Low alloy steel welded pipes buried in the earth were sent for failure analysis investigation. Failure of steel pipes had not been caused by 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 most probably attributed 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 instances cracks are emanating from the tip of these discontinuities. Chemical analysis, visual inspection, optical microscopy and SEM/EDS analysis were utilized 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 multiple secondary cracks on the HAZ area following intergranular mode. ? Presence of Zn inside the interior from the cracks manifested that HAZ sensitization and cracking occurred prior to galvanizing process.
Galvanized steel tubes are utilized in many 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 followed by 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 making use of 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 necessary just before hot dip galvanizing. Hot dip galvanizing is conducted 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 right after the installation) have resulted in leakage and a costly repair of the installation, were submitted for root-cause investigation. The subject of the failure concerned underground (buried in the earth-soil) pipes while faucet water was flowing in the tubes. Loading was typical for domestic pipelines working under low internal pressure of a few couple of bars. Cracking followed a longitudinal direction and it also 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 coupled with energy dispersive X-ray spectroscopy (EDS) were mainly utilized in the context in the present evaluation.
Various welded component failures related to fusion and heat affected zone (HAZ) weaknesses, like hot and cold cracking, absence of penetration, lamellar tearing, slag entrapment, solidification cracking, gas porosity, etc. are reported within the relevant literature. Insufficient fusion/penetration results in local peak stress conditions compromising the structural integrity from the assembly at the joint area, while the existence of weld porosity leads to serious weakness from the fusion zone , . Joining parameters and metal cleanliness are thought as critical factors to the structural integrity in 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 up to #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) then ethanol cleaning and hot air-stream drying.
Metallographic evaluation was performed using a Nikon Epiphot 300 inverted metallurgical microscope. Additionally, 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 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 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 for the longitudinal direction showing a straight pattern with linear steps. The crack progressed adjacent to the weld zone in the weld, most probably after the heat affected zone (HAZ). Transverse sectioning of the tube ended in opening in the with the wall crack and exposure of 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 identified 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 findings as well as 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 could be regarded as the main failure mechanism.