The Carburizing Behavior of High‐Temperature Short‐Time Carburizing Gear Steel: Effect of Nb Microalloying

Herein, the carburizing behavior of 20MnCr5 gear steel with different Nb contents after carburizing with the temperature from 900 to 950 °C and the corresponding holding time from 8 to 5 h are investigated. The theoretical results of carbon content and the depth of carburized layer for the two experimental carburizing processes should be the same according to the classical diffusion model. However, the experimental results show that the carbon content of the entire hardened layer increases, and the depth of carburized layer increases by 35% after high‐temperature carburizing. The carbon content of 0.061%Nb steel on the outer surface increases, but the depth of carburized layer remains unchanged. It is found that the theoretical D values at 900 °C and 950 °C are 2.0 × 10−7 and 3.3 × 10−7 cm2 s−1, respectively. However, the experimental D value is much larger than 5.0 × 10−7 cm2 s−1. The carbon diffusion is greatly promoted by high‐temperature carburizing because the number of excess carbides on the outer surface decreases. The diffusion coefficient decreases after the addition of Nb due to forming a large number of small particles of Cr7C3 and NbC, but the effect is only obvious on the outer surface.