Low alloy steel welded pipes buried in the earth were sent for failure analysis investigation. Failure of steel pipes was not caused 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 attributed to intergranular cracking initiating from the outer surface in the weld heat affected zone and propagated through the wall thickness. Random surface cracks or folds were found across the pipe. In some instances cracks are emanating from the tip of those discontinuities. Chemical analysis, visual inspection, optical microscopy and SEM/EDS analysis were utilized as the principal analytical methods 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 to the fracture area. ? Proof multiple secondary cracks in the HAZ area following intergranular mode. ? Presence of Zn within the interior of the cracks manifested that HAZ sensitization and cracking occurred before galvanizing process.
Galvanized steel tubes are used in lots of 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 as a raw material accompanied by resistance welding and hot dip galvanizing as the most appropriate manufacturing process route. Welded pipes were produced using resistance self-welding in the steel plate by using 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 before hot dip galvanizing. Hot dip galvanizing is carried out in molten Zn bath with a temperature of 450-500 °C approximately.
Several failures of HDPE Pipe fittings occurred after short-service period (approximately 1 year after the installation) have triggered leakage along with a costly repair from the installation, were submitted for root-cause investigation. The main topic of the failure concerned underground (buried within the earth-soil) pipes while plain tap water was flowing within the tubes. Loading was typical for domestic pipelines working under low internal pressure of a few number of bars. Cracking followed a longitudinal direction and it also was noticed on the weld zone area, while no macroscopic plastic deformation (“swelling”) was observed. Failures occurred to isolated cases, without any 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 utilized in the context of the present evaluation.
Various welded component failures related to fusion and/or heat affected zone (HAZ) weaknesses, such as cold and hot cracking, absence 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 from the assembly at the joint area, while the presence of weld porosity results in serious weakness of the fusion zone , . Joining parameters and metal cleanliness are considered 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, followed by fine polishing using diamond and silica suspensions. Microstructural observations carried out after immersion etching in Nital 2% solution (2% nitric acid in ethanol) accompanied by ethanol cleaning and hot air-stream drying.
Metallographic evaluation was performed using a Nikon Epiphot 300 inverted metallurgical microscope. Additionally, 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 employing an EDAX detector was also utilized 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 from 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 from the weld, most probably pursuing the heat affected zone (HAZ). Transverse sectioning of the tube ended in opening in the with the wall crack and exposure in the fracture surfaces. Microfractographic investigation performed under SEM using backscattered electron imaging revealed a “molten” layer surface morphology which had been brought on by the deep penetration and surface wetting by zinc, since it was recognized by Multilayer pipe analysis. Zinc oxide or hydroxide was formed as a consequence of the exposure of zinc-coated cracked face for the working environment and humidity. The aforementioned findings and 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 key failure mechanism.