Required Material Characteristics

Piston ring materials must possess the following characteristics in order for the ring to perform its functions under the most severe operating conditions. Moreover, these characteristics must be retained over the engine's entire operating life; not just when it is first brought into service!

Toughness

The material must withstand high forces and at the same time be able to deflect (move), in order to ensure conformability without breaking or fatiguing over time.

Elasticity

The material must be able to act like a spring when the ring is closed to its working diameter. This 'springiness' generates the tension that allows the ring to maintain its designed contact pressure on the cylinder wall.

Corrosion Resistance

The material must be resistant to chemical and physical changes caused by corrosive combustion gases and high temperatures.

Surface Treatment Characteristics

The chosen surface treatment must be compatible with the base material and show optimal wear, friction, and scuff-proof behaviors as established by the design requirements.

Light weight

Reduced reciprocating mass will limit energy losses during high-speed operation. Lighter weight also means lower inertial forces acting on the rings during operation.

Long Service Life

The selected piston ring material must be able to retain the above performance characteristics over the service life of the engine even under high temperature conditions.

From Cast Iron to Steel

Cast iron has been and still is a key piston ring material. However, engine design has continued to evolve. To meet the ever-increasing performance demands of modern engines, piston ring materials have been changing as well. In many applications, steel rings have replaced iron rings because:

1. Steel has high strength and great toughness that allows the ring to be made thinner and lighter.
2. Steel has greater resistance to heat which means less deformation and ring tension relaxation over time.
3. Steel is an excellent base material for a variety of surface treatments that reduce friction and prolong ring service life.

Let's compare the properties of cast iron and steel.

Strength

High strength is desirable because it allows a thinner piston ring to withstand the stresses of ring operation and movement in the groove. Even the highest grades of cast iron cannot match the strength of steel.

Thermal Fatigue Resistance

The ring material must be able to maintain its required tension for its entire service life. Higher resistance is desirable because the material becomes weaker over time when exposed to prolonged high temperature. Steel offers more superior thermal fatigue resistance than cast iron.

Modulus of Elasticity

A higher modulus value means that the material must be deflected less to produce the equivalent spring effect. Steel has higher elasticity (200 ~ 215 Mpa) compared to cast iron (157 ~ 167 Mpa), which allows for thinner and lighter ring design.

Corrosion Resistance and Surface Modification Characteristics

Not only must the base material be corrosion resistant, it must also be adaptable to variety of surface treatment processes. Since there are now a large number of surface treatment options for the designer, highly adaptable base materials are desired.