Analysis of the Correlation between Weld Microstructure and Cracking in Q355B Pipes

ZHANG Kun

doi:10.3969/j.issn.1004-4620.2025.04.012

Abstract：Regarding the transverse crack problem of square pipes made by flash butt welding of 8.25 mm thick Q355B steel strips,multi-dimensional detection and analysis were carried out using equipment such as direct-reading spectrometers,universal tensile testing machines,metallographic microscopes,and scanning electron microscopes.The results show that the chemical composition,mechanical properties,and inclusions of the base metal meet the GB/T 3274-2017 standard and are not the causes of cracks.The cracks originate from the heat-affected zone(HAZ)of the weld.In this zone,there are a large number of widmanstatten structures(grain size grade 6.5)and clustered Si,Mn oxide inclusions,with a hardness of 187-220 HBW,which is significantly higher than that of the base metal.The core causes are as follows:excessive welding power leads to coarse austenite grains;too fast air cooling after welding induces the formation of widmanstatten structures,which reduces the plasticity and toughness of the weld;inclusions in the molten pool are not completely discharged,for crack sources;and combined with the tensile stress from roll for correction,cracks eventually propagate.By optimizing the welding heat input,post-weld slow cooling,and upsetting parameters,the formation of widmanstatten structures and the residual of inclusions can be inhibited,and the crack problem can be solved.