Low alloy steel welded pipes buried in the ground were sent for failure analysis investigation. Failure of steel pipes was not due to 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 probably attributed 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 across the pipe. In some cases cracks are emanating from the tip of these discontinuities. Chemical analysis, visual inspection, optical microscopy and SEM/EDS analysis were utilised as the principal analytical approaches 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 in the HAZ area following intergranular mode. ? Presence of Zn in the interior from the cracks manifested that HAZ sensitization and cracking occurred prior to galvanizing process.
Galvanized steel tubes are employed in many 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 of 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 required prior to hot dip galvanizing. Hot dip galvanizing is carried out in molten Zn bath in 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 resulted in leakage and a costly repair of the installation, were submitted for root-cause investigation. The topic of the failure concerned underground (buried within the earth-soil) pipes while faucet water was flowing within 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 inside 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 used in the context of the present evaluation.
Various welded component failures associated with fusion and heat affected zone (HAZ) weaknesses, such as hot and cold cracking, lack of penetration, lamellar tearing, slag entrapment, solidification cracking, gas porosity, etc. are reported in the relevant literature. Lack of fusion/penetration results in local peak stress conditions compromising the structural integrity from the assembly in the joint area, while the existence of weld porosity brings about serious weakness in the fusion zone , . Joining parameters and metal cleanliness are thought as critical factors to the structural integrity from 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 utilizing 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 performed after immersion etching in Nital 2% solution (2% nitric acid in ethanol) followed by ethanol cleaning and heat-stream drying.
Metallographic evaluation was performed using a Nikon Epiphot 300 inverted metallurgical microscope. Additionally, high magnification observations from 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 utilizing an EDAX detector was employed 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. Because it is evident, crack is propagated for the longitudinal direction showing a straight pattern with linear steps. The crack progressed alongside the weld zone in the weld, probably after the heat affected zone (HAZ). Transverse sectioning from the tube led to 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 which was 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 to the working environment and humidity. The above mentioned findings and the detection of zinc oxide on the on the fracture surface suggest strongly that cracking occurred before galvanizing process while no static tensile overload during service may be considered as the key failure mechanism.