SPRING MANUFACTURING : WHICH MATERIALS ARE USED
Let’s make a rundown of the materials used in spring manufacturing and the way in which they are made; knowledge is a prerequisite to deepen the causes of failure related to surface defects.
Read also: WHY DOES A SPRING BREAK?
What are the features of the materials used for spring manufacturing and, more generally, for the elements with elastic characteristics? They must have high yield resistance, so that the Hooke diagram elastic area is as wide as possible.
In theory the ideal elastic material is the one in which the yield resistance coincides with the Rm, that is which one that maintains the proportionality between stress and deformation until it breaks down.
In spring manufacturing materials, the yield resistance (Rp02) is about 70% of the Rm, which is already a considerable value if compared to that one of other materials with non-elastic uses. The safety coefficients adopted for generic structural steels are much higher than 0.7. Just to be clear: K = 1 / 0.7 is the value to use to divide the rupture σ (Rm), to calculate the maximum σ which must not be exceeded using a certain material.
Materials for springs manufacturing have an extremely high Rm and therefore have an equally high Rp02.
For compression/traction springs for which the material stress is not axial (σ), but tangential, the tangential forces (τ) shall be considered, that is those which arise in the event of shear strain.
For these materials the conventional rule is to consider 45% of Rm as σ max, even if the responsible strain for yielding is τ.
Furthermore, the block settling operation, which forces the material to outreach its elastic-plastic area, reporting a permanent deformation, increases the limit of use by about 10%, reaching 55-56% of Rm.
As mentioned, spring materials must have a high Rm and therefore the question is: how to obtain materials with high RM? And what structural chemical characteristics must these materials have?
The prerequisite for materials with high Rm is the presence of carbon in the chemical analysis. Then the mechanical characteristics are obtained in two different ways:
- work hardening for metal drawing
- tempering
Metal Drawing
With this process the materials are pinched and pulled so that their surface layers thicken, reaching very high surface hardness values.
In this way the diameter of the wire is reduced. It starts from wires of higher diameter submitting them to several stages of reduction with machines called dies.
During the manufacturing cycle there are also heat treatments (patenting and distension), which are used to “refine” the steel structure and allow it to be cold worked.
The wires drawn by carbon have a hard “skin” and a softer heart. The effect is to elevate the wire’s Rm.
Quenching
Carbon alloyed steels with C% of about 0.5% are used in this field. The most commonly used alloying elements are Si, Cr, V, Ni. After a first machining reduction of the diameter, always by drawing, the steels receive a hardening and tempering treatment that gives them a martensitic structure. Contrary to drawn materials, pre-hardened wires are wires in which the hardness of the material is more constant along the wire section, due to the hardening effect.
Drawn wires’ materials for springs
1) Carbon steels for elastic uses (0.5% <C% <1%), called carbon patented wires.
Patenting is a heat treatment carried out a temperature of about 500 ° C in a lead salt bath that gives the material a “pearlitic” structure, which prevents the wire from breaking during its cold deformation, at the time of ‘winding / bending.
In Europe, these materials are regulated by EN 10270-1. Within this standard, the wires are classified by percentage of carbon L (low), M (medium) and H (high) and by type of use, static (S) and dynamic (D). Therefore, the following combinations SL, SM, SH, DM effectively not produced) and DH are obtained.
2) Stainless steels. The main feature is the presence of high percentages of Nickel (Ni) and Chromium (Cr) which confer resistance to corrosion (stainless). From a structural point of view, the materials can be austenitic, ferritic, martensitic and biphasic. Those that are most used in the spring sector are the austenitic ones, followed by the martensitics and biphasics.
As the name suggests, austenitic steels have an austenitic type structure. Until recently, the reference standard was EN 10270-3, then replaced by UNI ISO EN 6931-1.
These steels are obtained by drawing hardening. The most commonly used wire is 1.4310 (Aisi 302), both in the NS (normal strength) and HS (high strength) versions. HS are obtained by increasing the hardening by drawing or by adding alloying elements, typically Molybdenum (Mo). Generally, with the same diameter, the Rm of stainless steels are lower than those of carbon steels.
Hardened wires’ materials for springs
The pre-hardened wires for springs are included in the EN 10270-2 standard. They are classified by chemical analysis and type of use. We can have:
- FD, static uses.
- TD, moderately dynamic uses.
- VD, highly dynamic uses, typically in engine valves, V means Valve indeed.
From the chemical analysis point of view, there are simple carbon pre hardened steels and alloy steels. The alloying elements confer different properties.
The most common are SiCr, but there are also CrVs: they are used for high temperature applications (> 120 °) for their better yielding behavior (relaxation) under high temperature load.
SiCrVs combine the characteristics of high resistance to fatigue with those of yielding resistance high temperatures.
SiCrVNi are steels with very high fatigue resistance, used in combination with cold nitriding.
The predominant feature of pre-hardened products is their use in situations where very high fatigue strength is required. They were created for use in the engine sector for the return of the internal combustion engine valves (high temperatures and high number of work cycles). The first producer of pre-hardened products was Garphyttan (now Suzuki-Garpyttan) with the Oteva series, but after the patent expiration they were also implemented by other producers of spring wire.
Other elastic materials
There are also other materials used for elastic applications, but generally the main element of their use is other; elasticity is just a secondary requirement, even if necessary.
Therefore, where the conductivity of the current is the most important characteristic, copper alloys are used (work hardened brass, phosphor bronze, copper-beryllium alloys or cobalt beryllium copper) which represent a good compromise between conductivity and elasticity requirements.
If otherwise lightness is preferred, titanium alloys are used. Recently these alloys have found particular uses that exploit the shape memory of the material activated by temperature changes.
For aerospace applications, where the temperature conditions are very strict (low and high temperatures) or the environments are highly corrosive, Nickel alloys are used.
In this context, Inconel, Monel, Nimonic and Hastelloy are used.
In watchmaking industry, on the other hand, very high Rm is preferred because of the high strains due to limited dimensions. In this application field Ni-Co alloys, such as Phynox and Haynes, are used.
He graduated in electronic engineering from the Milan Polytechnic in 1992. Since 2000, he has been working at Mollificio Valli as technical sales manager.
Over the years, he has acquired extensive experience in the calculation and technical aspects of spring production.
He has always been passionate about mathematics and statistics, and has had the opportunity to apply his knowledge in statistical control techniques, metrological aspects and in general in the practical field of problem solving and continuous improvement.