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 also contains multiple intergranular secondary cracks. The failure is 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 around the Welded Steel Tube. Sometimes cracks are emanating through the tip of these discontinuities. Chemical analysis, visual inspection, optical microscopy and SEM/EDS analysis were utilised as the principal analytical techniques 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 within the interior in the cracks manifested that HAZ sensitization and cracking occurred prior to galvanizing process.
Galvanized steel tubes are employed in lots of 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 accompanied by resistance welding and hot dip galvanizing as the most suitable manufacturing process route. Welded pipes were produced using resistance self-welding from 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 required before 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 right after the installation) have led to leakage as well as a costly repair in the installation, were submitted for root-cause investigation. The topic of the failure concerned underground (buried inside the earth-soil) pipes while tap water was flowing inside the Seamless Boiler Water Wall Tubes. Loading was typical for domestic pipelines working under low internal pressure of some number of bars. Cracking followed a longitudinal direction plus it was noticed on the weld zone area, while no macroscopic plastic deformation (“swelling”) was observed. Failures occurred to isolated cases, without any other similar failures were reported in the same batch. Microstructural examination and fractographic evaluation using optical and scanning electron microscopy along with energy dispersive X-ray spectroscopy (EDS) were mainly used in the context of the present evaluation.
Various welded component failures related to fusion and heat affected zone (HAZ) weaknesses, such as hot and cold cracking, insufficient penetration, lamellar tearing, slag entrapment, solidification cracking, gas porosity, etc. are reported inside the relevant literature. Insufficient fusion/penetration contributes to local peak stress conditions compromising the structural integrity from the assembly on the joint area, while the presence of weld porosity results in serious weakness of the fusion zone , . Joining parameters and metal cleanliness are thought as critical factors for 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 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 up to #1200 grit, then fine polishing using diamond and silica suspensions. Microstructural observations performed after immersion etching in Nital 2% solution (2% nitric acid in ethanol) accompanied by ethanol cleaning and heat-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, 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 using an EDAX detector was 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 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 pursuing the heat affected zone (HAZ). Transverse sectioning in the tube ended in opening from the through 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 brought on by the deep penetration and surface wetting by zinc, since it was identified by EDS analysis. Zinc oxide or hydroxide was formed because of the exposure of Structure Lsaw Steel Pipe Price to the working environment and humidity. The above mentioned 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 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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