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 occurred 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 inside the weld heat affected zone and propagated with the wall thickness. Random surface cracks or folds were found around the Fashion Design Sanitary Pipe Fitting. Sometimes cracks are originating from 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 near the fracture area. ? Proof of multiple secondary cracks in the HAZ area following intergranular mode. ? Presence of Zn in the interior of the cracks manifested that HAZ sensitization and cracking occurred just before galvanizing process.
Galvanized steel tubes are employed 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 as 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 in the steel plate by making use of 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 needed 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 right after the installation) have triggered leakage as well as a costly repair from the installation, were submitted for root-cause investigation. The subject of the failure concerned underground (buried within the earth-soil) pipes while plain tap water was flowing inside the Hot Dipped Galvanized Steel Pipe. 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 in the weld zone area, while no macroscopic plastic deformation (“swelling”) was observed. Failures occurred to isolated cases, with no 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 utilized in the context of the present evaluation.
Various welded component failures related to fusion or heat affected zone (HAZ) weaknesses, such as cold and hot cracking, insufficient 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 of 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 thought 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 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, 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) then ethanol cleaning and hot air-stream drying.
Metallographic evaluation was performed using a Nikon Epiphot 300 inverted metallurgical microscope. Additionally, 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.
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 adjacent to 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 was due to the deep penetration and surface wetting by zinc, because it was recognized by EDS analysis. Zinc oxide or hydroxide was formed as a consequence of the exposure of 3pe Coating Ssaw Spiral Steel Pipe towards 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 before galvanizing process while no static tensile overload during service could 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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