Nonlinear Ultrasonic Detection And Characterization Of HR3C Steel Fireside Corrosion

Because of its good oxidation resistance and the lasting strength, HR3C steel has been used to fabricate boiler tube of ultra supercritical generator unit. Owing to containing a considerable number of elements such as S, Cl, Na, K in coal of our country,the fireside corrosion of the substrate HR3C occurs while the coal is burning.The fireside corrsion has a high speed,flourishes in clusters which will lead to thinning the the wall of a pipe and bursting pipe accident and then threaten the safe operation of a power plant.However.The conventional detection method can not finish the nondestructive detection and characterization of early fireside corrosion due to the limitation of the principle.At the same time,the nonlinear-ultrasonic detection has been applied widely on the detection of the early defects as a result of its sensitivity.Thus this paper does some reserch of nonlinear-ultrasonic detection on fireside corrosion and the results are as follows:I Simulate the sulfate/chloride corrosion of 20 steel and HR3C in the lab,and explore the relationship between the corrosion layer thickness and the corrosion time.Put forward that:when the oxide layer is hold,the parabolic shape will be better in fitting curves of the relationship between the corrosion layer thickness and the corrosion time,whlie the piecewise function wave will be better while oxide layer is peeled off;Ⅱ Due to the effect of so many infactors in the detection like the model of the signal,coupling mode and algorithm and so on,this paper adopted CSK-IA the sample block as the research object,systematically studied the effect of transmitted signal parameters on nonlinear-ultrasonic detection and detected fireside corrosion basing on the nonlinear-ultrasonic,thus put forward the principle of nonlinear-ultrasonic testing signal selection:the transmit signal should be used in the mode of pulse string, and the number of the string should be more and the voltage should be higher if possible and so on.III The results of the the HR3C and 20 steel fireside corrosion detection show that,(1) Under the action of sulfate,when the corrosion time is 20h,the corrosion layer is very thin and it combines with the metal base colsely with few porein the base matel,he growth of the nonlinear coefficient has a little change within 20% range of the original sample(30% for 20 steel);when the time increasing to 150h,the roughness of samples increases as well as the corrosion layer and the pore bacomes more in the metal base.As a result, the fundamental frequency decreases and the second harmonic generation increases, which will lead a big change of the nonlinear coefficient, and the growth will be 96.2% of the original sample(62.3% for 20 steel);When the corrosion time increases further to 200h, the corrosion layer thicken small as well as the pore,the growth of the nonlinear coefficien will be 158.5% of the original sample(595% for 20 steel of sulfate corrosion).(2) Under the action of sulfate and chlorine,the speed of HR3C fireside corrosion accelerates.It manifests as follows:the corrosion layer thicken obviously the thickness will be 1749.2um with oxide layer holden,the corrosion layer of the sample surface bursts seriously so that it can hardly combine with the metal base,the pore inside become more and more,which leads to significant nonlinear effect.From the incubation period to saturation period,the nonlinear coefficien changes substantially and the growth will be 378.1% of the original sample.With the different corrosion time, the nonlinear coefficient also changes. Therefore, the nonlinear coefficient can be a characteristic parameter which can have a a relatively complete and accurate non-destructive characterization and evaluation of fireside corrosion.