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Austenitic steels

valvesSo far in Race Engine Technology's coverage on valves, there have been a number of articles on valve materials for the more extreme applications, specifically pieces on Inconel valves and Nimonic materials used for turbocharged endurance applications. There have also been articles on the lightweight valves, made from low-density materials such as titanium and titanium aluminide.

Valves made from these materials are expensive for various reasons, some to do with the price of the raw material, some due to the immaturity of the technology and some due to the extra processing time required to produce the valves. While it is always interesting to take note of the latest materials technology, there are many people in our sport who can afford neither the cost nor the risk of running these materials, and would derive little real benefit from doing so anyway.

For these people, their requirements are that the parts should be of consistent quality, that they are inexpensive (compared to the options above) and reliable. For many in this group (which extends into many forms of world championship racing) the material of choice is austenitic valve steel.

Austenitic valve steels are so named because the material structure remains austenitic even down to room temperature and below. For many steels, the austenitic phase is stable only at elevated temperatures, and on cooling transforms to another structure. Usually, we can quench from the austenitic phase to form martensite, which is very hard and can be tempered to give the required combination of strength and ductility.

Austenitic valve steels, which are commonly used for valves, are characterised by containing large percentages of chromium and nickel. The austenitic valve steels are neither very hard nor particularly strong at room temperature, but they retain a high proportion of their strength at the temperatures at which racing valves operate.

One difference between these steels and standard 'stainless' steels - which are also austenitic steels - is the amounts of other alloying elements, with the valve materials often having a higher percentage of carbon than a common stainless steel such as 316. Most stainless steels contain less than 0.1% of carbon, but austenitic valve steels commonly contain between 0.3% and 1% of carbon.

There are cheaper materials that contain less of the expensive alloying elements, although they are not as strong as austenitic steels at higher temperatures and are not as corrosion-resistant as austenitic steels. For racing purposes then, where temperatures are higher than in normal series-production applications, austenitic valves are common. Their fatigue strength and tribological behaviour can be enhanced by treatments such as nitrocarburising, and such treatments are particularly effective in materials containing high percentages of chromium.

The high percentage of chromium also makes the materials suited to nitriding processes, and plasma nitriding is used by some valve manufacturers to excellent effect here. The surface hardness of the nitrided layers can be in excess of 70 HRc on these materials.

For older engines that still run cast-iron (rather than bronze) valve guides, it may be necessary to chrome-plate the material to prevent wear.

Fig. 1 - These austenitic steel valves are for a road application, but the material remains popular for racing

Written by Wayne Ward

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