Valve Materials. Special Valve Designs

The demands made on a valve include endurance strength at elevated temperatures, wear resistance, resistance to high-temperature corrosion, and oxidation and corrosion resistance.

The standard valve materials are the following:
- Ferritic-martensitic valve steels: X45CrSi93 is the standard choice for monometallic intake valves and is used exclusively as the material for the stem in bimetallic valves. X85CrMoV182 is a higher alloy and is used as an intake valve material where the thermal and mechanical loading does not permit the use of the Cr-Si material.

- Austenitic valve steels: Here the austenitic Cr-Mn steels have proven to be an economical solution. A widely used choice is the X53CrMnNiN21-9 (21-4N) alloy, which is deemed to be the classic exhaust valve material—for hollow valves, too.

- Valve materials with high nickel content: If the Cr-Mn steels no longer satisfy thermal requirements, then a transition to materials with high nickel content is the correct remedy. They are necessary where maximum operational reliability, and that means resistance to spalling and corrosion, are needed (in aviation engines, for racing use, in highly turbocharged diesel engines, and for using heavy oil as the fuel).

Valve steels prepared in a powder metallurgical (PM) process are available as special materials. In this way, material qualities are achieved that have a positive effect on strength and on resistance to hot corrosion.

Heat Treatment. Closely defined heat treatment makes it possible to further improve the technical characteristics of the valve steels. In many cases this can obviate the need for going to higher- quality alloys.

Martensitic valve steels are generally hardened. The hardness and strength of austenitic steels can be boosted by so-called structural (precipitation) hardening.

Surface Finishing. The following techniques may be used:
- Hard chrome plating for the valve stem: The manufacturing process, choice of materials, and operating conditions may make it necessary to chrome plate standard valves at the contact area along the stem. In standard bimetallic valves the chrome layer, from 3 to 7 μm thick, covers both valve materials. Thicker applications of chrome, up to 25 μm, may be employed in truck or industrial engines where there are high load levels or where there is more severe wear.

- Abrasive polishing: In all cases the stem has to be polished whenever the valve is chrome plated in order to remove any chrome nodules still present and to level out any unevenness. Roughness after the polishing operation is a maximum of Ra 0.2 (maximum Ra 0.4 for nonplated), which has a very favorable effect on valve guide wear and thus permits engineering for minimum clearance.

- Nitriding the valves: Bath immersion and plasma nitriding are used. The nitriding layers, approximately 10 to 30 μm in thickness, are extremely hard at the surface (approximately 1000 HV 0.025) and are particularly insensitive to wear. Like chrome-plated valves, immersion-nitrided valves are abrasively polished to finish them.

Special Valve Designs. Racing imposes the severest demands on the valve, and here it is a matter of withstanding extreme loading for relatively short periods of time.

Achieving engine speeds of some 18000 rpm requires a very free-running valve train and a lightweight valve.

The next step toward weight reduction, in addition to adopting hollow valves, is to choose more exotic materials—such as titanium. This material permits components that are about 40% lighter when compared with steel. It must be kept in mind, however, that titanium does not offer very good high-temperature strength.

That is why, when using titanium for exhaust valves, it is essential to ensure particularly effective heat dissipation. This is done with hollow valves in conjunction with seat rings exhibiting high thermal conductivity.

Exhaust Control Valves. Turbocharger regulation valves (overrun control valve):

The overrun control valve (also referred to as a “waste gate”) limits the charging pressure developed by the exhaust turbocharger and, in gasoline engines, can be intermittently exposed to temperatures of about 1000°C; the thermal load in the diesel engine is about 850°C. This is the criterion used when selecting engineering materials.

Diesel engines can usually get along with the 21-4N alloy (X53CrMnNiN21-91), while a material that can withstand high temperatures, such as Nimonic 80A (NiCr20TiAl), is used in gasoline engines. The overrun control valve is secured with screws or rivets. Typical embodiments are shown in Fig. 7-145.

Fig. 7-145. Configurations for overrun control valves

Exhaust gas return (EGR) valve: EGR valves have to cope with temperatures of up to about 800°C. Of the valve materials available for use, the 21-4N alloy (X53CrMnNiN21-9) has been found to be sufficient for this application since the valves are subjected to thermal stress only; they have moderate exposure to corrosive effects and very little mechanical loading.