What Are Engine Valves Made Of?

Engine valves are constructed from a variety of materials to withstand the demanding conditions inside a combustion engine. The most durable and heat-resistant valves are made from nickel-based alloys and ceramics, while the most widely used valves are stainless steel due to its optimal balance of properties and cost.

This article explores the different metals, ceramics and composites used for engine valves. It discusses the key factors considered when selecting valve materials such as heat resistance, strength and cost.

The manufacturing processes of forging, casting and powder metallurgy are also examined. Stainless steel alloys prove to be the best compromise between performance, durability and affordability for most intake and exhaust valves.

The article also covers the optimal materials for engine valve seats such as hardened steel alloys and Stellite.

Materials used in engine valves

Metals

Stainless steel and nickel alloys are the most widely used metals for engine valves.

• Stainless steel valves have excellent strength at high temperatures. The most common stainless steel alloys used are Silchrome-1 (11% chromium content) and Nimonic (20% chromium, 2.5% titanium content) which can withstand temperatures up to 1000°C. Stainless steel valves are durable but relatively heavy.
• Nickel-based alloys like Inconel offer improved heat resistance over stainless steel. Inconel valves can operate at temperatures exceeding 1090°C making them well suited for high performance engines. Inconel has good corrosion resistance but is difficult to machine.
• Titanium valves are around 45% lighter than steel with excellent strength. The lower weight allows titanium valves to follow aggressive cam profiles for better engine breathing. Ti-6Al-4V alloy combines lightness, strength and heat resistance up to 600°C. However, titanium valves are expensive and wear rapidly.

Ceramics

• Silicon nitride valves have high heat resistance allowing operation up to 1050°C. Silicon nitride is lightweight, around 65% less dense than steel. It has low thermal expansion and resists corrosion. However, ceramic valves are brittle and expensive to manufacture.
• Aluminum oxide ceramic valves can withstand temperatures up to 1200°C. Alumina valves are tough and resistant to wear but have lower strength than silicon nitride. Cost is the main barrier to widespread use.

Composite Materials

• Metal matrix composites like silicon carbide particulate reinforced aluminum alloy offer strength at high temperatures approaching 1000°C. They have lower density than metals but higher manufacturing costs.
• Carbon fiber reinforced plastics like carbon-carbon composites combine lightness with heat resistance above 1000°C. However, they are susceptible to oxidation requiring protective coatings. Cost is again a limiting factor.

Factors considered when choosing materials for engine valves

The key properties considered when selecting valve materials include:

• Strength and durability at high temperatures – Valves must withstand cyclic stresses and fatigue over long operating lifetimes. High strength alloys and ceramics are required.
• Heat resistance – Exhaust valves experience temperatures exceeding 800°C requiring materials that retain strength at high temperatures like Nimonic alloys. Inlet valves operate 100-150°C cooler than exhaust.
• Corrosion resistance – Valve materials must resist corrosion and oxidation from hot exhaust gases containing water vapor, carbon dioxide and sulfur compounds. Stainless steels and nickel alloys provide good corrosion resistance.
• Weight – Lighter valves allow higher engine speeds and more aggressive camshaft profiles. Density should be minimized where possible by using alloys like titanium or technical ceramics.
• Cost – Exotic alloys and composite materials are expensive. High volume production favors affordable materials like stainless steel.

Manufacturing Process

Forging

Most mass produced steel and titanium valves start as forged parts. Hot forging produces valves close to final shape requiring less machining. The grain flow improves strength. Precision die forging achieves tighter tolerances and thinner stems.

Casting

Some manufacturers use investment casting for complex valve geometries. Liquid metal is poured into a ceramic mold to form the valve. Cast titanium and stainless steel valves require extensive machining to finish the components.

Powder Metallurgy

Sintered powder metals compressed into shape can produce intricate or porous valve designs difficult to manufacture otherwise. Powder metallurgy is used for very high performance alloy valves. Ceramic valves are also made from powdered raw materials like silicon nitride.

Best Materials for Engine Valves

For intake valves, heat resistant stainless steel alloys like Silchrome-1 or Nimonic 80A are most commonly used. They offer an optimal balance of properties – heat resistance, strength, corrosion resistance and cost.

Exhaust valves experience higher temperatures and stresses. High performance nickel-chromium alloys like Inconel X-750 provide superior heat resistance but are expensive. Therefore, Silchrome-1 stainless steel with protective coatings or inserts is widely used for exhaust valves in most engines.

More advanced materials like titanium alloys, ceramics or composites are restricted to high end racing engines and specialty applications due to their higher manufacturing costs.

Engine Valve Seat Materials

The valve seat is the area in the cylinder head where the valve meets to form a gas-tight seal. It must withstand impact stresses from cyclic valve closing. Common valve seat materials include:

• Hardened steel alloys – used for compatibility with steel valves. They can be precision machined but have limited heat resistance.
• Stellite alloys – Cobalt or nickel-based alloys with properties between steel and ceramics. They offer high hardness, wear resistance and heat resistance up to 850°C.
• Cemented carbides – Extremely hard materials like tungsten carbide allow minimal valve recession but are brittle.
• Ceramics – Silicon nitride seats have high hardness and heat resistance but cost remains a barrier.

Stellite alloys provide the best combination of hardness, heat resistance and machinability for most automotive applications. Hardened steel seats are also very common.