Bolt Grades and Property Classes Guide

A metric bolt property class is a designation defined by ISO standards that indicates the mechanical properties of the bolt, typically shown as two numbers separated by a dot, such as 8.8, 10.9 or 12.9, and the numbers represent standard-defined tensile and yield strength ranges verified through specified testing methods.

Property class 8.8 is a metric bolt designation defined by ISO 898-1, where the first number indicates a nominal tensile strength of approximately 800 MPa (rounded value per the standard) and the second number indicates a yield strength ratio of 0.8 of the nominal tensile strength, resulting in a minimum yield strength of approximately 640 MPa as defined by the standard.

The difference between property class 8.8 and 10.9 bolts is in their standard-defined mechanical property ranges, where 10.9 has a higher nominal tensile strength and a higher yield strength ratio of 0.9, and the two classes are not interchangeable without engineering review, fastener standard confirmation, and matching nut and washer coordination.

The difference between property class 10.9 and 12.9 bolts is that 12.9 represents a higher strength level defined by ISO 898-1, with a higher nominal tensile strength and a yield strength ratio of 0.9, and 12.9 bolts require careful attention to material, thread processing, hardness, and hydrogen embrittlement risk during finishing.

SAE Grade 5 is a grade defined by SAE J429 for inch-series bolts, requiring a minimum tensile strength of 120,000 psi for diameters up to 1 inch as specified in the standard, and is commonly applied to inch-series hex bolts, cap screws, and related fasteners in general industrial and automotive applications.

Within the corresponding size ranges in SAE J429, Grade 8 has higher specified strength requirements than Grade 5. Exact values must be read from the applicable edition and diameter range, and selection must follow the assembly specification and matching hardware requirements.

SAE Grade 2 is a grade defined by SAE J429 for inch-series bolts with lower mechanical property requirements, commonly specified for low-strength general-purpose inch fasteners where the applicable standard does not require higher grades such as Grade 5 or Grade 8.

ASTM A307 covers carbon steel bolts and studs. Any Grade A minimum tensile strength statement must identify the applicable edition and size range, and selection must follow the project specification.

ASTM F3125 is an ASTM standard that covers high-strength structural bolts and replaces earlier standards, covering Grades A325 and A490, and it is used in structural steel connections where the project specification, bolt dimensions, nut grade, and washer requirements are confirmed before installation.

Class 8.8 and SAE Grade 5 belong to different classification systems and are not formally equivalent, and buyers should confirm the applicable standard, thread system, dimensions, mechanical requirements, and project specification before considering any substitution.

Metric and inch bolt grades cannot be substituted based on grade designation alone because the two systems differ in thread form, dimensional standards, mechanical property testing methods, and nut and washer matching requirements, and any substitution should be reviewed by engineering against the project specification.

ISO 898-2 provides matching guidance for nuts used with ISO 898-1 externally threaded fasteners. Final matching also depends on thread, nut style, dimensions, finish, and the project specification.

Hardened washers should be used with high-strength bolts when the applicable standard, project specification, or structural design requires them, and buyers should confirm the washer standard, hardness range, dimensions, and finish match with the bolt and nut requirements.

Grade markings can help identify a fastener grade, but requirements and exceptions depend on product type, dimensions, and the specified standard. Markings do not replace certificates or testing.

Buyers should include the bolt grade or property class, the applicable standard, thread system, diameter, length, material specification, finish, nut and washer grade requirements, marking requirements, and required inspection documents in the RFQ, and each of these items should be confirmed with the supplier before production.

Common grade-selection and substitution mistakes include treating metric property classes and SAE grades as direct equivalents, substituting grades without confirming dimensional standards and thread fit, mismatching nut and bolt grades, assuming higher-strength grades always improve safety, and substituting without reviewing the project specification and application requirements.

Stainless steel fastener property classes are defined by different standards such as ISO 3506 for metric sizes and follow a different designation system than carbon steel property classes, so stainless steel grades should not be compared or substituted with carbon steel grades based on designation alone without reviewing the applicable standard and project requirements.

For common ISO 898-1 property classes, the first part of the designation represents one hundredth of nominal tensile strength. The digit after the decimal represents one tenth of the ratio of nominal lower yield strength or specified plastic elongation strength to nominal tensile strength, with the exact definition, minimum values, and size ranges taken from the applicable tables.