radial Lines	Grade 5 Medium Carbon Steel, Quenched and Tempered	Over 1 thru 1-1/2	74,000	81,000	105,000
6 Radial Lines	Grade 8 Medium Carbon Alloy Steel, Quenched and Tempered	1/4 thru 1-1/2	120,000	130,000	150,000
Stainless markings vary. Most stainless is non-magnetic	18-8 Stainless Steel alloy with 17-19% Chromium and 8-13% Nickel	1/4 thru 5/8		40,000 Min. 80,000 – 90,000 Typical	100,000 – 125,000 Typical
		3/4 thru 1		40,000 Min. 45,000 – 70,000 Typical	100,000 Typical
		Above 1		40,000 Min. 45,000 – 70,000 Typical	80,000 – 90,000 Typical
Head Marking	Class and Material	Nominal Size Range (mm)	Mechanical Properties
Head Marking	Class and Material	Nominal Size Range (mm)	Proof Load (MPa)	Min. Yield Strength (MPa)	Min. Tensile Strength (MPa)
Metric bolts
8.8	Class 8.8 Medium Carbon Steel, Quenched and Tempered	All Sizes below 16mm	580	640	800
8.8	Class 8.8 Medium Carbon Steel, Quenched and Tempered	16mm - 72mm	600	660	830
10.9	Class 10.9 Alloy Steel, Quenched and Tempered	5mm - 100mm	830	940	1040
12.9	Class 12.9 Alloy Steel, Quenched and Tempered	1.6mm - 100mm	970	1100	1220
Stainless markings vary. Most stainless is non-magnetic. Usually stamped A-2	A-2 Stainless Steel alloy with 17- 19% Chromium and 8-13% Nickel	All Sizes thru 20mm		210 Min. 450 Typical	500 Min. 700 Typical
Tensile Strength: The maximum load in tension (pulling apart) which a material can withstand before breaking or fracturing. Yield Strength: The load at which a material exhibits a specific permanent deformation. Proof Load: An axial tensile load which the product must withstand without evidence of any permanent set. 1MPa = 1N/mm² = 145 pounds/inch²