Bolt grade explained by JM Hardware®
Bolt head markings what do they mean? how to identify bolt grade? Determining bolt grade and head marking.
In this video you will know about bolt grades or fastener grade of metric fasteners and how to identify bolt strength by bolt grade.
What does the bolt grade mean?
Property Class is a designation system which is used to defines the strength of metric fasteners i.e Bolts nuts as per ISO898- part 1. The metric fasteners are designated by a symbol comprised of 2 numbers, separated by a decimal. For example 4.6 and 8.8. The property class is normally stamped on the head of the bolt. The first digit of bolt grade represents (1/100) th part of the nominal tensile strength or proof load stress in mega pascals. For example, a bolt with property class 4.6 have nominal tensile strength equal to 4 x 100 = 400 MPa.
Bolt Grade Markings and Strength Chart
US Bolts
| Head Marking | Grade and Material | Nominal Size Range (inches) | Mechanical Properties | ||
| Proof Load (psi) | Min. Yield Strength (psi) | Min. Tensile Strength (psi) | |||
| 307A Low carbon steel | 1/4" thru 4" | N/A | N/A | 60,000 |
No Markings | Grade 2 Low or medium carbon steel | 1/4" thru 3/4" | 55,000 | 57,000 | 74,000 |
| 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 |
| 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 |
 | Grade A325 Carbon or alloy steel with or without boron | 1/2" thru 1-1/2" | 85,000 | 92,000 | 120,000 |
Stainless Markings Vary | 18-8 & 316 Stainless Steel alloy with chromium and nicke | All sizes thru 1" | N/A | 20,000 Min. 65,000 Typical | 65,000 Min. 100,000 – 150,000 Typical |
 | 651 Silicon bronze An alloy of mostly copper and tin with a small amount of silicon | 1/4" thru 3/4" | N/A | 55,000 | 70,000 |
| 7/8" thru 1-1/2" | N/A | 40,000 | 55,000 | ||
 | Aluminum 2024 Aluminum alloy with copper, magnesium and manganese; solution heat treated and age hardened | All sizes | N/A | 36,000 | 55,000 |
Metric Bolts
| Head Marking | Class and Material | Nominal Size Range (mm) | Mechanical Properties | ||
| Proof Load (MPa) | Min. Yield Strength (MPa) | Min. Tensile Strength (MPa) | |||
 | Class 8.8 Medium carbon steel, quenched and tempered | All sizes below 16mm | 580 | 640 | 800 |
| 16mm - 72mm | 600 | 660 | 830 | ||
 | Class 10.9 Alloy steel, quenched and tempered | 5mm - 100mm | 830 | 940 | 1040 |
 | Class 12.9 Alloy steel, quenched and tempered | 1.6mm - 100mm | 970 | 1100 | 1220 |
| Usually Stamped A-2 or A-4 | A-2 & A-4 Stainless Steel alloy with chromium and nickel | All sizes thru 20mm | N/A | 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 maximum 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
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