Continuous Cooling Transformation Behavior in Welding Coarse-Grained Heat-Affected Zone and Microstructure Evolution 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 ( t 8/5 ) from 800 to 500 °C on the continuous cooling transition (CCT) behavior of the welded coarse-grained heat-affected zone (CGHAZ) of G115 new martensitic heat resistant steel. Subsequently, the scanning electron microscope (SEM), the transmission electron microscope (TEM) and the microhardness tester were conducted to observe and measure the microstructure, the precipitates, as well as the microhardness. As revealed from the results, the B element increased A c1 and A c3 of G115 steel and decreased M s and M f . As a result, the CCT curve generally shifted down, and the downward amplitude 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 A c1 and A c3 increased. The adsorption of B atoms on the supercooled austenite grain boundaries improved their stability, thereby promoting the decrease in M s and M f . The decrease in the alloy content in supercooled austenite brought by the formation of alloy cementite was the main reason for the gradual increase of M s and M f in materials containing B when there was a large t 8/5 . The microhardness decreased with the increase in t 8/5 , and the increase in the prior-austenite grain size was the main reason for the decreased microhardness. In order to guarantee CGHAZ microstructure and properties of G115 steel, welding t 8/5 should be less than 500 s