Detection technique of oxide scales blockage in austenitic stainless steel tubes with weak ferromagnetism

LIU Feng; ZHANG Shuai; ZHANG Shuangnan; CAI Guixi; LI Jiankui

doi:10.7688/j.issn.1000-1646.2025.01.10

Abstract：[Objective]To improve power generation efficiency and accelerate the development of green and low-carbon economy,supercritical or ultra-supercritical units have gradually become the main units for thermal power generation in China.As a key component of supercritical or ultra-supercritical units,boiler tubes are mostly made of austenitic stainless steel.After being used in an environment of high temperature,high pressure,and steam for a long time,a boiler tube will face an oxidized inner wall,on which oxide scales are produced.Due to the start and stop of the unit and other factors,the oxide scales are easy to fall off and accumulate in the tube.When the accumulation reaches a certain degree,local blockage of the tube occurs,resulting in tube burst.The on-site quantitative detection of oxide scales is generally carried out by magnetic nondestructive detection.However,boiler tubes exhibit weak ferromagnetism due to cold working and long-term aging,which makes the results of existing magnetic quantitative detection method for oxide scales inaccurate.[Methods]To realize the accurate detection of oxide scale blockage in austenitic stainless steel tubes with weak ferromagnetism,this paper proposed a new magnetic detection method based on the enlargement of the magnetic circuit.The present study compared the simulation results of the existing magnetic detection method and the new one by finite element simulation and probed into the reasons why the existing method failed to accurately detect oxide scales in boiler tubes with weak ferromagnetism.[Results]The simulation results show that the absolute error of the proportion of the oxide scales blockage area is 138％-474％in boiler tubes with weak ferromagnetism when the existing magnetic detection method is employed,while that is 3.4％-6.7％in the case of using the new magnetic detection method.In short,the new magnetic detection method has a much smaller absolute error than the existing one,able to be applied to the quantitative detection of oxide scales in boiler tubes with weak ferromagnetism since which is less affected by tube magnetism.Oxide scales with different blockage area proportions in standard specimens with different wall thicknesses were detected using the new magnetic detection method.On this basis,correspondence was established between the detected signal values and the blockage area proportions in standard specimens with different wall thicknesses,verifying the correctness of the simulation results.The correctness of the calibration formula is verified by an experiment on field tubes containing oxide scales giving rise to different blockage area proportions,and the detection error of the new magnetic detection method is calculated.The experimental results show that the maximum absolute error of the proportion of oxide scale blockage area in specimens with weak ferromagnetism ranges from 1.2％to 5.6％,and the maximum absolute error is less than 5.6％in the specimen calibration experiment.In the field tube experiment,the range of this maximum absolute error is 1.5％-6.4％,and the maximum absolute error is less than 6.4％.[Conclusion]This indicates that the new magnetic detection method is less affected by the magnetism of the tube wall,and its error is acceptable in practical engineering.The proposed technique enables the nondestructive detection and evaluation of oxide scale blockage in austenitic stainless steel tubes with weak ferromagnetism.