Low alloy steel welded pipes buried in the earth were sent for failure analysis investigation. Failure of steel pipes was not 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 likely attributed to 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 around 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 used as the principal analytical approaches 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 to the fracture area. ? Proof multiple secondary cracks at 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 numerous 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 being a raw material accompanied by resistance welding and hot dip galvanizing as the best manufacturing process route. Welded pipes were produced using resistance self-welding of 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 needed just before hot dip galvanizing. Hot dip galvanizing is conducted 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 right after the installation) have resulted in leakage along with 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 plain tap water was flowing in the tubes. Loading was typical for domestic pipelines working under low internal pressure of some 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, and 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 used in the context from the present evaluation.
Various welded component failures related to fusion and/or heat affected zone (HAZ) weaknesses, like cold and warm cracking, insufficient 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 on the joint area, while the presence of weld porosity leads to serious weakness of the fusion zone , . Joining parameters and metal cleanliness are considered as critical factors towards 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 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, then fine polishing using diamond and silica suspensions. Microstructural observations completed after immersion etching in Nital 2% solution (2% nitric acid in ethanol) then ethanol cleaning and heat-stream drying.
Metallographic evaluation was performed utilizing a Nikon Epiphot 300 inverted metallurgical microscope. Additionally, 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 utilizing an EDAX detector was used 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. As it is evident, crack is propagated for the longitudinal direction showing a straight pattern with linear steps. The crack progressed adjacent to the weld zone in the weld, probably following the heat affected zone (HAZ). Transverse sectioning of the tube led to opening of 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 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 because of the exposure of zinc-coated cracked face to the working environment and humidity. The above 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 could be viewed as the main failure mechanism.