Multilayer Pipe – More FAQ’s..

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 region of the weld, that also contains multiple intergranular secondary cracks. The failure is 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 round the pipe. In some cases 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 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 to the fracture area. ? Evidence of multiple secondary cracks on the HAZ area following intergranular mode. ? Presence of Zn in the interior from 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 a raw material then 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 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 carried out 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 following the installation) have triggered leakage and a costly repair in the installation, were submitted for root-cause investigation. The main topic of the failure concerned underground (buried within the earth-soil) pipes while tap water was flowing inside the tubes. Loading was typical for domestic pipelines working under low internal pressure of a few 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, without any other similar failures were reported within 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 employed in the context from the present evaluation.

Various welded component failures attributed 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 inside the relevant literature. Insufficient fusion/penetration leads to local peak stress conditions compromising the structural integrity from the assembly at the joint area, while the actual existence of weld porosity results in serious weakness in the fusion zone [3], [4]. Joining parameters and metal cleanliness are considered as critical factors towards the structural integrity in 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 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 approximately #1200 grit, accompanied by 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 using a Nikon Epiphot 300 inverted metallurgical microscope. Additionally, high magnification observations of 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 using 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 of the representative fractured pipe (27 mm outer diameter × 3 mm wall thickness) is shown in Fig. 1. Since 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 probably pursuing the heat affected zone (HAZ). Transverse sectioning from the tube led to opening in 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 that was brought on by the deep penetration and surface wetting by zinc, as it was identified by Multilayer pipe analysis. Zinc oxide or hydroxide was formed caused by the exposure of zinc-coated cracked face towards the working environment and humidity. The above 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 could be viewed as the main failure mechanism.

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