Material Matters
The two factors most influential on the performance of piston rings are the nature of the ring’s geometry and the quality of the materials used in its manufacture.
Additionally, use of the correct fuels and lubricants enhances the optimum functioning of the rings but specifying high quality rings from bona de manufacturers will lead to a more reliable engine performance. An increasing variety of materials are proving their worth in the manufacture of piston rings.
Material Synergy
Materials used for the production of piston rings include cast irons, bronzes, steels, stainless steels & stellites (cobalt alloys).
However, it is increasingly recognised that rings manufactured from composite materials offer incremental benefit. One of the key areas of progress in this area is the use of special surface coatings, which when applied to the substrate material improve specific aspects of its performance – better wear standards, resistance to corrosion or heat stability, for example.
These advances in so-called ‘surface engineering’ have resulted in piston ring performance which exceeds anything achievable by use of a single material.
For the users of piston rings, this means longer ring life, reduced ring failure and of course, lower costs.
Modern Piston Ring Materials
Cast Irons
The majority of all piston rings are manufactured from grey cast iron – but not just any grey cast iron is appropriate. We make use of over 20 different grades of cast iron, each designed to match its particular application to give the best possible results.
Small changes in the chemical composition of the base material coupled with a ‘fine-tuning’ of casting techniques results in a range of cast irons with varying strengths and wear resistances.
The properties of a cast iron are directly influenced by the materials’ microstructure. Small changes in chemical composition, casting temperature and cooling conditions can dramatically affect the condition of this microstructure.
Only lengthy experience of foundry operation can guarantee the correct grade and quality of the resultant material. Quality control procedures are also vital – in our foundry each and every melt is thoroughly analysed for conformance to specification.
Grey cast iron microstructures are known as ‘heterogeneous’ meaning they contain many different, smaller micro structural components, which together form the entire microstructure. So cast iron may be thought of as a ‘natural composite material’. It contains the following elements:
Pearlite
Lamellar Pearlite is the basic matrix material which helps control wear, corrosion resistance, and material hardness.
Graphite Flake
The shape and distribution of the graphite flake is the dominant factor controlling the overall strength of the material.
Phosphide Eutectic
This helps smash abrasive particles into smaller, less damaging panicles, assisting in the reduction of abrasive wear.
Carbides
These very hard particles assist in protecting the sliding surface from damage during day-to-day piston ring operation.
To a metallurgist this is a comparatively simple microstructure, but it demonstrates the need for piston ring manufacturers to have absolute control and understanding of the base material In order to maximise the efficiency of the finished ring.
As little as 0.0015% of lead entering the melt is all that is necessary to undermine the integrity of the microstructure – can you afford to take this risk?
Special Alloys
Piston Rings UK chooses from about 20 different alloy additions for its cast irons, each one with a special function to perform. For instance, molybdenum is used to give improved resistance to thermally induced loss of tension, and increases the fatigue resistance of the material. Vanadium forms hard carbides within the microstructure which improves the ring’s wear resistance; copper helps give a good matrix structure, for improving the anti-corrosion characteristics of the material.
Surface Coatings
Cast irons are just one part of the recipe with which PRUK seeks improved piston ring performance. Surface coatings on the cast iron substrate material dramatically alter the properties of piston rings, and can be separated into two categories – hard coatings & soft coatings.
Hard coatings such as chrome plate are designed to improve the wear characteristics of the rings and increase their useful operational life. Soft coatings such as phosphate help protect the rings during running-in, the most likely time for ring damage to occur.
Hard Coatings
Hard coatings for piston rings are frequently specified in 4-stroke engines and are now becoming more widely adopted for use in 2-stroke engines. Hard chrome plate and plasma coated materials all help to improve the rings’ wear resistance by providing a hardened surface, which has both low friction and good thermal conductivity.
Hard Chrome Plate
PRUK uses three different types of hard chrome plate, selecting the most appropriate to suit the substrate material, and the ring face which is to be plate-ring side face or running face. The chrome layer may appear smooth to the naked eye, but it is actually full of tiny micro-cracks, which help retain lubrication oil to keep the ring protected even when oil starvation occurs.
Whilst hard chrome plate has proved itself to be one of the best coating material for piston rings, even this suffers when the temperatures and pressure begin to exceed its working limit.
Plasma Coatings
Plasma coatings are so called because of the technique used for surface treatment, rather than any particular material utilised. Hence great care should be taken to ensure the correct coating is selected. Molybdenum is used when scuffing is a problem, as is a mixture of molybdenum, chromium metal and compounds.
However, although porosity in the coating helps with oil retention it can also lead to internal oxidation and attrition. Hence plasma coated rings should only be used when chrome plated rings are no a viable alternative.
Soft Coatings
The purpose of soft coatings (or running-in coatings) is to protect the piston rings and help them form an effective seal during the initial period of operation. The coatings are normally one of two types chemical conversion coatings such as phosphate, or soft metallic coatings such as copper.
Of whatever type, the coatings wear down quickly, enabling the piston rings to conform to the cylinder bore. The material chosen must be highly scuff resistant, adhere firmly to the cast iron substrate and not cause damage to the cylinder liner or the piston.
Chemical Conversion Coatings
These coatings usually cover the whole of the ring surface, giving protection to the rings during storage as well as during running-in. Phosphating – the most popular process among PRUK’s customers – converts the outer surface of the ring to a manganese phosphate which is black in colour. Once treated, the rings are dipped in special viscous oil for extra protection. This oil gets trapped between the course crystalline structure aiding lubrication and preventing surface scuffing in the period immediately following installation.
Metallic Coatings
Soft metallic coatings are usually plated or sprayed onto the peripheral edge of the piston rings. The materials chosen are usually soft, the most common being copper, although tin, cadmium and even lead have been used. Copper is also a popular choice for ‘double-plating’ on top of a chrome plated Jing.
Copper coatings are especially effective when an engine’s cylinder liners are worn and no longer perfectly circular. The thin, swift wearing layer of copper quickly gives an effective seal, even when the bore is highly worn.
Conclusion
The wide choice of materials available to today’s piston ring manufacturers means increased reliability and better value for money for engine operators. However, the increased complexity of the manufacturing process means that there is also greater room for error and inferior quality.
By controlling both the cast iron production and surface treatments in house, PRUK can be sure of maintaining the quality of its products and offering the greatest choice of ring type to its customers.