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Rings of steel

ringsThe UN summit meeting in Copenhagen, COP 15, has come and gone. A meeting whereby 192 or so countries met together to fudge some kind of global agreement to reduce greenhouse gas emissions yet again attracted that section of the environmental lobbyists who seem to be determined to press their cause via mayhem and violence. It is little wonder therefore that the authorities seeking to maintain law and order, and according to journalist types, threw a 'ring of steel' around the venue to safeguard those present. The connotation to all this is clearly one of strength and therefore safety, and so it was almost certain that when looking towards increased performance, manufacturers of piston rings would eventually move towards the substance as well.

But what precisely is steel and how does it differ from that other popular piston ring material, cast or even ductile iron? Rather like the focus of the UN Summit meeting, the answer is all in that most politically sensitive of all materials - carbon. And whereas with greenhouse gas emission, the culprit is carbon dioxide - at levels currently around 380 ppm in the atmosphere, with steel that carbon manifests itself in slightly larger amounts but typically less than 1%.

Pure iron, in other words an iron that contains no carbon or impurities, has poor strength and is soft, softer even than aluminium. Silver-grey in appearance, in this form the material is highly reactive in air or moisture but the addition of only small amounts of carbon increases its strength substantially but to the detriment of ductility. When this amount of carbon present exceeds 2%, graphite flakes begin to be formed which when at 3% by weight gives us our typical grey cast iron. Producing a level of inherent lubrication at the surface, these flakes are also stress raisers which reduce the overall strength of the material and make it much more susceptible to brittle fracture. But for its poor strength however, cast iron makes an ideal piston ring material.

However as dynamic loading in the top ring increases - as a result of increases in engine speed, for instance, there comes a point when cast iron, even in its stronger and more ductile nodular carbon form, can no longer cope. The desire to reduce piston weight, especially around and above the top ring also calls for a stronger, thinner and therefore more flexible ring material. Steel in one of its many forms would therefore seem to be ideal having much better tensile and fatigue strengths (sometimes as much as five times of that of cast iron), together with improved hardness. As a result, rings can be made thinner and more flexible reducing both friction and wear. Whereas a typical cast iron top ring would be no thinner than 1.5 mm, its equivalent in steel is typically 1 mm or even less. But although the steel ring may be stronger, in order to minimise friction and wear, the working surfaces will need to be coated with one of any number of coating technologies.

Manufacturers are rarely keen to talk about exact materials but one such used - AINSI 9254, is described as a high-alloy steel with between 0.51 - 0.59% carbon. Classed as a medium to high carbon steel this material was chosen to give excellent wear characteristics with the hardness obtained at the expense of ductility and toughness.

But while UN Summit meeting itself dissolved into confusion and acrimony, somewhat ironically the carbon in the 'Ring of steel' kept all those inside safe from harm.

Written by John Coxon

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