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 related to 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 around the pipe. Sometimes cracks are emanating from the tip of those discontinuities. Chemical analysis, visual inspection, optical microscopy and SEM/EDS analysis were utilised as the principal analytical techniques 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 near to the fracture area. ? Evidence of multiple secondary cracks on 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 lots of 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 as 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 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 at a temperature of 450-500 °C approximately.
A series of failures of HDPE pipe fittings occurred after short-service period (approximately 1 year following the installation) have resulted in leakage and a costly repair of the installation, were submitted for root-cause investigation. The topic of the failure concerned underground (buried in 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 couple of bars. Cracking followed a longitudinal direction plus it 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 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 employed in the context from the present evaluation.
Various welded component failures related to fusion or heat affected zone (HAZ) weaknesses, such as 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 results in 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 of the fusion zone , . Joining parameters and metal cleanliness are viewed as critical factors to 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) accompanied by ethanol cleaning and heat-stream drying.
Metallographic evaluation was performed using a Nikon Epiphot 300 inverted metallurgical microscope. Furthermore, high magnification observations from 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 utilizing an EDAX detector was also utilized 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 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 next to the weld zone in the weld, most likely pursuing the heat affected zone (HAZ). Transverse sectioning from the tube led to 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 had been caused by the deep penetration and surface wetting by zinc, as it was identified by Multilayer pipe analysis. Zinc oxide or hydroxide was formed as a consequence of the exposure of zinc-coated cracked face to the working environment and humidity. The above mentioned findings and 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 may be viewed as the main failure mechanism.