45 steel belongs to GB699 (high quality) carbon structural steel, and 40Cr belongs to GB3077 alloy structural steel. The differences are after heat treatment.
The main role of chromium in heat treatment is to improve the hardenability of steel. Due to the improvement of hardenability, the mechanical properties such as strength, hardness and impact toughness of 40Cr quenched (or quenched and tempered) are significantly higher than 45 steel, but also due to the strong hardenability, the internal stress during quenching of 40Cr is greater than 45 steel. Under the same conditions, the cracking tendency of 40Cr steel is greater than that of 45 steel. Therefore, in order to avoid workpiece cracking, 40Cr quenching mostly uses oil with low thermal conductivity as the quenching medium, while 45 steel uses water with high thermal conductivity as the quenching medium.
45 steel is a high-quality carbon structural steel with a carbon content of 0.42-0.50%, a tensile strength of 610 MPa, and a yield point of 360 MPa. It is used for general shaft parts. 40Cr steel is an alloy structural steel with a carbon content of 0.37-0.45%, a chromium content of 0.8-1.1%, a tensile strength of 1000 MPa, and a yield point of 800 MPa. It is often used in mold manufacturing and has good hardness and wear resistance.
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