Piston Ring Handbook
Piston ring materials are subject to a complex stressing system under mainly dynamic mechanical and tribological loads. Such loads call for the use of materials with high strength (chiefly at temperatures of 200-300°C) especially in the elastic range, which also have the necessary wear characteristics for operation in normal and dry lubrication conditions. Additionally, thermophysical properties such as thermal conductivity and thermal expansion are a major factor in the performance of piston rings. Increasingly, corrosion resistance and resistance to microwelding are properties that also determine the characteristics of piston ring materials.
The preferred material for compression rings is a low-alloyed, heat-treated nodular cast iron (KV1/GOE 52). This material is characterized by a high bending strength of min. 1300 MPa and a high modulus of elasticity attributable to a martensitic microstructure and spherulitic graphite structure. For increased stresses a higher hardness is created by varying the martensite morphology (KV4/GOE 56).
In the 2nd groove, alloyed grey cast irons are used in a heat-treated condition (F14/GOE 32). Besides having a high bending strength and modulus of elasticity, an increased hardness of 320 to 470 HB is produced in order to obtain the required wear resistance in the uncoated condition.
The demand for high wear strength is also met by the use of a tempered, alloyed cast iron (GOE 44). This has the benefit of a high bending strength of min. 800 MPa and high modulus of elasticity. The good wear resistance results from the combination of a fine-pearlitic matrix structure and finely dispersed, precipitated secondary carbides.
Unalloyed grey cast iron is used for 2-piece oil rings in the 3rd groove. These ring materials (STD / GOE 12, GOE 13) are characterized by a fine-lamellar graphite structure in a pearlitic matrix and have good conformability due to a relatively low modulus of elasticity.
Reduced width piston rings in gasoline engines to match reductions in the overall height of pistons, and increasing combustion pressures in diesel engines call for materials with increased strength characteristics.
These challenges are met by the use of high-chromium alloyed steels and spring steels. The greater durability under increased stresses is demonstrated by the improved fatigue strength manifested as form stability, as shown in Fig. 23 in a comparison of S/N curves for different piston ring materials (spherulitic, heat-treated cast iron versus heat-treated 18% chromium steel).
The wear resistance derives from finely distributed chromium carbides of the type M23C6 and M7C3 embedded in the tempered martensite matrix. For improved wear resistance these steels are mainly used in a nitrided condition or with a peripheral coating.
The steels mentioned are used chiefly as compression ring materials for gasoline engines and truck diesel engines as well as for the steel rails and expander-spacers of oil control rings and for 2-piece profiled steel oil rings.
1) The fatigue strength is determined by plotting the Wöhler S/N curve at bending stresses which piston rings survive intact after 107 stress cycles. In the process the closure stress sM is overlaid with a threshold stress (stress amplitude) sA. The fatigue strength serves as a measure of the piston ring form stability under dynamic stressing .
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