Spring materials are some of the strongest materials used, they are chosen for their strength as they are designed to work to greater working stresses than most other components. This can be seen in a helically wound compression spring which can be stressed to 70% or more of the ultimate tensile strength of the material.
Many spring materials are required to work in extreme environments such as high or low temperatures and they will be pressurised under large loading.
Often used for their magnetic and electrical capabilities, there are many materials available in the production of a reliable and long-lasting spring. We will look at the most commonly used materials in wire springs (strip materials will be looked at in a later blog).
The most popular material of choice is high carbon spring steel, they are relatively low-cost and are widely available. They are also the strongest materials to be used in spring production.
If the wire size is 1.60mm the range of material grades taken from BS5216 could be used as follows:
The material grades also refer to the surface finish and the dynamic qualities are:
EN10270-1 is the most up to date specification for high carbon spring steel however BS5216 is still widely recognised and used in our industry.
During the drawing process the mechanical strength is obtained as the size of the wire decreases, as this happens the ultimate tensile strength of the material increases.
Some of the above grades can include a zinc or aluminium/zinc coating when pre-drawn. This will allow for sufficient corrosion protection for non-arduous applications, otherwise the components will need electroplating to produce a corrosive protection.
Pre-hardened and tempered steels
Low alloy or carbon pre-hardened and tempered steels are another option for spring materials, they can be drawn annealed and then hardened later on during the wire manufacturing process allowing them to be stronger than cold drawn materials above the size of 2.00mm.
The hardening process results in the mechanical strength of these materials allowing the ultimate tensile strength to not be dependent on wire size. Larger section materials can actually obtain a higher ultimate tensile strength.
With excellent static and dynamic properties they are prone to corrode easily without any surface protection.
If the materials are pre-hardened and tempered there are many standards relating to whether they are carbon steel or one of the many low alloy steels. The most popular alloys used are silicon chrome (BS2083 685A55) or chrome vanadium (BS2083 730A65).
Where the corrosive or relaxation resistance requirements are too great or the working temperatures are too high stainless steels are used.
With many grades of stainless steel available they vary in their mechanical properties and corrosion protection. They tend to be approx. 10% weaker than spring steels of the same size, however there are precipitation hardened grades that are nearly of equivalent strength.
The stainless grades used are usually 301S26, 302S26 both similar having 17%/18% chromium and 7%/8% nickel respectively. However, for greater corrosion resistance, especially in salt water, grades 316S33 and 316S42 are used as they have molybdenum added for improved resistance to chlorides. Stainless steel grades are covered by EN10270-3 however BS2056 is still widely used in our industry.
Stainless steels go through a cold drawing process, just as carbon steels, to increase their tensile strength. During this process the materials become slightly magnetic, if low magnetic permeability is required there are two stainless grades that can be used, these are 305S11 and 904S14,
Where greater strength is needed, precipitation-hardened stainless steels can be used. Once 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, 1.00mm wire has a minimum ultimate tensile strength of 1710 N/mm2, while after heat treatment this is increased to 2030N/mm*. A cost of this is slightly inferior corrosion performance to 302S26.
Now that you know about the correct materials for a spring, please do get in touch if you would like to know more, email us or call 01425 611517.
Next month our Director of Quality, Steve Blunt will look at ‘the production of compression springs’.
Jon Davies, Sales Manager
Southern Springs & Pressings manufacture a wide range of springs, wire forms, flat strip components and tailor made metal products to meet your needs. We also provide specialist services such as tooling, assembly and design solutions to help deliver your products to market.