Low alloy steel welded pipes buried in the earth were sent for failure analysis investigation. Failure of steel pipes was not brought on 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 most probably associated with 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 around the pipe. In some instances cracks are emanating from the tip of these 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 PEX-AL-PEX pipe 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 on the HAZ area following intergranular mode. ? Presence of Zn inside the interior in the cracks manifested that HAZ sensitization and cracking occurred prior to galvanizing process.
Galvanized steel tubes are employed in numerous 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 being a raw material then resistance welding and hot dip galvanizing as the most appropriate manufacturing process route. Welded pipes were produced using resistance self-welding from 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 needed just before hot dip galvanizing. Hot dip galvanizing is performed in molten Zn bath in a temperature of 450-500 °C approximately.
A number of failures of HDPE pipe fittings occurred after short-service period (approximately 1 year right after 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 inside the earth-soil) pipes while 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 and it was noticed at 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 utilized in the context in the present evaluation.
Various welded component failures associated with fusion and heat affected zone (HAZ) weaknesses, including cold and hot cracking, insufficient penetration, lamellar tearing, slag entrapment, solidification cracking, gas porosity, etc. are reported in the relevant literature. Absence of fusion/penetration results in local peak stress conditions compromising the structural integrity in the assembly on the joint area, while the presence of weld porosity results in serious weakness of the fusion zone , . Joining parameters and metal cleanliness are thought as critical factors for 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 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 up to #1200 grit, then fine polishing using diamond and silica suspensions. Microstructural observations carried out after immersion etching in Nital 2% solution (2% nitric acid in ethanol) followed by ethanol cleaning and heat-stream drying.
Metallographic evaluation was performed utilizing a Nikon Epiphot 300 inverted metallurgical microscope. In addition, high magnification observations in the microstructure and fracture topography were conducted to ultrasonically cleaned specimens, using 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 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 to the longitudinal direction showing a straight pattern with linear steps. The crack progressed next to the weld zone of the weld, probably pursuing the heat affected zone (HAZ). Transverse sectioning from the tube led to opening from 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 had been caused by the deep penetration and surface wetting by zinc, because it was identified by Multilayer 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 above 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 might be viewed as the key failure mechanism.