There are many different types of materials available to the spring designer. In this section we will deal with the more commonly used spring wire materials. Strip materials will be discussed later.
Cold Drawn Carbon Steels
For general engineering purposes spring steels are the best choice due to their relative low cost and their wide availability. They also are the strongest spring materials that the designer can specify.
For sizes lower than 2.00mm cold drawn carbon steels specified under standards such as BS5216, 5202, DEFlO6 are the highest strength. These materials come in a range of strengths and surface finishes that can be matched to the spring’s end requirements. For instance, for the wire size 1.60mm the range of material grades taken from BS5216 generally used are shown in Table 1.
Table 1: Material grades taken from BS5216
The grades also refer to the material surface finish and therefore dynamic qualities as shown in Table 2:
Table 2: Dynamic qualities of material grades
Due to the fact that the mechanical strength is obtained through the drawing process, as the size of wire increases, so the ultimate tensile strength of the material decreases. Some of the above grades are available pre-drawn with a zinc or aluminium/zinc coating that will give sufficient corrosion protection for non-arduous applications. Otherwise the above materials, like all carbon or low alloy steels will require some form of corrosive protection.
Pre-hardened & Tempered Steels
Other types of spring material are low alloy or carbon pre-hardened and tempered steels.
These materials are drawn annealed and are then hardened by the wire manufacturer to produce a high strength material. These are stronger than cold drawn materials above the size of 2.00mm. The mechanical strength for these materials is obtained through the hardening process, so the ultimate tensile strength does not depend on the wire size. In fact, it is possible to obtain a higher ultimate tensile strength with larger section materials than lower section.
They have excellent static and dynamic properties, but are prone to corrode readily without surface protection.
There are many standards covering pre-hardened and tempered materials, depending on whether it is a carbon steel or one of the many low alloy steels. Alloys such as silicon chrome (BS2083 685A55) or chrome vanadium (BS2083 730A65) are among the most widely used.
Stainless steels are used widely throughout manufacturing where the corrosive or relaxation resistance requirements are too great for normal spring steels, or the working temperatures are too high. There are many grades of stainless steel varying in their mechanical properties and corrosion protection. Generally stainless steels are about 20% weaker than spring steels of the same size, but there are precipitation hardening grades that are nearly of equivalent strength.
Stainless steel grades are covered by BS2056 1991, and the grades generally used are 301S26, 302S26 both similar having 17%/18% chromium and 7%/8% nickel respectively. These grades are used widely, but for greater corrosion resistance especially salt water, grades 316S33 and 316S42 are used, having molybdenum added for improved resistance to chlorides.
The stainless grades detailed above all get their strength from the cold drawing process. This process makes the materials slightly magnetic. If very low magnetic permeability is required there are two stainless grades that can be used. These are 305S11 and 904S14, which are virtually free from residual magnetism.
If greater strength is required, precipitation-hardening stainless steels can be used. After the springs are manufactured they are heat treated at 480°C. This causes small precipitates to grow through the material, increasing the ultimate tensile strength. For example, in the as drawn condition, 1.OOmm wire has a minimum ultimate tensile strength of 1710 N/mm2, while after heat treatment this is increased to 2030N/mm*. This increase is at the cost of a slightly inferior corrosion performance than 302S26 and 301S26.
Copper-based alloys are used where high electrical and thermal conductivity, nonmagnetic or good atmospheric resistance are required. There are three alloys that find a place in spring manufacture covered by BS2873. They are CZ107, a spring brass wire,
PB102, PB103 phosphorus bronze and CB102 beryllium copper.
Spring brass wire CZ107 and phosphorus bronze PB102, PB103 get their material strength from the work performed in cold drawing. Phosphorus bronze, with its high tin content, has the higher tensile strength, and due to this it is the most widely used copper alloy.
Beryllium copper CB102 is a precipitation hardening material. It can be purchased in a variety of hardnesses, depending the amount of heat treatment carried out at the mill. It is the most expensive copper alloy, but as it can be hardened it can be used to greater working stresses than the other copper alloys.
Stress Relieving Heat Treatment
It is generally recommended that all spring materials are subjected to a stress-relieving operation after forming. In the case of cold drawn spring steel this would be at a temperature between 220°C and 375°C for 10 minutes to 1 hour depending on the type of spring and its application. The object of this is to reduce the stresses introduced during coiling, especially in the case of compression and extension springs, as these stresses are not beneficial. Stress relieving also slightly increases the elastic limit of the material and stabilises the spring’s dimensions. The problem with stress relieving is that as the ‘coiled in’ stresses are removed, the spring will move and this leads to dimensional change. This dimensional change has to be taken into account by the spring maker before coiling.
Stress relieving is often not carried out on extension springs as the heat treatment reduces the amount of initial tension.
Article written by David Banks-Fear and published on MDF by kind permission of Southern Springs & Pressings Limited.
David Banks-Fear is a Mechanical Design Forum Group member. He is a technical author and consultant design engineer with nearly 40 years of experience. He and his design team are available to assist with any technical design issues with springs, pressings and precision engineered parts. Email: [email protected]