Low alloy steel welded pipes buried in the earth were sent for failure analysis investigation. Failure of steel pipes was not caused by tensile ductile overload but resulted from low ductility fracture in the area of the weld, which also 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 those discontinuities. Chemical analysis, visual inspection, optical microscopy and SEM/EDS analysis were used as the principal analytical techniques for the failure investigation.
Low ductility fracture of HDPE pipe during service. ? Investigation of failure mechanism using macro- and microfractography. Metallographic evaluation of transverse sections near the fracture area. ? Proof multiple secondary cracks at the HAZ area following intergranular mode. ? Presence of Zn within the interior from the cracks manifested that HAZ sensitization and cracking occurred prior to 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 suitable 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 in the welded tube in degreasing and pickling baths for surface cleaning and activation is needed prior to hot dip galvanizing. Hot dip galvanizing is carried out in molten Zn bath with 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 led to leakage as well as a costly repair in the installation, were submitted for root-cause investigation. The topic of the failure concerned underground (buried inside the earth-soil) pipes while tap water was flowing in the tubes. Loading was typical for domestic pipelines working under low internal pressure of a few couple of bars. Cracking followed a longitudinal direction and it was noticed at the weld zone area, while no macroscopic plastic deformation (“swelling”) was observed. Failures occurred to isolated cases, and no other similar failures were reported in 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 of the present evaluation.
Various welded component failures attributed to fusion or heat affected zone (HAZ) weaknesses, such as cold and warm cracking, lack of penetration, lamellar tearing, slag entrapment, solidification cracking, gas porosity, etc. are reported inside the relevant literature. Lack of fusion/penetration leads to local peak stress conditions compromising the structural integrity in the assembly at the joint area, while the existence of weld porosity leads to serious weakness of the fusion zone , . 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 approximately #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 hot air-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, 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 employing an EDAX detector was employed to gold sputtered samples for qfsnvy elemental chemical analysis.
A representative 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 from the weld, most probably after the heat affected zone (HAZ). Transverse sectioning from the tube ended in opening of the from 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 was due to the deep penetration and surface wetting by zinc, because it was identified by Multilayer pipe analysis. Zinc oxide or hydroxide was formed because of the exposure of zinc-coated cracked face towards the working environment and humidity. The above findings as well as 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 primary failure mechanism.