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PEX-AL-PEX Pipe – Seek Advice..

Posted on April 16, 2019 in Sponsors

Low alloy steel welded pipes buried in the earth were sent for failure analysis investigation. Failure of steel pipes had not been brought on 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 most probably attributed to 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. In some cases cracks are emanating from the tip of such 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 HDPE pipe fittings 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 at 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 used 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 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 from the steel plate by using constant contact pressure for current flow. Successive pickling was realized in diluted HCl acid bath. Rinsing from the welded tube in degreasing and pickling baths for surface cleaning and activation is necessary before 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 triggered leakage and a costly repair in 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 handful of bars. Cracking followed a longitudinal direction plus it was noticed on the weld zone area, while no macroscopic plastic deformation (“swelling”) was observed. Failures occurred to isolated cases, with 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 utilized in the context from the present evaluation.

Various welded component failures associated with fusion and/or heat affected zone (HAZ) weaknesses, such as hot and cold cracking, insufficient penetration, lamellar tearing, slag entrapment, solidification cracking, gas porosity, etc. are reported within the relevant literature. Absence of fusion/penetration leads to 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 [3], [4]. Joining parameters and metal cleanliness are viewed as critical factors to 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 employing 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, 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 using a Nikon Epiphot 300 inverted metallurgical microscope. Additionally, 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 using 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. Because it is evident, crack is propagated to the longitudinal direction showing a straight pattern with linear steps. The crack progressed alongside the weld zone in the weld, most likely pursuing the heat affected zone (HAZ). Transverse sectioning of the tube led to opening in the from 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 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 for the working environment and humidity. The aforementioned 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 might be regarded as the primary failure mechanism.