The piston ring is a key component in a marine combustion engine. High mechanical loads, relatively high temperatures and corrosive gases and liquids influence its performance in terms of sealing capacity, wear of cylinder liner and the ring itself. Base material of the ring, coating technology and ring geometry design are discussed in the article.

When tailoring cast iron materials suitable as piston ring base material two parameters are of importance; the morphology of the graphite and the constituents of the matrix. To optimize the properties of the cast iron a compromise is needed to achieve a satisfactory performance of the piston rings.

Daros Piston Rings AB is currently developing a second generation of chromium-ceramic coating the so called Z-chrome. The objective of this project has been to increase the maximum operating temperature of the coating and leave the other characteristics of the coating unaffected. The difference between the commercial Daros coating Tritor® and the Z-chrome is the ceramic component included in the coated layer.

Insufficient conformability of piston ring and liner geometry may produce a large local cylinder wall pressure which will destroy the oil film leading to uncontrolled wear and scuffing. Lack of conformability can also produce leakage paths for the combustion gases. Therefore a correct ring shape is of utmost importance. A new design philosophy designated OPCORE® has been developed and is presented here.

Cast iron is often used as the piston ring material in marine diesel engines. The mechanical properties of cast irons are to a great extent governed by the size, distribution and shape of the incorporated graphite particles. In a set of experiments, the mechanical properties of a pearlitic grey cast iron and a pearlitic compacted graphite cast iron are compared. Both cast iron grades have a eutectic composition. The experiments confirm the importance of micro-yielding of the matrix at the tip of the graphite particles on the macroelastic behaviour of the studied cast irons. This applies especially for the flake graphite cast iron where the graphite tip is sharper and the matrix bridges between the graphite particles are shorter than in the case of the compacted graphite cast iron resulting in micro-yielding at the graphite tip at a very low macro-stress and macro-strain. The high local stresses at the graphite tips also result in the opening of the graphite cavities which is much more severe in the flake graphite cast iron than in the compacted graphite cast iron. The mechanical properties of the eutectic flake graphite cast iron are largely affected by the size and amount of the graphite particles. The smaller the graphite particles (faster solidification) in the microstructure, the lower the values of the mechanical properties. In compacted graphite cast iron, the macro-elastic behaviour is influenced by the matrix and the overall coarseness of the microstructure to a greater extent and the effect of the incorporated graphite particle size is much less pronounced.

Nine grades of pearlitic cast iron containing different graphite morphologies (from flake, compacted and spheroidal) have been studied. The parameters investigated include the graphite aspect ratio, nodularity, graphite size and modulus of elasticity. These parameters have been correlated and compared with different existing bound and model equations. It has been found that the modulus of elasticity of the graphite phase increases as the aspect ratio and nodularity of the graphite increases, i.e.flake graphite gives a lower modulus of elasticity than spheroidal graphite. The experimental values of the modulus of elasticity show good agreement to bound and model equations, although flake graphite cast irons show higher deviation from the modelled values. An equation for the correlation between the graphite modulus of elasticity and the nodularity is presented. Introducing this linear correlation into an existing model for the determination of the effective modulus of elasticity gives a continuous function, including all grades of cast irons, with a very good agreement with experimental values. The modulus of elasticity of cast irons can be accurately predicted from both bound and especially model equations, using the aspect ratio and nodularity of the contained graphite particles. The fit is improved by using a modulus of elasticity of the graphite phase that is based on the graphite morphology, considering that the modulus of elasticity of the graphite is different in the basal and prismatic planes.

The deformation of metallic materials includes both an elastic and a plastic deformation. In the case of cast irons, the elastic region becomes less pronounced as the graphite changes from spheroidal to flake shaped, as observed in nodular and grey cast iron, respectively. The present study aims to correlate the shape of the graphite phase with the deformation behaviour, where the plastic deformation and other strain accommodating events are quantified by measurements of the acoustic emission events occurring in the interior of the material at loading. It also aims to explain how the appearance of cast iron stress–strain curves depends on the graphite morphology where, for instance, spheroidal graphite cast irons exhibit a seemingly linear elastic behaviour in contrast to flake graphite cast irons. The present study includes a series of pearlitic cast iron material grades with differences in nodularity and carbon equivalent, respectively. It is shown that as the roundness of the graphite phase increases, the ability to absorb energy increases. The measured acoustic emission indicates that plastic deformation occurs in the seemingly linear elastic region regardless of the cast iron grade, i.e. no cast iron grade exhibits perfect linear elasticity. The plastic deformation rate in the elastic region increases as the roundness of the graphite decreases and as the carbon equivalent increases. It is shown that the plastic deformation governs the resulting modulus of elasticity in all kind of cast irons, i.e. the modulus of elasticity decreases as the yielding of the material increases. The present study improves the understanding of the deformation behaviour in the elastic region of different cast irons. The survey shows that acoustic emission testing is a useful method when studying the deformation behaviour of cast irons.

The plastic deformation behavior of cast irons, covering the majority of graphite morphologies, has not been comprehensively studied previously. In this investigation, the effect of graphite morphology and graphite fraction on the plastic deformation behavior of pearlitic cast irons has been evaluated. The investigation is based on tensile tests performed on various different cast iron grades, where the graphite morphology and volume fraction have been varied. Pearlitic steel with alloying levels corresponding to the cast irons were also studied to evaluate how the cast iron matrix behaves in tension without the effects of the graphite phase. It is concluded that as the roundness of the graphite phase increases, the strain hardening exponent decreases. This demonstrates that the amount of plastic deformation is higher in the matrix of lamellar cast iron grades compared to compacted and nodular cast iron grades. Furthermore, this study shows that the strength coefficient in flake graphite cast irons increases as the graphite fraction decreases due to the weakening effect of the graphite phase. This study presents relationships between the strain hardening exponent and the strength coefficient and the roundness and fraction of the graphite phase. Using these correlations to model the plastic part of the stress-strain curves of pearlitic cast irons, we were able to calculate curves in good agreement with experimentally determined curves, especially for gray cast irons and ductile iron.

The piston ring is a key component in a marine combustion engine. High mechanical loads, relatively high temperatures and corrosive gases and liquids influence its performance in terms of sealing capacity, wear of cylinder liner and the ring itself. Base material of the ring, coating technology and ring geometry design are discussed in the article.

When tailoring cast iron materials suitable as piston ring base material two parameters are of importance; the morphology of the graphite and the constituents of the matrix. To optimize the properties of the cast iron a compromise is needed to achieve a satisfactory performance of the piston rings.

Daros Piston Rings AB is currently developing a second generation of chromium-ceramic coating the so called Z-chrome. The objective of this project has been to increase the maximum operating temperature of the coating and leave the other characteristics of the coating unaffected. The difference between the commercial Daros coating Tritor® and the Z-chrome is the ceramic component included in the coated layer.

Insufficient conformability of piston ring and liner geometry may produce a large local cylinder wall pressure which will destroy the oil film leading to uncontrolled wear and scuffing. Lack of conformability can also produce leakage paths for the combustion gases. Therefore a correct ring shape is of utmost importance. A new design philosophy designated OPCORE® has been developed and is presented here.

Cast iron is often used as the piston ring material in marine diesel engines. The mechanical properties of cast irons are to a great extent governed by the size, distribution and shape of the incorporated graphite particles. In a set of experiments, the mechanical properties of a pearlitic grey cast iron and a pearlitic compacted graphite cast iron are compared. Both cast iron grades have a eutectic composition. The experiments confirm the importance of micro-yielding of the matrix at the tip of the graphite particles on the macroelastic behaviour of the studied cast irons. This applies especially for the flake graphite cast iron where the graphite tip is sharper and the matrix bridges between the graphite particles are shorter than in the case of the compacted graphite cast iron resulting in micro-yielding at the graphite tip at a very low macro-stress and macro-strain. The high local stresses at the graphite tips also result in the opening of the graphite cavities which is much more severe in the flake graphite cast iron than in the compacted graphite cast iron. The mechanical properties of the eutectic flake graphite cast iron are largely affected by the size and amount of the graphite particles. The smaller the graphite particles (faster solidification) in the microstructure, the lower the values of the mechanical properties. In compacted graphite cast iron, the macro-elastic behaviour is influenced by the matrix and the overall coarseness of the microstructure to a greater extent and the effect of the incorporated graphite particle size is much less pronounced.

Nine grades of pearlitic cast iron containing different graphite morphologies (from flake, compacted and spheroidal) have been studied. The parameters investigated include the graphite aspect ratio, nodularity, graphite size and modulus of elasticity. These parameters have been correlated and compared with different existing bound and model equations. It has been found that the modulus of elasticity of the graphite phase increases as the aspect ratio and nodularity of the graphite increases, i.e.flake graphite gives a lower modulus of elasticity than spheroidal graphite. The experimental values of the modulus of elasticity show good agreement to bound and model equations, although flake graphite cast irons show higher deviation from the modelled values. An equation for the correlation between the graphite modulus of elasticity and the nodularity is presented. Introducing this linear correlation into an existing model for the determination of the effective modulus of elasticity gives a continuous function, including all grades of cast irons, with a very good agreement with experimental values. The modulus of elasticity of cast irons can be accurately predicted from both bound and especially model equations, using the aspect ratio and nodularity of the contained graphite particles. The fit is improved by using a modulus of elasticity of the graphite phase that is based on the graphite morphology, considering that the modulus of elasticity of the graphite is different in the basal and prismatic planes.

The deformation of metallic materials includes both an elastic and a plastic deformation. In the case of cast irons, the elastic region becomes less pronounced as the graphite changes from spheroidal to flake shaped, as observed in nodular and grey cast iron, respectively. The present study aims to correlate the shape of the graphite phase with the deformation behaviour, where the plastic deformation and other strain accommodating events are quantified by measurements of the acoustic emission events occurring in the interior of the material at loading. It also aims to explain how the appearance of cast iron stress–strain curves depends on the graphite morphology where, for instance, spheroidal graphite cast irons exhibit a seemingly linear elastic behaviour in contrast to flake graphite cast irons. The present study includes a series of pearlitic cast iron material grades with differences in nodularity and carbon equivalent, respectively. It is shown that as the roundness of the graphite phase increases, the ability to absorb energy increases. The measured acoustic emission indicates that plastic deformation occurs in the seemingly linear elastic region regardless of the cast iron grade, i.e. no cast iron grade exhibits perfect linear elasticity. The plastic deformation rate in the elastic region increases as the roundness of the graphite decreases and as the carbon equivalent increases. It is shown that the plastic deformation governs the resulting modulus of elasticity in all kind of cast irons, i.e. the modulus of elasticity decreases as the yielding of the material increases. The present study improves the understanding of the deformation behaviour in the elastic region of different cast irons. The survey shows that acoustic emission testing is a useful method when studying the deformation behaviour of cast irons.