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 happened from low ductility fracture in the area of the weld, that 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 Black Square Steel Tubes. In some cases cracks are emanating through the tip of these discontinuities. Chemical analysis, visual inspection, optical microscopy and SEM/EDS analysis were used 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 close to the fracture area. ? Evidence of multiple secondary cracks at the HAZ area following intergranular mode. ? Presence of Zn within the interior in the cracks manifested that HAZ sensitization and cracking occurred before galvanizing process.
Galvanized steel tubes are utilized 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 as 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 applying 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 required before hot dip galvanizing. Hot dip galvanizing is conducted in molten Zn bath with 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 as well as a costly repair of the installation, were submitted for root-cause investigation. The main topic of the failure concerned underground (buried inside the earth-soil) pipes while tap water was flowing within the P235gh Seamless 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 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 in conjunction with energy dispersive X-ray spectroscopy (EDS) were mainly used in the context in the present evaluation.
Various welded component failures related to fusion or heat affected zone (HAZ) weaknesses, like cold and warm cracking, absence of penetration, lamellar tearing, slag entrapment, solidification cracking, gas porosity, etc. are reported inside the relevant literature. Absence of fusion/penetration contributes to 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 of the fusion zone , . Joining parameters and metal cleanliness are considered as critical factors towards 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 up to #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 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 utilizing an EDAX detector have also been used 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 of 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 adjacent to the weld zone of the weld, most likely after the heat affected zone (HAZ). Transverse sectioning of the tube resulted in opening of the with the wall crack and exposure from 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, as it was recognized by EDS analysis. Zinc oxide or hydroxide was formed because of the exposure of X80 Carbon Ssaw Steel Pipe for the working environment and humidity. The above 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 may be regarded 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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Miao lin: [email protected]
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