cast iron because it is a brittle material so in case of damage in ring it give indication by sound.
For the different functionalities we use mainly two type of piston rings,i.e Compression ring and the Oil rings. In most cases piston rings are made up of Cast Iron. Cast iron easily adheres to the cylinder wall. In addition, cast iron can be easily coated with other materials to enhance its durability. If we consider Compression ring and Oil ring separately for a more clear picture, compression rings are those closest to the piston head. It creates a gas-tight barrier in between the piston and cylinder wall and seals leakage. It is exposed to the harshest conditions in terms of mechanical and thermal loading. The preferred material for this is low alloy, heat treated nodular cast iron with martensitic micro structure for higher hardness. Oil rings are nearer to the crankcase and performs the function of regulating the excess oil. It wipes off the excess oil from cylinder walls and return it back to crankcase. Unalloyed grey cast iron is used for these rings. Due to the pearlitic matrix of grey cast iron, it has low modulus of elasticity which ensures good adherence to the cylinder walls.
Piston rings are subject to wear as they move up and down the cylinder bore due to their own inherent load and due to the gas load acting on the ring. To minimize this, they are made of wear-resistant materials, such as cast iron and steel, and are coated or treated to enhance the wear resistance.
There are many piston materials that are in use like Cast iron , aluminium ,and it's alloys and also some times tungsten with some alloys But for efficient use we keep piston weight as possible as low so generally Aluminium alloys are preferred.
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