Evaluation of HIC Resistance for Thick-wall Welded Pipe
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Abstract:
It is required for the steel materials used in the sour environment to have sufficient resistance to hydrogen induced cracking(HIC). For line pipe steels, HIC resistance could be varied during pipe making process due to the large plastic deformation applied in the thick-wall pipe. In order to figure out such effect, HIC tests were performed not only in the plate condition but in the pipe condition and their results were compared in terms of cracking ratio. Test results demonstrated a detrimental effect of plastic deformation to HIC resulting in a substantial increase in the cracking ratio after pipe forming process. All of the cracks found in the pipe material were located in the outer layer of pipe where the tensile strain was resulted during pipe forming stage. In order to understand the HIC resistance of the pipe but in the plate condition, it was suggested to pre-strain the plate to some extent before the HIC test.Keywords:
Pipe
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Charpy impact test
Base metal
Pipe
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The microstructure and chemical composition of cracked water-cooling wall pipe used in power plant were analyzed,and its fracture was analyzed by SEM.The result shows that there are residual stress and inclusions in the structure at weld end between pipe and fin plate,and under working pressure the cracks initiate in weld end,then they propagate along tensile stress area and eventually result in fracture.
Pipe
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Brittleness
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The characteristics and causes of longitudinal cracking of IU X95 drill pipe with size ofφ101.6 mm×8.38 mm during using were analyzed.The results show that the drill pipe cracking was brittle,and the surface micro-crack suffered from contaminating of metal with low melting point,such as Pb and Sn,to result in poor property and crack extension easily.The dill pipe cracked when cracking extended to some degree under the action of temperature and tensile stress.
Drill pipe
Brittleness
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During the inspection of the heating surface of No. 2 boiler in a thermal power plant, it was found that the fins of the thin tube in the middle partition wall were cracked. In order to find out the cause of fin cracking, a comprehensive experimental study was carried out on the thin tube of middle partition wall with fin cracking by means of appearance analysis, mechanical property analysis, chemical composition analysis and metallographic microstructure detection. The results showed that the fin cracking of thin tube in the middle wall of Chen 2 boiler was due to the large stress on the thin tube in the middle wall caused by the poor expansion of the tube screen during the operation, and finally the fin weld root which was easy to produce stress concentration cracks. It was recommended to find out the causes of the poor expansion of the thinning pipe of the middle partition wall on site, and at the same time, focused on the root of the nearby tube panel fin weld to check whether there was any cracking, and replaced the defects in time to prevent the similar cracking failure from happening again.
Fin
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The cracking cause of corrugated pipe of stainless steel used in an electromechanical product was analyzed by means of SEM.energy spectrum and so on.The results showed that the welded joint region was sensitized during welding,subsequently,corrosion crack along the grain boundary formed by pickling,the corrugated pipe bore radial direction alternating load during the vibration test,the crack began to extend from the bottom of intergranular corrosion cracks,lastly,the crack penetrated the pipe wall.The fracture surface characterized with obvious cowrie pattern lines,that proved the corrugated pipe occurred fatigue failure.
Pickling
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The cracking reasons of a ZG15Cr1Mo1V-B2 steel high preesure outer cylinder which cracked when pumping were analyzed by means of residual stress test,mechanical properties test,fracture analysis and metallographic examination.The results show that the cracking started from the repairing welding area where loose defects concentrated and the ductility of the material didn't meet with the requirement.So the bearing capacity of the defect area decreased and the stress concentration was aggravated.At the same time,it has a high residual stress and a large hardness gradient at the repairing welding area.So it could be concluded that the main reasons for the cylinder cracking were that the casting defects were not cleaned thoroughly during repairing welding,the welding quality was not high enough and the stress elimination was inadequate.
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Hydrostatic test
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Line (geometry)
Weld line
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The inlet branch pipes connected the aggregate pipes and furnace tubes of a reformer, the outlines of them extended as “S” shapes, and they were made of austenitic steel 1Cr18Ni9Ti. After about 60,000 hours service, a lot of cracks were found near the elbows of inlet branch pipe and joints with the furnace tubes, many of them propagated through the pipe walls, and lead to leakage of raw material. In order to explore the failure causes, the microstructure of cracks and corrosion ingredients remained on fracture surface were analyzed by scanning electron microscopy (SEM), chemical composition, metallographic and mechanical properties of the branch-pipe steels were examined. The test result showed that the cracks propagated from the outside to inside of the pipe wall, the fracture surfaces were almost flat, there was no obvious deformation existed at the fracture section. Certain percent of chlorine and sulphur elements could be found and the secondary cracks could be observed on the microstructure fracture surfaces. The inlet branch pipes were elongated when they were connected with the aggregate pipes and furnace tubes, the stress were analyzed by finite element method (FEM) under the tensile deformation, the results showed that the stresses near the elbow were high. It could be concluded that the crack of the inlet branch pipe were stress corrosion cracks accelerated by the aggressive medium rich of CI and S ion and high stresses. Based on the causes analysis results, the corresponding countermeasures and improvements were suggested.
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A steam pressure pipe broke into pieces in the course of normal operation. The chemical composition and mechanical property analyses proved that the material of the pipe was good. However,the metallographic examination and fractographic observation showed that there were some defects in the weld joint of the pipe and the fixed bracket,where there was a zone of high stress concentration. Furthermore,mechanical analysis of the weld joint explained the reason why there was high peak stress and it indicated that the wall thickness of the pipe was too thin to endure the severe stress condition. In addition,the possibility of steam hammer or water hammer and their possible influence were discussed. Based on these analyses,it was concluded that the cracking occurred first in the weld joint between a steam pipe and a fixed bracket. And then it lead to the rupture of the whole pipe. The cracking resulted from poor quality of the welding and inapposite design of the pipe thickness as well as the structure of the joint.
Hammer
Pipe
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