Specifications and information
The information below is intended for informative purposes only and should only be regarded as such. A copy of any specification referred to should be consulted.
Property Classes for Bolts & Screws (ISO 898-1)
Externally threaded parts marking The number to the left of the dot (one or two digits) indicates 1/100 of the nominal tensile strength in megapascals. The number to the right of the dot indicates 10 times the ratio between the nominal yield strength and the nominal tensile strength. e.g. 8.8 will signify that the fastener has a nominal tensile strength of (8 x 100) 800 MPa and a yield strength ratio of (8 ÷ 10) 0.8. Multiplying 800 x 0.8 gives a nominal 640 MPa of stress at 0.2% non-proportional elongation, the same applies to G10.9 and G12.9. A 0 before the digit on the left of the dot, indicates reduced loadability.
Property Classes for Nuts (ISO 898-2)
Internal threaded nuts designation and marking
There are three styles of nuts
style 0 – thin nut with a minimum height of 0.45 x (nominal thread diameter) and maximum height 0.80 x (nominal thread diameter)
style 1 – nut with a minimum height of 0.80 x (nominal thread diameter) and a maximum height of 0.89 x (nominal thread diameter)
style 2 – nut with a minimum height of 0.89 x (nominal thread diameter)
style 0 has a number designation with a 0 as the first digit, indicating reduced loadability and the second number indicating 1/100 of the nominal stress under proof load in megapascals.
style 1 and style 2 have a number designation that corresponds with the number to the left of the dot for parts in ISO 898-1, the number denotes 1/100 of the nominal tensile strength of the mating bolt/stud/screw in megapascals.
e.g. a G8 nut will be mated with G8.8 bolt/screw/stud (note: for style 1 galvanized tapped oversize nuts, BS 3692 recommends that bolts should be mated with a nut of a higher property class).
The parts of ISO 898, that our products fall in to, deal with normative references, designation and design, the steel chemical composition, mechanical properties, proof loads and marking criteria. There is a section devoted to tests that can be carried out to ascertain that products meet the criteria specified.
Sizes above M39
ISO 898-2 applies up to and including M39, and ISO 898-1 likewise. Above that size the standard does not assign property classes or mechanical values at all. Annex B.2 of ISO 898-2 states that for nuts above D 39 mm the mechanical properties, the tests to be performed, and the relevant marking and labelling are to be specified by agreement between purchaser and supplier.
A designation such as “M48 property class 8” is therefore a trade description rather than a standardised one. The nut will be marked with an 8 to show that that is the intended mating class for the appropriate property class designation of bolt (i.e. a nut marked 8 is intended for a bolt/screw/stud marked 8.8). Where no values are specified at the time of order, common practice is to carry across the values given for M39, but this is a matter of agreement and not a requirement of any standard.
Stainless Steel Fasteners (ISO 3506)
ISO 3506-1 deals with externally threaded parts.
ISO 3506-2 deals with Internally threaded parts
Stainless fasteners use a letter-number system instead — A2-70, A4-80, and so on. The letter
refers to the steel grade (A2 is the common 304-type, A4 is the more corrosion-resistant
316-type). For externally threaded parts the number is the tensile strength in MPa ÷ 10.
For nuts under ISO 3506-2 the number denotes the stress under proof load rather than tensile
strength, so an A4-80 nut is rated by the load it must withstand, not by a tensile figure.
DIN 267-4 and BS 3692
DIN 267-4 Technical delivery conditions Property classes for nuts
DIN 267-4 was last updated in 1983 and has since been withdrawn, superseded by DIN EN 20898-2.
DIN 934 nuts are nonetheless still manufactured to it. Proof loads are lower compared to
ISO 898-2, therefore to differentiate between the two standards vertical bars are used either side of the
property class marking e.g. |8| and not 8
Like ISO 898-2, DIN 267-4 stopped short of the larger sizes, stating that the mechanical
properties and their test methods for nuts above 39 mm nominal thread diameter were to be
specified by agreement.
BS 3692 [2014] ISO metric precision hexagon bolts, screws and nuts
As with DIN 267-4, nuts specified in BS 3692 have a lower proof load value than ISO 898-2 and
vertical bars are used either side of the property class, or strength grade as BS 3692 names it.
Dimensional Standards (DIN 934, ISO 4032, etc.)
DIN 934 Hexagon nuts with metric coarse and fine pitch thread - Product grades A and B
Dimensions and mass; M1 - M160
ISO 4032 [2023] Hexagon regular nuts (style 1) - Product grades A and B
Dimensions, marking and designation; M5 - M39
The 2023 edition is the fifth, and cancels and replaces ISO 4032:2012. The scope was narrowed
from M1.6 - M64 to M5 - M39. Sizes below M5 and above M39 were moved into an informative
annex (Annex A) covering historical nuts that do not conform to ISO 898-2 or ISO 3506-2. It is
worth being aware of this when a drawing or enquiry calls for, say, an M48 nut “to ISO 4032”:
that size now sits outside the body of the standard.
ISO 8673 [2023] Hexagon regular nuts (style 1) with metric fine pitch thread - Product grades A and B
Dimensions, marking and designation; M8 - M39
DIN 936 / DIN 439 M8 to M52 and M8 x 1 to M52 x 3 hexagon thin nuts - Product grades A and B
ISO 4035 [2023] Hexagon thin nuts chamfered (style 0) - Product grades A and B
Dimensions, marking and designation; M5 - M39
ISO 8675 [2023] Hexagon thin nuts (style 0), with fine pitch thread - Product grades A and B
Dimensions, marking and designation; M8 - M64
Allthread / Stud Bolts (DIN 976-1)
DIN 976-1 [2016] Fasteners - Stud bolts - Part 1: Metric threadDIN 976-1 gives length and weight information, as well as normative references; M2 - M72. In Section 6 are the marking requirements. These specify that at sizes of M5 and greater, for steel property classes greater than G4.8 and austenitic stainless steel A2-70 or A4-70, be marked at one end with the respective symbols (Section 6.1) or using colour coding (Section 6.2), as an alternative to symbols.
Why We Hardness Test
If done properly a hardness test is useful, not only to confirm that the material being tested conforms to hardness specification for the appropriate standard, but also to give a good indication of tensile strength and other mechanical properties of the material. However it must not be relied upon as a given that if material hardness is within specification other mechanical properties will be within specification. A tensile test, to proof load and then increasing until fracture occurs is a more reliable way to confirm tensile strength and other properties of a material.
3.1 Certificates — What Should Be On Them?
The short answer is that the list lives in ISO 16228, Section 5.8, Table 3. It is not in EN 10204, which deals only with the type of document, and it is not in ISO 898 or ISO 3506, which specify the product rather than the paperwork. For an F3.1 test report the minimum results are chemical composition, the relevant strength property, hardness where the class is quenched and tempered, and thread acceptance. The detail is set out below.
ISO 898 parts 1 and 2, and ISO 3506 parts 1 and 2, state that fasteners produced shall be capable of conforming to the applicable requirements, when using the applicable tests and methods in the appropriate specification. They state that if a purchaser requires test results, and any additional or specific tests, these are agreed at the time of order. These specifications add that they do not mandate which of the tests the manufacturer shall perform.
As an example — in ISO 898-1 the relationship between yield and tensile strength is fixed by the designation itself, not by the particular steel used. The digit after the dot is ten times the ratio of nominal yield strength to nominal tensile strength, so property class 8.8 has a nominal tensile strength of 800 MPa and a nominal stress at 0.2% non-proportional extension (Rp0.2, the point at which steel will no longer return to its original shape when a load is removed) of 640 MPa. The specified minimum Rp0.2 for class 8.8 is 640 MPa up to and including M16, and 660 MPa above M16.
In practice, the quenched and tempered steels used for these classes sit some way above those minimums, and a measured tensile strength gives a reliable indication of where Rp0.2 will fall. A manufacturer working with known material, documented process control and a material inspection document is therefore able to demonstrate that the product conforms to the applicable requirements. What is actually tested, and what is reported on the certificate, comes from the product standard and from what is agreed at the time of order.
EN 10204[2004] Metallic products - Types of inspection documents
The European standard for 2.1, 2.2, 3.1, and 3.2 certificates, EN 10204 sets out the types
of certificate and a general overview of the meaning of the respective numbers. 2.1 and 2.2
are inspection documents based on non-specific inspection, and 3.1 and 3.2 are inspection
documents based on specific inspection. EN 10204 does not specify any test results that need
to be on a certificate.
This is generally the specification number that will be referenced on a 3.1 test
report/certificate.
EN 10168[2004] Steel products - Inspection documents
List of information and description, again a European standard. EN 10168 sets out code numbers and designations for products included in a certificate. e.g. A 02 is the type of inspection document, B 02 is the code for steel designation.
ISO 16228[2018] Fasteners - Types of inspection documents
The International standard that gives comprehensive information regarding minimum
requirements for what needs to be included on a certificate, and prefixes the type of
document with the letter F.
Section 5.7, Table 2, Minimum requirements for reporting of test/inspection results. This
section deals with methods of control, measurements shall be expressed as either min or max
or min/max and give measured values, attributes shall be expressed as Go or No-Go or
Go/No-Go and expressed as Conform, and inspection which will be expressed as Conform.
Section 5.8, Table 3, Minimum test/inspection results for fasteners. Results to be included
in F3.1 test reports are:-
ISO 898-1 Chemical composition, Tensile strength, Hardness for quenched and tempered property classes, and Thread acceptance.
ISO 898-2 Chemical composition, Proof load, Hardness for quenched and tempered property classes, and Thread acceptance.
ISO 3506-1 Chemical composition, Tensile strength and Elongation, Hardness for fasteners made of martensitic and ferritic stainless steel, and Thread acceptance.
ISO 3506-2 Chemical composition, Proof load, Hardness for fasteners made of martensitic and ferritic stainless steel, and Thread acceptance.
Section 6 Required content for each type of fastener inspection document.
Section 6.1 General, states other information and/or test/inspection results may be included
at the discretion of the supplier or if requested by the purchaser at the time of the order.
Section 6.4 Content of the fastener test report F3.1, Table 6, Required content for F3.1.
First section, General information related to the supplier.
Second section, General information related to the delivered fasteners.
Third section, Test/inspection results for material properties.
For fasteners made of steel or stainless steel
- the fastener reference standard (e.g. DIN 934, ISO 898-2) and date of publication;
- the material category, grade and/or designation (e.g. alloy steel for property class 10.9, A2 for stainless steel, etc.);
- the code identifying the test facility type where the chemical analysis has been performed: S (supplier), M (manufacturer), D (distributor), L (laboratory);
- the heat, cast, lot number of the material, or trace number to the material lot, the chemical analysis which shall include the chemical elements, the required chemical composition limit(s) and the test results. The elements specifically required by the reference standard shall be reported, however other elements may be included; the chemical analysis can also be a copy of the original material inspection document attached to the fastener inspection document.
Fourth section, Test/inspection results for mechanical, physical, dimensional, functional and/ finish-coating properties.
For each reported property
- the mechanical or physical property, its symbol, if any, the test method, the reference standard or technical specification and its date;
- the dimensional property and its symbol, if any;
- the code identifying the test facility type where the chemical analysis has been performed: S (supplier), M (manufacturer), D (distributor), L (laboratory);
- the quantity of tested parts, except when all test/inspection results are reported;
- the required limit(s), if any;
- the test/inspection results according to section 5.8, Table 3.
These items shall also be included in F3.1 when functional and/or finish-coating properties
are specified in the relevant product standard (and/or technical specification) and/or by
the purchaser.
Fifth section, validation.
From the contents of section 5.8, Table 3 in ISO 16228, it is clear which test results are required to satisfy the minimum requirements for a 3.1 test report/certificate, for the applicable material chemical and mechanical properties standard, and any other tests that may/may not be included.
Table 3 sets a floor, not a ceiling. Section 6.1 makes clear that further results may be included at the discretion of the supplier, or where the purchaser requests them at the time of the order. Anything beyond the minimum is therefore a matter for the order to settle, and is not an omission from the certificate.
Test series — FF and MP
It is wrongly assumed that test results should be included because they appear in a table in the appropriate specification, e.g. ISO 898-1 Tables 8-13, which are headed with Test series (FF 1-4, and MP 1 and 2). The assumption is that because the words test series appear, those tests must be carried out and the results included on the 3.1 certificate. In fact these designations describe the form of specimen a test is carried out on — the FF series covers tests performed on finished fasteners, the MP series tests performed on machined test pieces — and the tables set out which of those are feasible for a given size and property class. They are a classification of test methods and their applicability, not a schedule of tests that must be performed and reported.
Boron
We are often asked for Boron (B) results, as, rightly, boron appears in the ISO 898-1 chemical composition limits for property class 8.8 and greater. Whether it appears in the results depends on the steel actually used. Where the grade is boron-treated, a 30MnB4 for example, boron will be reported in the material inspection document and will appear as a figure on the 3.1 certificate. Where the grade is an alloy steel with no boron added, 42CrMo4 for example, boron is likely not to have been analysed at all, so it will not appear in the original material inspection document (the chemical analysis can also be a copy of that document) and may not be shown in the results of a 3.1 certificate.
Basic Thread Diameters (Metric)
The basic major, minor and pitch diameters are calculated using portions of the heights of theoretical triangles, from thread root to thread crest (In reality the root and crest points of the triangles are flattened), generated by the thread pitch, with a 60° thread angle, minor being 5/8 height and pitch 3/8 height. The following equations are simplified versions, but yield the same results: (D [internal]/d [external]) major diameter = nominal diameter i.e. M16 = 16mm (D1/d1) minor diameter = major diameter(D/d) - 2 x (0.54127 x thread pitch(P)) (D2/d2) pitch diameter = major diameter(D/d) - 2 x (0.32476 x thread pitch(P))
Thread Fit and Tolerance (Metric)
Internal thread fit is denoted by a capital letter e.g. 6H, external thread fit is denoted by a lowercase letter e.g. 6g. The thread fit is split into two parts, the first, the number, denoting the tolerance grade, values are given in ISO 965-1, Table 3 for external thread (Td) and Table 4 for internal thread (TD). Different thread pitches have a tolerance expressed in microns (µm) (there are 1000µm in a mm), e.g. 1mm thread pitch for tolerance grade 4 of external threads = 112µm, tolerance grade 6 = 180µm, and tolerance grade 8 = 280µm. External thread tolerance grades are split into three: 4,6,8 and are used to give tolerance for the major and pitch diameters. Internal thread tolerance grades are split into five: 4,5,6,7,8 and are used to give tolerance for the minor and pitch diameters. The second part of the thread fit, the letter, which denotes the tolerance position and gives the fundamental deviation, ISO 965-1 Table 1, again expressed in microns. Internal fundamental deviation is split in to two, G,H, external fundamental deviation is split into eight, a,b,c,d,e,f,g,h, and as with tolerance grade is thread pitch dependant. To find the tolerance value of a specific thread the tolerance grade value is added to or subtracted from a basic value plus the fundamental deviation value. The tolerance values for internal threads increase the diameter, and external tolerance values decrease the diameter. e.g. M16 x 2mm pitch 6H full nut Basic minor diameter = 13.835mm Basic pitch diameter = 14.701mm ISO 965-1, Table 1, 2mm thread pitch, fundamental deviation H = 0µm ISO 965-1, Table 2, 2mm thread pitch, minor diameter tolerance grade 6 = 375µm ISO 965-1, Table 4, 2mm thread pitch, pitch diameter tolerance grade 6, basic major diameter above 11.2mm, less than or equal to 22.4mm = 212µm Using this information the calculations are: Minor diameter valuesmin. = 0µm + 13.835mm = 13.835mmmax. = 13.835mm + 375µm = 14.210mm Pitch diameter valuesmin. = 0µm + 14.701mm = 14.701mmmax. = 14.701mm + 212µm = 14.913mm In the case of an M16 x 2mm pitch 6G full nut Fundamental deviation G = +38, therefore the calculations would be: Minor diameter valuesmin. = 38µm + 13.835mm = 13.873mmmax. = 13.873mm + 375µm = 14.248mm Pitch diameter valuesmin. = 38µm + 14.701mm = 14.739mmmax. = 14.739mm + 212µm = 14.951mm The majority of internal threaded nuts produced are 6H and external threaded bolts, screws, allthread etc. are 6g, 8g is also used, but is less common and for galvanised parts 6az and 6AZ (for even greater tolerance 6ax and 6AX). To work out the minor and pitch dimensions for tapped oversize nuts, the fundamental deviation of AZ (az, external) is worked out by adding 300 to 20 times the thread pitch (ax/AX is 220 times thread pitch minus 20) The standard which covers this is ISO 965, part 4 for external threads, and part 5 for internal threads . Again we will use an M16 x 2mm thread pitch nut: AZ = 300 + (20 x 2 = 40) gives a fundamental deviation of 340µm and applied as above calculations will give: Minor diameter valuesmin. = 340µm + 13.835mm = 14.175mmmax. = 14.175mm + 375µm = 15.550mm Pitch diameter valuesmin. = 340µm + 14.701mm = 15.041mmmax. = 15.041mm + 212µm = 15.253mm External threads are worked out in the same way, but instead of the minor diameter being the crucial value, the major diameter is, and the tolerances are subtracted. There may also be two tolerance designations e.g. 5g6g, the first tolerance is the pitch tolerance and the second the major diameter. In the case of 6az the diameter of the external thread would be further reduced — screwed undersize — to accommodate the galvanized coating thickness, crucially a 6H nut — often called ISO fit or tapped standard — would be used, not 6AZ. ISO 10684 also states, in Annex F, that assembly strength, for 6az, 6ax, 6AZ, and 6AX, will be reduced and provides methods to combat this. 6az: The bolt or screw should not be manufactured to the minimum tensile strength as specified in ISO 898-1. 6AZ: To fit nuts with a higher mating property class (e.g. G8.8 bolts with G10 Nuts) or use style 2 nuts of the same mating property class, and 6AX it says that in some national standards nuts of two mating property classes higher are mandated.