In the last installment we reviewed the two basic types of steel used in valves: martensitic and austenitic. In many offshore powerboats, especially for you guys with forced induction, exhaust gas temperatures get too high for 21-4N material. A good transition point to start looking at superalloy exhaust valves (with good fuel and spark calibration) is at the 650hp level on a 540ci engine.
Inconel
This takes us to the nickel-based superalloys (which are also austenitic steels), the first is Inconel 751. Considered to be the choice of blown offshore powerboaters, Inconel gets an earned and deserved reputation for reliability. Choose this for valve temps around 1500°F
Nimonic 80a
The top dog of the exhaust valve materials is Nimonic 80a. With a temperature capability of around 1600°F, this material can take almost anything you can throw at it. Nimonic 80a is not the strongest material at all temperatures, though. It becomes a toss up at lower temperatures, with Inconel 751 being the better choice at temperatures below 1400°F. In fact, below 1400°F, there isn’t a stronger material to pick than Inconel 751. If one were having valve failures due to mechanical stress rather than temperature, Inconel 751 might offer a robust solution against failure.
Temperature Checking
So one might ask, how do I know what my valve temperature is? In what is called a Temperature Check Study, valves of martensitic material get installed in the exhaust position and the engine is run at full load for two hours. Through understanding the tempering characteristics of the material, the hardness can be measured on the valves and the temperatures of those valves can be inferred. This test is key in selecting the appropriate material for a particular engine combination, but know that fueling, boost, spark, engine output, and many other factors all affect exhaust gas temperature and exhaust valve temperature.
Hardness Where Needed
As noted previously, austenitic valve materials trade hot strength for hardness at room temp. The lack of hardenability presents a challenge in a couple of areas – the valve tip and the valve seat face. The valve tip needs to be 52-54 HRc min to carry the contact stress of the rocker arm. Some solutions weld a martensitic stem to an austenitic head, making the stem hardenable. Another solution is to weld or press in a hardened tip to carry the load. A third option is to use a hardened lash cap. In any case, one of the solutions needs to be employed.
Hardfacing
In demanding applications, such as offshore powerboat exhaust valves, valve face wear may be a challenge. A countermeasure to this is the addition of a hardfacing operation. Hardfacing alloys have properties that bring a favorable combination of high hot hardness, corrosion resistance and wear resistance at high temperatures. Of the hardfacing materials, Stellite 6 is the most widely used. Stellites are cobalt-based Co-Cr-C-W alloys. A harder option is Stellite 1, which forms chromium and tungsten carbide particles upon hardening. Stellite F adds in a high nickel content which stabilizes the austenitic structure of the alloy and prevents phase changes in high temp environments. Other hardfacing materials outside the scope of this article are Tribaloy and several grades of proprietary alloys produced by Eaton Corporation (branded Eatonite).
Be sure to check out the next installment in this series, where we will talk about stem surface treatments, weight reduction and liquid cooling of exhaust valves.
