The crankshaft is one of the most critical components in an internal combustion engine. It endures a variety of mechanical stresses, including bending, torsion, shear, and tension, as well as impact loads and inertial forces during operation. These conditions place extremely high demands on the material's tensile strength, fatigue resistance, hardness, and wear resistance. While various manufacturing techniques can be used to enhance mechanical properties, selecting the right material remains crucial to ensure performance, reliability, and longevity.
Material selection for crankshafts depends on several factors, such as service safety, quality stability, process feasibility, surface strengthening potential, lightweight design, energy efficiency, environmental considerations, and cost-effectiveness. In practice, two main types of materials are commonly used: forged steel and ductile iron. Each has its own advantages depending on the application and power requirements.
Ductile iron, also known as nodular cast iron, has become a popular choice due to its ability to achieve complex shapes through casting, which reduces machining time and allows for more efficient lightweight designs. With advancements in casting technology, ductile iron can now match or even exceed the mechanical properties of some carbon steels. For example, QT800-6, a spheroidal graphite cast iron developed by Yuchai, offers high elongation, impact toughness, and mechanical strength. This material can be further enhanced through heat treatments like quenching and rolling, resulting in superior fatigue resistance and wear performance.
In contrast, forged steel crankshafts are typically preferred for high-power engines, especially those exceeding 160 kW. They offer higher strength and better fatigue resistance under extreme operating conditions, making them more suitable for applications with high burst pressures, often reaching up to 15 MPa. However, forged steel crankshafts are more expensive to produce and require more energy-intensive heat treatment processes.
While ductile iron is becoming increasingly viable for many applications, it still faces limitations in very high-stress environments. For this reason, forged steel remains the material of choice in heavy-duty and high-performance engines. The development of advanced casting techniques and improved material properties continues to push the boundaries of what ductile iron can achieve, but for the most demanding applications, forged steel is still the standard.
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