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 resulted from low ductility fracture in the area of 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 propagated through the wall thickness. Random surface cracks or folds were found across the Square Pipe. In some instances cracks are emanating from the tip of such 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 welded pipes during service. ? Investigation of failure mechanism using macro- and microfractography. Metallographic evaluation of transverse sections near to the fracture area. ? Proof multiple secondary cracks on 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 employed in numerous 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 accompanied by resistance welding and hot dip galvanizing as the best manufacturing process route. Welded pipes were produced using resistance self-welding in the steel plate by using 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 required prior to hot dip galvanizing. Hot dip galvanizing is carried out in molten Zn bath in a temperature of 450-500 °C approximately.
A number of 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 in the installation, were submitted for root-cause investigation. The main topic of the failure concerned underground (buried in the earth-soil) pipes while tap water was flowing inside the Rubber Elbow Pipe Fitting. 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 within 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 used in the context in the present evaluation.
Various welded component failures associated with fusion and/or heat affected zone (HAZ) weaknesses, such as cold and hot cracking, absence of penetration, lamellar tearing, slag entrapment, solidification cracking, gas porosity, etc. are reported within the relevant literature. Lack of fusion/penetration contributes to local peak stress conditions compromising the structural integrity in the assembly in the joint area, while the actual existence of weld porosity leads to serious weakness in the fusion zone , . Joining parameters and metal cleanliness are considered as critical factors for 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, 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 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 utilizing an EDAX detector was utilized 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 of the representative fractured pipe (27 mm outer diameter × 3 mm wall thickness) is shown in Fig. 1. Since it is evident, crack is propagated for the longitudinal direction showing a straight pattern with linear steps. The crack progressed alongside the weld zone in the weld, probably after the heat affected zone (HAZ). Transverse sectioning from the tube led to opening from 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 brought on by the deep penetration and surface wetting by zinc, since it was recognized by EDS analysis. Zinc oxide or hydroxide was formed as a consequence of the exposure of Ssaw Oil And Gas Pipe for the working environment and humidity. The above mentioned findings as well as 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 regarded as the primary failure mechanism.
Rise Steel consisted of subsidaries of Cangzhou Spiral Steel Pipe Factory, Hebei All Land Steel Pipe Factory, Hebei Yuancheng Steel Pipe Factory, Cangzhou Xinguang Thermal Insulation Pipe Factory .The company is located in Tianjin port, the largest comprehensive port and an important foreign trade port, engaging in the management of steel pipe production nearly 20 years.The company is a high-tech enterprise intigrated with independent production and sales business.We are committed to the concept of “innovation, technology and service”.
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Miao lin: [email protected]
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