Analysis of microstructural variation and mechanical behaviors in submerged arc welded joint of high strength low carbon bainitic steel

Microstructural variation in high strength low carbon bainitic steel weldment was investigated in detail by means of optical microscope, transmission electron microscope and scanning electron microscope equipped with electron backscattered diffraction. The results showed that the welded joint has various microstructures such as acicular ferrite, coarse granular ferrite and fine polygonal ferrite. The martensite-austenite (MA) constituent has a variable structure in each sub-zone, which includes fully martensite and fully retained austenite. Meanwhile, the fine grained heat affected zone has higher content of retained austenite than the welded metal (WM) and coarse grained heat affected zone (CGHAZ). The orientation relationship between retained austenite and product phases in the WM and CGHAZ is close to Kurdjumov-Sachs relationship. However, the polygonal ferrite in the fine grained HAZ has no specific orientation relationship with the neighboring retained austenite. The toughness of the coarse grained region is much lower than that of the WM because the coarse bainite contains many large MA constituents to assist the nucleation of microcracks and coarse cleavage facet lowers the ability to inhibit the crack propagation.