As with wire, there is a wide range of strip materials available to the spring manufacturer. As many parts produced in strip are not primarily used as a spring, many low strength alloys are used, generally for their formability and electrical conductivity.
Strip materials can be obtained in different grades of hardness, and some spring materials are able to be heat treated to increase their strength and hardness.
Due to the vast range of materials available this section will deal with carbon steels, stainless steels and copper alloys only.
Fig.1: Examples of flat strip spring products
There are a number of grades of carbon steel strip. These grades are classified according to the carbon content, the method of manufacture and whether a heat treatment is used. Annealed carbon steel strip is used where formability is required, a heat treatment after forming will increase the materials strength and hardness. Where formability is not an issue there are heat treated grades of spring steel and texture rolled materials. These materials are obtained in the hard condition and are used in applications such as clock springs and seat belt retaining springs.
British Standards for annealed spring steels include BS5770 Pt 2 CS50, CS70 and CS95 annealed. In the hard condition which is covered in the British Standards by BS5770 pt 3. there is available CS70HT, CS80HT, CS95HT. The number in the grade designates the percentage of carbon in the alloy e.g. CS70 has 0.7% carbon.
Texture rolled materials can be obtained. This material has good surface finish, uniformity of mechanical properties and precision thickness tolerances. It is a high strength material and is used widely in seat belt retainers.
If operating in a corrosive environment, carbon steel springs require some form of protection as these materials will corrode readily.
These materials are widely used for their corrosion resistance, their ability to withstand elevated temperatures and their resistance to relaxation.
Stainless steels are generally obtained in the hard rolled condition, strip components designed to be manufactured from stainless steels should take the effect of spring hardness into account.
Stainless steels are about 20% weaker than heat treated springs steels of the same size. As the hardness of stainless steel is generated during the cold rolling process, the work hardening will cause the stainless steel to be slightly magnetic.
British Standards covering stainless steel strip materials include BS5770 Pt 4, 302525, 301521, 316516. All these grades can be obtained in varying levels of spring hardness, depending on the thickness of the material.
Copper-based alloys are used where high electrical and thermal conductivity and or where being non-magnetic is a priority. Copper-alloys also exhibit good atmospheric corrosion resistance, but as the majority of copper-alloy strip components are used as electrical contacts many copper parts are electro-plated.
There are three alloys that find a place in spring manufacture covered by EN 1654. They are CuZn36, a spring brass strip, CuSn5, CuSn6 phosphorus bronze and CuBe2 beryllium copper.
Spring brass strip CuZn36 and phosphorus bronze CuSn5, CuSn6 get their material strength from the work performed in cold working. 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 CuBe2 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 precipitation hardened it can be used to greater working stresses than the other copper alloys.
Table 1: Flat Strip Spring material reference table
Fatigue Performance of Flat Strip Materials
The fatigue performance of strip materials is greatly affected by the edge and surface condition. It is possible to purchase some strip materials with a dressed or rounded edge which greatly improves the fatigue performance, but if the components are punched out of the material, the edge finish will depend of the performance of the tooling.
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]