Spring Steel Types Used in Spring Making
The selection of the correct spring steel that best meets the needs for a spring or wire form design is usually a compromise among all the materials available. Here's a good general guideline-
- If the volume of material is large, the choice of material is highly important. Why? It will be the major cost element in the spring.
- If the material quantity is small, availability rather than cost often dictates the material choice.
It is best to consult with a spring manufacturer before making your final decision.
Here is a link to a chart that shows a good cross reference of spring steel types and their specification numbers-
Summary of Spring Material Cross Reference.
To get a good explanation of the various specifications mentioned in this chart go to this great web page-
Wire Terms Used for Spring Wire.
Now, let's look at spring steel in general. This type of wire is made for high duty. You could even call it a specialty steel.
The standards and controls used to hold the quality of the material made is determined by the steelmaker's equipment, knowledge, and integrity.
The most important qualities for this wire are accuracy, ductility, hardenability, finish, high fatigue value, and uniformity in response to tools and heat treatment from one lot to another.
Accuracy in size depends on uniformity of hot rod and of annealing during the cold-rolling process. Keeping the mills and rolls in proper conditon also controls this process.
Ductility is obtained by selecting a raw material that has inherent characteristics for ductility. Also, the combination of annealing and rolling used will effect this process. The size and arrangement of the carbide particles and the ferrite areas in the steel create the ductility. Production must be designed to bring about the carbide structure at the final size for the ductility required. High ductility simplifies the manufacture of the spring or form and also makes impossible parts possible to make.
Hardenability is really set in the original melting of the steel. This is done by varying the deoxidation process which results in shallow hardening fine-grained steels or deep hardening coarse-grained steels.
Fine-grained steels are tougher and preferred for thinner sized wire. Coarser grained steels are usually desirable for heavier sizes. Good hardenability controls whether the final physical properties are acheived uniformly in heat treatment.
Finish is dependent on the quality and maintenance of the roll surfaces during cold processing. The original surface of the hot rolled steel may also have some slight effect. Finish means freedom from surface defects which may cause failure, and also a general good appearance of the wire.
High fatigue values are controlled by the following-
- Fineness of grain created from the hardening characteristics of the spring steel
- Any surface condition that would lower the ultimate strength or interrupt the continuity of the surface(this will become the location of maximum stress)
- Internal defects such as pipes, laminations, large inclusions, or banded structures will act as internal stress raisers and break the metal down prematurely.
Here is a good chart that list the main wire types used for springs and wire forms showing their typical properties-
Typical Properties of Common Spring Materials
Strength levels used for round wire vary with the wire size. This chart below shows that.
Flat spring steel parts and springs have normal hardness ranges as shown in the graphic. High hardness ranges are used only where there is no alternative because of the dangers of increased notch sensitivity.
Choice of Materials for Heat Treatment of Springs
Springs are usually either treated with low temperature heat applied for pre-strengthened material, or high temperature heat to strengthen annealed material.
The most common is using pre-strengthened material and heating with low temperature. This is normally called stress relieving. The temperature range for this is normally from 400 degrees to 800 degrees farenheit. The purpose of this treatment is to relieve excessive residual stresses and to stabilize the part dimensionally. It is also performed to raise the yield strength of cold-drawn material.
Stress relieving is also done to springs that are electroplated to reduce the possibility of failure from hydrogen.
Many spring-like parts require some very intricate forming. This forming is sometimes impossible to do with apre-hardened spring steel. Soft or annealed material is used instead and heat treated to spring like properties after forming.
High carbon and alloy steels are hardened by quenching to full hardness then tempering to the desired level. Negligible residual stress results from this unless a set-out operation follows. Note that this practice can very easily create distortion in the parts.
The are some spring steels, like beryllium copper and 17-7PH that are actually strengthened by a simple heating process after forming. These type of materials are called age hardenable. Note that this process fully stress relieves the part but here also distortion may occur unless special techniques are used.
Most stress relieving and age hardening is done in air and a moderate amount of oxide forms on part surfaces because of this. This is not detrimental. Actually, there is some belief that this increases corrosion resistance. There are special protective atmosphere or vacuum furnaces that can be used if no surface oxides are permitted.
Unlike stress relieving, hardening methods for carbon and low alloy steels are always done in protective atmosphere or molten salt to prevent decaburization. The tempering process which follows is done in air. So, some oxides still do result with this method.
In summation, when deciding on the proper heat-treating methods it is best to consult with the wire manufacturer.
Here is a link to a chart showing a comparison of hardness to tensile strength -
Various Hardness to Tensile Strength Conversions
If the springs need to be non-magnetic this must be specified. The spring maker should be consulted on this also. Ferrous spring materials are all highly magnetic. Even 18-8 stainless steel is magnetic at spring temper (it is only non-magentic in the annealed condition).
Also note that many spring materials are susceptible to stress corrosion failure. Ferrous materials can be protected with a variety of finishes.
Stay tuned for updated information related to compression springs and other spring types. Our team and visitors to spring-makers-resource.net will be contributing.
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