Spring steel

What defines the spring steel family?

Spring steel is a medium- to high-carbon steel that is hardened and tempered to produce a high elastic limit. It usually contains silicon, manganese, chromium or vanadium. A component made from spring steel returns to its original shape after loading instead of retaining a permanent set. The ratio of elastic limit to tensile strength distinguishes this steel family. This ratio determines how far a spring can deflect and store energy without yielding.

Spring design primarily addresses fatigue rather than a single load. Steel cleanliness, surface finish and decarburisation control are therefore as important as chemical composition. Rolled-in defects and decarburised surface layers can cause premature failure. ASTM A689 covers hot-wrought steel bars for general-purpose coil, torsion and leaf springs.

The Laxcon Steels grade reference lists one spring steel grade with its full composition.

Effects of alloying elements

Carbon can increase hardness, but hardness alone does not provide the high elastic limit required in a spring. Silicon, manganese, chromium and vanadium provide the principal alloying effects in spring steels.

Silicon contents well above one percent sharply increase the elastic limit. Silicon also raises the temperature at which tempering begins to soften the steel. This effect improves resistance to permanent set. SAE 9260 contains 1.8 to 2.2 percent silicon. It is the reference grade of this type and a standard material for leaf springs and heavy coil springs.

Manganese increases hardenability. It allows a thick leaf or heavy bar to harden through its section instead of only at the surface. Chromium also increases hardenability and improves temper resistance. SUP-9 contains 0.52 to 0.60 percent carbon, 0.65 to 0.95 percent chromium and 0.65 to 0.95 percent manganese. It is the chromium-manganese spring steel designation in the Japanese system. Its applications include suspension coil springs and stabiliser bars. Vanadium refines the grain and increases fatigue strength. Chromium-vanadium spring steels are therefore used for valve springs and other high-cycle applications.

Silicon grades serve leaf springs and heavy sections where resistance to permanent set is the main requirement. Chromium-manganese grades serve general suspension applications. Chromium-silicon and chromium-vanadium grades serve applications with very high cycle counts or elevated operating temperatures.

Surface condition and processing

Fatigue cracks initiate at the surface. Decarburisation control, shot peening and pre-setting therefore have a major effect on spring life.

Decarburisation can occur when spring steels are held at the austenitising temperature. Silicon-bearing grades are particularly susceptible to surface carbon loss. The resulting low-carbon surface layer is soft and lies in the region of highest bending stress. Furnace atmosphere control and specified limits on decarburised layer depth are therefore part of spring bar procurement.

Shot peening introduces residual compressive stress into the surface layer. Applied tensile stress must overcome this compression before a crack can open. Peening typically produces a greater increase in fatigue life than selection of a higher grade.

Pre-setting loads a finished spring once beyond its working range. The most highly stressed surface yields and develops a favourable residual stress state. The spring then behaves elastically throughout its working range.

Steel cleanliness also affects fatigue life. Springs represented by identical certificates can differ in life by an order of magnitude when one is produced from cleaner steel. Non-metallic inclusions near the surface are the usual sites of crack initiation.

Questions about spring steel grades

Where the spring steel family is not the right answer

Spring steels are unsuitable as general engineering materials. They are selected for elastic limit rather than toughness or machinability. Their carbon contents make them difficult to machine, susceptible to quench cracking and poor under shock loading. Medium-carbon and chromium-molybdenum grades are more appropriate for shafts and machined components.

Spring steels are unsuitable for welded components. The heat-affected zone can harden and crack, while welding destroys the temper on which spring performance depends. Brackets and eyes are formed or clamped instead of being welded to the spring.

Spring steels are also unsuitable for corrosive service. Corrosion fatigue is a common failure mechanism in vehicle springs. A corrosion pit initiates a crack, and the spring can then break under a load that it was designed to carry indefinitely. Where environmental exposure cannot be controlled, suitable alternatives include stainless spring materials such as AISI 302 or 17-7 PH. Nickel alloys are used for hot corrosive duty. These materials are selected instead of applying a coating over a carbon spring steel.