Failure analysis of a thermally insulated pipeline in a district heating system

•Failure of the thermally insulated pipe welded joint in district heating system was observed.•Crack was propagated along the fusion line in the transversal direction with the brittle fracture.•Lower mechanical properties were investigated by indentation, tensile and slow-strain rate tests.•Stress concentration on the toe region was attributed to the crack initiation.

Failure analysis of a thermally insulated pipeline which failed during operation in a district heating system was performed by utilizing optical and electron microscopy in addition to various mechanical tests. The fractographic investigation showed that a crack initiated at the inside of the pipe and propagated to the outside. The crack was initiated at the toe region of the weldment and propagated along the transverse direction of the weldment. In the mechanical tests, the low toughness and hydrogen sensitivity of the weldment affected the failure of the thermally-insulated pipeline. In short, the interaction of the multiple effects in the HAZ, including the lower mechanical properties, stress concentration, and hydrogen susceptibility, led to pipe failure.