The number is the strength
8.8 means 800 MPa tensile and 640 MPa yield; 10.9 means 1000 and 900. The nut carries the first number: 8 or 10.
Reference tool — not a test
Read the bolt before it goes in an RFQ.
Fastener marking
Class, material, pair.
Choose the part and type the marking. Below the examples, the marking of an offered replacement can be checked against it: the property class or grade. The drawing and the RFQ line update with every change.
Examples:
Replacement check
On the markings alone: the joint requirements, the coating and the tightening torque decide.
All 130 engineering tools
A bolt head, a stud end or a nut face carries a property class or a material grade, with the maker’s mark beside it. Metric fasteners are marked to ISO 898-1 and ISO 3506, inch bolts with SAE J429 lines, flange bolting with ASTM grades. Type what is stamped, and the reader explains each mark, names the matching nut and writes the RFQ line.
A marking is the maker’s statement, not a test. Where the class or grade matters, ask for the material certificate.
How a fastener is read
Number, group, grade.
Stainless has its own marks
A2-70 and A4-80 are ISO 3506: A for austenitic steel, 2 or 4 the group, 70 or 80 the tensile strength in tens of MPa.
Flange bolting is marked ASTM
B7 studs take 2H nuts, B8M studs 8M nuts. The maker’s mark, and often a heat number, stand beside the grade.
| Class | Tensile, MPa | Yield, MPa | Ratio, % |
|---|---|---|---|
| 4.6 | 400 | 240 | 60 |
| 4.8 | 400 | 320 | 80 |
| 5.6 | 500 | 300 | 60 |
| 5.8 | 500 | 400 | 80 |
| 6.8 | 600 | 480 | 80 |
| 8.8 | 800 | 640 | 80 |
| 9.8 | 900 | 720 | 80 |
| 10.9 | 1000 | 900 | 90 |
| 12.9 | 1200 | 1080 | 90 |
Nominal values, from what the class numbers stand for. The standard’s minimum values depend on the diameter. ASTM studs: B7 with 2H nuts, B7M with 2HM, B16 with 7, B8 with 8, B8M with 8M.
Sources
ISO 898-1 classes per the summaries of TR Fastenings, EngineerFix and Fastenal (the first number × 100 is the tensile strength, the second the yield ratio). ISO 3506 stainless fasteners (A2 usually 304, A4 316, the number the strength in tens of MPa) per Torqbolt, SteelConstruction AD 342 and Sakysteel. SAE J429 lines per Portland Bolt and AFT Fasteners. ASTM A193, A320 and A194 grades per WhatIsPiping, American Fastener and Portland Bolt.
Before you send the fastener
Questions about fasteners in an RFQ
What does 8.8 on a bolt mean?
A property class of ISO 898-1: the first number × 100 is the nominal tensile strength, 800 MPa, and the second is the share of it at which the bolt yields, 80 %, so 640 MPa. It goes with a class 8 nut.
What is the difference between A2-70 and A4-80?
Both are ISO 3506 stainless fasteners. A2 is usually 304-type steel and A4 316-type, with molybdenum; 70 and 80 give the tensile strength, 700 and 800 MPa.
What do the lines on a bolt head mean?
Radial lines mark SAE J429 grades of inch bolts: three lines grade 5, six lines grade 8. A head without lines is usually grade 2, but a bare head proves nothing.
What are B7 and 2H?
ASTM marks on flange stud bolts and their nuts: B7 is a Cr-Mo alloy steel stud to ASTM A193, quenched and tempered, and 2H the A194 heavy hex nut that goes with it. Stainless studs are B8 (304) or B8M (316), with 8 or 8M nuts.
What else should the request include?
The thread (M16, 5/8-11 UNC) and length, the head or nut type, the finish, the quantity and, for flange bolting, the flange size and rating. Ask for the material certificate where the class or grade matters.
Fastener read?
Send the line with the photographs.
A decoded marking is a cross-check, not proof. Attach photographs of the head and the thread, and Vessel Core checks them against each other before an offer goes back.
Buyer tools
