What Fastener Surface Markings Actually Tell You

What Symbols Appear on Fastener Heads

Fastener markings are small engravings or stamped characters located on the head, shank, or collar of a bolt, nut, or screw. These characters are not decorative. They identify the material grade, the applicable technical standard, and sometimes the manufacturer. The most common marking is a two-part code, such as a letter or number followed by a dot and another number.

These codes follow standardized formats. A code like 8.8 tells you the material is a hardened steel with specific tensile and yield strength ratios. A code like A2-70 indicates a stainless steel grade with a defined strength level. The location of the stamp matters. Standards specify where the marking must appear for a given fastener type and size. If a bolt has a hex head, the stamp is usually on one of the faces. For washers, the marking might be on the flat face or the edge.

Readers often assume that any number on a bolt is a size. This is a common error. Size refers to the thread diameter or nominal length. The grade is a mechanical property. A 12mm bolt and a 12.9 bolt are completely different concepts. One is a dimension. The other is a strength specification. Confusing these two things is the most frequent reason for rejected shipments or field failures.

How Grade Codes Define Material Strength

The two-part number system used for carbon and alloy steel fasteners is the most widespread format. The first number represents the tensile strength in 100-megapascals units. The second number is the yield strength in 100-megapascals units. The ratio between the two numbers is called the proof load factor.

For example, a grade 10.9 bolt has a minimum tensile strength of 1000 megapascals and a minimum yield strength of 900 megapascals. This means the material can withstand 900 megapascals before it undergoes permanent deformation. The 10.9 code tells the engineer that the bolt has high strength. It also implies the material has been heat-treated and quenched. This process increases hardness but reduces ductility.

Stainless steel markings follow a different logic. They use letter prefixes like A, B, or M. A2-70 means the material is austenitic stainless steel with a tensile strength of 700 megapascals. The “A” indicates the type of alloy structure. The “2” distinguishes it from the “1” grade, which usually has lower strength or different corrosion resistance. This coding system helps buyers match the fastener to the environment. A marine installation might require A4-80 for higher corrosion resistance, even if the tensile strength is lower than a carbon steel equivalent.

Head Stamping vs. Laser Marking

Head stamping is the traditional method of applying fastener markings. A die presses characters into the metal. The depth of the stamp is critical. If the stamp is too shallow, the characters are hard to read. If it is too deep, it can weaken the head geometry. Most standards specify a minimum depth, often around 0.1 millimeters for larger fasteners.

Laser marking has become common for smaller fasteners or for applications where stamping is difficult. Laser etching creates a mark by removing material or altering the surface color. It allows for finer details and smaller fonts. However, laser marks can wear off more easily than stamped marks if the fastener is subjected to abrasive contact.

The method of marking affects the verification process. A stamped mark is raised or indented. You can feel it with a fingernail. A laser mark is flat or slightly recessed. Visual inspection under good lighting is the primary check. Magnifying glasses are useful for small nuts and screws. If a marking is missing or illegible, the fastener is non-conforming. It cannot be accepted for critical applications.

Sourcing Decisions Based on Markings

When you source fasteners, the marking is the first line of defense against substitution. A supplier might offer a cheaper bolt that looks identical but has a lower grade. The marking on the head is the quickest way to spot this. If the drawing calls for 12.9 and the box contains 8.8, the shipment is wrong.

Buyers should require that the marking matches the purchase order exactly. This includes the font style and spacing, if specified by the standard. Some industries require a specific mark on the shank as well. This is common for high-tension bolts used in bridges or towers.

Cost and lead time for certified stainless bolt sets often reflect the complexity of the marking and the required certifications. A simple A2-70 bolt is easier to source than a high-strength alloy steel bolt. The latter requires heat treatment and rigorous testing. The marking serves as a promise of that process. If you cannot verify the marking, you cannot verify the quality.

Practical Example: Verifying a Grade 12.9 Bolt

Imagine you are installing a heavy-duty machinery base. The engineering drawing specifies M20 x 1.5 x 90 mm grade 12.9 bolts. You receive a crate of bolts. You pick one up and check the head. You see “12.9” stamped on the face. This is a good sign.

Next, you check the shank. Some standards require a mark there for large fasteners. You look for “12.9” or a manufacturer logo that corresponds to the grade. If the head says 12.9 but the shank is blank, you might still accept it if the standard allows it. However, you should document this.

Now, consider a scenario where the head says “12.9” but the material feels light. Grade 12.9 bolts are heavy for their size due to the alloy content. If the bolt feels suspiciously light, it might be made of a lower-grade steel that was painted or marked incorrectly. You would take a sample to a lab for a hardness test. If the hardness is too low, the batch is rejected. The marking was a red flag that triggered the test.

This example shows that markings are not just labels. They are the trigger for quality verification. Without them, you have no way to distinguish a high-grade bolt from a low-grade one without destructive testing.

Common Errors in Reading Markings

Buyers and inspectors make several mistakes when reading fastener markings. The first is confusing the size with the grade. An M10 bolt is 10 millimeters in diameter. A 10.9 bolt is a grade. They look similar but mean different things.

The second error is ignoring the standard prefix. A “12.9” bolt under standard EN 109-2 is different from a “12.9” bolt under an American standard. The mechanical properties might be the same, but the manufacturing tolerances and testing methods differ. You must check the standard reference on the drawing.

The third error is accepting markings that are painted over. Some cheap suppliers paint the head to make it look like it has a marking, or they stamp a low grade and paint over it to look like a higher grade. This is a form of fraud. You must check the edge of the head and the shank. If the paint is thick or chipping, the mark underneath may be false.

Final Checks Before Acceptance

Before you accept a delivery, perform a final check. Look at the marking under bright light. Use a magnifier if needed. Compare the font and depth with a known good sample from a previous approved lot. If you have a supplier qualification file, use it.

If the fastener is for a critical application, such as a pressure vessel or a high-rise building, the visual check is not enough. You need the material certificate. The certificate should confirm the heat number and the test results. The marking on the bolt should match the heat number or the grade listed in the certificate.

If the marking is missing, do not assume it is a minor defect. It is a major defect. A fastener without a grade marking is an unknown material. You cannot trust it. Return it. This protects your project and your liability.