Valve Materials for Extreme Applications (2)
In the last article, the author looked briefly at the kinds of materials that we might use for high specific output applications such as supercharged or turbocharged engines. In the article we mentioned a couple of the popular 'superalloys' which are used for poppet valves, namely 'Nimonic' and 'Inconel' alloys. Although not as widely used as the Nimonic materials, Inconel is used in the same sorts of applications, and Wayne Ward recently discussed its use as a valve material with Californian-based Supertech, a leading valve manufacture specialist who uses this material.
One part of the discussion with Managing Director Willy Tagliavini concerned manufacture of Inconel valves, and how this differs from a more normal austenitic valve material. The cost of an Inconel valve is around 40 - 50% more than an austenitic valve of the same design, and I asked where this premium comes from. It was claimed that the material itself costs between three and four times as much as a typical stainless grade and that the machining is more complex, both during the turning and grinding stages, requiring special tooling and different techniques. "It is necessary to use special tool inserts for high temperature alloys with a special composition for higher durability and slower machining speeds" said Tagliavini. It was clear from this response that their typical Inconel valve, which is made of Inconel Alloy 751, takes longer to make than a stainless valve. Coupled with the premium that they pay for the material, we can see where the extra cost comes from.
The company uses a basic rule of thumb when advising it's customers on choosing a high temperature material over a stainless item. When the output reaches or exceeds 130hp per litre of displacement, then the recommendation is for the exhausts to be made of a high-temperature material. Although they make valves from both Inconel and Nimonic alloys, it is claimed that "due to commercial reasons and availability" the company have settled on Inconel 751 as their high temperature material of choice. The 130hp per litre rule is not by any means set in stone, as there are many factors which can influence the requirement for a high-temperature valve material. One important factor is the rate of heat transfer between the valve and the seat, where typically 75% of the heat from the valve is released through the contact with the seat, so the contact area and time of contact have a big influence.
Owing to the ability of Inconel materials to run at higher temperatures than a stainless alloy, some customers take advantage of this and use Inconel valves with a narrower seat contact. The narrower seat means that the valve will run at a higher temperature which the material has within its capability, but more importantly allows a better flowing exhaust valve which may be a very significant advantage in some applications.
The above company has had a number of customers who have had bad experiences with sodium-filled OEM hollow valves, and that some of their trade in Inconel valves is supplying replacements for the hollow parts with customers willing to accept the weight penalty for the prize of increased reliability.
Fig. 1 - Inconel valves for extreme applications (Courtesy Supertech Performance Inc)
Written by Wayne Ward