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 area of the weld, which contains multiple intergranular secondary cracks. The failure is probably related to intergranular cracking initiating from the outer surface within 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 those 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 near the fracture area. ? Proof 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 numerous 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 of 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 necessary before hot dip galvanizing. Hot dip galvanizing is performed in molten Zn bath at a temperature of 450-500 °C approximately.
Several failures of HDPE Pipe Welding Machine occurred after short-service period (approximately 1 year following the installation) have led to leakage as well as a costly repair of the installation, were submitted for root-cause investigation. The subject of the failure concerned underground (buried within the earth-soil) pipes while faucet water was flowing within the tubes. Loading was typical for domestic pipelines working under low internal pressure of some number of bars. Cracking followed a longitudinal direction and 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 in conjunction with energy dispersive X-ray spectroscopy (EDS) were mainly used in the context in the present evaluation.
Various welded component failures related to fusion or heat affected zone (HAZ) weaknesses, like cold and hot cracking, insufficient penetration, lamellar tearing, slag entrapment, solidification cracking, gas porosity, etc. are reported within the relevant literature. Insufficient fusion/penetration contributes to local peak stress conditions compromising the structural integrity from the assembly at the joint area, while the existence of weld porosity brings about serious weakness from the fusion zone , . Joining parameters and metal cleanliness are considered 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 as much as #1200 grit, accompanied 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. In addition, 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 employing 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 of the representative fractured pipe (27 mm outer diameter × 3 mm wall thickness) is shown in Fig. 1. Because it is evident, crack is propagated for the longitudinal direction showing a straight pattern with linear steps. The crack progressed alongside the weld zone in the weld, most probably following the heat affected zone (HAZ). Transverse sectioning in the tube led to opening of the through the wall crack and exposure from the fracture surfaces. Microfractographic investigation performed under SEM using backscattered electron imaging revealed a “molten” layer surface morphology that was brought on by 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 and also 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 might be considered as the key failure mechanism.