Low alloy steel welded pipes buried in the ground were sent for failure analysis investigation. Failure of steel pipes was not due to tensile ductile overload but resulted from low ductility fracture in the area of the weld, which contains multiple intergranular secondary cracks. The failure is probably associated with intergranular cracking initiating from the outer surface in the weld heat affected zone and propagated with the wall thickness. Random surface cracks or folds were found across the Steel Pipe Welding. In some cases cracks are emanating from the tip of these discontinuities. Chemical analysis, visual inspection, optical microscopy and SEM/EDS analysis were utilized 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 the fracture area. ? Proof multiple secondary cracks in the HAZ area following intergranular mode. ? Presence of Zn within the interior of the cracks manifested that HAZ sensitization and cracking occurred before galvanizing process.
Galvanized steel tubes are utilized 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 best manufacturing process route. Welded pipes were produced using resistance self-welding from 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 required prior to hot dip galvanizing. Hot dip galvanizing is conducted in molten Zn bath at 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 led to leakage along with a costly repair of the installation, were submitted for root-cause investigation. The subject of the failure concerned underground (buried inside the earth-soil) pipes while faucet water was flowing in the Lsaw Straight Welded Pipe. Loading was typical for domestic pipelines working under low internal pressure of some couple of bars. Cracking followed a longitudinal direction and it 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 coupled with energy dispersive X-ray spectroscopy (EDS) were mainly employed in the context in the present evaluation.
Various welded component failures related to fusion and heat affected zone (HAZ) weaknesses, including hot and cold cracking, insufficient penetration, lamellar tearing, slag entrapment, solidification cracking, gas porosity, etc. are reported in the relevant literature. Insufficient fusion/penetration contributes to local peak stress conditions compromising the structural integrity in the assembly on the joint area, while the presence of weld porosity leads to serious weakness in the fusion zone , . Joining parameters and metal cleanliness are considered 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 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 as much as #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) followed by ethanol cleaning and hot air-stream drying.
Metallographic evaluation was performed utilizing a Nikon Epiphot 300 inverted metallurgical microscope. In addition, high magnification observations in 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 employing 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 from the representative fractured pipe (27 mm outer diameter × 3 mm wall thickness) is shown in Fig. 1. Because 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 following the heat affected zone (HAZ). Transverse sectioning in 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 had been caused by the deep penetration and surface wetting by zinc, since it was identified by EDS analysis. Zinc oxide or hydroxide was formed as a consequence of the exposure of 3pe Coating Steel Pipes to 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 before galvanizing process while no static tensile overload during service might be considered 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]
Amy Shi: [email protected]
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