Causes of Cracks Occurred in Welding of X80 pipeline steel

When the welding process of X80 pipeline steel is unreasonable, or the number of inclusions in the base metal is large, the shape and distribution are uneven, cracks are easy to form in the HAZ near the fusion line at the weld toe.

The microstructure at the crack origin of X80 pipeline steel is coarse granular bainite and lath bainite; the MA islands on the bainitic ferrite are larger in size, and there are larger inclusions. Therefore, improving the purity of the base metal of X80 pipeline steel, reducing the content of inclusions, and optimizing the welding process of X80 pipeline steel to obtain a fine and uniform structure in the HAZ can effectively avoid the occurrence of cracks.

The shape and distribution of MA islands on the matrix are the main factors affecting the performance of the HAZ coarse-grained region, and the coarse MA islands are easy to initiate cracks. The greater the welding heat input of X80 pipeline steel, the easier it is to form boundary nucleation ferrite, which increases the hardness of MA island, and the easier it is for cracks to nucleate and crack along the junction of MA island and matrix.

In addition, under the influence of residual stress generated by welding X80 pipeline steel, higher and higher working pressure and complex conveying medium, the welded joints of X80 pipeline steel are prone to stress corrosion cracking and hydrogen-induced cracking.