The effect of Cr in 40Cr steel is mainly to improve the hardenability of 40Cr steel, and slightly increase the tempering stability of 40Cr steel at low and high tempering temperature. 40Cr steel has tempering brittleness and should be cooled quickly after high temperature tempering.
Tempering temperature of 40Cr steel is lower than that of 35CrMo steel after quenching and tempering to the same hardness (or the same tensile strength Rm), so plasticity and toughness of 40Cr steel are worse than that of 35CrMo steel.
The role of Cr in 35CrMo steel is the same as that in 40Cr steel. The effect of adding Mo into 35CrMo steel is to further improve the hardenability and tempering stability of 35CrMo steel, eliminate or reduce the second kind of tempering brittleness, and refine the grain. Therefore, 35CrMo steel has higher hardenability and better tempering stability than 40Cr steel, and its grain growth tendency is small when quenched and heated, and there is no tempering brittleness tendency. Small parts can be air-cooled after tempering, and large parts should be quickly cooled after tempering, such as water cooling or oil cooling. Because Mo is a rare high melting point metal, the price of 35CrMo steel is higher than that of 40Cr steel.
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