Multilayer Pipe – More FAQ’s..

Low alloy steel welded pipes buried in the ground were sent for failure analysis investigation. Failure of steel pipes had not been due to 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 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 round the pipe. In some cases cracks are emanating from the tip of these 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 to the fracture area. ? Proof of multiple secondary cracks at the HAZ area following intergranular mode. ? Presence of Zn inside the interior of the cracks manifested that HAZ sensitization and cracking occurred just before galvanizing process.

Galvanized steel tubes are utilized 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 as being 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 from 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 required before hot dip galvanizing. Hot dip galvanizing is conducted in molten Zn bath with 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 resulted in leakage and a costly repair of the installation, were submitted for root-cause investigation. The subject of the failure concerned underground (buried inside the earth-soil) pipes while faucet 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 plus it was noticed on 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 attributed to fusion and/or heat affected zone (HAZ) weaknesses, like cold and warm cracking, absence of penetration, lamellar tearing, slag entrapment, solidification cracking, gas porosity, etc. are reported within the relevant literature. Insufficient fusion/penetration leads to local peak stress conditions compromising the structural integrity from the assembly on the joint area, while the actual existence of weld porosity leads to serious weakness from the fusion zone [3], [4]. Joining parameters and metal cleanliness are considered as critical factors for the structural integrity of 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 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 performed after immersion etching in Nital 2% solution (2% nitric acid in ethanol) accompanied 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, 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 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 towards the longitudinal direction showing a straight pattern with linear steps. The crack progressed alongside the weld zone in the weld, most likely following the heat affected zone (HAZ). Transverse sectioning of the tube ended in opening in 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 caused by the deep penetration and surface wetting by zinc, as it was recognized by PEX-AL-PEX pipe analysis. Zinc oxide or hydroxide was formed as a consequence of the exposure of zinc-coated cracked face for 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 before galvanizing process while no static tensile overload during service might be considered as the key failure mechanism.