Spring Materials & Surface Finishing

An Introduction to the materials used in helical and flat spring design including options for material surface finishing and plating processes

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Spring Technology Experts

We specialise in designing, developing and manufacturing high performance, quality-driven springs, wireforms and pressings. Customers select Springtech as a supply partner because they trust us to consistently deliver innovatively designed, precision-made components at a competitive price and shipped on time.

• Full-service solution: design, tooling, manufacture & finishing

• Specialised product & material engineering expertise

• Design & development for new concepts or existing products

• Rigorous quality & inspection management

• Extensive inventory of high-quality materials

We are proud to offer all our customers a fully integrated service spanning product and tooling design, engineering, manufacture and finishing services all certified to ISO9001.

Materials Our Engineers have a wealth of experience in designing products for demanding applications across a number of performance-critical industries. As well as carbon and stainless steels, we also hold inventories of Hastelloy, Inconel, Titanium, Monel, Tantalum, Nimonic etc.

Products We supply a huge range of wire and flat products from tailor-made, small batches to high volume call-off orders. Our spring range includes compression, extension, torsion, garter, wave and flat springs. Wireforms and pressings are custom designed to customer specifications and tooled in-house.

Engineering As advances in design technology enable ever more sophisticated products to be conceived, we are continually investing in new engineering and production plant to raise capacity and optimise throughput.

Quality Commitment to, and implementation of, exacting quality standards are hard-wired into all our processes. When sourcing materials we never purchase from suppliers who could compromise our product performance or quality.

We serve a highly diverse and truly international customer-base. Many of our customers require our specialist engineering expertise to ensure that product design and material selection are fit for purpose for applications operating in highly demanding working environments.

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Material Selection

Selecting the most appropriate material for a specific application is essential to ensure that the spring behavioural dynamics are efficient, robust and matched to the anticipated working life of the application. There are a wide range of materials available to the spring designer, when selecting the material for a specific application the following requirements should be considered:

• Resistance to corrosion and/or oxidisation

• Working temperature (high or low)

• Resistance to acids & chemicals

• Working environment e.g. salt water exposure

Typical Materials – Industry Applications

Imperial Metric Material lb/in3 kg/mm3 Industry/Applications Aluminium 0.0979 0.00000271 Pressed & cast components Beryllium Copper 0.298 0.00000825 Automotive, Aerospace, Electronics Brass 0.307 0.0000085 Automotive, Aerospace, Electronics Copper 0.322 0.0000893 Automotive, Aerospace, Electronics Elgiloy / Phynox 0.2999 0.0000083 Medical, Marine, Petro-Chemical Hastelloy B 0.333 0.00000922 Chemical Processing Hastelloy C 22 0.314 0.00000869 Nuclear Fuel Processing, Chlorination Systems Hastelloy C 276 0.321 0.000089 Marine, Pollution Control, Waste Treatment Food & Chemical Processing, Furnace Inconel 600 0.3046 0.00000843 Components, Nuclear Engineering Inconel 625 0.305 0.00000844 Marine, Aerospace, Nuclear, Chemical Processing Inconel X750 0.298 0.00000825 Aerospace, Gas Turbines, Nuclear Reactors Monel 400 0.319 0.00000883 Marine, Valves, Pumps, Heat Exchangers Monel K 500 0.306 0.00000847 Marine, Musical Instruments, Pumps Nickel 0.317 0.0000088 Various Aerospace, Thermal Processing, High Nimonic 90 0.298 0.00000825 Temperature Applications Ni-Span C 0.2941 0.00000814 Watches, Weighing Machines Phosphor Bronze 0.3197 0.00000885 Automotive, Aerospace, Electronics Spring Steel 0.2829 0.00000783 Various Stainless Steel 0.2829 0.00000797 Various Stainless Steel 0.2854 0.0000079 Precipitation Hardenable 17/7 Titanium 0.163 0.0000042 Aerospace, Jewellery www.springs.co.uk [email protected]

Material Definitions

Materials Definitions Aluminium is a soft, durable, lightweight, malleable metal with appearance ranging from silvery to dull grey, depending on the surface roughness. Aluminium is non- magnetic and non-sparking. It is also insoluble in alcohol, though it can be soluble in Aluminium water in certain forms. It is ductile, and easily machined, cast, drawn and extruded. Corrosion resistance can be excellent due to a thin surface layer of aluminium oxide that forms when the metal is exposed to air.

Austenitic steels have austenite as their primary phase (face centred cubic crystal). These are alloys containing chromium and nickel (sometimes manganese and Austenitic nitrogen), structured around the Type 302 composition of iron, 18% chromium, and Stainless Steel 8% nickel. Austenitic steels are not hardenable by heat treatment. The most familiar stainless steel is probably Type 304, Type 304 ‘surgical’ stainless steel is austenitic steel containing 18-20% chromium and 8-10% nickel.

Beryllium copper, also known as copper beryllium is a metal alloy of copper and 0.5 to 3% beryllium, and sometimes with other alloying elements. Beryllium Copper combines high strength with non-magnetic and non-sparking qualities. It has Beryllium excellent metalworking, forming and machining qualities. It has many specialized Copper applications in tools for hazardous environments, musical instruments, precision measurement devices, ammunition, and aerospace. Beryllium-containing alloys create an inhalation hazard during manufacturing due to their toxic properties.

Copper is easily worked, being both ductile and malleable. The ease with which it can be drawn into wire makes it useful for electrical work in addition to its excellent electrical properties. Copper can be machined, although it is usually necessary to Copper use an alloy for intricate parts, such as threaded components, to get really good machinability characteristics. Good thermal conduction makes it useful for heatsinks and in heat exchangers. Copper has good corrosion resistance, but not as well as gold. It has excellent brazing and soldering properties and can also be welded. Chromium is a steel-grey, lustrous, hard metal that takes a high polish and has a high melting point. It is also odourless, tasteless, and malleable. The name of the element is derived from the Greek word "chrōma" (χρωμα), meaning colour, because many of its compounds are intensely coloured. Chromium was regarded with great interest because of its high corrosion resistance and hardness. The steel Chrome could be made highly resistant to corrosion and discoloration by adding chromium to Vanadium form stainless steel. This application, along with chrome plating (electroplating with chromium) are currently the highest-volume uses of the metal. Chromium and ferrochromium are produced from the single commercially viable ore, chromite, by silicothermic or aluminothermic reaction or by roasting and leaching processes

Hastelloy categorised as “super alloys” or “high-performance alloys”. The predominant alloying ingredient is typically the transition metal nickel. Other alloying ingredients are added to nickel in each of the subcategories of this trademark Hastelloy designation and include varying percentages of the elements molybdenum, chromium, cobalt, iron, copper, manganese, titanium, zirconium, aluminium, carbon, and tungsten. The primary function of the Hastelloy super alloys is that of effective survival under www.springs.co.uk [email protected]

high-temperature, high-stress service in a moderately to severely corrosive, and/or erosion prone environment where more common and less expensive iron-based alloys would fail, including the pressure vessels of some nuclear reactors, chemical reactors, distillation equipment and pipes and valves in chemical industry. Inconel alloys are oxidation and corrosion resistant materials well suited for service in extreme environments. When heated, Inconel forms a thick, stable, passivating oxide layer protecting the surface from further attack. Inconel retains strength over a Inconel wide temperature range, attractive for high temperature applications where aluminum and steel would succumb to creep as a result of thermally-induced crystal vacancies. Inconel's high temperature strength is developed by solid solution strengthening or precipitation strengthening, depending on the alloy. There are different types of stainless steels: when nickel is added, for instance, the austenite structure of iron is stabilized. This crystal structure makes such steels non- magnetic and less brittle at low temperatures. For greater hardness and strength, more carbon is added. When subjected to adequate heat treatment, these steels are used as razor blades, cutlery, tools, etc. Significant quantities of manganese have been used in many stainless steel compositions. Manganese preserves an austenitic structure in the steel as does nickel, but at a lower cost. Austenitic, or 300 series, stainless steels make up over 70% of total stainless steel production. Superaustenitic stainless steels, such as alloy AL-6XN and 254SMO, exhibit great resistance to chloride pitting and crevice corrosion due to high molybdenum content (>6%) and nitrogen additions, and the higher nickel content ensures better resistance to stress-corrosion cracking versus the 300 series. The higher alloy content of superaustenitic steels makes them more expensive. Ferritic stainless steels generally have better engineering properties than austenitic grades, but have reduced corrosion resistance, due to the lower chromium and nickel content. They are also usually less expensive. Martenitic Martensitic stainless steels are not as corrosion-resistant as the other two classes Stainless but are extremely strong and tough, as well as highly machineable, and can be Steel hardened by heat treatment. Precipitation-hardening martensitic stainless steels have corrosion resistance comparable to austenitic varieties, but can be precipitation hardened to even higher strengths than the other martensitic grades. Duplex stainless steels have a mixed microstructure of austenite and ferrite, the aim usually being to produce a 50/50 mix, although in commercial alloys the ratio may be 40/60. Duplex steels have improved strength over austenitic stainless steels and also improved resistance to localised corrosion, particularly pitting, crevice corrosion and stress corrosion cracking. Duplex grades are characterized into groups based on their alloy content and corrosion resistance. Lean duplex refers to grades such as UNS S32101 (LDX 2101), S32304, and S32003. The standard duplex is 22% chromium with S31803/S32205 known as 2205 being the most widely used. Super duplex refers to 25% chromium grades such as S32760 (ZERON 100), S32750 (2507), and S32550 (Ferralium). Hyper duplex refers to higher chromium grades such as S32906. The properties of duplex stainless steels are achieved with an overall lower alloy content than similar-performing super-austenitic grades, making their use cost-effective for many applications.

Monel is very difficult to machine as it work-hardens very quickly. It needs to be turned and worked at slow speeds and low feed rates. It is resistant to corrosion and Monel acids, and some alloys can withstand a fire in pure oxygen. It is commonly used in applications with highly corrosive conditions. www.springs.co.uk [email protected]

Alloy 90 (Nimonic 90) is a precipitation hardenable nickel-chromium-cobalt alloy strengthened by the addition of Titanium and Aluminium. Alloy 90 has a high stress rupture strength and creep-resistant at temperatures to about 920°C. Alloy 90 is Nomonic typically used on such things as turbine blades, hot working tools and high- temperature springs. Due to its ability to withstand very high temperatures, Nimonic is ideal for use in aircraft parts and gas turbine components such as exhaust nozzles on jet engines where the pressure and heat are extreme. Tantalum is a chemical element with the symbol Ta and atomic number 73. A rare, hard, blue-grey, lustrous transition metal, tantalum is highly corrosion resistant and occurs naturally in the mineral tantalite, always together with the chemically similar niobium - its main use today is in tantalum capacitors in electronic equipment. The major use for tantalum, as the metal powder, is in the production of electronic Tantalum components, mainly capacitors and some high-power resistors. Because of the size and weight advantages, tantalum capacitors are attractive for portable telephones, personal computers, and automotive electronics. Tantalum is inert against most acids except hydrofluoric acid and hot sulphuric acid, also hot alkaline solutions cause tantalum to corrode. This property makes it an ideal metal for chemical reaction vessels and pipes for corrosive liquids.

Titanium is used in steel as an alloying element to reduce grain size and as a deoxidizer, and in stainless steel to reduce carbon content. Applications for titanium can be found in industrial, aerospace, recreational, and emerging markets. Titanium alloys are used in aircraft, armour plating, naval ships, spacecraft, and missiles. Due to its high corrosion resistance to sea water, titanium is used to make propeller shafts and rigging and in the heat exchangers of desalination plants, in heater- chillers for salt water aquariums, fishing line and leader, and for divers' knives. Titanium Titanium is used to manufacture the housings and other components of ocean- deployed surveillance and monitoring devices for scientific and military use. Due to its superior strength and light weight when compared to other metals traditionally used in firearms (steel, stainless steel, and aluminium), and advances in metalworking techniques, the use of titanium has become more widespread in the manufacture of firearms. Primary uses include pistol frames and revolver cylinders. For these same reasons, it is also used in the body of laptop computers

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Nace® Compliant Spring Materials

Springs manufactured utilising Nace® compliant materials are ideal for industrial applications operating in harsh and challenging environments, specifically where corrosion is a potential high-risk factor.

The Nace® specification covers materials for applications used in environments containing hydrogen sulphide (oil & gas production equipment): NACE MR01-75 / ISO 15156. The specification details the materials which can be utilised for various different components (e.g. bolts, piping, springs etc.) and also indicates the specific material properties for each application.

Although the specification covers a significant range of materials (including carbon and alloy steels, stainless steels, nickel alloys etc.) not all of these materials are suitable for use for all possible components.

Nace® Specification for Springs

The material choices are as follows:

Inconel X-750: Cold worked and age hardened condition, maximum hardness of 50 HRC

Nimonic 90: Cold worked and age hardened condition, maximum hardness of 50 HRC

Elgiloy (Phynox): Cold worked and age hardened condition, maximum hardness of 60 HRC

MP35N: Cold worked and age hardened condition, maximum hardness of 55 HRC

References:

MR0175/ISO15156: Part 3, Table A.1, Springs (Tables A.36 & A.39)

Table A.36 – nickel based alloys UNS N07750 (Inconel X-750) & N07090 (Nimonic 90)

Table A.39 – cobalt based alloys R30003 (Elgiloy/Phynox) & R300035 (MP35N)

Nace® Specification for Snap Rings

Table A.1 also covers compliant materials for use in the manufacture of Snap Rings.

Stainless Steel S15700 (15-7MO) Precipitation hardened stainless steel, annealed and aged condition, maximum hardness of 30 HRC

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Common Surface Finishes

Rumbling

Rumbling is used to remove burrs and sharp edges and improve overall surface finish.

A Rumbling (vibratory finishing) Machine is an open-topped tub or bowl, mounted on spring and usually lined with polyurethane. The vibratory action is created by a special motor assembly attached to the bottom of the bowl.

Bowl Vibrators are round, bowl-shaped where the container is mounted on springs. The vibratory action is imparted to the bowl by eccentric weights mounted on a vertical shaft located at the centre of the bowl. When activated the parts and media move in a spiral motion. A major advantage of Bowl Vibrators is that they can be set-up to automatically separate the media from the components.

Shot Peening

Shot peening is a cold work process, the main purpose of which is to strengthen the component in order to prevent fatigue stress whilst also cleaning the surface. During the peening process components are bombarded with spherical shot in order to create compressive stresses which avoid the generation of micro-cracking and increase fatigue lifetime.

Hydrogen Embrittlement

When carbon steel is pickled in preparation for plating or during some electroplating processes, hydrogen can become absorbed into the material. Cracks can develop in the pickling or processing bath, more often they appear when the plated parts are in service.

Hydrogen Embrittlement is more severe when there is high hardness, carbon content and/or high stress concentration. Hydrogen Embrittlement can be removed by baking the plated parts immediately after processing to drive the hydrogen out of the material.

Common Plating Processes

Copper Plating

Rack or barrel bright copper electroplating for components. Copper plating is used extensively in the electronics, electrical and water sectors for electrical conductance and corrosion resistance. Copper is also used widely as a decorative finish.

Under flash: Copper plating is commonly used as a base coat for secondary electroplating or finishing examples of which include cadmium, nickel, chromium or tin.

Gold Plating

For decorative purposes and electronics applications. Gold plating is used in the electronics industry due to its good properties of conductivity (electrical and thermal), wear resistance and anti-fretting.

Nickel Plating

Bright or dull nickel plating using either rack, barrel or wire processes. Typically used before chrome plating, nickel also has wide applications in the electronics industry due to its ease of solderability. www.springs.co.uk [email protected]

Silver Plating

For decorative purposes and electronics applications. Silver plating is used in the electronics industry due to its good properties of conductivity (electrical and thermal), wear resistance and anti-fretting.

Tin Plating

Tin is mainly used in the electronics industry because of its ease of solderability and good electrical conductance. Tin also offers increased corrosion resistance in harsh operating environments including applications subject to chemical exposure.

Zinc Plating

Zinc Plating is the most common anti-corrosion finish used on springs. The table below gives an indication of salt spray resistance obtainable from common deposit thicknesses with post passivation colours:

Passivation µm Hours µm Hours µm Hours µm Hours None 5 24 8 48 12 90 25 192 Black 5 48 8 72 12 N/A 25 N/A Blue 5 48 8 72 12 N/A 25 N/A Clear 5 48 8 72 12 N/A 25 N/A Olive Drab 5 72 8 120 12 192 25 360 Yellow 5 72 8 120 12 192 25 360 Based on a 5% Neutral Salt Spray

British Standards for Zinc Plating

Trivalent Process (Thin, Clear, or Colour Passivate): BS EN12329:2000 FeZn

Trivalent Process (Thick Film, or Full Passivate): BS EN6338:2000 FeZn

Older Hexavalent Specifications: BS1706:1990 FeZn

This is only a small selection of the possible finishes available, there are many other options including plastics, paints and organic coatings.

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Contact the spring experts today:

Springtech Ltd Unit 1, Fairview Industrial Estate Beech Road Wycombe Marsh High Wycombe Buckinghamshire HP11 1RY United Kingdom

Tel: +44 (0) 1494 556700

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DISCLAIMER

The information contained in this white paper is provided for reference only, without warranty. Whilst every precaution has been taken in the preparation, neither the authors or Springtech Limited shall have any liability to any person or entity with respect to any loss or damage caused or alleged to be caused directly or indirectly by the instructions, specifications, materials and products described in this paper.

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