40Cr Steel Vs 45# Steel

40Cr steel 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. The content of Si and Mn is equivalent, and the content of Cr is 0.80% ~ 1.10%. In the case of hot rolling supply, the 1% Cr basically does not work, and their mechanical properties are roughly the same. Because the price of 40Cr is about half of that of 45# steel, 40Cr is not needed for the purpose of economy.
 
45# steel is called in GB, S45C in JIS, 1045 in ASTM; 45# steel is high-quality carbon structural steel, chemical composition: carbon content is 0.42-0.50%, Si content is 0.17-0.37%, Mn content is 0.50-0.80%, Cr Content & It; = 0.25%. Cold and hot processing properties are good, mechanical properties are good, and low price, wide source, so widely used. Its biggest weakness is that the workpieces with low hardenability, large section size and high requirements should not be used.
 
Heat Treatment of 40Cr Steel and 45# Steel
Quenching and tempering 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 mechanical properties of 40Cr allloy after quenching (or quenching and tempering) are significantly higher than that of 45# steel, but also due to the strong hardenability, the internal stress of 40Cr during quenching is larger than that of 45# steel, and the cracking direction of 40Cr material is larger than that of 45# steel under the same conditions. Therefore, in order to avoid workpiece cracking, oil with low thermal conductivity is mostly used as quenching medium (sometimes double liquid quenching method is also used, as the saying goes, water quenching oil cooling), while water with high thermal conductivity is used as quenching medium for 45# steel. Of course, the choice of water and oil is not absolute. It is also closely related to the shape of the workpiece. Water quenching can also be used for simple 40Cr parts, while oil quenching or even salt bath may be used for complex 45# steel parts.
Recommended temperature for heat treatment of 45 Steel: normalizing 850℃, quenching 840℃, tempering 600℃.

Quenching and tempering of 45# steel: the quenching temperature of 45# steel is A3 + (30 ~ 50) ℃. In practice, the upper limit is generally taken. The higher quenching temperature can accelerate the heating speed of the workpiece, reduce the surface oxidation and improve the work efficiency. In order to homogenize the austenite of the workpiece, enough holding time is needed. If the actual charging capacity is large, it is necessary to extend the holding time properly. Otherwise, the hardness may be insufficient due to uneven heating. However, if the holding time is too long, the defects of coarse grains and serious oxidative decarburization will also occur, which will affect the quenching quality. In our opinion, if the charging capacity is greater than the requirements of the process documents, the heating and holding time needs to be extended by 1/5. Because the hardenability of 45# steel is low, 10% brine solution with high cooling rate should be used.

The highest hardness of medium carbon steel after heat treatment is about HRC55 (hb538), and σ B is 600-1100mpa. Therefore, medium carbon steel is widely used in all kinds of medium strength applications. In addition to being used as building materials, it is also widely used in manufacturing various mechanical parts.
 

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