VALVE MATERIALS AND DESIGNS (Part 1)

VALVE MATERIALS AND DESIGNS

Choosing the correct valve
for specific applications

by Mike Mavrigian

both photos by author

Choosing the optimum valve for a specific application can sometimes be confusing considering the various choices available today. In this article, we attempt to explain the various materials and designs in an effort to better understand today’s valve offerings.

STAINLESS STEEL VALVES

Although stainless steel valves may be offered in varying grades/alloy recipes, high-performance stainless steel valves are most commonly made of material referred to as EV8 (a more expensive heavy-duty stainless alloy material) and are made from a one-piece forging. In addition, some valve makers offer a stronger stainless steel formula that offers higher heat resistance (Manley’s XH-428 is an example). Some makers use EV8 only for their exhaust valves, while others utilize this material for both intake and exhaust valves. High-quality performance stainless valves should feature hard stellite tips (since stainless is not hardenable, a hardened tip must be welded onto the stem) and hard-chrome-plated stems (not cheap flash chroming) to reduce guide wear. Undercut stems contribute to slight weight reduction and benefit flow characteristics. Note: If a particular brand of stainless steel valves does not feature a hard tip, the use of lash caps will be required.

TITANIUM VALVES

Titanium (chemical symbol Ti) offers the highest strength-to-weight ratio of any known metal. In an un-alloyed condition, Ti is as strong as some steel materials but about 45-percent lighter. When used to manufacture automotive valves, titanium is alloyed with small percentages of various materials, including copper and molybdenum. Titanium is a fairly hard material and can be challenging to machine as it can gall if tooling isn’t hard and sharp enough, and if the material isn’t cooled properly during machining.

Titanium was discovered independently by a couple of guys, including a British amateur geologist and a German chemist, in the late 1700s. The German, Martin Heinrich Klaproth, reportedly named the material titanium for the Titans of Greek mythology. Starting in the 1950s, titanium began to see serious use by both the United States and the Soviet Union for military applications, including submarines and jet aircraft.

Many titanium valves are produced by starting with a forging, then machined to final shape, but some are produced using a two-piece inertia-welded design. Citing Xceldyne as an example of this approach, it utilizes an inertia welding process to attach a partially machined valve head and stem together. During this process, the two previously machined parts are fused together using stat-of-the-art equipment that uses inertia and a force to weld the two pieces into one solid component.

Once the valve blank is welded, it’s heat-treated to alter the grain structure of the titanium through precision heating and cooling at varying temperatures, taking into account the properties of the alloys and the specific application (intake or exhaust). According to Xceldyne, this process is so effective that inertia welded valves have been certified as having a superior grain structure as compared to a one-piece forged design. The valve is then CNC-machined and, in many cases, undercut in the stem area to allow a bed for the inlay of a coating. The valve is then plasma-moly-coated. Specific sections of the valve are further machined and the stem is ground, leaving the plasma-moly coating over only the desired stem area. The head, stem and keeper grooves are then final machined. Stem-grinding is then finalized to establish dimensional tolerance to within 0.0002″. The valve is then precision-polished to reduce the potential for carbon buildup.

Three styles of valve tips are generally available, which includes a hardened steel tip or a ceramic-coated tip (ceramic tips are to be used in conjunction with lash caps) and thin-film technology such as a PVD (plasma vapor deposition) coating.
As we noted earlier, titanium is a relatively soft material requiring a protective contact surface at the stem tips, usually requiring hardened lash caps. Xceldyne noted that when valves feature stem diameters smaller than 5/16″ (7 mm or less in diameter), a specialized hard coating is applied to the stem tip in order to protect the tip from lash-cap friction.

The ceramic coating is a durable hard coating intended to protect the titanium from the friction caused by the lash cap. Other coatings such as a PVD treatment, a CrN (chrome nitride) treatment, CVD (chemical vapor deposition) or DLC (diamond-like carbon) or other highly specialized protective application may be applied to the tips. This hardened feature at the tip prevents material transfer or galling between the tip and lash cap.

Hollow titanium valves are also available, either with hollow stems or with a combination of hollow stems and hollow heads. Hollow stem designs reduce valve weight by about 10 percent. The hollow head design is a proprietary process that removes an additional 6-8 grams of weight (of course, depending on valve size). As part of the proprietary process, the inside of the valve head may be reinforced to provide a support structure for strength and rigidity.

When a stem is gun-drilled, according to Xceldyne’s Scott Highland, careful attention is paid to achieving a consistent precision surface finish and concentricity in the I.D. to obtain uniform stem wall thickness. Sonic measurement technology (and other proprietary methods) are employed to monitor the I.D. operations.

Highland noted that the commonly used lock design for titanium valves is the “super 7″ style, commonly referred to as a seven-degree lock, which is actually closer to 8 degrees. Lock grooves are square grooved or radiused for superior lock engagement as well as reduced potential for stress risers. Xceldyne notes that it applies a specialized thin-film PVD coating to the locks and retainers to prevent material galling between titanium/titanium materials. Highland mentioned that lock-to-retainer interface is perhaps the biggest galling-potential issue that must be addressed.

Highland also noted that while undercutting is employed on many titanium valve designs, there are occasions when exhaust valves may feature an overcut in order to provide the required additional cross-sectional mass needed for some extreme applications.

Some precautions concerning the handling and use of titanium valves include:
• Do not touch the valve surface with your bare hands since fingerprint acids may affect the coating. Use gloves or coat the valve with oil before handling.
• Never use a lapping compound or any abrasive material when the valve is coated with a PVD-style coating.
• Valve seats should be replaced during each and every rebuild in order to ensure a proper valve-to-seat contact. The width of the contact zone (valve face to valve seat) should be at least 1 mm.
• New valve seats should be a relatively soft material, such as bronze or nodular iron (heat-treated to Rockwell RC32 or less).
• Unless directed otherwise by the valve maker, always use hardened lash caps on titanium valves. Some makers offer valves built with friction-welded hardened tips. Bare, unprotected titanium tips are relatively soft and will mushroom when exposed to rocker arm forces.

We’re told that if the titanium valve features a stellite tip (stellite tipped valves don’t require lash caps), during valve service, the stellite tips can be ground, with caution. You should be able to safely remove approximately a maximum of 0.015″ to 0.020″.

As far as valve seats are concerned, again remember to keep in mind that titanium is a relatively soft material. A traditional cast or hard seat can beat a groove into the valve face, so a nickel bronze seat material is recommended.

KPMI (Kibblewhite Precision Machining Inc.), to cite an example, recommends the use of its Ampco 45 seat material for titanium valve applications. This material’s high 80 percent copper content provides excellent thermal qualities and the 5 percent nickel provides just enough hardness to prevent pounding-out. The remaining aluminum content provides the degree of softness to prevent damage to the titanium valve face.

Titanium valves are extremely lightweight and are designed for applications where valvetrain weight needs to be reduced, for high-rpm and extended high-rpm applications, since titanium valves allow for higher engine speeds and will accommodate highly aggressive camshaft profiles. The lighter weight contributes to minimized wear on rocker arms and improved valve spring life. As valve weight is reduced, lighter springs can be used. As spring force is reduced, this reduces frictional loads between the lifters and cam lobes, so the use of titanium valves offer both higher engine speeds, quicker engine acceleration and reduced friction throughout the valvetrain. While lighter weight and the resulting ability to achieve higher engine speed is of obvious benefit in any form of racing, the ability of the engine to produce quicker acceleration is extremely beneficial in a drag-racing application.

It should be obvious that titanium valves are designed for higher engine speeds, which is fine for higher top-end power. However, for extreme temperature situations (blown, turbo, nitro engines), titanium may not be the ideal choice.

Also, for many street applications, titanium may not be a good choice for an engine that doesn’t need to rev as highly, and for an engine that will be buttoned up and not torn down and serviced regularly. In other words, it’s probably best to reserve the use of titanium for naturally aspirated race or inlet-side forced induction applications where valvetrain weight and sustained high-rpm use is paramount.

INCONEL VALVES

Inconel is a registered trademark of Special Metals Corp., referring to a family of nickel-based superalloys. Inconel alloys are oxidation- and corrosion-resistant materials designed for use in high-heat environments. Inconel retains strength over a wide temperature range. As opposed to steel or aluminum, Inconel doesn’t creep (change dimension) as much under high heat use. Inconel is commonly used in high-stress aircraft applications such as high-speed airframe and jet engine components.

Five grades of Inconel are in common use, including 600, 625, 690, 718 and 939. As an interesting side note, a special Inconel X material was used in the makeup of the skin for the legendary X-15 rocket plane.

Basically, the benefits of Inconel include light weight, extreme resistance to temperature, high strength and resistance to thermal dynamics.
Inconel alloy makeup (depending on the specific alloy mix) can include carbon, manganese, silicon, phosphorous, sulfur, nickel, cobalt, chromium, iron, aluminum, molybdenum, titanium, boron and copper, with the heaviest material concentration accounted for with nickel and chromium.

Inconel valves offer extremely high thermal resistance and are designed for high heat applications as found in turbocharged, supercharged and nitrous applications.

NIMONIC 90 VALVES

Nimonic is a nickel-chromium alloy. A specific grade of this material, Nimonic 90, is used by some makers for producing high performance valves. Nimonic 90 is a “super” alloy comprised of nickel-chromium-cobalt, which offers high strength and especially an ability to withstand extremely high temperatures, reportedly well within the 2,000 degree F range, without distortion. This material is also widely used in aerospace industries for applications such as valves in turbo motors, and blades and discs in gas turbines. Manley reports that it’s seen success in extreme applications such as nitromethane and high-boost turbo applications such as multiple-turbo tractor-pull engines.

STELLITE

Stellite is a hard coating applied to valve stem tips and faces to provide a hard surface to minimize wear. Stellite alloy is a non-magnetic and non-corrosive cobalt-chromium alloy that may also contain a tungsten element. It resists embrittlement and annealing at higher temperatures. Interestingly, the term Stellite was derived from the name of a Scottish racehorse. Stellite is often applied to steel or stainless steel valves.

Tags: INCONEL, KPMI, NIMONIC, STAINLESS STEEL VALVES, STELLITE, TITANIUM VALVES, VALVE MATERIALS, VALVES, XCELDYNE

 Print This Post