How to select the right steel for engine and transmission components

Selecting the right steel for engine and transmission components is a critical decision for automotive manufacturers, especially for Indian MSMEs focused on producing durable and high-performing parts. Given the harsh operating conditions these components endure—high temperatures, mechanical loads, friction, and wear—the choice of steel significantly impacts performance, reliability, and lifespan. 

This article outlines the key considerations and guidelines for selecting the appropriate steel for engine and transmission components. 

1. Understand the operating environment 

Engine and transmission components operate under extreme conditions, including: 

• High temperatures: Exhaust valves, piston rings, and cylinder liners are exposed to intense heat. The steel used must have excellent thermal stability to resist deformation and retain strength at high temperatures. 

• Mechanical stress: Crankshafts, connecting rods, and transmission gears endure high loads and cyclic stresses. The steel needs to have high tensile strength and fatigue resistance to prevent failure under these demanding conditions. 

• Friction and wear: Components such as gears, camshafts, and valve seats experience constant friction. Choosing steel with high wear resistance is crucial for maintaining the functionality and longevity of these parts. 

The selection process must begin by considering these factors to ensure the steel can withstand the specific stresses faced by each component. 

2. Assess the required mechanical properties 

Each engine and transmission component has unique mechanical demands. The key properties to focus on include: 

• Strength: High tensile strength is essential for components like crankshafts, connecting rods, and transmission shafts, which face intense mechanical loads. 

• Toughness: The steel must absorb energy without fracturing, especially for parts like gears and camshafts that face impact loads. 

• Fatigue resistance: Repeated stress can lead to fatigue failure. Selecting steel with good fatigue resistance is critical for components such as crankshafts and connecting rods. 

• Wear resistance: Components subjected to constant friction, like gears and valve seats, require steel that resists wear and maintains performance over time. 

• Thermal stability: For parts exposed to high temperatures, such as exhaust valves and piston rings, steel with excellent heat resistance and minimal thermal expansion is crucial. 

3. Consider heat-treatable steels for enhanced properties 

Many engine and transmission components are made from heat-treatable steels. These steels undergo controlled heating and cooling processes to enhance their mechanical properties, such as hardness, toughness, and wear resistance.

Common heat treatments include: 

• Quenching and tempering: this process increases both hardness and toughness, making it ideal for crankshafts, connecting rods, and gears. 

• Surface hardening: techniques like carburising and nitriding create a hard outer layer while keeping the core tough. This is especially useful for gears, camshafts, and valve seats, which need wear resistance on the surface but toughness in the core. 

• Normalising: normalising improves the strength and uniformity of steel, making it suitable for components like cylinder liners and transmission shafts. 

Selecting a heat-treatable steel ensures that the component can withstand the operational stresses of modern engines and transmissions.

4. Select the appropriate alloy steel 

Alloying steel with other elements, such as chromium, molybdenum, and nickel, can enhance its properties, making it better suited for specific applications. Here are some common alloy steels used in engine and transmission components: 

• a. Chromium-molybdenum (Cr-Mo) steel 

This steel is widely used for high-stress components such as crankshafts, connecting rods, and gears. Chromium improves hardness and wear resistance, while molybdenum increases strength and toughness. Cr-Mo steel also has good heat resistance, making it ideal for components subjected to high temperatures and mechanical loads. 

• b. Nickel-chromium (Ni-Cr) steel 

Ni-Cr steel offers excellent toughness and fatigue resistance, making it a suitable choice for gears, camshafts, and transmission shafts. Nickel improves toughness, while chromium increases wear resistance. Ni-Cr steel also responds well to heat treatment, further enhancing its mechanical properties. 

• c. Stainless steel 

For components exposed to corrosion and high temperatures, such as exhaust valves and turbocharger components, stainless steel is a preferred choice. It offers excellent corrosion resistance and maintains its strength and durability at elevated temperatures. Types like 316L stainless steel are commonly used in these applications. 

• d. Carbon-manganese (C-Mn) steel 

C-Mn steel is known for its excellent wear resistance and toughness. It is often used in components like gears and cylinder liners that require a balance of strength and wear resistance. It also responds well to heat treatments, which can further improve its mechanical properties. 

5. Evaluate cost and availability 

While high-alloy steels offer superior performance, MSMEs must also consider the cost-effectiveness of the materials. Carbon-manganese steels or lower-alloy steels may provide an adequate balance of performance and cost for many applications.

Moreover, ensuring that the steel grades are readily available through reliable suppliers, such as the JSW One platform, can help MSMEs streamline procurement and reduce lead times. 

6. Review specific applications of steel in engine and transmission components 

• Crankshafts 

Crankshafts must endure high torsional loads and cyclic stresses. Cr-Mo steel or Ni-Cr steel is typically used for crankshafts due to their strength, toughness, and fatigue resistance. These steels are often heat-treated to enhance hardness and durability. 

• Connecting rods 

Connecting rods face significant compressive and tensile forces, making C-Mn steel or Cr-Mo steel ideal for their construction. Heat treatment further increases their fatigue resistance and ensures long-term performance under high loads. 

• Gears 

Transmission gears are constantly exposed to friction, torque, and impact. Surface-hardened Ni-Cr steel or C-Mn steel is commonly used, as it offers excellent wear resistance while maintaining core toughness. 

• Valves and valve seats 

Exhaust valves are subjected to extreme temperatures and corrosive gases, making stainless steel the best choice. 316L stainless steel is a popular option, known for its corrosion resistance and ability to withstand high temperatures. For valve seats, surface-hardened steel ensures a durable seal and prolonged lifespan. 

• Cylinder liners 

Cylinder liners need to resist wear from friction and the high temperatures generated by combustion. C-Mn steel, with its excellent wear resistance and toughness, is frequently used for this application. Heat-treated C-Mn steel ensures a long-lasting liner that minimises wear over time.