45# steel tube belongs to high-quality carbon structural steel, 40Cr steel pipe belongs to alloy structural steel. The difference between 40Cr steel pipe and 45# steel tube is after heat treatment.

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 steel pipe after quenching (or quenching and tempering) are significantly higher than that of 45# steel tube. But also due to the strong hardenability, the internal stress of 40Cr steel pipe during quenching is larger than that of 45# steel tube, and the cracking tendency of 40Cr steel pipe is larger than that of 45# steel tube under the same conditions.

Therefore, in order to avoid workpiece cracking, oil with low thermal conductivity is mostly used as quenching medium when quenching 40Cr steel pipe, sometimes double liquid quenching method is also used, as the saying goes, water quenching oil cooling. While 45# steel tube uses water with high thermal conductivity as quenching medium.