Study On Microstructure Evolution At High Temperature And Welding Repair Process Of Reformer Furnace Tube

Hydrogen reformer furnace tube is an important part of hydrogen production equipment by steam reforming of hydrocarbons.Under the service environment of high temperature and high pressure for a long time,material deterioration often occurs and these tubes can not be used any more.Therefore,it must be replaced and maintained.While hot cracks and other defects often appear in the welding operation during replacement and maintenance,which makes it difficult to guarantee the maintenance quality.In order to solve these welding problems existing in the replacement and repair of furnace tubes,this paper firstly observed the microstructural characteristics,morphology and distribution of precipitates in welded joints of HP40Nb reformer furnace tubes deteriorated in service,and the performance parameters such as strength,hardness and impact toughness were measured.Secondly,the reformer furnace tubes with material deterioration were heat treated by integral heating in electric furnace.The morphology and change of the microstructure of the base metal after heat treatment was observed and analyzed.Thirdly,the effect of welding arc on the microstructure and mechanical properties of the welding heat affected zone on the service degraded reformer tube and the effect of welding repair were studied by using thermal simulation technology.Fourthly,TIG arc was used to conduct surface remelting treatment on the welded groove and its effect on microstructure of the service degraded reformer furnace tubes was analyzed.Finally,TIG arc remelting groove treatment combined with surfacing welding process were adopted to carry out the welding repair test.Through the comparison of the repairing effects of different welding processes,an appropriate welding repairing method for the deteriorated reformer furnace tubes was obtained.The results show that:(1)Lots of carbides are precipitated in the base metal of serviced degraded HP40Nb furnace tubes and aggregated on the austenite grain boundary of austenite to form a network chain structure,and only a small amount of carbides precipitated out from the deposited metal of the in-service welded joint;the tensile strength of the joints decreased only slightly,but its elongation and impact toughness have been seriously reduced.(2)After the whole heat treatment at 980?for 6 hours,only the grain boundary carbides were partially dissolved,which could not effectively eliminate the grain boundary precipitated phase.(3)In the thermal simulation experiments of welding heat affected zone,the precipitation degree of austenite grain boundary and intra-grain precipitates in the zone decreases gradually with the increase of thermal cycle peak temperature,and the proportion of chromium-rich phase M₂₃C₆ decreases by 59%.When the peak temperature is1250°C,a large number of short rod-like structures appear inside the grain boundary precipitates,and the intragranular precipitates almost disappear,the G phase dissolves completely to form Nb C.The material elongation increases to about 2%under the simulated welding thermal cycle at a peak temperature of 1250°C,but still far below the standard value(8%).The tensile strength in the area with the peak temperature of 1150°C or above is higher than the standard value,but the tensile strength and elongation appears the lowest value at the peak temperature of 1050°C.(4)TIG arc groove surface remelting treatment can obtain the fine grain area with the maximum depth up to 2.0 mm,the original precipitates in austenite grains of HAZ are dissolved completely,and the precipitates at grain boundary are dissolved in large quantities,which are in the form of mesh sieve.(5)TIG arc remelting groove combined with surfacing welding process can effectively inhibit the generation of cracks in the HAZ during the welding repair of the service degraded reformer furnace tubes.