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What are the Executive Standards of 40Cr steel?

45 steel belongs to gb699 "(high quality) carbon structural steel", and 40Cr steel belongs to gb3077 "alloy structural steel". The difference is after heat treatment. The main function of Cr 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 steel after quenching (or quenching and tempering) treatment are also significantly higher than those of 45 steel.

Chinese executive standard: GB / T3077-2018
American executive standard: ASTM a29 / A29m-04
Japanese executive standard: JIS G4053-2018
German executive standard: EN 10083 / 1-2018
British executive standard: EN 10083 / 1-2018
French executive standard: EN 10083 / 1-2018
Russian executive standard: TOCT 4543-71

However, due to the strong hardenability, the internal stress of 40Cr steel is greater than that of 45 steel during quenching. 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 steel is mostly quenched with oil with low thermal conductivity as the quenching medium (sometimes double liquid quenching method is also used, as the saying goes, water quenching oil cooling), while 45 steel uses water with high thermal conductivity as the quenching medium.