Low alloy steel welded pipes buried in the ground 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 region of the weld, that also 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 cases 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 techniques 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 near the fracture area. ? Proof multiple secondary cracks at the HAZ area following intergranular mode. ? Presence of Zn within the interior in 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 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 suitable manufacturing process route. Welded pipes were produced using resistance self-welding from the steel plate by using 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 before hot dip galvanizing. Hot dip galvanizing is conducted 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 right after the installation) have led to leakage as well as a costly repair from the installation, were submitted for root-cause investigation. The subject of the failure concerned underground (buried inside the earth-soil) pipes while faucet 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 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 associated with fusion and/or heat affected zone (HAZ) weaknesses, like cold and hot cracking, lack of penetration, lamellar tearing, slag entrapment, solidification cracking, gas porosity, etc. are reported within the relevant literature. Absence of fusion/penetration contributes to local peak stress conditions compromising the structural integrity in the assembly on the joint area, while the existence of weld porosity brings about 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 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) then ethanol cleaning and hot air-stream drying.
Metallographic evaluation was performed employing a Nikon Epiphot 300 inverted metallurgical microscope. Additionally, 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 utilizing an EDAX detector have also been 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 in the representative fractured pipe (27 mm outer diameter × 3 mm wall thickness) is shown in Fig. 1. As it is evident, crack is propagated to the longitudinal direction showing a straight pattern with linear steps. The crack progressed adjacent to the weld zone from the weld, most probably following the heat affected zone (HAZ). Transverse sectioning from the tube resulted in opening in the through the wall crack and exposure of 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 PEX-AL-PEX pipe analysis. Zinc oxide or hydroxide was formed because 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 just before galvanizing process while no static tensile overload during service could be considered as the main failure mechanism.