The crankshaft is a critical component in an engine, responsible for converting the linear motion of the pistons into rotational motion. It experiences high impact loads, inertial forces, and alternating stresses such as bending, torsion, shear, and tension during operation. Because of these harsh working conditions, the crankshaft must meet extremely high requirements in terms of tensile strength, fatigue resistance, hardness, and wear resistance. Although various material processing techniques can be used to achieve desired mechanical properties, the final selection must still satisfy all operational demands.
Material selection for crankshafts depends on factors like service safety, quality stability, process feasibility, surface strengthening potential, lightweight design, energy efficiency, environmental protection, and cost-effectiveness. There are different criteria depending on the operating conditions of the engine.
Currently, crankshaft materials are mainly divided into two categories: forged steel and ductile iron. Casting allows for more complex shapes, which is beneficial for lightweight design and reduces machining allowances as casting technology improves. Both ductile iron and steel can undergo similar heat treatments, including surface hardening, to enhance fatigue strength, hardness, and wear resistance. Ductile iron crankshafts are less sensitive to stress concentration, making them widely used in many applications globally.
However, for high-power engines above 160 kW, ductile iron may not be sufficient, and forged steel crankshafts are generally preferred due to their superior mechanical properties.
**Common Crankshaft Material Types**
**Ductile Iron Crankshaft**
Since its introduction in 1947, ductile iron has seen significant improvements in strength and ductility. Today, it can reach tensile strengths of 700–900 MPa with elongation rates between 2% and 6%, comparable or even exceeding that of carbon steel. This makes it a viable alternative to forged steel for crankshaft manufacturing.
Cast iron offers advantages in achieving ideal structural shapes, supporting lightweight design, and reducing machining needs. Yuchai has developed a spheroidal graphite cast iron crankshaft called QT800-6, which has been patented and successfully applied in high-strength environments.
QT800-6 (normalized ductile iron) is a new type of crankshaft material developed by Guangxi Yuchai. It features high elongation and impact toughness, offering excellent performance. This material can be strengthened through "shaft quenching + rolling" to achieve higher fatigue strength, even surpassing some quenched and tempered steels.
Traditionally, ductile iron was normalized to improve strength. However, modern advancements allow for pearlitic ductile iron crankshafts that can achieve high mechanical properties directly from the cast state, eliminating the need for normalization. This simplifies the production process, lowers costs, and reduces internal stresses, minimizing deformation risks after surface hardening.
The trend toward using as-cast ductile iron crankshafts reflects technological progress. To ensure quality, strict control over chemical composition and key processes like spheroidization, inoculation, and solidification is essential. A comprehensive production management system is necessary across all stages, from raw materials to final inspection.
**Quenched Steel Crankshaft**
Quenched and tempered steel crankshafts are typically made from high-quality carbon steel, low-alloy steel, or medium-carbon alloy steel, such as 45, 40Cr, or 42CrMo. The heat treatment process involves quenching and high-temperature tempering, which enhances strength, toughness, and overall mechanical performance.
While this method provides excellent results, it is also time-consuming, energy-intensive, and complex. Forged steel crankshafts, though more expensive to process, are often used in lower-power engines. However, in high-power engines and diesel engines where burst pressure can reach up to 15 MPa, ductile iron may not meet the required fatigue strength and reliability. In such cases, only forged steel crankshafts are suitable.
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