Industry News
How to read bolt head markings? Strength grades, material designations and maker's marks
2026-10-14
Compiled by JIALI Industrial (164.hk) — a Hong Kong fastener supplier specialising in screws, bolts, nuts and selection know-how.
Why head markings matter - the first line of defence in inspection
When a batch of bolts arrives, read the head first. A fastener head marking is permanent information stamped or embossed on the top of the bolt head, usually carrying two elements: the property class and the maker's identification mark. The requirement is set by GB/T 3098.1 (identical to ISO 898-1): bolts and screws with nominal diameters of 5 mm and above must be clearly marked with their property class, while GB/T 3098.6 (ISO 3506-1) requires stainless fasteners to carry the steel group and strength designation such as A2-70 or A4-80. Why does this matter? On a warehouse shelf, grade 8.8 and grade 10.9 carbon steel, or zinc-plated and stainless parts, look almost identical; once batches are mixed, joints may be under-strength at best and fail suddenly under load at worst. Head markings act as the fastener's identity card: checking the marks against the purchase order and the EN 10204 3.1 mill certificate is the fastest way to catch mixed lots, counterfeits or downgraded parts. When a quality dispute arises, the maker's mark also allows tracing back to the production batch, a key basis for claims and failure analysis.
Reading carbon-steel strength grades and stainless designations
Carbon-steel bolt head markings are simple to decode: the digit before the dot times 100 gives the nominal tensile strength in MPa, and the digit after the dot times the first digit times 10 gives the nominal yield strength. A grade 8.8 bolt therefore has a tensile strength of 800 MPa and yield of 640 MPa; grade 10.9 means 1040 MPa tensile and 940 MPa yield, exactly as specified in GB/T 3098.1 and ISO 898-1. Marks may be raised, recessed or dot-patterned; grades 10.9 and above often use dot embossing to avoid stress concentration. Stainless markings follow a steel-group and strength-class format: A2-70 means austenitic 304 group steel with 700 MPa tensile strength, while A4-80 means the 316 group at 800 MPa. Under ISO 3506-1 ordinary austenitic grades come only in classes 50, 70 and 80; suppliers claiming A2-90 or A2-100 should be treated as suspect until certified. Duplex steels carry their own designations, usually explained on the mill certificate. Three checks are essential: the head marking must be legible, it must match the ordered specification, and it must match the certificate grade - any mismatch means the batch should be quarantined.
Nut markings, maker's marks and the certificate-check chain
Nut markings follow the same logic in a simpler form: GB/T 6170 and ISO 898-2 require style-1 steel nuts to be marked with property classes 6, 8, 10 or 12, with corresponding classes for high (style-2) nuts. Match the nut class to the bolt: a class 8 nut pairs with an 8.8 bolt and a class 10 nut with a 10.9 bolt, so that in overload the bolt breaks rather than the nut stripping. Besides the grade, the head should carry a maker's identification mark - a letter or symbol tracing the part to a specific manufacturer; ISO 898-1 requires the property class and maker's mark to be applied together, so a part with no maker's mark is itself a warning sign. Run a four-step incoming chain: sample and check markings per GB/T 2828.1 at general inspection level II, AQL 2.5 - a grade or mark mismatch quarantines the whole lot; match the head markings against the EN 10204 3.1 certificate line by line for heat number, size and class; where necessary send samples for third-party tensile and hardness testing, especially for grades 10.9 and above; and store parts classified by grade and batch to prevent re-mixing. Need fasteners with clear head markings, traceable maker's marks and 3.1 certificates? Contact JIALI Industrial (164.hk) with sizes, grades and service environment for a matched proposal.
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