| With the rapid development of the petrochemical industry,high temperature centrifugal casting furnace tubes have long been widely used in cracking furnace and manufacturing hydrogen furnace of petrochemical plants.Since the working conditions of tube are very bad,working temperature up to 700℃-1050℃,interal pressure up to 1.8MPa and because of overheating or other errors,furnace tubes would have been severely damaged or destructed before they reach the designed life span.To be repaired,a new tube is about to be welded to the previous one.But the weldability of the service tube turns weak,the side of it is pretty easy to crack after welded,hence makes the welding repair difficult.This paper,under one Manauritea 900 hydrogen manufacturing furnace of a petrochemical enterprise,which has served 40,000 hours at a high- temperature, focuses chiefly on the microstructure alteration of the base material and welding seam pro and after its running to analysis the relationship between the damage and the weldability of the furnace tube.And secondly,adopt solution treatment to improve service furnace tube's weldability,and then apply target technology fulfilling the welding repair,providing scientific theory and practice basis for welding repair.Following are some major research contents and result obtained.(1) There are no obvious deformation through sampling and macroexaming the service furnace tube.However,a 0.5mm dark brown oxide layer has been formed on the outer side of the service-aged tube.But there are no cracks found through penetration testing at the surface both inside and outside of the tube.(2) Results of base material metallographic microstructure analysis note that the organizational structure,morphology and distribution of the service tube are significantly different from that of the new one's,the pattern of eutectic carbide has changed from frame shape to massive or chain;And the primary carbide and the secondary carbide which precipitated from the austenite previously coarsened.The damage of service tube,according to an associated document ranks 3.5,which makes the welding very difficult to do.(3) The mechanical properties of the service-aged furnace tube,such as the hardness,the tensile strength,the elongation percentage and the impact toughness at room temperature,have been determined.Result shows that the hardness was decreased,the mechanical properties becomes worse,the tensile strength is obviously lower than standard,the elongation percentage is almost close to zero,and the impact toughness has been turned very low under long time high temperature condition.The performance of base material has been affected significantly after long time service.(4) The crack which appeared in the heat affected zone near the fusion line is judged liquation crack.The Gleeble hot-ductility testing reveales that the liquefied crack's resistance of Manaurite 900 is very weak,and is very easy to crack after long time service.(5) After solution treatment,the percentage of secongary carbide decreased, the pattern of primary carbide turns frame shaped and nomogeneous microstructure, the grain boundary become narrow,the mechanical property improved,the plasticity and toughness increased,and the impact fracture is in ductile rupture.The solution treatment substantially improved the performance of the long term served furnace tube.(6) The cracking susceptibility and cracking in welding repair can be affectively avoided through applying this proposed welding procedure.The resule shows that the welded furnace meets the requirement of normal functioning and the repairing is a great success.
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