Low alloy steel welded pipes buried in the earth were sent for failure analysis investigation. Failure of steel pipes was not 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 probably associated with 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. Sometimes 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 methods 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 at the HAZ area following intergranular mode. ? Presence of Zn within the interior in 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 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 making use of 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 just before hot dip galvanizing. Hot dip galvanizing is performed in molten Zn bath in a temperature of 450-500 °C approximately.
A number of failures of HDPE Pipe Welding Machine occurred after short-service period (approximately 1 year following the installation) have led to leakage and a costly repair from the installation, were submitted for root-cause investigation. The main topic of the failure concerned underground (buried within the earth-soil) pipes while faucet water was flowing within the tubes. Loading was typical for domestic pipelines working under low internal pressure of a few number 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 coupled with energy dispersive X-ray spectroscopy (EDS) were mainly employed in the context in the present evaluation.
Various welded component failures attributed to fusion or 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. Insufficient fusion/penetration results in local peak stress conditions compromising the structural integrity from the assembly on the joint area, while the existence of weld porosity brings about serious weakness from the fusion zone , . Joining parameters and metal cleanliness are considered as critical factors towards the structural integrity from 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 up to #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 heat-stream drying.
Metallographic evaluation was performed utilizing a Nikon Epiphot 300 inverted metallurgical microscope. In addition, 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 have also been employed 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 in the representative fractured pipe (27 mm outer diameter × 3 mm wall thickness) is shown in Fig. 1. As it is evident, crack is propagated to the longitudinal direction showing a straight pattern with linear steps. The crack progressed adjacent to the weld zone from the weld, probably pursuing the heat affected zone (HAZ). Transverse sectioning of the tube resulted in opening from the from the wall crack and exposure in the fracture surfaces. Microfractographic investigation performed under SEM using backscattered electron imaging revealed a “molten” layer surface morphology which had been due to the deep penetration and surface wetting by zinc, as it was identified by PEX-AL-PEX 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 may be regarded as the primary failure mechanism.