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 region of the weld, which contains multiple intergranular secondary cracks. The failure is probably associated with intergranular cracking initiating from the outer surface within the weld heat affected zone and propagated through the wall thickness. Random surface cracks or folds were found across the pipe. Sometimes 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 near the fracture area. ? Proof multiple secondary cracks at the HAZ area following intergranular mode. ? Presence of Zn within the interior of the cracks manifested that HAZ sensitization and cracking occurred before 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 suitable manufacturing process route. Welded pipes were produced using resistance self-welding in the steel plate by making use of 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 needed prior to hot dip galvanizing. Hot dip galvanizing is carried out in molten Zn bath at a temperature of 450-500 °C approximately.
Several failures of HDPE pipe fittings occurred after short-service period (approximately 1 year after the installation) have led to leakage as well as a costly repair of 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 within the tubes. Loading was typical for domestic pipelines working under low internal pressure of a few number of bars. Cracking followed a longitudinal direction and it was noticed on the weld zone area, while no macroscopic plastic deformation (“swelling”) was observed. Failures occurred to isolated cases, with no other similar failures were reported in 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 from the present evaluation.
Various welded component failures attributed to 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 in the relevant literature. Insufficient fusion/penetration contributes to local peak stress conditions compromising the structural integrity of the assembly at the joint area, while the actual existence of weld porosity leads to serious weakness from 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 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, followed by fine polishing using diamond and silica suspensions. Microstructural observations carried out 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. 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.
An agent 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. Since it is evident, crack is propagated for the longitudinal direction showing a straight pattern with linear steps. The crack progressed alongside the weld zone of the weld, most likely following the heat affected zone (HAZ). Transverse sectioning in the tube ended in opening from 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 which had been brought on 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 because of the exposure of zinc-coated cracked face to the working environment and humidity. The aforementioned 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 may be regarded as the key failure mechanism.