Heat treatment process of alloy structural steel 40Cr steel
40Cr belongs to GB3077 "alloy structural steel". The carbon content of 40Cr steel is 0.37% ~ 0.44%, which is slightly lower than that of 45 steel, and the content of Si and Mn is equivalent, containing Cr0.80% ~ 1.10%. In the case of hot rolling supply, this 1% Cr basically does not work, and the mechanical properties of the two are roughly equivalent.
Tempering treatment of 40Cr steel: The main role of Cr in heat treatment is to improve the hardenability of steel. Due to the improvement of hardenability, the strength, hardness, impact toughness and other mechanical properties of 40Cr after quenching (or tempering) treatment are also significantly higher than 45 steel, but also due to the strong hardenability, the internal stress of 40Cr during quenching is larger than 45 steel, and the cracking tendency of 40Cr material is larger than that of 45 steel material under the same conditions.
Therefore, in order to avoid cracking of the workpiece, 40Cr quenching mostly uses oil with low thermal conductivity as the quenching medium (sometimes also uses double liquid quenching method, commonly known as water quenching oil cold), while 45 steel uses water with high thermal conductivity as the quenching medium. Of course, the choice of water and oil is not absolute, and the shape of the workpiece is also closely related. Simple 40Cr parts can also use water quenching, while complex 45 steel parts may have to use oil quenching or even salt bath.
40Cr workpiece tempering quenching and tempering, various parameters of the process card are specified, in actual operation:
(1) The 40Cr workpiece should be cooled by oil after quenching, the hardenability of 40Cr steel is better, the cooling in oil can be hardened, and the deformation and cracking tendency of the workpiece is small. In the case of tight oil supply, the workpiece with an uncomplicated shape can be quenched in water, and no cracking is found, but the temperature of water and water must be strictly mastered.
(2) The hardness of 40Cr workpiece is still high after tempering, and the second tempering temperature should be increased by 20~50 ° C, otherwise, the hardness is difficult to reduce.
(3) After the high-temperature tempering of 40Cr workpieces, the complex shape is cooled in oil, and the simple one is cooled in water, in order to avoid the influence of the second type of tempering brittleness. Temper the workpiece after rapid cooling, and then apply stress relief treatment if necessary.
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