Bolt Grades And Markings
UNIFIED BOLTS
| Head Marking | Grade & Material | Nominal Size Range (Inches) | Proof Load (PSI) | Min. Yield Strength (PSI) | Min. Tensile Strength |
|---|---|---|---|---|---|
 | GRADE 2 Low or medium carbon steel | 1/4 thru 3/4 | 55,000 | 57,000 | 74,000 psi 510 MPa |
| GRADE 2 Low or medium carbon steel | Over 3/4 thru 1-1/2 | 33,000 | 36,000 | 60,000 psi 414 MPa |
 | GRADE 5 Medium Carbon Steel, Quenched and Tempered | 1/4 thru 1 | 85,000 | 92,000 | 120,000 psi 828 MPa |
| GRADE 5 Medium Carbon Steel, Quenched and Tempered | Over 1 thru 1-1/2 | 74,000 | 81,000 | 105,000 psi 724 MPa |
 | 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 |
 | GRADE 7 Medium Carbon Alloy Steel | 1/4 thru 1-1/2 | 120,000 | 130,000 | 133,000 psi 917 MPa |
 | GRADE 8 Medium Carbon Alloy Steel, Quenched and Tempered | 1/4 thru 1-1/2 | 120,000 | 130,000 | 150,000 psi 1034 MPa |
 | 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 preceding 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 × 100 = 800 MPa
- Yield Strength or Stress at 0.2% permanent Set: 8 × 8 × 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 × 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
| Head Marking | Grade & Material | Nominal Size Range | Proof Load (MPa) | Min. Yield Strength (MPa) | Min. Tensile Strength |
|---|---|---|---|---|---|
| CLASS 4.6 Low or Medium Carbon Steel | 5mm - 68mm | 225 | 235 | 400 MPa 58015 psi |
| CLASS 4.8 Low or Medium Carbon Steel | 1.6mm - 68mm | 310 | 315 | 430 MPa 62366 psi |
| CLASS 5.8 Low or Medium Carbon Steel | 5mm - 68mm | 380 | 392 | 520 MPa 75419 psi |
 | CLASS 8.8 Medium Carbon Steel, Quenched and Tempered | All Sizes below 16mm | 580 | 628 | 800 MPa 116030 psi |
| CLASS 8.8 Medium Carbon Steel, Quenched and Tempered | 16mm - 72mm | 600 | 628 | 830 MPa 120381 psi |
 | CLASS 9.8 Medium Carbon Steel, Quenched and Tempered | 1.6mm - 68mm | 650 | 660 | 900 MPa 130533 psi |
 | CLASS 10.9 Alloy Steel, Quenched and Tempered | 5mm - 68mm | 830 | 833 | 1040 MPa 150839 psi |
 | CLASS 12.9 Alloy Steel, Quenched and Tempered | 1.6mm - 68mm | 970 | 1060 | 1220 MPa 176946 psi |
 | CLASS 14.9 Alloy Steel, Quenched and Tempered | 1.6mm - 68mm | 1090 | 1236 | 1400 MPa 203052 psi |
STAINLESS BOLTS
| Head Marking | Grade & Material | Class | Nominal Size Range | Proof Load (MPa) | Min. Yield Strength (MPa) | Min. Tensile Strength |
|---|---|---|---|---|---|---|
 | A2 Stainless Steel alloy with 17-19% Chromium and 8-13% Nickel | Class 50 | 1.6 - M39 | 500 | 210 Min. | 500 MPa 72518 psi |
| A2 Stainless Steel alloy with 17-19% Chromium and 8-13% Nickel | Class 70 | 1.6 - M24 | 700 | 450 Min. | 700 MPa 101526 psi |
| A2 Stainless Steel alloy with 17-19% Chromium and 8-13% Nickel | Class 80 | 1.6 - M24 | 800 | 600 Min. | 800 MPa 116030 psi |
 | A4 Stainless Steel alloy with 17-19% Chromium, 8-13% Nickel and molybdenum 3% | Class 50 | 1.6 - M39 | 500 | 210 Min. | 500 MPa 72518 psi |
| A4 Stainless Steel alloy with 17-19% Chromium, 8-13% Nickel and molybdenum 3% | Class 70 | 1.6 - M24 | 700 | 450 Min. | 700 MPa 101526 psi |
| A4 Stainless Steel alloy with 17-19% Chromium, 8-13% Nickel and molybdenum 3% | Class 80 | 1.6 - M24 | 800 | 600 Min. | 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.
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