40Cr steel - alloy structure steel
40Cr belongs to GB3077 "alloy structure steel". The carbon content of 40Cr steel is 0.37% to 0.44%, which is slightly lower than that of 45 steel. The content of Si and Mn is comparable, containing Cr0.80% to 1.10%. In the case of hot rolling supply, these 1% of Cr basically does not work, and their mechanical properties are roughly the same. Since the price of 40Cr is about half that of that of 45 steel, 40Cr is not used for economic reasons.
Conditioning treatment of 40Cr steel: The main function of Cr in heat treatment is to improve the quenchability of steel. Due to the improvement of quenchability, the strength, hardness, impact toughness and other mechanical properties of 40Cr after quenching (or tempering) treatment are also significantly higher than those of 45 steel, but also because of the strong quenchability, the internal stress of 40Cr is also greater than that of 45 steel during quenching. Under the same conditions, the fracture tendency of the workpiece of 40Cr material is also higher than that of 4. 5 The workpiece of steel material is large. Therefore, in order to avoid cracking of workpieces, 40Cr quenching mostly uses oil with low thermal conductivity as the quenching medium (sometimes double-liquid quenching method, commonly known as water quenching oil cold), while 45 Gang uses water with higher thermal conductivity as the quenching medium. Of course, the choice of water and oil is not absolute, and it is also closely related to the shape of the workpiece. Simple-shaped 40Cr parts can also be quenched with water, while 45 steel parts with complex shapes may have to use oil quenching or even salt baths.
The tempering tempering of 40Cr workpieces is stipulated in various parameter process cards. In practice, we experience:
(1) After the 40Cr workpiece is quenched, oil cooling should be adopted. The quenchability of 40Cr steel is good, the cooling energy can be hardened in the oil, and the deformation and cracking tendency of the workpiece is small. However, under the condition of tight oil supply, small enterprises can quench workpieces with incomplex shapes in water, and no cracks have been found, but operators should strictly grasp the temperature of incoming and outgoing water based on experience.
(2) The hardness of the 40Cr workpiece is still high after tempering, and the second tempering temperature will increase by 20~50 °C. Otherwise, it will be difficult to reduce the hardness.
(3) After high-temperature tempering of 40Cr workpieces, complex shapes are cooled in oil and simply cooled in water, in order to avoid the influence of type II tempering brittleness. After tempering and cooling, the artefact will be applied to eliminate stress if necessary.
The maximum hardness that can be achieved after heat treatment of medium carbon steel is about HRC55 (HB538), and σb is 600 to 1100MPa. Therefore, among various applications at the medium strength level, medium carbon steel is most widely used. In addition to building materials, it is also widely used to manufacture various mechanical parts. As long as the temperature of medium carbon steel is enough and the insulation time is enough, it is generally possible to reach this hardness value. If it is not deformed, it is impossible. The first is to have a processing margin, and then be processed on the grinder, and the other is surface quenching.
