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 weld, which also contains multiple intergranular secondary cracks. The failure is most likely attributed 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 Seamless Stainless Steel Tube. In some cases cracks are originating from the tip of these discontinuities. Chemical analysis, visual inspection, optical microscopy and SEM/EDS analysis were utilized 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. ? Proof multiple secondary cracks at the HAZ area following intergranular mode. ? Presence of Zn inside the interior from the cracks manifested that HAZ sensitization and cracking occurred just before galvanizing process.
Galvanized steel tubes are employed 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 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 in 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 prior to hot dip galvanizing. Hot dip galvanizing is performed in molten Zn bath in a temperature of 450-500 °C approximately.
Several failures of underground galvanized steel pipes occurred after short-service period (approximately 1 year following the installation) have led to leakage along with a costly repair of the installation, were submitted for root-cause investigation. The topic of the failure concerned underground (buried within the earth-soil) pipes while faucet water was flowing in the Threaded Galvanized Pipe. Loading was typical for domestic pipelines working under low internal pressure of some couple 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, 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 used in the context from the present evaluation.
Various welded component failures attributed to fusion and heat affected zone (HAZ) weaknesses, including cold and hot cracking, insufficient 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 from the assembly in the joint area, while the existence of weld porosity brings about serious weakness of the fusion zone , . Joining parameters and metal cleanliness are thought as critical factors to 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 carried out 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. Additionally, 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.
An agent sample from failed steel pipes was submitted for investigation. Both pipes experience macroscopically identical failure patterns. A characteristic macrograph of the representative fractured pipe (27 mm outer diameter × 3 mm wall thickness) is shown in Fig. 1. Because it is evident, crack is propagated to the longitudinal direction showing a straight pattern with linear steps. The crack progressed alongside the weld zone of the weld, probably after the heat affected zone (HAZ). Transverse sectioning from the tube led to opening from the from the wall crack and exposure of the fracture surfaces. Microfractographic investigation performed under SEM using backscattered electron imaging revealed a “molten” layer surface morphology which had been caused by the deep penetration and surface wetting by zinc, because it was identified by EDS analysis. Zinc oxide or hydroxide was formed as a consequence of the exposure of House Building Material Steel Pipe for 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 regarded as the primary failure mechanism.