The Mechanism Research On Improving Impact Toughness Of Simulated Heat-affected Zone In HSLA Steels By Addition Of Aluminum

Due to the high efficiency, convenient operation and automation, large heat inputwelding technology is used widely with the trendy of large-scale steel structure. Theimpact toughness of heat-affected zone (HAZ) of high strength low alloy (HSLA) steelstends to be deteriorated by high heat input welding. The element aluminum has greatinfluence on the microstructure of HAZ of HSLA, but the mechanism is still unknown.In this thesis, the internal microstructure, the fraction and the morphology ofmartensite-austenite (M-A) constituents of coarse-grained heat-affected zone (CGHAZ)in three different Al content (0.023%,0.038%and0.070%) steels were investigated bywelding simulation with different heat inputs (20kJ/cm,100kJ/cm,200kJ/cm), theimpact toughness of CGHAZ with different Al-containing HSLA were also investigated.The internal microstructure of M-A constituents was observed using transmissionelectron microscope (TEM), the fraction and the morphology of M-A constituents wereobserved by Image-Pro and scanning electron microscope, the martensite and retainedaustenite were identified by electron diffraction pattern.The influence of varying aluminum content on the microstructure and mechanicalproperties of CGHAZ showed that the proportion of residual austenite in M-Aconstituents improved, the volume fraction of M-A constituents with different sizedecreased, in particllarly, the volume fraction of M-A constituents with big sizedecreased obviously. Because of the clamping and dragging the carbon-rich region withferritic growing, with increasing Al content, the volume fraction of massive M-Aconstituents first decreased and then increased. The absorbed energy of sample withincreasing Al from0.027%to0.070%, the absorbed energy increased from78to299J.The transmission electron microscopy (TEM) was used to analyze the second phaseparticle in CGHAZ through X-ray spectra. It could not play the role of pinning grainboundary to prevent grain growth because of the large size of precipitated phase.The influence of varying heat input on the microstructure and mechanical propertiesof CGHAZ with0.038%Al content showed that the size of M-A constituents became big, the volume fraction of all M-A constituents and massive M-A constituents increased.With increasing heat input, the volume fraction of M-A constituents with average sizeless than3mm first increased and then decreased, but the influence of heat input on theM-A constituents with average size more than3mm was opposite. The absorbed energydecreased from299J to176J with increasing heat input from20kJ/cm to200kJ/cm.This research indicates that adding Al element can improve the ferritic phasetransformation point and inhibite carbide precipitation, promote carbon atoms migratedfrom ferrite matrix to residual austenite. Adding Al element could increase the fraction ofretained austenite, reduce the fraction of M-A constituents and modify the morphologyand size of M-A constituents, which can significantly improve the impact toughness ofCGHAZ of HSLA steel. Therefore, adding appropriate Al element is a solution to protectHSAL steel from being deteriorated by high heat input welding.