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 area of the weld, that also contains multiple intergranular secondary cracks. The failure is most probably attributed to intergranular cracking initiating from the outer surface inside the weld heat affected zone and spread with the wall thickness. Random surface cracks or folds were found across the Seamless Drilling Tubes. In some instances cracks are originating through 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 close to the fracture area. ? Evidence of multiple secondary cracks in 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 used in numerous 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 being a raw material then resistance welding and hot dip galvanizing as the most suitable manufacturing process route. Welded pipes were produced using resistance self-welding in 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 prior to hot dip galvanizing. Hot dip galvanizing is performed in molten Zn bath with a temperature of 450-500 °C approximately.
A number of failures of underground galvanized steel pipes occurred after short-service period (approximately 1 year after the installation) have resulted in leakage and a costly repair of the installation, were submitted for root-cause investigation. The topic of the failure concerned underground (buried inside the earth-soil) pipes while faucet water was flowing within the Seamless Boiler Steel Pipe. Loading was typical for domestic pipelines working under low internal pressure of a few handful of bars. Cracking followed a longitudinal direction and it was noticed at the weld zone area, while no macroscopic plastic deformation (“swelling”) was observed. Failures occurred to isolated cases, with no other similar failures were reported inside 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 utilized in the context from the present evaluation.
Various welded component failures attributed to fusion and heat affected zone (HAZ) weaknesses, like cold and hot 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 actual existence of weld porosity results in 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 analysis of 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, 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) then ethanol cleaning and hot air-stream drying.
Metallographic evaluation was performed utilizing 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 employing an EDAX detector was also employed 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 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 to the longitudinal direction showing a straight pattern with linear steps. The crack progressed next to the weld zone of the weld, most probably pursuing the heat affected zone (HAZ). Transverse sectioning in the tube resulted in opening of 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 caused by the exposure of Carbon Welded Steel Pipe towards the working environment and humidity. The aforementioned findings and also 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 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]
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