Low alloy steel welded pipes buried in the ground were sent for failure analysis investigation. Failure of steel pipes had not been due to 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. Sometimes cracks are emanating from the tip of such discontinuities. Chemical analysis, visual inspection, optical microscopy and SEM/EDS analysis were utilized as the principal analytical methods for the failure investigation.
Low ductility fracture of HDPE pipe during service. ? Investigation of failure mechanism using macro- and microfractography. Metallographic evaluation of transverse sections close to the fracture area. ? Evidence of 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 just before galvanizing process.
Galvanized steel tubes are utilized in many 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 being a raw material followed by resistance welding and hot dip galvanizing as the best manufacturing process route. Welded pipes were produced using resistance self-welding in the steel plate by applying 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 before hot dip galvanizing. Hot dip galvanizing is conducted in molten Zn bath in 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 led to leakage along with a costly repair from the installation, were submitted for root-cause investigation. The subject of the failure concerned underground (buried within the earth-soil) pipes while plain tap 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 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 within the same batch. Microstructural examination and fractographic evaluation using optical and scanning electron microscopy along with energy dispersive X-ray spectroscopy (EDS) were mainly used in the context from the present evaluation.
Various welded component failures attributed to fusion and heat affected zone (HAZ) weaknesses, including hot and cold cracking, absence 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 of the assembly in the joint area, while the presence of weld porosity results in serious weakness of the fusion zone , . Joining parameters and metal cleanliness are considered as critical factors for the structural integrity of 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 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 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) followed by ethanol cleaning and hot air-stream drying.
Metallographic evaluation was performed using a Nikon Epiphot 300 inverted metallurgical microscope. Additionally, 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 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 in the representative fractured pipe (27 mm outer diameter × 3 mm wall thickness) is shown in Fig. 1. Because it is evident, crack is propagated towards the longitudinal direction showing a straight pattern with linear steps. The crack progressed next to the weld zone of the weld, most likely pursuing the heat affected zone (HAZ). Transverse sectioning of the tube led to opening of the with 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 was due to the deep penetration and surface wetting by zinc, because it was identified by Multilayer pipe analysis. Zinc oxide or hydroxide was formed caused by the exposure of zinc-coated cracked face for the working environment and humidity. The above mentioned findings as well as 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 may be considered as the primary failure mechanism.