Property classes 4.6 to 12.9 and SAE/ASTM grades: what the head marking tells you

The marking on a bolt head tells you the strength class and who made it. It does not tell you the dimensions, the coating or whether a given lot was tested. Reading it correctly starts with the difference between nominal and minimum values.

Reading a metric class

Under ISO 898-1, the number before the dot is one hundredth of the nominal tensile strength in megapascals. The number after the dot is ten times the ratio of nominal yield strength to nominal tensile strength. So 8.8 means a nominal tensile strength of 800 MPa and a nominal yield of 0.8 × 800 = 640 MPa.

Nominal is not minimum

The nominal values name the class. The values a bolt must actually meet are the minimums in ISO 898-1, and for several classes they are higher. Minimum tensile strength and minimum yield (lower yield or 0.2% proof strength), in MPa:

• 4.6: nominal 400; minimum tensile 400, yield 240
• 4.8: nominal 400; minimum tensile 420, yield 340
• 5.6: nominal 500; minimum tensile 500, yield 300
• 8.8, d ≤ 16 mm: nominal 800; minimum tensile 800, yield 640
• 8.8, d > 16 mm: nominal 800; minimum tensile 830, yield 660
• 10.9: nominal 1000; minimum tensile 1040, yield 940
• 12.9: nominal 1200; minimum tensile 1220, yield 1100

A common error is to quote 900 MPa as the yield strength of 10.9. That is the nominal value; the minimum is 940 MPa.

What ISO 898-1 requires on the head

• Hexagon and hexalobular head bolts and screws, and socket head cap screws, of diameter 5 mm and above must carry the manufacturer's identification mark and the property class, preferably on the top or side of the head. The dot may be left out.
• A zero in front of the class, such as 08.8, means reduced loadability, for example a bolt with a low head.
• Left-hand threads of 5 mm and above carry a left-turning arrow, or on hexagon heads an alternative groove marking.

If you need proof that the delivered lot meets the class, ask for an inspection document under EN 10204 or ISO 16228 when you order.

Nuts

ISO 898-2 numbers nut classes after the highest bolt class they mate with: a class 8 nut for bolts up to 8.8, class 10 up to 10.9, class 12 up to 12.9. A higher-class nut may replace a lower one, except for prevailing-torque nuts. Thin nuts (classes 04 and 05) have reduced loadability and are not designed to prevent thread stripping.

Inch grades: SAE J429 and ASTM

• SAE J429: Grade 2 carries only the manufacturer's identification, Grade 5 has three radial lines and Grade 8 has six. Minimum tensile strength is 74 ksi for Grade 2 (1/4 to 3/4 in), 120 ksi for Grade 5 (1/4 to 1 in, 105 ksi above 1 in to 1-1/2 in) and 150 ksi for Grade 8 (1/4 to 1-1/2 in).
• ASTM: grades are marked with letters and numbers and the manufacturer's identification. Structural bolts under ASTM F3125, which replaced A325 and A490, are still marked A325 or A490.
• Three lines on a large bolt: ASTM A449 shared the three-line marking with Grade 5 until 2006, when it received its own A449 marking. Grade 5 stops at 1-1/2 in, so an older three-line bolt above that size is A449.

Nearest equivalents are approximate

Charts often pair Grade 5 with 8.8 and Grade 8 with 10.9. The numbers are close (120 ksi is about 827 MPa; 150 ksi is about 1034 MPa) but the standards, sizes and test requirements differ. Order the standard your drawing names.

What to send

1. Class or grade as written on the drawing, with the product standard.
2. Size, length and finish.
3. The nut class and style, if nuts are included.
4. The inspection document you need.