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 area of the weld, which contains multiple intergranular secondary cracks. The failure is most likely associated with intergranular cracking initiating from the outer surface inside the weld heat affected zone and spread through the wall thickness. Random surface cracks or folds were found across the Carbon Steel Elbow. In some instances cracks are originating from the tip of such discontinuities. Chemical analysis, visual inspection, optical microscopy and SEM/EDS analysis were used 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. ? Proof multiple secondary cracks in the HAZ area following intergranular mode. ? Presence of Zn in the interior from the cracks manifested that HAZ sensitization and cracking occurred just before galvanizing process.
Galvanized steel tubes are utilized in many 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 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 carried out in molten Zn bath in a temperature of 450-500 °C approximately.
Several failures of underground galvanized steel pipes occurred after short-service period (approximately 1 year following the installation) have resulted in leakage along with a costly repair in the installation, were submitted for root-cause investigation. The subject of the failure concerned underground (buried inside the earth-soil) pipes while plain tap water was flowing in the Rubber Elbow Pipe Fitting. Loading was typical for domestic pipelines working under low internal pressure of a few handful 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, and no 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 utilized in the context of the present evaluation.
Various welded component failures related to fusion and heat affected zone (HAZ) weaknesses, including hot and cold cracking, insufficient penetration, lamellar tearing, slag entrapment, solidification cracking, gas porosity, etc. are reported within the relevant literature. Absence of fusion/penetration leads to local peak stress conditions compromising the structural integrity in the assembly on the joint area, while the existence of weld porosity leads to serious weakness of the fusion zone , . Joining parameters and metal cleanliness are considered as critical factors to 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 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 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, 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 utilizing 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. Since it is evident, crack is propagated for the longitudinal direction showing a straight pattern with linear steps. The crack progressed next to the weld zone of the weld, most probably after the heat affected zone (HAZ). Transverse sectioning of the tube resulted in opening in the from the wall crack and exposure in the fracture surfaces. Microfractographic investigation performed under SEM using backscattered electron imaging revealed a “molten” layer surface morphology which had been due to the deep penetration and surface wetting by zinc, since it was recognized by EDS analysis. Zinc oxide or hydroxide was formed caused by the exposure of Erw Welded Steel Pipe for the working environment and humidity. The above mentioned findings and 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 could be considered 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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