Low alloy steel welded pipes buried in the earth 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 also 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 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 methods for the failure investigation.
Low ductility fracture of HDPE pipe fittings during service. ? Investigation of failure mechanism using macro- and microfractography. Metallographic evaluation of transverse sections close to the fracture area. ? Proof of multiple secondary cracks at the HAZ area following intergranular mode. ? Presence of Zn in the interior of the cracks manifested that HAZ sensitization and cracking occurred before galvanizing process.
Galvanized steel tubes are utilized 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 then resistance welding and hot dip galvanizing as the most suitable manufacturing process route. Welded pipes were produced using resistance self-welding of the steel plate by making use of 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 at a temperature of 450-500 °C approximately.
A number of failures of HDPE Pipe Welding Machine occurred after short-service period (approximately 1 year after the installation) have led to leakage along with a costly repair from the installation, were submitted for root-cause investigation. The topic of the failure concerned underground (buried within the earth-soil) pipes while plain tap water was flowing in the tubes. Loading was typical for domestic pipelines working under low internal pressure of some couple of bars. Cracking followed a longitudinal direction plus it was noticed at the weld zone area, while no macroscopic plastic deformation (“swelling”) was observed. Failures occurred to isolated cases, with no other similar failures were reported within the same batch. Microstructural examination and fractographic evaluation using optical and scanning electron microscopy along with energy dispersive X-ray spectroscopy (EDS) were mainly utilized in the context in the present evaluation.
Various welded component failures associated with fusion or 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 inside the relevant literature. Lack of fusion/penetration results in local peak stress conditions compromising the structural integrity from the assembly on the joint area, while the presence of weld porosity leads to serious weakness in the fusion zone , . Joining parameters and metal cleanliness are thought as critical factors for the structural integrity in 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 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 as much as #1200 grit, then fine polishing using diamond and silica suspensions. Microstructural observations completed after immersion etching in Nital 2% solution (2% nitric acid in ethanol) accompanied by ethanol cleaning and heat-stream drying.
Metallographic evaluation was performed employing a Nikon Epiphot 300 inverted metallurgical microscope. Furthermore, high magnification observations in 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 used 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 towards the longitudinal direction showing a straight pattern with linear steps. The crack progressed alongside the weld zone in the weld, most probably pursuing the heat affected zone (HAZ). Transverse sectioning from the tube ended in opening from the with 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 caused by the deep penetration and surface wetting by zinc, as it was identified by PEX-AL-PEX pipe analysis. Zinc oxide or hydroxide was formed as a consequence of the exposure of zinc-coated cracked face towards the working environment and humidity. The aforementioned findings and also 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 could be regarded as the primary failure mechanism.