EFFECT OF SILICON ON CGHAZ TOUGHNESS AND MICROSTRUCTURE OF MICROALLOYED STEELS

The aim of this article is to present the beneficial effect of a reduc tion of silicon content on coarse-grained heat-affected zone (CGHAZ) t oughness. This study was achieved with experimental and industrial E35 5 structural steels. These 0.09 wt pet C steels were Ti-microalloyed w ith silicon contents ranging from 0.05 to 0.5 wt pet. First, we demons trate that the CGHAZ toughness is predominantly affected by the volume fraction of retained austenite (gamma r). Second, we show that the ex istence of retained austenite pertains only to its carbon enrichment. This enrichment is promoted essentially by an increase of the silicon level due to the retarding action of silicon on the formation of carbi des in ferrite as well as in austenite. In the same way, the increase of silicon content slows down the decomposition of retained austenite into pearlite. The reduction of the silicon content of the steel great ly increases the ductility of the CGHAZ through the decrease of the vo lume fraction of retained austenite.