Low alloy steel welded pipes buried in the earth 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 area of the weld, which also contains multiple intergranular secondary cracks. The failure is most likely associated with intergranular cracking initiating from the outer surface in 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 utilised as the principal analytical methods 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 multiple secondary cracks on 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 used 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 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 using constant contact pressure for current flow. Successive pickling was realized in diluted HCl acid bath. Rinsing in 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 right after the installation) have triggered leakage and a costly repair in the installation, were submitted for root-cause investigation. The main topic of the failure concerned underground (buried in the earth-soil) pipes while faucet water was flowing within the tubes. Loading was typical for domestic pipelines working under low internal pressure of a few handful of bars. Cracking followed a longitudinal direction and it also was noticed in the weld zone area, while no macroscopic plastic deformation (“swelling”) was observed. Failures occurred to isolated cases, with no other similar failures were reported inside 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 of the present evaluation.
Various welded component failures related to fusion or heat affected zone (HAZ) weaknesses, including cold and warm cracking, absence 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 in the assembly in the joint area, while the existence of weld porosity results in serious weakness in 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 approximately #1200 grit, accompanied by fine polishing using diamond and silica suspensions. Microstructural observations completed 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 of the microstructure and fracture topography were conducted to ultrasonically cleaned specimens, working with 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 have also been 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. As it is evident, crack is propagated towards the longitudinal direction showing a straight pattern with linear steps. The crack progressed adjacent to the weld zone in the weld, most probably following the heat affected zone (HAZ). Transverse sectioning from the tube ended in opening in the from 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, because it was identified 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 as well as 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 considered as the key failure mechanism.