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STEEL FOR FASTENERS

International fastener standards do not specify the acceptable guidelines for steel to be used as base material. For example, ISO 898-1, which dictates the chemical and mechanical properties of externally threaded steel fasteners, only stipulates limits for the chemical composition of steels and the minimum tempering temperatures for the different property classes of bolts, screws and studs.
 
Whereas it is common in Europe to refer to European standards (EN standards) for the steel types, other standards are applicable outside of Europe. In general, it is left to the manufacturer to decide on the type of base material to apply, as long as the products comply with all requirements in the relevant fastener standards.
 
In this publication, reference is made to EN-standardized steel types, but this should not be considered as a limitation of the steel types used worldwide for DIN- or ISO-standardized fasteners.
 
Non-alloy quality steels are often used in the manufacturing of fasteners, like anchor rods or pins. The standards for this type of steel are dictated in EN 10025-2, which designates the yield strength of the steel, e.g., S355. The yield strength of this type of steel is 355 MPa (N/mm2) for diameters up to 16 mm, and it diminishes with larger diameters. Added letters and numbers inform about the required impact strength and the test temperature; for example, S235J2 requires an impact strength of 27J at -/- 20°C. Older designations, like St37-2 or St52-3, are occasionally still used. Their numbering (37 and 52) indicates the minimum tensile strength in kgf/mm2, which converts to approximately 360 MPa and 510 MPa, respectively.
 
As most fasteners are produced by cold heading, standards have been developed for the steels that are used in these processes. The standard series EN 10263 (parts 1-5) explains the technical delivery conditions and properties of steels, the steels not intended for heat treatment after forming, case-hardening steels, quenched and tempered (QT) steels and stainless steels. All types of steel included in EN 10263 must be killed (completely deoxidized). After casting, the steel gradually solidifies without gas bubbling out, which is a result of adding aluminum, ferrosilicon and manganese to create a stronger structure.
 
Steels addressed in EN 10263-2 should all be non-alloy or alloy quality steels. EN 10027-1 explains steel naming, such as C8C or C10C. The number indicates the carbon content; the content is roughly the name’s number divided by 100. For example, C10C has a carbon content of about 0.10%. Alloying elements are Si, Mn, P, S and Al. The aluminum may be replaced by other elements that have a similar (deoxidizing) effect. These steels are used in the manufacturing of low-strength fasteners like rivets.
 
EN 10263-3 deals with non-alloy quality steels and alloy special steels that are intended for case hardening. In their designations, the main alloying elements are mentioned, e.g., 22MnB4. The first numbers specify the carbon content multiplied by 100 (to use the same example, C ± 0.22%). The abbreviations of the main alloying elements follow, and the last numbers indicate the content of the most important alloying element(s) multiplied by a factor that is dependent on the alloying element:
  • Multiplied by 4 for Cr, Co, Mn, Ni, Si, W
  • Multiplied by 10 for Al, Be, Cu, Mo, Nb, Pb
  • Multiplied by 100 for Ce, N, P, S, C
  • Multiplied by 1000 for B.
To demonstrate, there is approximately 4/4 = 1% Mn present in 22MnB4. Generally, case-hardening steels feature a relatively low carbon content but acquire a very hard wear-resistant surface when carbon is added to the surface as part of the heat treatment. These steel types are commonly used in the production of fasteners like pins, thread-forming screws, sheet metal screws, tapping screws etc.
 
EN 10263-4 contains information on steels for quenching and tempering, which must be non-alloy or alloy special steels; they are designated engineering steels. Because of their chemical composition, they are perfectly suitable for hardening and have suitable toughness at a given tensile strength in the quenched and tempered (QT) condition. This part of EN 10263 includes steels with names like 25CrMo4 and 42CrMo4 or boron-containing steels like 28B2 and 20MnB4. The chemical composition can again be obtained from the names: 42CrMo4 (or 1.7225) contains ± 0.42% carbon and ± 1% chromium, while 28B2 (or 1.5510) contains ± 0.28% chromium and 0.002% boron.
 
The last part of EN 10263 informs about stainless steels. These are all alloy steels with a chromium content ≥ 10.5% and a maximum carbon content of 1.2%. Their steel names all begin with the letter X to indicate that the content of at least one alloying element is over 5%. Of course, one of the elements is Cr, but with austenitic stainless steels also the nickel content is more than 5%. For these steels, the naming convention with multiplication factors is not valid – the numbers at the end of the name indicate the average percentage of the main alloying elements. In this way, X5CrNi18-10 contains ± 0.05% carbon, ± 18% chromium and ± 10% nickel, while X3CrNiCuMo17-11-3-2 is composed of ± 0.03% C, ± 17% Cr, ± 11% Ni, ± 3% Cu and ± 2% Mo.
 
Other steel types for fasteners are, for example, free-cutting steel and cold- or heat-resistant steels. Free-cutting steels are standardized in EN 10087. This type of steel is characterized by suitable machinability and short chip breaking, which is achieved by deliberately increasing the sulfur content to a maximum of 0.34%, sometimes with the addition of lead. A popular type of steel for fasteners is 11SMn30, which has a carbon content ≤ 0.13% and ± 0.3% sulfur, or the leaded variant 11SMnPb30. Fasteners are machined from steel in the cold-drawn condition. The method of machining on automatic lathes is no longer commonly used for the mass production of fasteners, but this process is still used for small quantities and for products that are difficult to cold form due to complex shapes. Some free-cutting steel types can be case hardened, but, generally speaking, free-cutting steels have limited strength properties.
 
Austenitic stainless steels have specific properties that make them suitable for low temperature use (down to -200°C). Other alloy steels may be used at temperatures up to 700°C, and these steels are quenched and tempered or have an austenitic structure. Nickel-based alloys (with nickel content of ≥ 50%) are also suited for high temperature applications, though they do not strictly qualify as steel since the main element is nickel and not iron.
 
For these special fastener materials, the technical and mechanical properties are standardized in DIN 267-13, while EN 10269 defines their chemical composition and the heat treatment requirements.
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