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 resulted from low ductility fracture in the weld, that also contains multiple intergranular secondary cracks. The failure is probably attributed to intergranular cracking initiating from the outer surface in the weld heat affected zone and propagated through the wall thickness. Random surface cracks or folds were found around the Lsaw Welded Black Pipe Sizes Hot Sale. In some cases cracks are originating through the tip of these discontinuities. Chemical analysis, visual inspection, optical microscopy and SEM/EDS analysis were used 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 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 prior to galvanizing process.
Galvanized steel tubes are employed in lots of outdoors and indoors application, including hydraulic installations for central heating system units, water supply for domestic and industrial use. Seamed galvanized tubes are fabricated by low alloy steel strip as 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 of 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 necessary prior to hot dip galvanizing. Hot dip galvanizing is performed 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 after the installation) have triggered leakage and a costly repair of the installation, were submitted for root-cause investigation. The main topic of the failure concerned underground (buried within the earth-soil) pipes while tap water was flowing within the Pre Insulated Pipe. Loading was typical for domestic pipelines working under low internal pressure of some couple of bars. Cracking followed a longitudinal direction plus it 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 coupled with energy dispersive X-ray spectroscopy (EDS) were mainly used in the context in the present evaluation.
Various welded component failures attributed to fusion and heat affected zone (HAZ) weaknesses, such as cold and hot cracking, insufficient penetration, lamellar tearing, slag entrapment, solidification cracking, gas porosity, etc. are reported inside the relevant literature. Insufficient fusion/penetration results in local peak stress conditions compromising the structural integrity of the assembly on the joint area, while the actual existence of weld porosity results in serious weakness from the fusion zone , . Joining parameters and metal cleanliness are viewed as critical factors to the structural integrity from the welded structures.
Chemical analysis of 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, then fine polishing using diamond and silica suspensions. Microstructural observations performed 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. Additionally, 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 using an EDAX detector was also 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. Since 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 following the heat affected zone (HAZ). Transverse sectioning in the tube ended in opening in 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 that was brought on by the deep penetration and surface wetting by zinc, as it was identified by EDS analysis. Zinc oxide or hydroxide was formed because of the exposure of Seamless Line 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 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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Hamer Chen:[email protected]
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Miao lin: [email protected]
Amy Shi: [email protected]
Hamer Chen:+86 18202505824
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Amy Shi: +86 18630426996