Low alloy steel welded pipes buried in the ground were sent for failure analysis investigation. Failure of steel pipes was not brought on by tensile ductile overload but occurred from low ductility fracture in the area of the weld, which also contains multiple intergranular secondary cracks. The failure is most probably related to intergranular cracking initiating from the outer surface within the weld heat affected zone and spread with the wall thickness. Random surface cracks or folds were found around the 90 Degree Elbow Pipe Fitting. Sometimes cracks are originating through the tip of such 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 welded pipes during service. ? Investigation of failure mechanism using macro- and microfractography. Metallographic evaluation of transverse sections near the fracture area. ? Evidence of multiple secondary cracks on the HAZ area following intergranular mode. ? Presence of Zn in the interior from the cracks manifested that HAZ sensitization and cracking occurred just before galvanizing process.
Galvanized steel tubes are utilized in many 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 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 of the steel plate by applying 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 needed prior to 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 underground galvanized steel pipes 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 topic of the failure concerned underground (buried inside the earth-soil) pipes while faucet water was flowing inside the Erw Line Pipe Manufacturers. Loading was typical for domestic pipelines working under low internal pressure of some 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 within the same batch. Microstructural examination and fractographic evaluation using optical and scanning electron microscopy along with energy dispersive X-ray spectroscopy (EDS) were mainly utilized in the context from the present evaluation.
Various welded component failures attributed to fusion and heat affected zone (HAZ) weaknesses, including hot and cold cracking, absence of penetration, lamellar tearing, slag entrapment, solidification cracking, gas porosity, etc. are reported in the relevant literature. Lack of fusion/penetration leads to local peak stress conditions compromising the structural integrity in the assembly on the joint area, while the existence of weld porosity leads to serious weakness in the fusion zone , . Joining parameters and metal cleanliness are thought 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 using 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, followed by fine polishing using diamond and silica suspensions. Microstructural observations carried out after immersion etching in Nital 2% solution (2% nitric acid in ethanol) followed by ethanol cleaning and heat-stream drying.
Metallographic evaluation was performed using a Nikon Epiphot 300 inverted metallurgical microscope. In addition, 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 utilizing an EDAX detector have also been employed to gold sputtered dkmfgb for local elemental chemical analysis.
A representative 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. Since it is evident, crack is propagated towards the longitudinal direction showing a straight pattern with linear steps. The crack progressed adjacent to the weld zone in the weld, most probably following the heat affected zone (HAZ). Transverse sectioning of the tube ended in opening of the with 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 recognized by EDS analysis. Zinc oxide or hydroxide was formed caused by the exposure of Pe Coating Spiral Steel Pipes for the working environment and humidity. The above mentioned 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 might be viewed as the key failure mechanism.