From the Web:
CHOOSING THE "RIGHT" REPLACEMENT VALVE SEAT
The original equipment manufacturers use a variety of seat materials, including cast iron, iron alloys, nickel alloys, cobalt alloys (stellite) and powdered metal (which generally contain no chrome or nickel, only vanadium and iron). Most OE seats in passenger car aluminum heads are a high grade of cast iron or powder metal. The better (more expensive) materials are usually found in high output and turbocharged engines, with hard seats and stellite being used mostly in diesels and industrial engines.
When replacing a seat, you should use one that is at least as good as the original if not better. Hard seats are a must for high temperature, high load and dry fuel (propane or natural gas applications). In fact, most seat suppliers have special alloys specifically designed for dry fuel applications. But hard seats are not required for light duty passenger car applications. Even so, many aftermarket seats are made of premium grade alloys or heat treated iron to provide improved longevity and performance.
As one seat supplier put it, "considering the insignificant difference in price between a so-so seat and a good seat, would not you rather sleep at night?"
Seat and valve materials must be compatible with one another as well as suited for the application. A hard valve generally requires a hard seat and vice versa. A stellite faced valve in an industrial engine, for example, would require a stellite seat. A titanium racing valve, on the other hand (which is relatively soft), would require a soft cast iron or beryllium-copper seat.
Brian Bender of S.B. International (J-LOY), said his company generally follows the OE material specifications on replacement seats. "If they use iron, nickel or cobalt in a given engine, we do the same."
Material compatibility is very important, said Bender, especially with fuels such as propane and natural gas. "We have our special Star Series inserts that provides extra hot hardness. It is a nonmagnetic nickel based alloy and contains some cobalt. This type of seat can really take the heat, especially if the air/fuel ratio or timing is off."
Bender said that most of the seat failures he sees are the result of abnormal engine operation (overheating, detonation, wrong air/fuel mixture, etc.) or because someone chose the wrong type of replacement seat.
"Some people are still trying to use plain cast iron seats in unleaded engines. It just will not work because it is too soft. They are a thing of the past so that is why we have discontinued plain cast iron seats in our line."
