What Is The Best Material For A Crankshaft?

Crankshaft Materials Overview

Crankshafts are key parts of engines. The material used affects performance, durability, and cost. The right material choice depends on the engine type, operating conditions, and performance needs.

Steel Alloys: The Backbone of Crankshafts

Crankshafts are made from various steel alloys because of their strength, toughness, and wear resistance. Here’s a look at the most common steel alloys used:

• Carbon Steel: A simple, cost-effective choice with low carbon content (under 0.3%). It has good strength and ductility but lacks hardness and wear resistance. It’s best for low-stress, low-speed applications.
• Alloy Steel: These steels include elements like manganese, chromium, molybdenum, and nickel, improving toughness, hardness, and corrosion resistance. Common alloy steels used for crankshafts are:
  • SAE-4340: A versatile alloy known for its fatigue resistance. It’s ductile and impact-resistant. This steel is used in high-performance applications, offering tensile strength between 100,000 and 110,000 psi.
  • 32-CrMoV-13 (32CDV13): Known for its exceptional strength and fatigue resistance, this alloy works well for extreme applications like aerospace and racing engines. It maintains top performance in intense conditions.

Key Alloy Compositions

• 4340: Contains 0.4% carbon, 0.75% manganese, and 1.85% nickel. It is alloyed with nickel to achieve deep and consistent hardness.
• 32-CrMoV-13: Made up of 0.34% carbon, 3% chromium, and 0.9% molybdenum. This alloy provides high strength and resists tempering and softening.

Performance Comparison: SAE-4340 vs 32-CrMoV-13

• SAE-4340: Known for its fatigue resistance and ability to handle high RPMs, SAE-4340 is ideal for performance engines that need both strength and durability.
• 32-CrMoV-13: This alloy outperforms SAE-4340 in fatigue resistance, making it perfect for endurance applications requiring the highest performance level.

Forged vs Cast Crankshafts

• Forged Crankshafts: Made from steel bars and roll-forged for extra strength. These crankshafts are lighter, more compact, and provide better damping. Vanadium micro-alloyed steels are often used because of their strength and durability.
• Cast Crankshafts: Typically made from ductile iron, these crankshafts are more cost-effective. They’re used in engines with lower performance needs and are common in standard production engines.

Other High-Strength Alloys

• 300M (AMS 6419): A version of 4340 with improved strength (285 ksi) and excellent fatigue resistance. It’s used in components that face high stress.
• EN-30B: An air-hardening nickel-chrome-moly alloy used in high-end billet crankshafts. It’s known for its high-strength impact resistance and is used in heavy-duty applications like rock drilling.

Choosing the right Crankshaft material is essential for ensuring top engine performance and longevity, especially for high-performance or extreme-condition applications.

Common Materials Used for Crankshafts

Crankshafts are made from different materials, each chosen for its performance qualities like strength, durability, and resistance to wear. The most common materials used are carbon steels, alloy steels, and forged alloys. Each material offers unique benefits depending on the engine type and application.

Carbon Steels

• Carbon Content: Less than 0.3%.
• Properties: Carbon steels provide good strength and flexibility. However, they are not as hard or wear-resistant as other materials.
• Applications: Best suited for low-stress, low-speed engines, such as those in agricultural and industrial machines.

Alloy Steels

• Carbon Content: Ranges from 0.4% to 0.6%.
• Added Elements: Manganese, chromium, molybdenum, nickel, silicon, cobalt, vanadium, and sometimes aluminum and titanium.
• Properties: Alloy steels are stronger, tougher, more resistant to corrosion, and better at handling fatigue. They also maintain good flexibility and impact resistance, even in high-strength conditions.

Examples of Alloy Steels:

• SAE-4340: A nickel-chrome-moly alloy with great strength and resistance to fatigue. It’s commonly used in high-performance applications.
  • Chemical Composition:
    • Carbon: 0.4%
    • Manganese: 0.75%
    • Chromium: 0.82%
    • Nickel: 1.85%
    • Molybdenum: 0.25%
    • Silicon: 0.25%
    • Vanadium: 0.07%
• 32-CrMoV-13: A strong French alloy known for its toughness and resistance to softening.
  • Chemical Composition:
    • Carbon: 0.34%
    • Manganese: 0.55%
    • Chromium: 3%
    • Nickel: <0.30%
    • Molybdenum: 0.9%
    • Silicon: 0.25%
    • Vanadium: 0.28%
• EN-30B: A high-quality nickel-chrome-moly alloy used in billets for high-performance crankshafts.
  • Chemical Composition:
    • Carbon: 0.3%
    • Manganese: 0.55%
    • Chromium: 1.2%
    • Nickel: 4.15%
    • Molybdenum: 0.3%
    • Silicon: 0.22%

Forged vs. Cast Crankshafts

Crankshafts can be made using either forging or casting methods. Forged crankshafts are preferred for high-performance engines because of their superior strength. Cast crankshafts are more common in lower-cost, lower-stress engines.

Cost and Availability

Material choice also depends on the cost and availability of alloys. High-performance alloys like SAE-4340 and EN-30B offer better mechanical properties but are more expensive. Manufacturers often balance performance with cost based on the engine’s needs.

Specific Applications

The best material for a crankshaft depends on factors like engine power, operating temperature, fuel type, and design. For high-performance engines or those under heavy stress, stronger alloys with better fatigue resistance are typically chosen.

Specific Alloy Steels for Crankshafts

Crankshafts are key parts of engines that need materials with high strength, fatigue resistance, and impact resistance. SAE-4340 is a widely used alloy steel for crankshafts because of its strong mechanical properties.

SAE-4340 Alloy Steel for Crankshafts

SAE-4340 is known for its high strength and fatigue resistance, which makes it a top choice for crankshafts. This steel also offers good ductility and impact resistance, especially at high strength levels. These qualities are important for crankshafts, as they endure repeated stresses and heavy loads during engine operation.

Chemical Composition of SAE-4340

• Carbon: 0.40-0.43%
• Manganese: 0.65-0.90%
• Silicon: 0.15-0.35%
• Chromium: 0.70-0.90%
• Nickel: 1.65-2.00%
• Molybdenum: 0.20-0.30%

These elements contribute to SAE-4340’s overall strength, toughness, and resistance to corrosion. These qualities help keep the crankshaft working well under tough conditions.

Mechanical Properties

• Tensile Strength: 108 – 161 ksi
• Yield Strength: 68.8 – 125 ksi
• Elongation: 12.2 – 22.0%

These properties show that SAE-4340 can handle a lot of stress before breaking, making it a reliable material for crankshafts in heavy-duty engines.

Heat Treatment for SAE-4340

SAE-4340 is usually used as through-hardened steel, with heat treatments like quenching and tempering to reach a hardness above HRC 50. It can also be carburized or induction-hardened for specific uses, but it is not recommended for post-heat treatment machining due to its increased hardness.

Applications in Crankshafts

SAE-4340 is commonly used in industries like aerospace, automotive, and oil & gas, where high-strength crankshafts are needed. Its toughness and fatigue resistance make it perfect for high-stress engine parts such as crankshafts, connecting rods, and other structural components. SAE-4340 is also used in machinery, aerospace, and pressure vessels, which all require reliability and strength.

Comparison to Other Alloys

When compared to other alloy steels like 4140 and 4130, SAE-4340 generally offers superior hardness and is regarded as the standard for ultra-high-strength alloys used in crankshafts and similar applications.

Industry Use

Thanks to its excellent properties, SAE-4340 is found in aircraft components, automotive engines, and machinery manufacturing. Its ability to handle high stress and resist fatigue makes it essential in industries that need reliability and top performance.

Crankshaft Manufacturing Processes and Types

Crankshafts are an essential part of any engine, converting the linear motion of Pistons into rotational motion. The material and manufacturing process chosen for a crankshaft affect its strength, durability, and performance. Below are the main types of crankshafts and the processes used to create them:

Cast Iron Crankshafts

Cast iron crankshafts are the most affordable option but offer the lowest strength and ductility.

• Tensile Strength: 65,000-80,000 psi
• Applications: Best suited for stock or mildly modified street applications.
• Characteristics: Cast iron crankshafts are durable but lack the toughness needed for high-performance or heavy-duty engines.

Nodular Iron Crankshafts

Nodular iron crankshafts are a stronger version of cast iron, offering better properties.

• Tensile Strength: Up to 100,000 psi
• Applications: Used in applications where more strength than cast iron is needed, such as modified street engines.
• Characteristics: The addition of magnesium forms graphite nodules, improving the material’s strength and ductility.

Cast Steel Crankshafts

Cast steel crankshafts offer better strength than cast iron, but they still do not match the performance of forged or billet crankshafts.

• Tensile Strength: 65,000-100,000 psi, depending on the alloy.
• Applications: Suitable for applications up to 500 horsepower.
• Characteristics: Cast steel is stronger than cast iron but lacks the uniform grain structure found in forged or billet crankshafts, resulting in lower strength.

Forged Crankshafts

Forged crankshafts are known for their superior strength and durability.

• Tensile Strength: 140,000-150,000 psi for high-quality forged crankshafts.
• Materials: Commonly made from 4130 and 4340 steel alloys, known for their strength and toughness.
• Manufacturing Process: Forging involves heating a large billet to around 2,500-2,700 degrees Fahrenheit and shaping it using high-pressure dies. This process compresses the grain structure, resulting in a more uniform and stronger crankshaft.
• Post-Forging Treatments: These crankshafts are often shot peened to reduce surface stress, heat-treated to increase strength, and nitrided for better wear resistance.

Billet Crankshafts

Billet crankshafts are machined from a solid steel billet and are well-known for their precision and strength.

• Tensile Strength: Comparable to or stronger than forged steel.
• Applications: Primarily used in high-performance and racing applications.
• Manufacturing Process: The crankshaft is machined from a solid billet using CNC equipment. This allows for precise design with minimal post-machining adjustments, ensuring high strength and durability.
• Material Quality: High-strength steels like SAE-4340 (AMS-6414) are commonly used because of their excellent fatigue resistance and impact strength.

These different manufacturing methods and materials make each type of crankshaft suited to specific engine needs, from standard street vehicles to high-performance racing engines.

Factors Influencing Material Choice

Choosing the right material for a crankshaft is crucial for its performance and durability. Several factors determine the best choice for each engine.

Power Output and Torque

The crankshaft must be strong enough to handle the engine’s power and torque. High-performance engines often use alloy steels like 4130, 4140, and 4340. These alloys have a tensile strength of 100,000 to 110,000 psi, making them ideal for powerful engines.

Engine Speed and Load

Materials must withstand different engine speeds and loads. For low-speed engines, welded crankshafts made from hardened low-alloy Chrome-Molybdenum steel are often the best choice. This material resists fatigue well and has lower inertia, which is ideal for engines with slower speeds.

Fuel Type and Quality

The type of fuel used can affect the crankshaft material. Engines running on fuels with high ethanol content need materials that resist corrosion and wear. The material must be compatible with these fuels to ensure long-lasting durability.

Operating Temperature and Environment

Crankshaft materials must perform well at high temperatures and when exposed to corrosive substances. For demanding applications, air-hardening nickel-chrome-moly alloys like EN-30B are often used. These alloys provide strong impact resistance, high strength, and stability in tough environments.

Cost and Availability

Material costs and availability are also important factors. Carbon steel is the most affordable and widely available, but it is only suitable for low-stress, low-speed applications. Alloy steels are more expensive but offer better performance, especially in high-performance engines.

Design and Geometry

The crankshaft’s design and geometry affect material choice. For example, the journal diameter and overall design impact the material selection. Welded crankshafts, which allow larger journal diameters, can reduce pressure on the Bearings, making them a good option for advanced engine designs.

In Conclusion, selecting the ideal crankshaft material depends on factors like engine power, speed, fuel type, and design. Materials like carbon steel, alloy steels, and specialized alloys such as EN-30B are chosen based on specific engine needs.

Choosing the best material for a crankshaft depends on the engine’s specific needs and its intended use. Materials like SAE-4340, EN-30B, 4330-M, and 300M are strong options for high-performance crankshafts because they offer excellent strength, fatigue resistance, and impact resistance. These materials work best in high-performance engines and racing applications, where strength and durability are crucial.

Key Material Options for Crankshafts

• SAE-4340: This material is perfect for ultra-high-strength needs, with a tensile strength of around 140,000 psi and great fatigue resistance. It’s commonly used in racing applications.
• EN-30B: Known for its high strength and impact resistance, making it ideal for stressed components.
• 4330-M: Offers high hardness and impact resistance, making it ideal for tough-duty crankshafts.
• 300M: A modified version of 4340, offering very high strength (285 ksi) and fatigue properties. It’s preferred in top-tier racing applications.

Material Suitability by Application

• High-Performance Engines: SAE-4340, EN-30B, and 300M are ideal for their balance of strength and fatigue resistance.
• Racing Crankshafts: Forged steel and billet steel, such as 4130/4140 chromoly and SAE-4340, are the go-to materials for their toughness and durability.
• Stock and Modified Street Engines: Cast steel and nodular iron perform well for stock or moderately modified engines.

The best material for a crankshaft depends on the engine’s horsepower, usage intensity, and durability requirements. Forged steel and billet steel are the top choices for high-performance and racing applications.