Material of crankshaft

Most marine large-scale low-speed diesel engines use semi combined crankshafts. Medium carbon high-quality carbon structural steels 30, 40 and 45 are often used for main journals, and cast steel is often used for cranks, such as zg270-450, ZG310-570 and zg25mnv. Generally, medium and high-speed diesel engines use integral crankshafts, medium carbon high-quality carbon structural steels or alloy quenched and tempered steels, or ductile iron ot600-03, ot700-02, etc.

The manufacturing method of crankshaft burr depends on the size, material and production batch of crankshaft. Die forging is adopted for small crankshafts with large production volume; Large and medium-sized integral crankshafts adopt free forging; The production quantity of large semi combined crankshafts is relatively small. The cranks and main journals can be made of cast steel or forged steel blanks. After machining, they can be connected into an integral crankshaft by the red sleeve process or the hydraulic sleeve process. Ductile iron crankshaft is formed by casting.

Heat treatment of crankshaft

The performance requirements of the crankshaft are met by heat treatment. In order to make the crankshaft have comprehensive mechanical properties, the structure of the crankshaft material must be s' or S. Common heat treatments include the following:

① Carbon steel crankshaft forgings shall be normalized and annealed after rough machining;

② Alloy steel crankshaft forgings shall be annealed first, and then quenched and tempered after rough machining. For those requiring high hardness on the journal surface, surface quenching or nitriding treatment can be adopted;

③ The cast steel crank shall be subject to two times of normalizing treatment and low-temperature annealing treatment, or high-temperature diffusion annealing + normalizing treatment + tempering treatment.

④ The nodular cast iron crankshaft is normalized and annealed after rough machining. The hardness is hb220 ~ 229. When the hardness of the journal surface is required to be high, high-frequency quenching shall be carried out.

Generally speaking, due to poor hardenability of carbon steel crankshaft, final normalizing can replace quenching and tempering, which can also eliminate structural defects and improve cutting performance. Alloy steel mainly adopts quenching and tempering to obtain tempered sorbite to meet the requirements of comprehensive mechanical properties.

Surface strengthening treatment of crankshaft journal

The surface strengthening treatment of crankshaft journal can greatly improve the wear resistance and fatigue strength of crankshaft. The main methods adopted are:

① Nitriding treatment, commonly used gas nitriding, gas soft nitriding, ion nitriding, etc;

② Shot peening and rolling strengthening can improve the fatigue strength of crankshaft.