Martensitic vs. Austenitic steel
In the world of engine valves, they are made of one of two types of steel: martensitic and austenitic. Martensitic steels are predominantly martensite, and feature a grain structure that resembles a collection of needles. Martensitic steels harden well and tend to be brittle before they are tempered. Once tempered, they are strong and durable.
https://en.wikipedia.org/wiki/Martensite#/media/File:Steel_035_water_quenched.png
Austenitic steels contain predominantly austenite. An austenitic steel will be soft, somewhere around 30-35 HRc. Austenitic valve steels present superior hot strength.
As an aside, the performance aftermarket oftentimes won’t tell the customer what type of material he’s buying. Whether it be they think that information is too confusing or some other reason we can’t be sure. Like valvesprings, many of the companies that sell valves don’t actually manufacture them, nor are they experts in that area. It could be less that they don’t want their customers to know, but more that they themselves don’t fully understand the information I’ll be telling you in this and the following articles on this topic.
Martensitic – Intake
Martensitic steels have the benefit of being hardenable and have good strength properties at room temperatures. This makes them a good choice for intake valves, which run cool, most often a few hundred degrees. Silchrome-1 is a very popular material used in engines for intake valves. Having a hardenable material is nice, since the valve tip needs to be 52-54 HRc min to carry the contact stress of the rocker arm. The valve can be hardened locally in the tip area. The rest of the valve can be left softer and ductile
Austenitic – Exhaust
The most common materials used for exhaust valves in passenger car and high performance street engines 21-2N and 21-4N. These materials have been around for decades. 21-2N offers a cost advantage over 21-4N due to the reduced nickel content (that’s what the “N” stands for). 21-2N contains 2% nickel and 21-4N contains nearly 4% nickel. Of the two, opt for 21-4N, it has higher temperature capability (around 1420°F). It appears that many intake valves in the performance aftermarket are made from an austenitic steel also. As we’ll discuss later, this drives an additional processing step since the valve tip cannot be left soft, yet cannot be hardened directly as with martensitic steels.
Join us for the next installment in this series, where we will discuss superalloys and their applications in offshore powerboat engines.
