Simulated HAZ continuous cooling transformation diagram of a bogie steel of high-speed railway

Simulated HAZ continuous cooling transformation (SH-CCT) diagram presents the start and end points of phase transformation and the relationships of the microstructures of HAZ, temperature and cooling rates. It is often used to assess the weldability of materials. In this paper, a weathering steel Q345C which is widely used in the bogies manufacturing was studied. The cooling times from 800${}^{\circ }$C to 500${}^{\circ }$C ($t_{8/5}$) were from 3 s to 6000 s, aiming to study the microstructures under different cooling rates. Different methods such as color metallography were used to obtain the metallography images. The results show that ferrite nucleates preferentially at the prior austenite grain boundaries and grows along the grain boundaries with a lath-like distribution when $t_{8/5}$ is 300 s. Austenite transforms into ferrite, pearlite and bainite with decreasing $t_{8/5}$. Pearlite disappears completely when $t_{8/5}=150$ s. Martensite gradually appears when $t_{8/5}$ decreases to 30 s. The hardness increases with decreasing $t_{8/5}$. The SH-CCT diagram indicates that the welding input and $t_{8/5}$ should be taken into consideration when welding. This work provides the relationships of welding parameters and microstructures.