In some instances a spring is required to operate in a corrosive environment which means some form of surface protection is needed. The options of protection available will depend on the application of the spring.
If cost is an issue, or where the material needs to be of a required strength, than it may not be possible to design a spring with materials that will not corrode.
An obvious choice is the use of nickel alloys, they are excellent for corrosion resistance, however the cost can be prohibitive. A simple method is to simply oil or grease the springs, this should give a sufficient level of corrosion protection for springs in transit or when in storage (as long as the storage conditions are not too testing).
Another effective method of protecting springs from corrosion is through either plastic coating or painting. The only issue with using either of these methods is that the protection is only effective until it is damaged, it will then be liable to corrosion underneath the finish.
The easiest option is to manufacture the spring from carbon steel, wire drawn with a galvanised coating which should be sufficient enough in most circumstances, if not, a better protection is required. A popular method is to use a metallic finish which can be obtained by electroplating the spring. In order to ensure the maximum corrosion resistance it’s vital to use the correct electroplated metal.
Zinc plate and cadmium (rarely used due to its toxicity) corrode in preference to steel and will protect the surface even when the coating is damaged.
Nickel, copper and chromium plate will lead to the steel corroding when damaged so is not recommended. Nickel plate is only recommend when the component needs to undergo soldering and so it’s widely used in the electronics industry. There is a risk of hydrogen embrittlement when electroplating which can lead to component failure when it’s loaded.
A de-embrittlement process must be carried out in order to minimise the risk, this process involves the component being held at an elevated temperature of 190-200°C for up to 24 hours to allow the hydrogen to dissipate.
If low alloy spring steels such as BS2083 685A55 are to be used then they should not be electroplated under any circumstances due to the high risk of hydrogen embrittlement.
In contrast, a mechanical zinc or zinc alloy plate will provide zero risk of hydrogen embrittlement and an equally effective corrosion resistance, all be it costly.
The other options available include coating the spring with a resin impregnated with zinc flakes, they can have either a black or silver finish. They provide a superior protection to mechanical or electroplating and avoid the risks associated with hydrogen embrittlement.
One of the key factors that a spring user requires is reliability, there has been much research into the behaviour of springs under fluctuating loads to ensure a spring is long-lasting.
If a spring will only be operated for less than 10,000 cycles during its lifespan then fatigue does not need to be taken into account. Any spring that will be performing for more than 10,000 cycles can be affected by fatigue and this needs to be factored in during the design process.
The factors that can affect the fatigue performance in springs include working stress, material surface quality and wear.
When in operation, springs will generally work between two fixed positions. In order to predict the working life of the spring the working stress at these positions can simply be calculated.
A common cause of failure in an extension spring is when the loop of the spring breaks off in the area where the hook meets the body of the spring, this is the area of highest stress. Loops are subjected to bending and torsion stress, when they are formed in extension springs small tooling marks are unavoidable. These marks can increase the likelihood of a failure at this point, sometimes loops are formed using bends that are too small and a small radius can increase stress. Different fatigue performance can be caused by different types of end loop, this can be solved by using a loop with a transition radius between the spring body and the end loop of approximately the body radius.
When an extension spring is required to work dynamically, it must be noted that an extension springs can have around 20% lower performance with regard to fatigue than compression springs.
Material surface quality
The material surface quality is very important when trying to avoid the risk of spring failure, fatigue cracks generally originate from the surface of the material, so the greater the surface quality the more chance of improved fatigue performance.
There are a number of methods that can improve the surface quality, the most popular of these is shot-peening. This involves firing small rounded beads of material at the surface of the spring which will lead to a small residual compressive stress this then lowers the chance of a fatigue crack appearing.
The process of shot-peening is generally carried out only on compression springs and large leaf springs, otherwise the shot could become trapped in the coils of close wound torsion and extension springs. Also, the inside face of the coils would not be peened and this would eliminate the benefits of the process.
The wear of springs can be caused in a manner or ways, when operating it’s important that the maximum deflection of the spring should not exceed 85% of the available deflection. If a spring is working close to its solid (coil bound) length, the number of active coils will reduce due to coils coming into contact with each other and there is a chance that this could cause wear to the contacting faces.
Another cause of wear is when a spring works over a shaft or in a bore, if a spring is able to contact either the shaft or the wall of the bore then the wear can lead to early fatigue. If the inside diameter of the spring is worn this can increase the failure as this is where the working stresses are at their greatest.
Torsion springs have a lower fatigue performance in comparison to compression and extension springs, mainly due the friction and wear between the spring and that shaft. It’s hard to eliminate this with torsion springs however it can be reduced with an effective design.
There are other factors that can affect the fatigue performance such as corrosion, material cleanliness and speed of operation.
Just like wire, there are a wide range of materials available in the production and design of flat strip products.
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. We will look at carbon steels, stainless steels and copper alloys only in this blog as there are such a vast range of materials out there.
The different grades of carbon steel strip are classified according to the carbon content, the manufacturing method, and if heat treatment is used in the process.
When formability is need, annealed carbon steel strip is used. After forming, if a heat treatment is used this will increase the materials strength and hardness. If formability is not required there are heat treated grades of spring steel.
Clock springs and seat belt retaining springs utilise these materials in their hard condition. They provide a good surface finish, uniformity of mechanical properties and precision thickness tolerances. Carbon steel springs will corrode readily so they do require some form of protection if working in a harsh environment.
Stainless steels are widely used for their corrosion resistance, their ability to withstand elevated temperatures and their resistance to relaxation.
They are generally obtained in the hard rolled condition, there is a need to take into account the effect of spring hardness when strip components are designed and manufactured from stainless steels.
Stainless steels are around 20% weaker than heat treated springs steels of the same size. During the cold rolling process the hardness of stainless steel is produced causing the stainless steel to be slightly magnetic.
When it is necessary to have high electrical and thermal conductivity and/or non-magnetic properties it is essential to use copper based alloys.
Another benefit of copper alloys is their good atmospheric corrosion resistance, as the majority of copper-alloy strip components are used as electrical contacts, many copper parts are electro-plated. With its high tin content, phosphor bronze has the high tensile strength in comparison to other copper alloys resulting in it being the most commonly used.
Beryllium copper is a precipitation hardening material and can be purchased in a variety of harnesses, depending on the amount of heat treatment carried out. As one of the most expensive copper alloys, it can be precipitation hardened and used to greater working stresses than the other alternatives.
Next month our Director of Quality, Steve Blunt will look at ‘tooling materials’.
Jon Davies, Sales Manager
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.