Fastener strength classes: bolts, screws, studs and nuts

What the two numbers in a class mean

The property class of a bolt, screw or stud made of carbon or alloy steel is written as two numbers separated by a point. The first, multiplied by 100, gives the nominal tensile strength in MPa. The second, multiplied by 10, shows what percentage of that strength the yield strength makes up.

Class 8.8 has a tensile strength of 8 × 100 = 800 MPa and a yield strength of 800 × 0.8 = 640 MPa. For 10.9 these are 1000 and 900 MPa, for 5.8 they are 500 and 400 MPa. The product of the numbers gives the same result: 8 × 8 × 10 = 640.

Yield strength is the stress after which the bolt no longer returns to its original length. For classes up to 6.8 it is the lower yield strength Re, and for 8.8 and above, where there is no clear yield plateau, the proof stress Rp0.2 is used: the stress that leaves 0.2 % permanent elongation.

Fasteners of class 8.8 and above are made of quenched and tempered steel. So bolts 5.8 and 8.8 look the same but differ in metal. The class is a property of the fastener, not the grade of rolled steel, and the two scales must not be mixed.

Bolt, screw and stud classes in numbers

Nominal values in the table are the decoding of the class; minimum values are the ones a fastener from a batch must not fall below.

For classes up to 6.8 the Re / Rp0.2 columns give the lower yield strength Re, for 8.8 and above the proof stress Rp0.2. The standard limits class 9.8 to small diameters (up to M16); its minimum values are taken from the text of the standard. In a project, minimum values are checked against the current edition of the standard.

What these numbers say about how a bolt behaves

Two patterns stand out. The ratio of yield to tensile strength grows from 0.6 for class 4.6 to 0.9 for 10.9 and 12.9: a high-strength bolt works closer to its limit and gives less warning before it breaks. The elongation after fracture on test pieces shows the same: 22 % for 4.6, 12 % for 8.8 and only 8–9 % for 10.9 and 12.9.

Nuts: class and the pair for the bolt

A nut class is written as one number: 5, 6, 8, 10, 12. Multiplied by 100, it roughly equals the proof load stress in MPa. The nut is made stronger than the shank, so that under overload the bolt breaks rather than the thread stripping.

The pair is chosen by the first number of the bolt class. DBN V.2.6-198 states it directly: class 5 nuts for bolts 5.6 and 5.8, class 8 for 8.8, class 10 for 10.9. If the bolt works in shear only, the code permits a nut one step lower (4, 6 and 8 respectively). The same principle applies to the other classes.

Nuts of a higher, a lower and a thin class

A nut of a higher class than needed does no harm, although it does not make the bolt stronger. A nut of a lower class wrecks the joint, because its thread strips first. Thin nuts of classes 04 and 05 (lock nuts) stand apart: they are rated for a lower load and do not replace the main nut.

Markings: what to look for on the head and the nut

The class and the manufacturer must be marked on the fastener itself, not only on the box.

• the hexagon head of a bolt carries the manufacturer’s mark and the class, for example 8.8; the mark is on the top face of the head or on the side, and on socket head screws on the head;
• studs of larger diameters are marked on the end face or on the plain shank, because they have no head;
• on nuts the class is a number on the face or flat (8, 10); small nuts use a conventional mark like a «clock face», which the standard explains;
• left-hand thread is marked with an arrow on the bolt head and notches on the nut flats;
• hot-dip galvanized nuts with oversize-tapped threads carry the letter Z or X after the class, and bolts with undersized threads the letter U (more in the section on coatings);
• stainless fasteners carry the steel grade and class with a hyphen: A2-70, A4-80;
• a bolt with no maker’s mark, or with blurred or crooked figures, is doubtful: the digits 8.8 can be stamped on a head made of ordinary steel. Without a document, such a fastener does not go into a load-bearing joint.

Which class for which joint

In load-bearing joints the bolt class is not chosen by eye: calculation gives it, together with the diameter and the number of bolts. DBN V.2.6-198 names the permitted classes (5.6, 5.8, 8.8, 10.9, 12.9, depending on whether the bolt works in tension or shear and whether the joint is checked for fatigue) and gives characteristic values for them: tensile strength 500, 500, 800, 1000 and 1200 MPa, yield strength 300, 400, 640, 900 and 1080 MPa (Table D.4). Below is the general logic of choice only.

Fastener coatings and hot-dip galvanizing of high-strength bolts

A plain black fastener without a coating rusts quickly in a damp environment, so outdoors it is protected. The coating is chosen by the environment and by the class: for high-strength bolts it also matters whether the method of application puts hydrogen into the steel.

The table lists the main options, without naming manufacturers. These are general differences: the actual service life of the protection depends on operating conditions and on the standard for the specific coating.

Galvanizing of fasteners should be clarified for the specific item: the class, diameter and the protection needed are set by the project.

Hot-dip galvanizing of bolts: what changes in the thread

Hot-dip galvanizing gives a layer over 40 µm, and an ordinary thread will not assemble with such a layer. So the set is made with the thread provided for in advance. There are two methods, and they must not be mixed. In the first, the bolt keeps an ordinary thread and the nut is tapped oversize after galvanizing; such nuts carry the letter Z or X after the class. In the second, the bolt is made with an undersized thread before galvanizing (marked U) and the nut is tapped after it to the ordinary tolerance.

A nut tapped oversize on a bolt with an undersized thread gives a high risk of thread stripping. The standard does not permit retapping a nut, and the thread of a galvanized bolt cannot be checked by stripping the coating and measuring it: the galvanizing process itself dissolves some of the steel.

The standard for hot-dip galvanized fasteners covers coarse threads from M8 to M64. Smaller diameters and pitches below 1.25 mm are not recommended for hot-dip galvanizing. We galvanize poles and steel structures by the hot-dip method in our own bath; for threads see design for galvanizing, for the coating thickness zinc coating thickness, for the methods hot-dip, electroplated or cold galvanizing.

Classes 8.8, 10.9 and 12.9 and hot-dip galvanizing

Classes 8.8 and below are hot-dip galvanized in the ordinary way, with the thread provided for. Class 10.9 is included in the standard for hot-dip galvanized fasteners, but with conditions. Class 12.9 is not: for bolts, screws and studs the scope ends at 10.9, for nuts at class 12.

The reason is hydrogen. During acid cleaning it is absorbed by the steel and does not always leave completely in the zinc bath, which can lead to brittle fracture: the bolt snaps under load hours or days after tightening. The higher the strength and hardness, the greater the risk. Our article on design for galvanizing says that steels with a yield strength above 650 MPa are more prone to cracking; 8.8 sits at that limit (640–660 MPa), while 10.9 and 12.9 are well above it (900–1100 MPa).

After ISO 10684:2004; the ordinary bath runs at 455–480 °C, the high-temperature one at 530–560 °C. A Ukrainian edition of this standard could not be confirmed by search.

High-strength sets for preloading

For bolts with controlled tension there are separate standards for sets (a bolt, nut and washer from one batch): DSTU EN 14399, systems HR and HV. In European practice such sets of class 10.9 are hot-dip galvanized, and acid pickling is replaced by mechanical cleaning. So they are bought ready-made, with the set manufacturer’s documents, rather than bolts being sent for galvanizing together with the structure.

DBN gives a warning: the design resistances of bolts 8.8 and 10.9 apply to bolts without coating, for example without galvanizing. If galvanized bolts are needed in the project, the project sets the class and the method of protection; sometimes it is simpler to take a bolt of a lower class and a larger diameter, but that is the designer’s decision.

Stainless: A2-70 and A4-80

Stainless bolts are marked differently: a letter, a digit and a number after a hyphen. The letter A stands for austenitic steel (most stainless fasteners are made of it), the digit after it for the group by composition, and the number after the hyphen for one tenth of the minimum tensile strength in MPa. So 70 means 700 MPa and 80 means 800 MPa.

A2 is steel with chromium and nickel, A4 is the same with molybdenum added. The 70 or 80 speaks of strength, not corrosion resistance: A2-80 and A4-80 are equally strong. The difference is in the environment. Molybdenum makes A4 more resistant to chlorides (sea, de-icing salts), while A2 near salt soon turns dark.

Stainless fasteners are not hardened, so A2-70 is below class 8.8 in tensile strength (700 against 800 MPa) and its yield strength is considerably lower. 8.8 must not be replaced by A2-70 without a recalculation. Two more practical points: the thread of stainless bolts tends to gall when tightened, so it is lubricated and the nut is not run on at high speed, and in a damp environment next to carbon steel the corrosion concentrates on the carbon steel part beside the stainless one.

Inspection and certificate

The class is confirmed by more than the figures on the head. A batch should come with a manufacturer’s document: a certificate or data sheet with the batch number, the product designation, the property class and a reference to the standard. On receipt, check that the marking on every bolt matches the document, not on every tenth.

If a batch looks doubtful (no maker’s mark, uneven figures, a price well below the market), it goes to a laboratory. A tensile test and a hardness measurement show the actual class. For critical joints the project sets the requirements for documents and incoming inspection.

Where mistakes are most common

• a bolt 8.8 with a class 5 nut: the nut thread strips first, and the load the bolt would carry never reaches it; in effect the nut is what holds;
• class and steel grade: S245 or S345 is the steel of sections, plates and our anchor cages, while 5.8 or 8.8 is a fastener class. The phrase «a bolt of S345 steel» does not set a class; for steel grades see steel grade selection;
• changing the class «because there is none in stock»: the diameter and the number of bolts in the project are chosen for a specific class, and 5.8 instead of 8.8 with the same thread will not hold the joint without a recalculation;
• galvanizing the bolt and the nut «as ordinary ones»: a nut tapped before galvanizing will not run onto a galvanized bolt, and a nut tapped oversize on a bolt with an undersized thread will strip. The set is taken by one method;
• welding fasteners: the wish to weld a nut or a bolt to a plate so that it does not turn is common. In classes 8.8 and above the heat changes the structure of the hardened steel, and next to the weld the metal softens or crumbles. Against loosening there are lock nuts, locking washers and other locks; welding is the designer’s decision;
• overtightening: a bolt tightened past its yield strength is permanently stretched, and can no longer be relied on. The needed torque depends on the diameter, the class, the lubrication and the coating, so the project sets it, not the feel of the wrench.

What to state on the drawing and in the request

In KM and KMD drawings fasteners go into a schedule separate from the rolled steel, and the class of the bolt and the nut is written with the diameter, length and coating. The bolt + nut pair should be named directly, not just «M20 bolt». If a joint is mixed (some connections bolted, some welded), that is marked as well.

We fabricate steel structures to the customer’s drawings and prepare KM and KMD drawings: KM and KMD design, steel fabrication. Erection is done by our partner construction company. Sizes and masses of bolts, nuts and washers are in the fastener range tables, other sections in the rolled steel handbook. Anchor bolts for poles and masts are covered in the article «Anchor bolts for poles and masts», and anchor cages in the article on foundation anchor cages.

Standards and validity

The properties of bolts, screws and studs by property class are set out in DSTU EN ISO 898-1:2022 «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» (current, in force from 31.12.2023). The choice of classes and bolt + nut pairs in load-bearing steel structures is made under DBN V.2.6-198:2014 «Steel structures. Design standards» (sections 6 and 7, tables D.3 and D.4).

Hot-dip galvanizing of steel structures is covered by DSTU EN ISO 1461:2024, which we follow (see the article on coating thickness). High-strength bolting sets for preloading are covered by DSTU EN 14399-1:2019 (general requirements), DSTU EN 14399-3:2024 and DSTU EN 14399-4:2024 (systems HR and HV). Spun hot-dip galvanizing of fasteners is covered by the international standard ISO 10684:2004 (confirmed in 2021, a second edition is in preparation), and electroplated coatings on fasteners by ISO 4042:2022.

Nuts and stainless fasteners are covered by the international standards ISO 898-2:2022 and ISO 3506-1:2020 (bolts, screws, studs), ISO 3506-2:2020 (nuts). Which current DSTU reproduces ISO 10684, ISO 4042, ISO 898-2 and ISO 3506 is checked before they are cited in project documents. The figures in this article are for orientation; a project uses the current text of the standards.