Low alloy steel welded pipes buried in the ground were sent for failure analysis investigation. Failure of steel pipes had not been due to tensile ductile overload but resulted from low ductility fracture in the area 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. In some cases cracks are emanating from the tip of those discontinuities. Chemical analysis, visual inspection, optical microscopy and SEM/EDS analysis were used 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 to the fracture area. ? Evidence of multiple secondary cracks on the HAZ area following intergranular mode. ? Presence of Zn inside the interior of the cracks manifested that HAZ sensitization and cracking occurred just before galvanizing process.
Galvanized steel tubes are employed 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 followed by resistance welding and hot dip galvanizing as the most suitable 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 in the welded tube in degreasing and pickling baths for surface cleaning and activation is required 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 following the installation) have led to leakage as well as a costly repair of the installation, were submitted for root-cause investigation. The subject of the failure concerned underground (buried within the earth-soil) pipes while faucet water was flowing inside 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 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 inside the same batch. Microstructural examination and fractographic evaluation using optical and scanning electron microscopy along with energy dispersive X-ray spectroscopy (EDS) were mainly employed in the context in the present evaluation.
Various welded component failures related to fusion or heat affected zone (HAZ) weaknesses, including cold and warm cracking, insufficient penetration, lamellar tearing, slag entrapment, solidification cracking, gas porosity, etc. are reported within the relevant literature. Insufficient fusion/penetration leads to local peak stress conditions compromising the structural integrity from the assembly on the joint area, while the actual 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 research into the fractured components was performed using standard optical emission spectrometry (OES). Low-magnification inspection of surface and fracture morphology was performed using a Nikon SMZ 1500 stereomicroscope. Microstructural and morphological characterization was conducted in mounted cross-sections. Wet grinding was performed using successive abrasive SiC papers as much as #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) then ethanol cleaning and hot air-stream drying.
Metallographic evaluation was performed employing a Nikon Epiphot 300 inverted metallurgical microscope. Furthermore, high magnification observations of the microstructure and fracture topography were conducted to ultrasonically cleaned specimens, employing 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 have also been used 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 of 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 adjacent to the weld zone of the weld, probably after the heat affected zone (HAZ). Transverse sectioning from the tube resulted in opening from the through the wall crack and exposure in the fracture surfaces. Microfractographic investigation performed under SEM using backscattered electron imaging revealed a “molten” layer surface morphology that was caused by the deep penetration and surface wetting by zinc, since it was recognized by PEX-AL-PEX pipe analysis. Zinc oxide or hydroxide was formed because of the exposure of zinc-coated cracked face to the working environment and humidity. The above findings and the detection of zinc oxide on the on the fracture surface suggest strongly that cracking occurred prior to galvanizing process while no static tensile overload during service may be viewed as the primary failure mechanism.