We ask a manufacturer and a material supplier to reveal the state of the art in valve seats. By Ian Bamsey with research by Stewart Mitchell
Claude Holguin is co-owner of CHE Precision, a company renowned for its race engine valve seats. We asked him: what needs to be considered in terms of the interaction between the valve seat and the cylinder head?
He says, “First of all, the valve seat is a critical engine component, as it is the foundation of the valvetrain. Seats with higher thermal conductivity will enable better heat transfer away from the valve to the cylinder head. For competition engines, we have come up with a proprietary copper-based seat used in an aluminum cylinder head; it seems to work the best for most race applications.
“Now, if you’re talking about a production engine with a cast-iron cylinder head, copper-based seats would not be recommended. As soon as you put the copper into the cast iron, it lessens its thermal conductivity benefit: the seat’s ability to transfer the heat is lost.
“With an aluminum head, no matter which copper alloy is used for the seat, the copper will absorb more temperature than the aluminum. Consequently, you have to think of the valve seat as a heat sink.
“The only place a valve can get rid of its heat are where it touches its seat and its guide. It is important for the seat to absorb that heat as best as possible – that’s why we continue to tell engine builders that they should really go deeper with seats, allowing more surface area to transfer the heat to the aluminum.
“No matter which copper allow you use, make sure you have a thick enough seat so that the valve’s life can be improved. A deeper seat not only gives you better thermal conductivity, it also provides better grip length; it has less tendency to fall out.
“Plus, it is best to have the seat at least 0.050-0.100 in larger in diameter than the valve head. If you have really high spring pressure – as is often the case in drag racing for example – the compression that closes the valve is pretty aggressive, and if there isn’t support around the edges, the whole thing is going to cave in or crack the OD of the seat.
“Often the cam designer is trying to obtain extreme acceleration and deceleration rates. The higher the rpm, the more rapidly the valve is coming down on the seat, and it can start to bounce. A copper material will give it more of a cushion, so that the impact isn’t so harsh on the valve. It’s like when a marble is dropped onto concrete and it jumps right back up into your hand. If you put a bit of cushion in there, it can control that bounce.
“Valve bounce is something that can happen to anybody, with any spring, whether it’s a motorcycle engine that’s going to 12,000 rpm or a 500 cu in a V8 going to 10,000 rpm. One problem with American V8s is that, because of the size of the valves they use, they can end up with an interlock.
“The problem is that the exhaust seat has combustion heat expanding it, while the intake seat has a cold charge coming in every fourth cycle to cool it. It is necessary to relieve the space between them so that the exhaust seat doesn’t push the intake seat out, which means you will have a leak that costs horsepower.
“The relief you need is a matter of trial and error. You can do a leak-down test when the engine is cold and you have all the compression in the world, but then when it gets hot there is no compression. That is when you realise the exhaust seat is pushing the intake seat out. If you take a close look with a glass, you can actually see that the exhaust seat has moved the intake seat out of the way.”
We asked if there is an appropriate ratio for seat depth as a function of diameter. “We don’t look at it as a set ration,” Holguin says. “We look at what is available.
“In most engines the exhaust seat is not able to go deeper, as the ledge to support the seat is quite thin. Also, the short side radius is small, so going deeper with the seat would shorten this radius and affect flow adversely.”
You mentioned that copper alloys provide a cushioning effect. Do you ever specify valve seat materials that don’t have this compressive aspect to them?
“No. If you consider the production engine seats that come out of the likes of Detroit, they are usually sintered. They are intended to be long-lasting, not performance based. This seat is a powdered metal. At one time we tried making them but there were still voids in the seat, even when using a 100 ton press.”
Can you do anything to a copper alloy to reduce the amount of expansion of the exhaust side to make sure the intake seat doesn’t get pushed out?
“Copper is copper. By itself, you can’t really do too much with it. However, when you alloy different materials with the copper, you have more flexibility to create what is needed. Over the years, in our company, we have used five different alloys for various seat applications. Some engine builders want the seat to simply be harder. Truth be told, you don’t necessarily want it harder. There is a big difference between hard and tough.
“You actually want the seat to be tougher rather than harder. You want it to stand up to the pressure, even if it contours a little bit, that isn’t the same problem as having it break like a piece of glass.
“When a material is tough, you can get a nick in it and the seat will still perform, and maybe lose only 2-3% of its strength. You can finish a race like that, but you certainly can’t finish a race when a seat has broken.
“That’s why we push for tougher alloys. When CHE creates alloys there are a couple of mills that will pour them for us – a bit of this and a dash of that – which gives us better workability. We are looking at it from the racers’ point of view, whereas Detroit is looking at getting 200,000 miles out of a seat. A drag racer might only need minutes of running time!”
What are the elements of the alloy that enhance toughness?
“That information can’t be revealed, but you would be surprised at the possible outcomes. What you put in the cake recipe and make the cake do different things. You can make the cake softer and more moist, or you can make it a little more dense and longer lasting. There is always a trade-off.”
Understandably Holguin won’t reveal the recipes his company uses for its well-respected valve seats. So we asked Dave Krus of Materion Performance Alloys which materials his company supplies to other companies making race engine valve seats?
“Copper beryllium has been the industry standard for a very long time,” Krus replies. “The first record I can find of [Materion Supplying] it for valve seats was in 1984; many teams still use it. Primarily we supply two copper-beryllium alloys – one is a high-strength alloy used for intake seats, the other is a higher conductivity alloy that has a better strength retention at temperature, which is used for exhaust seats.
“Another material we offer for seats is PerforMet alloy, which is the copper-based material we are now offering for piston rings and cylinder lines [see Grid in RET 115, December/January 2019].”
Beryllium-free PerforMet is a nickel-silicide strengthened copper alloy containing (nominally by weight) 7% nickel, 2% silicon and 1% chromium, with the rest being copper. Krus says, “PerforMet has good strength retention at temperature. It is harder and stronger than high-conductivity copper beryllium, and is more thermally conductive and has better strength retention at high temperature than high-strength copper beryllium.
“There are advantages to using one material that works well for both intake and exhaust seats, so for that reason as well some prefer PerforMet. It is now used quite a bit in NASCAR and NHRA for intake and exhaust seats. It does seem advantageous, especially with respect to durability on the exhaust side.
“Our Alloy 10X copper-beryllium alloy has the best strength retention at temperature of all the copper alloys we produce. It was originally produced for use in nuclear generators. It has a 480,000 Pa yield strength and 5% elongation at 800 F/427 C.
“Copper alloys generally lose their strength and get softer as the temperature rises. Eventually, they also will go through a ductility minimum, meaning they become more brittle at first then eventually become more ductile.
“The reason we decided to use Alloy 10X was because somebody using our C17510 high-conductivity copper beryllium alloy for valve seats was suffering cracking on the exhaust side. The phenomenon is called heat checking, where tiny cracks are generated by overheating on the surface of the component.
“That was about 10 years ago. I offered them Alloy 10X and the problem went away. However, it hasn’t taken off like we thought it would. Some of the engine builders who have used say they can overwhelm even its properties. If you get any valve seat material hot enough, Alloy 10X included, the seat doesn’t necessarily suffer cracking but recession occurs simply because the material softens. That said, we still have some loyal users who say they can’t find anything better for exhaust seats.”
We asked Krus: what is the phenomenon that occurs when the metallurgy starts to break down? “We call it over-ageing,” he replies. “In its soft state, before the final heat treatment, the metal is commonly hot-worked and/or cold-worked into its form and then we heat-treat it.
“Ageing of copper beryllium and many other copper alloys involves holding the material at a controlled elevated temperature for an extended period of time then the material will reach a peak in strength, and it will start to come down after that. The material’s ductility changes in the opposite way. At its peak strength the material is at its least ductile.
“Those are two separate phenomena. One question is, ‘What are the properties of a material within minutes of raising it to the component operating temperature?’ The other question is, ‘Does long-term exposure at the operating temperature cause permanent changes in the material’s properties?’
“Take a C17200 seat and a PerforMet seat, for example. At room temperature, C17200 alloy is one of the strongest copper alloys available, with 25% higher yield strength than PerforMet. Hold both seats at a temperature normally seen in racing intake seats, below something like 300 C. Within minutes, both materials are softer and weaker than they are at room temperature, but no permanent changes will happen even with very long exposures.
“Now take them to 400 C. Again, both are softer than at room temperature and 300 C. However, at this point, the C17200 seat is now softer than the PerforMet seat. Hold the two at 400 C for several hours and the C17200 seat will become progressively softer, while the PerforMet seat’s hardness will not change.
“Hold for long enough at 400 C, and when the two samples are cooled back to room temperature, the PerforMet seat’s hardness will be the same as it was, while the C17200 seat will be significantly softer than it was originally at room temperature. The C17200 seat has over-aged after a long time at 400 C; the PerforMet seat has not.
“For applications such as Top Fuel drag racing, some teams use the strongest alloys they can to avoid recession in the short term. They usually destroy the valve seats in a very short time anyway, so long-term exposure is not one of their main considerations. On the other hand, it is ideal if the exhaust valve seat has high conductivity, as that allows you to distribute the heat away from the combustion chamber into the head. That reduces hot-spots.”
What are some of the characteristics of the alloys that prevent excessive softening, and what are the trade-offs? “In order to have some strength at high temperature, you need a relatively lean alloy with fewer alloying elements in it,” Krus says. “It’ll then have a higher conductivity. Alloy C17200 was used traditionally as a copper beryllium for intake seats, and C17510 for exhaust valve seats; C17200 has a 1.8% beryllium and the C17510 about 0.3%.
“With the lower beryllium content, you have a higher conductivity but less strength at room temperature. PerforMet doesn’t contain any beryllium, it uses a different hardening alloy.
“You need something in a valve seat alloy that allows it to retain strength at temperature, or you run the risk of recession. C17510 and C17200 are cold-worked and then the material is taken into heat treatment to achieve the desired properties and internal structure.”
What kind of structure are you looking for, and how far up the scale of heat treatment do you go?
“Because we are talking about different hardening mechanisms, it gets a bit challenging to isolate the exact level to which the hardening process needs to be completed for every valve seat application. With copper beryllium, a coherent structure forms hard particles that strengthen the material – because they are coherent with the matrix – but the lattice spacing is smaller in this form.
“That means the particles pull the lattice together, and that puts the entire component in compression. Therefore, when you try to stress them, you have to overcome that first before you get to the zero point.
“For PerforMet, the strengthening is a combination of shrinking the lattice spacing and the formation of hard, nickel-silicide particles. The lattice strain is less than with C17200, which explains the room temperature strength difference. The nickel silicides, however, provide wear resistance and strengthening that is more resistant to temperature changes. You might say the alloy looks a bit like a composite material in that way.”
Is additive manufacturing used for valve seats? “We are looking at additive manufacturing,” Krus says. “Most of our alloys are produced by melting and casting, followed by metalworking and heat treatment, sometimes in multiple steps. The process is necessary to get the properties, and it needs to be done before the material is ready to be machined into components such as seats.
“The beauty of additive manufacturing is efficient production of near-net shapes. If the additively manufactured component requires metalworking or significant heat treatment after forming, that efficiency is wasted.
“It is difficult. We are thinking about ways to trick the system, although we haven’t perfected any yet.”
So it is that the technology of valve seats continues to progress behind the scenes, while on the stage of competition events it is the engine builders who hark the advice of Holguin and Krus who are most likely to find success.
Read more in Race Engine Technology – Issue 118