HDPE Pipe Fittings – Common Questions..

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, that also contains multiple intergranular secondary cracks. The failure is most probably associated with 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. 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 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. ? Proof of multiple secondary cracks on the HAZ area following intergranular mode. ? Presence of Zn within the interior from 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 being a raw material followed by resistance welding and hot dip galvanizing as the most suitable manufacturing process route. Welded pipes were produced using resistance self-welding from the steel plate by applying 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 number of failures of HDPE Pipe Welding Machine occurred after short-service period (approximately 1 year right after the installation) have resulted in leakage as well as a costly repair from 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 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 on the weld zone area, while no macroscopic plastic deformation (“swelling”) was observed. Failures occurred to isolated cases, without any 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 employed in the context in the present evaluation.

Various welded component failures associated with fusion and heat affected zone (HAZ) weaknesses, including hot and cold cracking, lack of penetration, lamellar tearing, slag entrapment, solidification cracking, gas porosity, etc. are reported inside the relevant literature. Absence of fusion/penetration results in 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 of the fusion zone [3], [4]. Joining parameters and metal cleanliness are considered as critical factors for the structural integrity from 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 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. Furthermore, high magnification observations from 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 employing an EDAX detector have also been 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 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 for the longitudinal direction showing a straight pattern with linear steps. The crack progressed next to the weld zone of the weld, most probably after the heat affected zone (HAZ). Transverse sectioning in the tube led to opening in 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 which had been brought on by the deep penetration and surface wetting by zinc, as it was identified by PEX-AL-PEX pipe analysis. Zinc oxide or hydroxide was formed caused by the exposure of zinc-coated cracked face towards the working environment and humidity. The aforementioned 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.

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