Low alloy steel welded pipes buried in the ground were sent for failure analysis investigation. Failure of steel pipes was not brought on by tensile ductile overload but resulted from low ductility fracture in the region of the weld, which also contains multiple intergranular secondary cracks. The failure is most likely related to intergranular cracking initiating from the outer surface within the weld heat affected zone and propagated through the wall thickness. Random surface cracks or folds were found around the pipe. In some instances cracks are emanating from the tip of such discontinuities. Chemical analysis, visual inspection, optical microscopy and SEM/EDS analysis were utilized as the principal analytical approaches for the failure investigation.
Low ductility fracture of HDPE pipe during service. ? Investigation of failure mechanism using macro- and microfractography. Metallographic evaluation of transverse sections near the fracture area. ? Evidence of multiple secondary cracks in 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 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 making use of 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 needed just before hot dip galvanizing. Hot dip galvanizing is performed in molten Zn bath with a temperature of 450-500 °C approximately.
A series of failures of HDPE Pipe fittings occurred after short-service period (approximately 1 year right after the installation) have triggered leakage as well as a costly repair from the installation, were submitted for root-cause investigation. The topic of the failure concerned underground (buried in the earth-soil) pipes while plain tap water was flowing inside the tubes. Loading was typical for domestic pipelines working under low internal pressure of some number of bars. Cracking followed a longitudinal direction and it was noticed on the weld zone area, while no macroscopic plastic deformation (“swelling”) was observed. Failures occurred to isolated cases, and no other similar failures were reported within the same batch. Microstructural examination and fractographic evaluation using optical and scanning electron microscopy in conjunction with energy dispersive X-ray spectroscopy (EDS) were mainly utilized in the context from the present evaluation.
Various welded component failures attributed to fusion or heat affected zone (HAZ) weaknesses, including cold and warm cracking, lack of penetration, lamellar tearing, slag entrapment, solidification cracking, gas porosity, etc. are reported in the relevant literature. Insufficient fusion/penetration contributes to local peak stress conditions compromising the structural integrity of the assembly in the joint area, while the existence of weld porosity leads to serious weakness of the fusion zone , . Joining parameters and metal cleanliness are viewed as critical factors to the structural integrity of 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 approximately #1200 grit, accompanied by 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 hot air-stream drying.
Metallographic evaluation was performed using a Nikon Epiphot 300 inverted metallurgical microscope. In addition, high magnification observations of the microstructure and fracture topography were conducted to ultrasonically cleaned specimens, using 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 employing an EDAX detector was also utilized to gold sputtered samples for qfsnvy elemental chemical analysis.
A representative sample from failed steel pipes was submitted for investigation. Both pipes experience macroscopically identical failure patterns. A characteristic macrograph of the representative fractured pipe (27 mm outer diameter × 3 mm wall thickness) is shown in Fig. 1. As it is evident, crack is propagated to the longitudinal direction showing a straight pattern with linear steps. The crack progressed adjacent to the weld zone of the weld, probably after the heat affected zone (HAZ). Transverse sectioning of the tube resulted in opening of 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 that was brought on by the deep penetration and surface wetting by zinc, since it was recognized by PERT-AL-PERT pipe analysis. Zinc oxide or hydroxide was formed because of the exposure of zinc-coated cracked face towards the working environment and humidity. The above mentioned 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 might be viewed as the key failure mechanism.