Continuous Cooling Transformation Behavior in Welding Coarse-Grained Heat-Affected Zone of G115 Steel With the Different Content of Boron

In this study, the welding thermal simulation was performed to examine the effects of the boron content and the cooling time (t8/5) from 800 °C to 500 °C on the continuous cooling transformation (CCT) behavior of the welded coarse-grained heat-affected zone (CGHAZ) of G115 new martensitic heat-resistant steel. Subsequently, the microstructure, the precipitates, and the microhardness were observed and measured using the scanning electron microscope (SEM), the transmission electron microscope (TEM) and the microhardness tester. As revealed from the results, the B element increased Ac1 and Ac3 of G115 steel and decreased Ms and Mf. As a result, the CCT curve generally shifted down, and the decline increased with the increase in the B content. Through the segregation of B atoms at the prior-austenite grain boundaries and the formation of high melting point carbides, the nucleation and growth of austenite were inhibited, the austenitizing process was delayed, and Ac1 and Ac3 increased. The adsorption of B atoms on the undercooled austenite grain boundaries improved their stability, thereby promoting the decrease in Ms and Mf. The decrease in the alloy content in undercooled austenite brought by the formation of alloy cementite was the main reason for the gradual increase of Ms and Mf in materials containing B when there was a large t8/5.