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Material choice for springs

Selecting the proper material for an application requires a general knowledge of what is available for use and depends on the environment in which the spring/retaining ring is to be used. The experts in spring technology at TFC offer some advice.

Specifying the correct material for a spring or retaining ring can prevent additional cost and failure in operation. Carbon steel is the most commonly specified material. Stainless steels, although more costly than carbon steel, provide far superior corrosion resistance and have higher temperature operating limits. So let's consider the range of materials available and their characteristics.

SAE 1070-1090 high carbon tempered spring steel is a standard material for spiral retaining rings and wave springs. Tensile strength and yield strength are maximised as a result of the oil tempered martensitic structure. SAE 1060-1075 High carbon cold drawn spring steel is a standard material for snap rings. Hard drawn carbon steel has no scale as it receives its strength from the drawing process.

In either temper, carbon steel is best suited in applications having a protected environment as it corrodes if not lubricated or atmospherically sealed. Additional corrosion protection can be added with special finishes. Rings and springs are normally supplied with an oil dip finish providing protection during shipment and for shelf storage. Carbon steel is highly magnetic. Maximum recommended operating temperature is 121°C.

302 stainless steel AMS-5866302 is the standard stainless steel for spiral retaining rings. This widely used material is specified because of its combination of corrosion resistance and physical properties. 302 obtains its spring temper condition by cold working. Though it is categorised as being a non-magnetic stainless, 302 becomes slightly magnetic as a result of cold working. It is not hardenable by heat treatment. Maximum recommended operating temperature is 204°C.

316  stainless steel ASTM A313 is nearly identical in physical properties and heat resistance to 302, but provides additional corrosion resistance, particularly against pitting, due to its molybdenum chemical content. 316 is generally used in food, chemical and sea water applications. 316 exhibits less magnetism than 302, however, as with 302, magnetism increases as the wire is cold reduced. This stainless grade is also not hardenable by heat treatment. Maximum operating temperature is 204°C.

17-7 Ph/C stainless steel is similar in corrosion resistance to type 302. This alloy is used almost exclusively for wave springs, yet offers both high tensile and yield strengths for special ring applications. In fatigue and high stress applications, 17-7 out performs even the finest grade of carbon steel. Spring properties are achieved by precipitation hardening Condition C to Condition CH-900. 17-7 PH C/CH-900 exhibits magnetism similar to high carbon steel. Maximum recommended operating temperature is 343°C.

Inconel X-750 is a nickel-chromium alloy used most commonly in high temperature and corrosive environments. Most commonly, Inconel X-750 is precipitation heat treated to a spring temper condition. The National Association of Corrosion Engineers (NACE) approves this hard temper to specification MR-0175 (Rc50 maximum) for spiral retaining rings and wave springs. It can also be tempered to Rc35 maximum, AMS-56991. This requires a longer heat treatment than spring temper and has a lower tensile strength. Maximum recommended operating temperature in both cases is 371°C. Alternatively it can be tempered to AMS-5698 to give a maximum operating temperature of 538°C. This material is typically used for retaining rings requiring corrosion and heat resistance.

Both the spring temper and the alternative tempers exhibit no magnetism and may be heat treated in either an open air or atmosphere controlled furnace. Open air heat treatment may produce oxidation, which often results in a slight black residue. An atmosphere controlled environment eliminates oxidation and produces a component with no residue.

Elevated temperatures
A286 alloy (AMS-5810) exhibits similar properties to Inconel X-750. Its spring temper condition is obtained by precipitation hardening. A286 may be heat treated similar to the Inconel tempering processes. The material exhibits no magnetism and has a maximum recommended operating temperature of 538°C.

Elgiloy (AMS-58761 - conforming to NACE standard MR-01-75) is known for its excellent resistance to corrosive environments, no magnetism, and use at elevated temperatures. Commonly used in oil industry applications, Elgiloy shows improved reliability over other NACE approved materials by resisting sulphide stress cracking. Additionally, Elgiloy is said to out perform "over 600% better than 17-7 PH in load retention at 343°C and to provide over 100% more cycles (in fatigue resistance) than carbon steel without breakage." Maximum operating temperature is 427°C.

Beryllium copper alloy, normally specified in a hard temper, produces excellent spring properties due to a combination of low modulus of elasticity and high ultimate tensile strength. The alloy gains its physical properties by precipitation hardening. In contrast to other copper alloys, beryllium copper has the highest strength and offers remarkable resistance to loss of physical properties at elevated temperatures. Beryllium copper is non-magnetic. Its electrical conductivity is about 2-4 times as great as phosphor bronze. Maximum recommended operating temperature is 204°C.

Phosphor bronze offers fair spring properties, fair electrical conductivity and is rated a step below beryllium copper in performance. It is purchased in a spring temper condition to maximise spring characteristics. Phosphor bronze is hardenable only by cold working. This material is also non-magnetic.

The finish of a part can also play an important role in its resistance to corrosive environments. Finishes are also very popular in creating an aesthetically pleasing product. Black oxide, for example, provides a flat black finish. Black oxide is intended more for cosmetic appearance than for corrosion resistance. Cadmium plating is used on carbon steel to increase the corrosion resistance of the product. The process of cadmium plating spiral retaining rings is costly and subjects the ring to the possibility of hydrogen embrittlement. Oil dip, meanwhile, is the standard finish for all products produced from carbon steel. The oil provides resistance to corrosion in transport and normal storage. The oil dip finish should not be considered a permanent finish.

Passivation is an optional cleaning operation for stainless steel. It provides a bright finish and increased corrosion resistance. Passivation dissolves iron particles and other substances, which have become embedded in the surface of stainless steel during production. If not dissolved, these foreign particles could promote rusting, discolouration or even pitting. The corrosion resistance of stainless steel is due to the thin, invisible oxide film that completely covers the surface of the ring and prevents further oxidation. Removing the contaminates prevents breaks in the oxide film for optimum corrosion resistance.

Zinc phosphate finish, sometimes referred to as 'Parkerizing', appears gray-black in colour. The corrosion resistance of phosphate is superior to black oxide but inferior to cadmium plating or stainless steel. Phosphate can not be applied to stainless steel.

Another finish is vapour degrease and ultrasonic clean - the standard clean and finish for stainless steels. The process removes oil and other organic compounds from the surface by use of a chlorinated solvent. The solvent removes oil and grease from the exposed surfaces of the ring or spring. Ultrasonics are used in forcing the solvent to act between the turns of the ring.
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