Effect of Si content on the microstructures and the impact properties in the coarse-grained heat-affected zone (CGHAZ) of typical weathering steel

摘要

This study aims to make clear the content limit of silicon (Si), which normally favors weathering steel but probably impairs its impact properties in the coarse-grained heat-affected zone (CGHAZ) induced by the conventional arc welding. Thermal simulation with a Gleeble 3500 simulator was thus carried out to reproduce the CGHAZ of this type of steel with differing silicon (Si) content from 0.12% to 0.48%-0.75%. Charpy V-notch impact tests and microstructure observations using optical microscopy, transmission and scanning electron microscopy (TEM and SEM), and electron backscatter diffraction (EBSD) were performed. The results showed that a microstructure composed of granular bainite (GB), acicular ferrite (AF), lath bainite (LB), and martensite/austenite (M/A) constituents formed In 0.12%Si and 0.48%Si samples, while the LB scarcely appeared in 0.75% Si sample. When the Si content increased from 0.12% to 0.48%-0.75%, the LB decreased and in contrast, the GB increased. In addition, the area fraction (average size) of the M/A constituents increased from 5.8% (0.70 mu m) to 9.2% (1.32 mu m), while the number fraction (mean size) of effective grains with high-angle grain boundaries (HAGBs) decreased (increased) from 27% (3.77 mu m) to 10% (5.71 mu m). Accordingly, the Charpy impact energy of the CGHAZ decreased sharply from 208 to 42 J. The increased large M/A constituents significantly enable the microcrack nucleation, while the reduced HAGBs inefficiently impede the microcrack propagation, finally leading to a deteriorated toughness in the CGHAZ of 0.75%Si steel. The silicon content in this steel should not exceed 0.48%.