Low alloy steel welded pipes buried in the ground were sent for failure analysis investigation. Failure of steel pipes was not caused by tensile ductile overload but happened from low ductility fracture in 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 through the wall thickness. Random surface cracks or folds were found across the Ssaw For Used Oil Steel Pipe. In some instances cracks are originating from the tip of such discontinuities. Chemical analysis, visual inspection, optical microscopy and SEM/EDS analysis were utilised as the principal analytical techniques 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 close to the fracture area. ? Evidence of multiple secondary cracks at the HAZ area following intergranular mode. ? Presence of Zn inside the interior of 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 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 most suitable 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 necessary just before hot dip galvanizing. Hot dip galvanizing is performed in molten Zn bath with a temperature of 450-500 °C approximately.
Several failures of underground galvanized steel pipes occurred after short-service period (approximately 1 year right after the installation) have resulted in leakage along with a costly repair of the installation, were submitted for root-cause investigation. The topic of the failure concerned underground (buried in the earth-soil) pipes while faucet water was flowing in the Ductile Iron Pipes. 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 on the weld zone area, while no macroscopic plastic deformation (“swelling”) was observed. Failures occurred to isolated cases, without any 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 employed in the context of the present evaluation.
Various welded component failures attributed to fusion and heat affected zone (HAZ) weaknesses, like cold and warm cracking, insufficient penetration, lamellar tearing, slag entrapment, solidification cracking, gas porosity, etc. are reported within the relevant literature. Lack of fusion/penetration results in local peak stress conditions compromising the structural integrity of the assembly on the joint area, while the presence of weld porosity leads to serious weakness from 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 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 as much as #1200 grit, then 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 employing a Nikon Epiphot 300 inverted metallurgical microscope. In addition, high magnification observations from the microstructure and fracture topography were conducted to ultrasonically cleaned specimens, working with 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 used 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 in 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, probably pursuing the heat affected zone (HAZ). Transverse sectioning of the tube ended in opening of the with 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 brought on by 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 Welded Structure Steel Pipe for the working environment and humidity. The above findings and 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 might be viewed as the key 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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