45# steel belongs to high-quality carbon structural steel, and 40Cr steel belongs to GB3077 alloy structural steel. The difference is after heat treatment. The main role of Cr in heat treatment is to improve the hardenability of steel. Due to the increase in hardenability, the mechanical properties of 40Cr steel after quenching (or quenching and tempering), such as strength, hardness, and impact toughness, are also significantly higher than those of 45# steel. But also because of the strong hardenability, the internal stress of 40Cr during quenching is greater than that of 45# steel. Under the same conditions, the cracking tendency of 40Cr material is greater than that of 45# steel.
Therefore, in order to avoid cracking of the workpiece, the oil with lower thermal conductivity is mostly used as the quenching medium during 40Cr quenching (sometimes the double liquid quenching method is also used, as the saying goes, water quenching oil cooling). And 45# uses water with higher thermal conductivity as the quenching medium.
After quenching and low temperature tempering, the 40cr alloy plate is used to manufacture parts with heavy load, low impact and wear resistance, and the solid thickness of the cross section is below 25mm, such as worms, spindles, shafts, collars. After quenching and tempering and high-frequency surface quenching, 40cr steel is used to manufacture parts with high surface hardness and wear resistance without great impact, such as gears, sleeves, shafts, main shafts, crankshafts, spindles, pins, Connecting rods, screws, nuts, intake valves.