Technical information of spring materials
Say ”spring steel”, and most of us think of a material that is really hard. If you ever tried drilling a hole in a flat spring or cutting off a thin spring wire with a pliers, you can confirm that it is true.
Springs store energy when deflected by an external load. This energy is stored in the material as stress and strain and the higher the elastic limit of the material, the more energy can be stored.
The elastic limit of a material is called the yield strength. With a high yield strength follows a high tensile strength and a high tensile strength is equal to a high hardness.
YIELD STRENGTH AND DUCTILITY
But a high yield strength is not the only requirement on a material for springs. Most springs are cold formed from a pre-hardened material. The ductility of the material need to be sufficient for the bending operations required for the desired geometry. In many cases, some yield strength need to be sacrificed to gain ductility and in other cases, hardening to final strength is done after forming.
Dynamic loads and fatigue life
All springs are stressed in either bending or torsion mode and – just like many other mechanical components – springs therefore experience the highest stress levels on the surface of the material. Given the high hardness and the dynamic nature of the loads on the springs, the quality of the surface is of great importance for the fatigue life of a spring. The materials used for springs with the highest requirements on dynamic loads and fatigue life are developed and processed to ensure the best possible surface quality.
TWO WAYS TO ACHIEVE CORROSION RESISTANCE
Corrosion resistance is a common requirement for springs. This can be achieved either by using a spring material that is inherently resistant to the given environment, or by applying a surface treatment. Corrosion resistant spring materials are mainly the austenitic stainless spring steels, but also duplex (austenitic-ferritic) steels and martensitic stainless steels are used in some cases. Among the nickel or cobolt based materials, we also find a lot of alloys that are more resistant to corrosion than any stainless steel and can be used in severely corrosive environments.
OTHER SPRING REQUIREMENTS
There are also other demands on a spring that requires other types of materials. Copper alloys are used where high electrical conductivity and/or a non-magnetic material is required. Super-alloys, which are mainly based on nickel, chromium, cobalt and molybdenum, are used for high working temperatures. Titanium alloys are used either for their corrosion resistance or because a few of the titanium alloys are actually the only materials that can give a spring with lower weight compared to a steel spring.
OPTIMIZED MATERIAL GROUPS
The material groups that we mainly work with are:
- Cold drawn and cold rolled carbon steels
- Patented and cold drawn carbon steel wire
- Low alloy steels for hardening after forming
- Oil- or induction hardened martensitic spring wire
- Stainless spring steels, which includes a lot of austenitic, duplex and precipitation-hardening grades, medical grades etc…
- Super alloys (often known under tradenames like Inconel, Nimonic, Hastelloy etc)
- Copper alloys
- Titanium alloys