| P91 steel is the primary material for steam pipelines in thermal power generating units owing to its exceptional high temperature strength,creep resistance,and good weldability.With the continual progress of China’s power industry towards ultra-supercritical coal-fired power generation technology,high-capacity,high-steam-parameter thermal power units pose greater challenges to the welded joints of high temperature and pressure pipelines.The width of the heataffected zone(HAZ)of the welded joints is too narrow to be prepared the traditional mechanical samples to accurately judge the role of each subzone in the overall failure process.In the present study,the microstructure and mechanical properties of domestic P91 steel welded joints with50000 hours in service along the radial direction are first investigated,Then the heat treatment method is used to simulate the welded heat cycle of domestic P91 steel base metal to obtain the heat treatment process parameters for preparing bulk samples in each subregion of the heataffected zone of the welded joints.Finally,the microstructure properties of the thermal simulated sample accelerately thermally aged at 650℃ for 200 hours aging are studied.The research are results as follows.The interior welded joint of domestic P91 steel pipe used in the main steam pipeline with50000 hours of service only has clear lath martensite orientation in the over tempered zone,while the lath martensite in the middle and exterior wall of the pipe has recovery decomposition only in the fine grain zone,where the morphological characteristics of lath martensite are not obvious.And the ferrite grain size in inter critical zone was 31% larger than that in the middle and exterior walls of the pipe.Additionally,larger precipitates were observed in the interior of the pipe,resulting in the lowest strength but highest ductility.And the smaller precipitates were observed in the middle wall of the pipe,resulting in the highest strength at room temperature and intermediate ductility.The exterior wall of the pipe exhibited the best high temperature strength but the lowest ductility.The hardness of the base metal and inter critical zones in the middle of the tube is higher than that of the inner wall of the tube,and the corresponding position of the outer wall of the tube is about 5%,while the coarse-grain zone is about 6% lower than that of the interior and the exterior wall.With the widest regions of low hardness in the inter critical zone and fine grain zone prone to creep damage.Therefore,it is determined that the weak zone of the welded joint in different positions along the radial direction of domestic thick wall P91 steel pipes is located on the exterior wall of the pipe.The domestic P91 steel base metal was treated with normalizing at 800 ℃,850 ℃,900 ℃,950 ℃,1000 ℃,1050 ℃,1100 ℃,1150 ℃,1200 ℃ and 1300 ℃ and tempering at 700 ℃,755 ℃,800℃.The specimens were scrutinized for their microstructure and attributes under varying heat treatment conditions.The outcomes manifest: the microstructure did not undergo significant alterations after normalizing at 800℃.Normalizing at 850℃ and 950℃ led to the formation of ferrite particles.Normalizing at 950℃ and 1000℃ yielded finely grained lath martensite,whereas,at 1050 ℃,the size of the martensitic laths expanded.Normalizing at1100 ℃,1150 ℃,and 1200 ℃ resulted in remarkably coarser lath martensite.Eventually,normalizing at 1300 ℃ brought forth the development of δ ferrite.Based on the findings,the heat-affected zone of welded joints in domestic P91 steel was partitioned into six regions,namely,the over tempered zone,inter critical zone,fine grain zone,grain size transformation zone,martensitic coarse grain zone and δ ferrite-martensitic coarse grain zone.The domestic P91 steel normalizing at 800 ℃,850 ℃,950 ℃,1300 ℃ & tempering at 755℃ was accelerated aging at 650 ℃ for 200 hours is equivalent to 54267 hours at 569 ℃.The microstructure and properties of thermal simulation of heat-affected zone P91 steel are well matched with the over tempered zone,inter critical zone,fine grain zone and coarse grain zone of the 50000 hours in service of the domestic P91 welded joint exterior walls of the pipe.After that,thermal simulation samples were accelerated ageing for 500 hours when the Laves phases begin to appear.At this time,the hardness of inter critical decreases to around 162 HBW,which is close to the allowable hardness for welded joints of domestic P91 steel and may induce Type IV cracking.The over tempered zone,fine grained zone,and coarse grained softening zone only experience a hardness reduction of approximately 185 HBW after exceeding the service life of the relevant power unit(30 years).Moreover,the grain size transition zone,martensite coarse grained region,and δ ferrite-martensite coarse grained region require an equivalent service duration of up to 77 years to achieve a hardness reduction to 185 HBW. |
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