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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 weld, which contains multiple intergranular secondary cracks. The failure is most probably attributed 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 Anti-Corrosive Steel Pipe. In some instances cracks are emanating from the tip of such 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. ? 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 employed in many 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 appropriate 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 in the welded tube in degreasing and pickling baths for surface cleaning and activation is needed prior to hot dip galvanizing. Hot dip galvanizing is carried out in molten Zn bath in 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 as well as a costly repair in the installation, were submitted for root-cause investigation. The main topic of the failure concerned underground (buried within the earth-soil) pipes while plain tap water was flowing inside the Transportation Steel Pipe. Loading was typical for domestic pipelines working under low internal pressure of a few number of bars. Cracking followed a longitudinal direction and it also was noticed at the weld zone area, while no macroscopic plastic deformation (“swelling”) was observed. Failures occurred to isolated cases, and no other similar failures were reported in 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 from the present evaluation.

Various welded component failures attributed to fusion or heat affected zone (HAZ) weaknesses, including cold and hot cracking, absence of 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 from the assembly at the joint area, while the existence of weld porosity results in serious weakness in the fusion zone [3], [4]. Joining parameters and metal cleanliness are viewed as critical factors towards the structural integrity in 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, followed by fine polishing using diamond and silica suspensions. Microstructural observations completed after immersion etching in Nital 2% solution (2% nitric acid in ethanol) then ethanol cleaning and hot air-stream drying.

Metallographic evaluation was performed employing a Nikon Epiphot 300 inverted metallurgical microscope. Furthermore, high magnification observations from the microstructure and fracture topography were conducted to ultrasonically cleaned specimens, employing 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 utilized 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 in the representative fractured pipe (27 mm outer diameter × 3 mm wall thickness) is shown in Fig. 1. Since it is evident, crack is propagated to the longitudinal direction showing a straight pattern with linear steps. The crack progressed adjacent to the weld zone in the weld, probably pursuing the heat affected zone (HAZ). Transverse sectioning of the tube led to opening in 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 that was due to the deep penetration and surface wetting by zinc, as it was identified by EDS analysis. Zinc oxide or hydroxide was formed caused by the exposure of Erw Square Structure Steel Tube for the working environment and humidity. The aforementioned findings as well as 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 may be viewed as the main 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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