Low alloy steel welded pipes buried in the ground were sent for failure analysis investigation. Failure of steel pipes had not been brought on by tensile ductile overload but resulted from low ductility fracture in the region of the weld, which contains multiple intergranular secondary cracks. The failure is probably related to intergranular cracking initiating from the outer surface in the weld heat affected zone and propagated from the wall thickness. Random surface cracks or folds were found round the pipe. Sometimes cracks are emanating from the tip of such discontinuities. Chemical analysis, visual inspection, optical microscopy and SEM/EDS analysis were used as the principal analytical methods for the failure investigation.

Low ductility fracture of HDPE pipe fittings during service. ? Investigation of failure mechanism using macro- and microfractography. Metallographic evaluation of transverse sections near the fracture area. ? Proof multiple secondary cracks on the HAZ area following intergranular mode. ? Presence of Zn inside the interior from the cracks manifested that HAZ sensitization and cracking occurred prior to galvanizing process.

Galvanized steel tubes are utilized in numerous outdoors and indoors application, including hydraulic installations for central heating system units, water supply for domestic and industrial use. Seamed galvanized tubes are fabricated by low alloy steel strip being a raw material accompanied by resistance welding and hot dip galvanizing as the best manufacturing process route. Welded pipes were produced using resistance self-welding in the steel plate by applying constant contact pressure for current flow. Successive pickling was realized in diluted HCl acid bath. Rinsing of the welded tube in degreasing and pickling baths for surface cleaning and activation is necessary just before hot dip galvanizing. Hot dip galvanizing is performed in molten Zn bath at a temperature of 450-500 °C approximately.

A series of failures of HDPE Pipe fittings occurred after short-service period (approximately 1 year after the installation) have led to leakage as well as a costly repair from the installation, were submitted for root-cause investigation. The subject of the failure concerned underground (buried inside the earth-soil) pipes while tap water was flowing inside the tubes. Loading was typical for domestic pipelines working under low internal pressure of some handful of bars. Cracking followed a longitudinal direction plus it was noticed in the weld zone area, while no macroscopic plastic deformation (“swelling”) was observed. Failures occurred to isolated cases, with no other similar failures were reported within the same batch. Microstructural examination and fractographic evaluation using optical and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (EDS) were mainly employed in the context in the present evaluation.

Various welded component failures related to fusion and heat affected zone (HAZ) weaknesses, such as hot and cold cracking, insufficient penetration, lamellar tearing, slag entrapment, solidification cracking, gas porosity, etc. are reported inside the relevant literature. Lack of fusion/penetration results in local peak stress conditions compromising the structural integrity of the assembly in the joint area, while the presence of weld porosity leads to serious weakness in the fusion zone [3], [4]. Joining parameters and metal cleanliness are thought as critical factors towards the structural integrity in the welded structures.

Chemical analysis of the fractured components was performed using standard optical emission spectrometry (OES). Low-magnification inspection of surface and fracture morphology was performed employing a Nikon SMZ 1500 stereomicroscope. Microstructural and morphological characterization was conducted in mounted cross-sections. Wet grinding was performed using successive abrasive SiC papers up to #1200 grit, then fine polishing using diamond and silica suspensions. Microstructural observations carried out after immersion etching in Nital 2% solution (2% nitric acid in ethanol) followed by ethanol cleaning and heat-stream drying.

Metallographic evaluation was performed utilizing a Nikon Epiphot 300 inverted metallurgical microscope. Furthermore, high magnification observations in the microstructure and fracture topography were conducted to ultrasonically cleaned specimens, working with a FEI XL40 SFEG scanning electron microscope using secondary electron and back-scattered imaging modes for topographic and compositional evaluation. Energy dispersive X-ray spectroscopy using an EDAX detector was also employed to gold sputtered samples for qfsnvy elemental chemical analysis.

An agent sample from failed steel pipes was submitted for investigation. Both pipes experience macroscopically identical failure patterns. A characteristic macrograph in the representative fractured pipe (27 mm outer diameter × 3 mm wall thickness) is shown in Fig. 1. Since it is evident, crack is propagated towards the longitudinal direction showing a straight pattern with linear steps. The crack progressed next to the weld zone of the weld, most probably after the heat affected zone (HAZ). Transverse sectioning from the tube resulted in opening from the from the wall crack and exposure from the fracture surfaces. Microfractographic investigation performed under SEM using backscattered electron imaging revealed a “molten” layer surface morphology which was caused by the deep penetration and surface wetting by zinc, since it was identified by PEX-AL-PEX pipe analysis. Zinc oxide or hydroxide was formed caused by the exposure of zinc-coated cracked face for the working environment and humidity. The above findings as well as the detection of zinc oxide on the on the fracture surface suggest strongly that cracking occurred just before galvanizing process while no static tensile overload during service could be considered as the main failure mechanism.

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