Relationship between Microstructure and Properties under Simulated On-Fire Processing Conditions of Q345R Boiler Steel

Abstract

Series of simulated on-fire processing experiments were conducted for Q345R boiler steel at 550, 700, 800, 950°C for a time of 0.5h, 2h, 5h respectively, followed by air cooling. Then the Brinell hardness, impact energy and tensile strength of the specimens were tested to study the variation of mechanical properties with processing conditions. Furthermore, characteristics of the banded structure and pearlite were observed by OM and SEM to investigate the effects of the processing on microstructure and properties comprehensively. The results are as follows: the comprehensive properties of surface hardness, tensile strength and impact energy were almost keeping the same when tested temperature was below 550°C. However, most of the properties were decreased to somewhat extent near 700 °C. The properties under simulated on-fire temperatures were not sensitive to the variation of holding time and the original banded structure. The formation of banded structure originated from previously heterogeneous segregation of alloy elements in dendritic solidification. Thus the higher processing temperature and adequate holding time were able to decrease the banded level through full diffusion of alloy elements, but didn’t contributed to those mechanical performances a lot. Further microstructure analysis showed that degeneration of pearlite near 700 °C, which was induced by interaction of both subcritical annealing and conventional spherical annealing, was the primary reason for the degradation behavior of Q345R steel. It was precisely at this temperature that the degradation of pearlite occured, leading to the decline of properties. Consequently, some non4inear mathematical models of different mechanical performances are established to predict relevant performances under other conditions in practical application.