Steel is an essential topic in the fastening world, as it is the most common material for fasteners. Steel refers to metal alloys containing iron (Ferro, Fe) and carbon (C).
There are many types of steel, though forgeable alloys are primarily used for fasteners because cold forming (also known as cold heading) is the typical means of producing fasteners.
Some products, however, are made from different steels, like cast iron or spring steel. Steel fasteners with high mechanical properties, such as classes 8.8, 10.9 or 12.9, use quenched and tempered steels.
The corrosion resistance of most steel alloys is low, so they are often coated with a protective metallic surface layer (e.g., zinc) to improve the lifetime of the fasteners.
Steel fasteners are often stocked in different property classes. The property class identifies the strength of a fastener.
As per European Standard EN 10020, the definition of steel is a “material which contains by mass more iron (Fe) than any other single element, having a carbon (C) content generally less than 2% and containing other elements. A limited number of chromium steels may contain more than 2% of carbon, but 2% is the usual dividing line between steel and cast iron.”
While steel is generally composed of an alloy of iron (Fe) and carbon (C), the other elements in steel may influence its characteristics and properties. Since fasteners are mostly made of steel, speaking of “iron fasteners,” for instance, is therefore an improper designation.
EN 10020 offers an elaborate classification system and distinguishes among three basic types of steel:
- Non-alloy steels – steel grades in which none of the elements present in the steel exceed the limit values from the below table.
- Stainless steels – steels with at least 10.5% chromium and a maximum of 1.2% carbon.
- Other alloy steels – steels that do not comply with the definition of stainless steels and in which at least one element from the below table is present in a concentration over the value limit in the below table.
|Element||Limit value (by mass)|
|Others (except carbon, phosphorus, sulfur, nitrogen) (each)||0.1%|
Non-alloy steels are further divided into non-alloy quality steels and non-alloy special steels. Quality steels have specified properties such as toughness, formability and grain size control, while special steels generally have a higher level of cleanliness in respect to non-metallic inclusions. Special steels are particularly suited for heat treatment (quenching and tempering, case hardening) as their chemical composition and production process are precisely controlled. Their phosphorus and sulfur content may be limited to a maximum value so that they comply with specific requirements after heat treatment. Therefore, special steels ensure that specified requirements, such as yield, tensile and impact strength, are met after cold forging and heat treatment.
Stainless steels are further subdivided by two criteria:
- Nickel content
- Less than 2.5% nickel
- More than 2.5% nickel
- Main properties
- Corrosion resistance
- Heat resistance
- Creep resistance
Other alloy steels are similarly differentiated into quality and special alloy steels. Alloy quality steels are not generally intended for quenching and tempering or surface hardening, but requirements exist for several factors, including toughness, formability and grain size control.
All other alloy steels are considered to be special alloy steels in that they share the same cleanliness and precise mechanical properties as special non-alloy steels.
The steel designation system is standardized in EN 10027. This standard is composed of two parts. Part 1 specifies rules for designating steels by means of a symbolic alphanumeric code (like C10C, S235JR or 34CrNiMo6), known as the steel name. This steel name specifies the application and principal characteristics (chemical composition, mechanical properties etc.). Part 2 provides a numerical system that consists of a material group number (one digit), a point and six more numbers (a steel group number of two digits and a sequential number of maximum four digits).
This system is based on the German “Werkstoff” numbering system. The last two digits, which define the steel production method and the heat treatment condition, are often omitted. Steel numbers that correspond with the steel names above are 1.0214, 1.0038 and 1.6582, respectively.