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 area of the weld, which contains multiple intergranular secondary cracks. The failure is probably related to intergranular cracking initiating from the outer surface within the weld heat affected zone and propagated from the wall thickness. Random surface cracks or folds were found across the pipe. In some cases 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 PEX-AL-PEX pipe during service. ? Investigation of failure mechanism using macro- and microfractography. Metallographic evaluation of transverse sections near the fracture area. ? Proof of multiple secondary cracks on the HAZ area following intergranular mode. ? Presence of Zn in the interior of the cracks manifested that HAZ sensitization and cracking occurred prior to galvanizing process.
Galvanized steel tubes are utilized in lots of 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 as 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 from the welded tube in degreasing and pickling baths for surface cleaning and activation is necessary before hot dip galvanizing. Hot dip galvanizing is performed in molten Zn bath in a temperature of 450-500 °C approximately.
Several failures of HDPE Pipe Welding Machine occurred after short-service period (approximately 1 year following the installation) have led to leakage along with a costly repair in 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 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, and no other similar failures were reported in 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 of the present evaluation.
Various welded component failures associated with 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 within the relevant literature. Insufficient fusion/penetration leads to local peak stress conditions compromising the structural integrity in the assembly at the joint area, while the actual existence of weld porosity results in serious weakness from the fusion zone , . Joining parameters and metal cleanliness are viewed as critical factors towards the structural integrity from 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 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, followed by fine polishing using diamond and silica suspensions. Microstructural observations performed after immersion etching in Nital 2% solution (2% nitric acid in ethanol) accompanied by ethanol cleaning and heat-stream drying.
Metallographic evaluation was performed using 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 using an EDAX detector was 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 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 alongside the weld zone of the weld, most likely after the heat affected zone (HAZ). Transverse sectioning from the tube led to opening in the through 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 caused by the deep penetration and surface wetting by zinc, as it was recognized by PEX-AL-PEX pipe analysis. Zinc oxide or hydroxide was formed caused by the exposure of zinc-coated cracked face to 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 may be considered as the key failure mechanism.