Valve Selection: Hot Valve Materials for Hot Engines
What kind of valve alloys should you use when building a performance engine?
By Larry Carley
Are stock valve materials good enough, or do you need to upgrade to valves that are made of a more durable alloy? If so, what kind of alloy? These are questions every engine builder must answer when selecting valves for performance engine applications.
To the naked eye, most valves look pretty much the same. Unless you're Superman and have spectral x-ray vision, one alloy looks pretty much the same as another - with the exception of coated titanium valves or steel valves that have a black nitride coating. But even with these valves, the visual difference is the coating material not the alloy.
To understand valve alloys, you need to know something about basic metallurgy. There are essentially two basic types of steel used to make valves. One is "martensitic" steel and the other is "austenitic" steel. The difference is in the microstructure of the steel and how the various ingredients in the alloy interact when the molten steel is cast and cooled. This affects not only the hardness and strength of the steel, but also its corrosion resistance and magnetic properties. As a rule, martensitic steels are magnetic while austenitic steels are non-magnetic.
In martensitic steel, the steel is "quenched" (cooled) very quickly from a molten state to freeze the grain structure in a particular configuration. Under a microscope, the grain structure has a needle-like (acicular) appearance. This makes the steel very hard but also brittle. Reheating and cooling the steel (a process called "tempering") allows some of the martensite crystals to rearrange themselves into other grain structures which are not as hard or brittle. By carefully controlling the heat treatment and quenching process, the hardness and tensile strength of the steel can be fine tuned to achieve the desired properties.
Steel alloys with a martensitic grain structure typically have a high hardness at room temperature (35 to 55 Rockwell C) after tempering, which improves strength and wear resistance. These characteristics make this type of steel a good choice for applications such as engine valves.
But as the temperature goes up, martensitic steel loses hardness and strength. Above 1000° F or so, low carbon alloy martensitic steel loses too much hardness and strength to hold up very well. For this reason, low carbon alloy martensitic steel is only used for intake valves, not exhaust valves. Intake valves are cooled by the incoming air/fuel mixture and typically run around 800° to 1000° F, while exhaust valves are constantly blasted by hot exhaust gases and usually operate at 1200 to 1450° F or higher.
To increase high temperature strength and corrosion resistance, various elements may be added to the steel. On some passenger car and light truck engines, the original equipment intake valves are 1541 carbon steel with manganese added to improve corrosion resistance. For higher heat applications, a 8440 alloy may be used that contains chromium to add high temperature strength. For many late model engines (and performance engines), the intake valves are made of an alloy called "Silchrome 1" (Sil 1) that contains 8.5 percent chromium.
Exhaust valves may be made from a martensitic steel with chrome and silicon alloys, or a two-piece valve with a stainless steel head and martensitic steel stem. On applications that have higher heat requirements, a stainless martensitic alloy may be used. Stainless steel alloys, as a rule, contain 10 percent or more chromium.
The most popular materials for exhaust valves, however, are austenitic stainless steel alloys such as 21-2N and 21-4N. Austenite forms when steel is heated above a certain temperature which varies depending on the alloy. For many steels, the austenitizing temperature ranges from 1600° to 1675° F, which is about the temperature where hot steel goes from red to nearly white). The carbon in the steel essentially dissolves and coexists with the iron in a special state where the crystals have a face-centered cubic structure. By adding other trace metals to the alloy such as nitrogen, nickel and manganese, the austenite can be maintained as the metal cools to create a steel that has high strength properties at elevated temperatures. Nitrogen also combines with carbon to form "carbonitrides" that add strength and hardness. Chromium is added to increase corrosion resistance. The end product is an alloy that may not be as hard at room temperature as a martensitic steel, but is much stronger at the high temperatures at which exhaust valves commonly operate.
Though austenitic stainless steel can handle high temperatures very well, the steel is softer than martensitic steel at lower temperatures and cannot be hardened by heat treating. To improve wear, a hardened wafer tip may be welded to the tip of the valve stem. Or, on some applications an austenitic stainless valve head may be welded to a martensitic stem to create a two-piece valve that has a long wearing stem and heat resistant head. The only disadvantage with a two-piece valve is that it doesn't cool as well as a one-piece valve. The junction where the two different steels are welded together forms a barrier that slows heat transfer up the stem.
21-2N alloy has been around since the 1950s and is an austenitic stainless steel with 21 percent chromium and 2 percent nickel. It holds up well in stock exhaust valve applications and costs less than 21-4N because it contains less nickel. 21-4N is also an austenitic stainless steel with the same chromium content but contains almost twice as much nickel (3.75 percent), making it a more expensive alloy. 21-4N is usually considered to be the premium material for performance exhaust valves. 21-4N steel also meets the "EV8" Society of Automotive Engineers (SAE) specification for exhaust valves.
SAE classifies valve alloys with a code system: "NV" is the prefix code for a low-alloy intake valve, "HNV" is a high alloy intake valve material, "EV" is an austenitic exhaust valve alloy, and "HEV" is a high-strength exhaust valve alloy.
Unfortunately, you can't always tell what kind of alloy a valve is made from because different valve suppliers use different alloys as well as their their own proprietary names for their valve materials. Thus one manufacturer may call their intake valve material a "422 stainless alloy" while another refers to it as an "NK-842 stainless intake material." Without a thorough metallurgical analysis, you can't really compare one manufacturer's valve material to another's. But do you really need such a comparison? As long as the alloy does what it is supposed to do, it doesn't matter what they call it.
The bottom line here is that intake valves and exhaust valves both require different types of alloys. The same alloy can be used for both intake and exhaust valves (say 21-2N or 21-4N, for example), but the best results are usually obtained when different alloys are selected for the intake and exhaust valves. Why? Because an exhaust alloy that has good high temperature strength and corrosion resistance really isn't needed on the intake side, and it may not have the hardness and wear resistance of an intake alloy at lower temperatures. Even so, some companies sell the same alloy for both intake and exhaust valves while others offer different alloys for intake and exhaust valves.
Intake valves run cooler and are washed with fuel vapors which tend to rinse away lubrication on the valve stem. So for intake valves, wear resistance may be more important than high temperature strength or corrosion resistance if the engine will be involved in any kind of endurance racing. Exhaust valves, on the other hand, run much hotter than intake valves and must withstand the corrosive effects of hot exhaust gases and the weakening effects of high temperatures. Consequently, a premium valve material is an absolute must on the exhaust side - especially in turbocharged and supercharged engines and those that inject nitrous oxide to boost power.
As combustion temperatures go up, valve alloys that work fine in a stock engine may not have the strength, wear or corrosion resistance to hold up in a performance application. If you want the valves to last, especially in a highly modified racing engine, upgrading to better valve alloys will be a must.
The best advice is to follow the valve alloy recommendations of your valve supplier, and to rely on their expertise when it comes to picking the best valve material for a performance application. If a stock valve alloy is holding up well enough in a performance application, there's no need to upgrade. But if an engine is experiencing valve burning or premature valve failure, then an upgrade to a better material may be needed to solve the problem.
Performance Valve Alloys
Materials that may be used for performance valve applications include carbon steel alloys, stainless steels, high-strength nickel-chromium-iron alloys and titanium. The alloys that are most commonly used for performance engines include various high chromium stainless alloys for intake valves, and 21-4N (EV8) for exhaust valves.
Inconel® refers to a family of trademarked high-strength austenitic nickel-chromium-iron alloys (a "superalloy" material) that is sometimes used for exhaust valves because of its superior high temperature strength. Inconel is a nickel base alloy that is sometimes thought of as a super-stainless steel, with 15 to 16 percent chromium and 2.4 to 3.0 percent titanium. Inconel 751 is classified as an HEV3 alloy by SAE. This alloy has been used for the exhaust valves in some late model GM medium duty truck engines (to prevent premature valve erosion), but is not commonly used in performance exhaust valves. For most performance applications, the exhaust valve material of choice is 21-4N - or titanium.
Titanium is often viewed as the ultimate valve alloy material because of its lightness. Titanium is about 40 percent lighter than steel, making it a good alternative for high revving engines. Lighter valves also allow more radical cam profiles that open and close the valves more quickly for better off the line performance and low end torque. The durability of titanium is similar to that of stainless steel. But from a cost standpoint, titanium is way beyond any steel alloy. A single titanium valve may cost $70 to $90 or more. Spending $1,200 or more for a set of valves may be peanuts to a professional racer with deep pockets, but for the average guy that's a lot of money. Yet titanium valves are being used in many street performance engines as well as everything else. Titanium valves are even being used in some production motorcycle engines these days.
One supplier of titanium valves said they use the same alloy for both intake and exhaust valves: a 6242 alloy that contains 6 percent aluminum, 2 percent moly, 4 percent zirconium and 2 percent tin. But a different heat treatments are used for the intake valves and exhaust valves. The heat treatment is very important because it determines the ultimate strength and hardness of the metal.
Titanium valves are often coated with moly or another friction-reducing surface treatment to reduce the risk of stem galling. Coated valves are recommended for street performance applications, but may not be necessary in drag racing or circle track applications where engines are torn down and inspected frequently.
Titanium valves will work with stock valve guides and seats, but for the best results they should be used with copper beryllium seats (to improve heat transfer and cooling) and manganese or silicone bronze valve guides.
Valves often have stem and/or head coatings to enhance performance. Stock valves as well as performance valves usually have chrome-plated stems to protect the stem from galling when the engine is first started. Chrome-plating also helps reduce valve seal wear on engines that use positive valve seals.
The thickness of the chrome plating can vary from a thin flash of .0002? to .0007? up to a hard plating of as much as .001?. It's interesting to note that chrome plating actually produces a rougher, not smoother, surface. But microscopic cracks in the surface of the chrome retain oil and improve lubrication to reduce wear.
Many Japanese OEMs use a black nitride coating on the valves instead of chrome plating. The nitride coating, which is applied in a salt bath treatment, protects the stems against scuffing and wear. Nitriding creates a thinner but harder surface layer that also does an excellent job of reducing wear.
Some performance valves may also have the stems treated with a special dry film lubricant to reduce friction and wear. With titanium valves, a dry film lubricant coating can also reduce the effects of valve erosion caused by the hot exhaust gases as they exit the combustion chamber. Dry film lubricants on the stem and inside of the valve head can also reduce the build up of carbon deposits that can create turbulence in the incoming air/fuel mixture and exiting exhaust gases.
As for the valve face, various coatings may be used to increase heat and wear resistance in valves made of steel or Inconel. Stellite is a hard facing material that's often required for heavy-duty diesel and gasoline exhaust valve applications, and may be used in some Top Fuel applications. Stellite is a cobalt base material with a high chromium content. It is applied to the valve face to protect against oxidation and corrosion. It may also be used on the stem tip for added wear resistance.
Ceramic thermal barrier coatings may also be applied to the combustion side of the valve head to reflect heat back into the combustion chamber. The theory here is that a heat reflective coating helps the valves run cooler. This helps the exhaust valves run cooler and last longer, and reduces heat transfer from the intake valves to the incoming air/fuel mixture for a denser, more powerful mixture. Heat reflected back into the combustion chamber also improves burning efficiency and power.