Bolt Grade Markings and Strength

What a grade actually guarantees

A bolt grade is a material and processing specification, not a general seal of quality. What it commits to is narrow and mechanical: a minimum tensile strength, a minimum yield (or proof) strength, and a hardness range, each verified by a standard test method. Those numbers tell you how much static load the shank will carry before it stretches or breaks. What a grade does not tell you is just as important. It says nothing about corrosion resistance, temperature behaviour, plating adhesion, or thread fit — those are governed by separate specifications. Nor does a marking prove any given batch was made correctly; that is the job of the accompanying material certificate (MTR). Read the marking as a statement about strength, and nothing more.

Reading an inch grade from the head

Inch-series bolts produced to SAE J429 carry a radial-line code on the head. No lines means Grade 2, the general-purpose low-carbon choice. Three radial lines mean Grade 5, a quenched-and-tempered medium-carbon steel. Six radial lines mean Grade 8, a quenched-and-tempered medium-carbon alloy steel at higher strength. More lines mean more strength, but also a harder part more sensitive to hydrogen from plating. Count the lines you can see, not the class you assume. The full strength values sit in the inch-grades table on this page, so use it instead of memorising. Remember that the same line code can appear on parts written to other standards — a drawing should name the specification, not just “Grade 8”.

Reading a metric property class from the head

Metric bolts carry a two-digit property class stamped on the head, defined in ISO 898-1. The first digit is one-tenth of the nominal tensile strength in megapascals; the second is the ratio of yield to tensile strength, also expressed as a tenth. So 8.8 means a minimum tensile of 800 MPa and a yield of 0.8 × 800 = 640 MPa, while 12.9 means 1200 MPa tensile and 1080 MPa yield. The classes run from 4.6, a soft plain low-carbon steel, through 12.9, an alloy steel that is quenched and tempered. The metric-property-classes table on this page lists each class with its strength. The second digit is a ratio, not a separate strength: 10.9 and 12.9 share the 0.9 ratio but differ in carbon and heat treatment.

Structural bolting: A325, A490 and F3125

For structural steel connections in building frames and bridges, the two workhorse specifications were ASTM A325, the medium-strength bolt for most joints, and ASTM A490, the higher-strength bolt for slip-critical and heavier-loaded joints. Because the two overlapped and were usually specified together, ASTM consolidated them into ASTM F3125, which now carries A325 and A490 as grades alongside weathering and other steel grades. In practice an engineer still writes “A325” or “A490” out of habit; F3125 is the umbrella that holds them. These parts use their own marking scheme — a manufacturer mark plus the grade — rather than radial lines or a property-class number. When a drawing names A325 or A490, quote to the matching F3125 grade. The strength figures for the structural grades appear in the inch-grades table on this page.

Stainless classes and why they are marked differently

Stainless bolts are marked differently because the reason you buy them is different. The ISO 898-1 classes are written around strength, so a stronger bolt means harder, more brittle steel. Stainless steel is bought for corrosion resistance, and its strength is set by the alloy and the amount of cold work, not by quench-and-temper hardening. ISO 3506-1 therefore uses its own code: a letter for the alloy family — A2 for the 18-8 type, A4 for the molybdenum-bearing type — followed by a number for strength. A2-70 and A4-80 are the common classes, the number being one-tenth of the tensile strength in MPa: 700 and 800. The stainless-classes table on this page shows how the letter-and-number code maps to strength. Do not substitute an ISO 898-1 class number for a stainless code; a Grade 8 and an A2-80 are not interchangeable.

Matching a grade to your application

Choose the grade for the load, then check every other requirement separately. Low-stress assembly and general hardware often use Grade 2 or classes 4.6 and 5.8. Machinery and automotive joints under moderate load and vibration lean on Grade 5 or class 8.8. High-strength, weight-sensitive joints move to Grade 8 or classes 10.9 and 12.9. Outdoors, in washdown areas or near salt, stainless A2 or A4 wins even at lower strength, because a rusted Grade 8 fails long before an A4-80. Two cautions: high-strength parts are more sensitive to hydrogen embrittlement from plating, and a soft nut under a hard bolt caps the assembly at the nut’s strength. When the choice is unclear, send the loads and environment with your request for a quote and we will help you settle it.

Producing a marked part to a grade

A grade is a process specification the customer names, not something we assign. When your drawing calls for a specific class, we produce to that class’s material and process requirements: the correct starting alloy, the specified heat treatment route, and the head marking the standard requires. Starting stock comes from the materials we run daily — carbon steel 1045, 1215 and 12L14; alloy steel 4130 and 4140; and stainless 301, 303, 304, 304L, 316, 316L, 410, 416 and 17-4PH — with heat treatment and plating handled in-house or through our vetted partner network. We hold ±0.01 mm on features that locate, seal or fit, with ISO 2768-m as the general tolerance. What we ship is the material certificate (MTR) for the lot and pre-shipment photos — the record that the raw material matches the grade you named. See our materials, custom bolts and tolerances pages for the drawing-to-part flow.