Heat, salt water and magnets: choosing the right material for a spring or washer

Disc spring material selection comes down to four questions: how hot, how corrosive, how magnetic, and how thick the part is. Carbon spring steels cover standard duty. 51CrV4 handles thicknesses above 1.25 mm. X22 CrMoV 12-1 works to 500 degrees C but is not corrosion resistant. Chromium nickel steels handle corrosion, Inconel handles up to 600 degrees C, and CuSn8 bronze is the anti-magnetic option with lower spring forces.

A spring that is correctly sized and wrongly specified will still fail. Heat relaxes it, corrosion pits it, and the wrong alloy near a sensor puts a magnetic field where nobody wanted one. Here is how the material choice actually gets made, in the order the questions come up.

1. Start with thickness, not environment

For standard duty the split is set by section thickness. Cold rolled carbon spring steels C67S (1.1231) and C75S (1.1248) to DIN EN 10132-4 are used for group 1 disc springs below 1.25 mm. Above 1.25 mm the material is the alloyed chrome vanadium steel 51CrV4 (1.8159), used in rolled condition, and in forged form above 6 mm.

Serrated safety washers and load washers are a separate case. Those use C60S (1.1211) carbon spring steel to DIN EN 10132-4, which is used exclusively for that family of parts.

2. How hot does it get?

Temperature is where most specification mistakes happen, because a standard spring steel loses force long before it looks damaged. Two options cover high temperature disc springs:

• X22 CrMoV 12-1 (1.4923) to DIN EN 10269 or 10088-2. A chromium molybdenum vanadium steel effective to 500 degrees C. Important caveat: it is not a corrosion resistant grade.
• Inconel X718 (2.4668) and X750 (2.4669). Nickel chromium alloys with excellent corrosion resistance, suitable up to 600 degrees C. Inconel disc springs are the choice when heat and a corrosive medium arrive together. X718 is virtually cobalt free, which is why it is often used in nuclear reactor technology.

One behaviour to design around at high temperature: creep under load. Exposure to a combination of temperature, time and tension can cause a gradual loss of installation height or spring force. If the joint depends on a fixed preload for years, that has to be in the calculation rather than discovered in service.

3. How corrosive is the environment?

For stainless steel disc springs there are two chromium nickel spring steels to DIN EN 10151. X10 CrNi 18-8 (1.4310) is available up to about 3.0 mm thickness, and X7 CrNiAl 17-7 (1.4568) is precipitation hardened and available up to roughly 2.5 mm. Both are cold formed, and cold forming makes them magnetic, which is worth noting if the assembly sits near instrumentation.

On the washer side, X5 CrNiMo 17-12-2 (1.4401) austenitic stainless steel to DIN EN 10151 offers excellent corrosion resistance, with higher corrosion resistance and lower magnetisability than the chromium nickel grades above.

4. Does magnetism matter?

If the answer is yes, the option is CuSn8 bronze (2.1030) to DIN EN 1654, a copper tin alloy that keeps its spring characteristics through cold forming. It is the material used for the original SCHNORR family of non-magnetic washers. The trade off is real and needs designing in: strength values, and the spring forces that follow from them, are considerably lower than with standard material.

5. Then pick the surface

Coating is a second, separate decision. Available surfaces include phosphating, blackened, mechanical zinc, zinc flake, nickel plated and electroless finishes.

• Phosphate and oil. The standard process for disc springs in low alloy steels. A zinc phosphate layer is impregnated with corrosion protection oil.
• Blackened. On safety washers, a black surface gives the best securing effect.
• Zinc, mechanical or flake. Good protection, and on safety washers no increase in initial torque is needed, because the added friction of the serrations is offset by the lubricating effect of the coating.
• Nickel plated and electroless. Used where appearance or a specific chemical exposure calls for it.

A short specification order

Thickness, then temperature, then corrosion, then magnetism, then surface. Working in that order avoids the common trap of choosing a stainless grade for a hot joint that then turns out to be limited on thickness, or choosing a heat resistant grade for a joint that also sees salt spray.

If you are between two grades, that is the point to ask. Standard parts are warehoused in Ann Arbor, and beyond the standard range SCHNORR develops tailor made solutions that go further than DIN or EN, including special sizes.

• C67S (1.1231) and C75S (1.1248) carbon spring steels are used for disc springs under 1.25 mm thickness. 51CrV4 (1.8159) is used above 1.25 mm.
• C60S (1.1211) is used exclusively for SCHNORR serrated safety washers and load washers.
• X22 CrMoV 12-1 (1.4923) is effective to 500 degrees C but is not corrosion resistant.
• Inconel X718 (2.4668) and X750 (2.4669) suit temperatures up to 600 degrees C with excellent corrosion resistance. X718 is virtually cobalt free and often used in nuclear reactor technology.
• Creep under a combination of temperature, time and tension can cause loss of installation height or spring force.
• X10 CrNi 18-8 (1.4310) and X7 CrNiAl 17-7 (1.4568) are corrosion resistant but become magnetic through cold forming. X5 CrNiMo 17-12-2 (1.4401) offers higher corrosion resistance and lower magnetisability.
• CuSn8 bronze (2.1030) is used for non-magnetic parts, with considerably lower strength and spring forces than standard material.
• Surfaces available include phosphating, blackened, mechanical zinc, zinc flake, nickel plated and electroless.