Bolt Classification (Grades) – Steel
Bolt classification is the means by which standardised fasteners are identified with respect to their material properties and strength characteristics. A thorough understanding of bolt classification is necessary for any mechanical engineer. This page is divided into the following sections:
- The ISO Metric Bolt Classification
- The Classification / Designation System
- Material Composition of Metric Bolt Classification
- Mechanical Properties of ISO Metric Bolt Classifications
- Physical Identification of Bolt Classification
- ISO 898-1:2013 – Mechanical Properties of Fasteners Made of Carbon Steel and Alloy Steel – Part 1: Bolts, screws and studs with specified property classes – Coarse thread and fine pitch thread
- ISO 724:1993 – ISO general-purpose metric screw threads – Basic dimensions
- BS 3643-1:2007 – ISO metric screw threads. Principles and basic data
- EN 1993-1-8:2005 – Eurocode 3 – Design of Steel Structures – Part 1-8 – Design of Joint
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The following sections describe the material properties of typical standardised steel metric fasteners found in use throughout the world and in particular, the European Economic Area (EEA). The ISO metric bolt classification is introduced and the varying properties between the different classifications discussed.
2. The ISO Metric Bolt Classification
The material properties and therefore the inherent strength of standard metric threaded fasteners are specified according to ISO 898-1:2013 . The properties defined in ISO 898-1 are limited to bolts used in an ambient temperature range of −50 °C to +150 °C, which covers most typical environments [RFE.0001]. The material properties are limited to fasteners in the size range M1.6 to M39. When noting this size limitation it is worth considering the following:
- Structural bolts are only provided in sizes greater than M12. Structural bolts are those bolts typically available “off-the-shelf” as mass produced items and are used in most heavy structural and mechanical applications. They are generally very widely available.
- At diameters/sizes > M39, the through-thickness material properties may be affected. The material properties of bolts > M39 should be verified with the manufacturer before use in design. [RFE.0002]
The ISO Metric bolt classification system in ISO 898-1:2013  assumes that the fastener thread form is produced in accordance with ISO 724:1993  and BS 3643 Part 1: 2007 . Further information on the basics of fastener thread form and fastener standardisation can be found here.
3. The Bolt Classification / Designation System
The strength of bolts in accordance with ISO 898  is specified using a two digit grading system, with the two digits separated by a dot. For example, a very commonly used class (probably the most common in the EEA) is denoted “Class 8.8”. The first digit, to the left of the dot is equivalent to 1/100th of the Ultimate Tensile Strength (UTS) of the bolt material. The second digit is equal to ten times the ratio of the nominal yield strength to the UTS. The nominal yield strength is either the lower yield stress of the material or the 0.2% proof stress, or the stress at 0.0048d non-proportional elongation (see Table 3).
The multiplication of the nominal tensile strength and the yield strength ratio provides the nominal yield strength in MPa. For example, a Class 8.8 bolt therefore has a UTS of 800MPa and a yield strength of 0.8 x UTS = 640MPa.
Table 1 presents the nominal and minimum tensile and yield stresses for each bolt classification in accordance with ISO 898-1. You will notice that the nominal stresses are lower than the minimum stresses. This is because the nominal stresses refer to the values listed in the class designation system. In any case, it is normal to use the nominal values for design. For example, the nominal values are provided for use in Eurocode EN 1993-1-8:2005 (Design of Joints) .
Table 1 – Tensile and yield stress values of bolt classification in accordance with ISO 898-1
NOTES ON TABLE 1:
- *For convenience, the table uses the notation for ultimate tensile stress and yield stress provided in Eurocode EN 1993-1-8:2005 (Design of Joints) . It is assumed that this is one of the most common design standards relevant to users of this website.
- **The yield strength is taken from either the material yield stress, the 0.2% proof stress or the stress at 0.0048d non-proportional elongation in accordance with ISO 898-1:2013 . Where the lower yield strength of a bolt material cannot be determined from the material stress-strain curve, the stress at 0.2% or 0.0048d proportional elongation is used instead. The relationship between yield stress and proof stress is described at the following page [LINK]. Please refer to Table 3 for further details.
4. Material Composition of Metric Bolt Classification
The table below provides details of chemical composition of the steels and the minimum tempering temperatures used in each bolt class to achieve the required properties.
Table 2 – Material composition of metric bolt classification in accordance with ISO 898-1
NOTES ON TABLE 2:
- Caution is advised when considering the use of property classifications 10.9/12.9. The capability of the fastener manufacturer, the service conditions and the wrenching methods should be considered. Environmental effects and coating methods may cause stress corrosion cracking of fasteners.
- Carbon (C) content dictates the hardness and strength of the steel, as well as response to heat treatment (hardenability). Ductility, forgeability, machinability and weldability decrease as the amount of carbon increases.
- Phosphorus (P) has the effect of increasing the tensile strength of steel and in some cases improving corrosion resistance. However, at higher concentrations phosphorus generates an embrittling effect and is therefore generally considered an undesirable impurity.
- Sulphur (S) improves machineability; however at higher content sulphur makes the steel hard, brittle and has a detrimental effect on impact properties. Sulphur concentration is therefore kept to a minimum.
- Boron (B) is useful as an alloying element in carbon (C) and low alloys steels due to its hardening effect by enhancing hardenability (the material’s potential to be hardened by thermal treatment).
5. Mechanical Properties of ISO Metric Bolt Classifications
The table below provides the detailed mechanical / physical properties of the different bolt classification in accordance with ISO 898-1:2013 . A lot of detail is provided in this table, much of which may not actually be useful for the typical design activity; however it is provided for information. The most interesting and immediately useful information for use in bolt design calculations is presented in Table 1, above.
Table 3 – Mechanical / physical properties of metric bolt classification in accordance with ISO 898-1
5.1 Fastener Strength
For detailed information on the strength / capacity of fasteners, please see the page on bolt strength.
6. Physical Identification of Bolt Classification
A common method of marking typical hex-head bolts is presented in Figure 1. This method of marking and other methods are detailed in ISO 898-1:2013 , along with a number of other standard identification methods.
Figure 1: Standardised identification marking of a typical hex-head bolt in accordance with ISO 898-1:2013
- a. denotes the Manufacturer’s marking
- b. denotes the property class of the fastener
Further information/expertise is requested regarding the properties of fasteners for use outside the nominal environmental conditions specified in ISO 898-1 , particularly at temperatures -50C <T < 150C
Further information/expertise is requested regarding the properties of fastener sizes >M39 and therefore not covered by ISO 898-1 . I.e. which standard(s) is(are) applicable to such fasteners and what are the effects of increased material thickness on impact, yield and fatigue strength.