Bolt Grades And Markings

	UNIFIED BOLTS
				MECHANICAL PROPERTIES
HEAD MARKING	GRADE AND MATERIAL		Nominal Size Range (inches)	Proof Load (psi)	Min. Yield Strength (psi)	Min. Tensile Strength
NO MARKINGS	GRADE 2 Low or medium carbon steel		1/4 thru 3/4	55,000	57,000	74,000 psi 510 Mpa
			Over 3/4 thru 1-1/2	33,000	36,000	60,000 psi 414 Mpa
3 RADIAL LINES	GRADE 5 Medium Carbon Steel, Quenched and Tempered		1/4 thru 1	85,000	92,000	120,000 psi 828 Mpa
			Over 1 thru 1-1/2	74,000	81,000	105,000 psi 724 Mpa
3 RADIAL LINES	GRADE 5.2 Medium Carbon Alloy Steel, Quenched and Tempered		1/4 thru 1-1/2	120,000	130,000	120,000 psi 828 Mpa
5 RADIAL LINES	GRADE 7 Medium Carbon Alloy Steel		1/4 thru 1-1/2	120,000	130,000	133,000 psi 917 Mpa
6 RADIAL LINES	GRADE 8 Medium Carbon Alloy Steel, Quenched and Tempered		1/4 thru 1-1/2	120,000	130,000	150,000 psi 1034 Mpa
6 RADIAL LINES	GRADE 8.2 Medium Carbon Alloy Steel, Quenched and Tempered		1/4 thru 1-1/2	120,000	130,000	150,000 psi 1034 Mpa

ISO property classes for steel bolts, screws, studs and nuts

The strength of standard ISO metric steel bolts, studs and nuts is readily identified by means of a numerical code.

Property classes for steel bolts, screws and studs

The code is comprised of two numbers separated by a dot. This dot is not a decimal marker but is merely a means of separating the two parts of the code. The number to the left of the dot when multiplied by 100 provides an indication of the Ultimate Strength in Mega Pascals (MPa) while the number to the right when multiplied by 10 times the preceeding number gives the Yield Strength (Mega Pascals - MPa) or Stress at 0,2% Permanent Set (MPa) depending on the property class; e.g. 8,8

Ultimate Strength 8 x100 = 800MPa

Yield Strength or Stress at 0,2% permanent Set 8 x 8 x 10 = 640 MPa

Property class Designation for Nuts

The corresponding code for nuts is a single number and is derived from the left hand number of the property class of the bolt, screw and stud. This number when multiplied by 100 indicates the Proof Load Stress of the nut in MPa. Thus, for the bolt of property class 8,8 in the above example a nut of property class 8 is required. This nut will have a Proof Load Stress of 8 x 100 = 800 MPa

In practice only standard threaded fasteners in the high tensile property classes, i.e. property class 8,8 and above are required to be marked with the appropriate code.

METRIC BOLTS
				MECHANICAL PROPERTIES
HEAD MARKING	GRADE AND MATERIAL		Nominal Size Range (mm)			Min. Tensile Strength
NO MARKINGS	CLASS 4.6 Low or Medium Carbon Steel		5mm - 68mm	225	235	400 MPa 58015 psi
NO MARKINGS	CLASS 4.8 Low or Medium Carbon Steel		1.6mm - 68mm	310	315	430 Mpa 62366 psi
NO MARKINGS	CLASS 5.8 Low or Medium Carbon Steel		5mm - 68mm	380	392	520 Mpa 75419 psi
8.8	CLASS 8.8 Medium Carbon Steel, Quenched and Tempered		All Sizes below 16mm	580	628	800 Mpa 116030 psi
			16mm - 72mm	600	628	830 Mpa> 120381 psi
9.8	CLASS 9.8 Medium Carbon Steel, Quenched and Tempered		1.6mm - 68mm	650	660	900 Mpa 130533 psi
10.9	CLASS 10.9 Alloy Steel, Quenched and Tempered		5mm - 68mm	830	833	1040 Mpa 150839 psi
12.9	CLASS 12.9 Alloy Steel, Quenched and Tempered		1.6mm - 68mm	970	1060	1220 MPa 176946 psi
14.9	CLASS 14.9 Alloy Steel, Quenched and Tempered		1.6mm - 68mm	1090	1236	1400 MPa 203052 psi

A2 Stainless

Steel alloy with 17- 19%
Chromium and 8-13% Nickel

500 Mpa

72518 psi

700 Mpa

101526 psi

800 Mpa

116030 psi

A4 Stainless

Steel alloy with 17- 19%
Chromium, 8-13% Nickel
and molybdenum 3%

500 Mpa

72518 psi

700 Mpa

101526 psi

800 Mpa

116030 psi

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.