| Erosion corrosion commonly occurs in mining machinery plants,hydro-turbine and delivery systems for liquid-solid particle slurries,such as ore,sand,and coal.Because of its synergistic effect,erosion corrosion has been shown to generate much greater weight loss than the sum of the weight losses in the individual pure erosion and pure corrosion processes.The delivery pipes suffering serious damage of erosion corrosion during dredging process,and the Q235B pipes usually become invalid in three years.The erosion corrosion behavior of steels in sea-water slurry dredging process were investigated,the laboratory evaluation system of steels was established,the effects of material characteristics and dredging conditions on erosion corrosion properties,mechanism were systematical analyzed,and the experimental results provided the basis for the development and application of erosion corrosion resistant steels.New erosion corrosion testing device was designed,the parameters applied for erosion corrosion evaluation in sea-water slurry and simulated alkaline slurry were determined by serial testing,and the laboratory evaluation system of steels was established,which provided the foundation for the erosion corrosion mechanism and research of erosion corrosion resistant steels.The testing results indicated that the repeatability was very good,the components of erosion corrosion can be measured also,and the results were consistent well with experience from practical applications.The erosion corrosion weight loss(W,mg/cm2)increased with temperature(T,?)according to a nearly linear relationship W=k×T-+a,where a is constant,k is coefficient of temperature,value of k is 0.5 when temperature below 20? and 3 between 20?40?.The effects of composition and mechanical properties of tested steels on erosion corrosion were investigated.It was found that the rust layer formed on the surface of weathering steel removed in erosion corrosion process,which led to more weight loss.The addition of small amount of Cr is beneficial to the raise of corrosion potential and suppress corrosion process,then improved the resistance of erosion corrosion.Carbon present in steel can transform into carbide and act as a cathode because of its higher potential,thereby accelerating the corrosion rate and leading to more erosion-corrosion weight loss.The erosion corrosion weight loss increased with Rm according to a linear relationship(W =-0.0086Rm+24.042).So steel with Rm strength of 1500 MPa is hoped to obtain two times resistance of erosion corrosion as carbon steel.The erosion corrosion resistance of different microstructure was followed as F<F+P<F+M+y<M.The resistance of erosion corrosion can be greatly improved as strength increased considering corrosion resistance.Tensile strength more than 1400MPa BMS-1 steel with excellent erosion corrosion resistance was developed,The erosion corrosion resistance of which was approximately two times as that of Q235B.The experimental results indicate that the weight loss increases with the linear velocity according to a nearly exponential relationship(W=KVn),the value of n increasing with increasing velocity.When the linear velocity changed from 2.2 m/s to 3.3 m/s,n increased from 1.01?1.30 to 1.55?1.83 in alkaline slurry.Whereas the corrosion weight loss contributed a higher percentage of the total weight loss in sea-water slurry,so the value of n decreased accordingly.Micrographs of the erosion-corroded surfaces were obtained via SEM.The results shown that the surface can usually observed different numbers of craters resulting from the combined effects of corrosion and erosion,craters were produced during the initial tens of seconds.Smaller numbers of craters were observed on the surfaces of carbon steels than that of high strength steels,and the craters even contact each other.Many gouges and platelets were observed also,there were strips formed by corrosion on the back surfaces of specimens.The gouges in carbon steels was deep and wide whereas narrow and shallow in high strength steels.The gouges,platelets and craters were considered to be the main mechanism of material removal for carbon steel,the platelets removal and particles impingement were the two reasons of the formation for craters in carbon steel.Craters and platelets are the weight loss modes for high strength steel.A deformed layer can be seen clearly in carbon steel surfaces,which tends to be cracked by subsequently striking particles,leading to the appearance of craters and enhancing the removal of material.So the weight loss of carbon steel is higher than that of high strength steel.The evolution of erosion corrosion components and synergy of two carbon steels(Q235B,Q345B)and two high strength steels(BW300TP,BMS-1)was investigated respectively in sea-water slurry.Pure erosion was responsible for the largest fraction of the total degradation in the erosion-corrosion experiment,representing more than 57%of the weight loss,strength and composition were important factors effecting the erosion corrosion resistance;Whereas pure corrosion component was less than 1%.The weight loss resulting from erosion-corrosion synergy was significant,approximately 21?40%of the total,and 30.6%in BMS-1 steel.The percentage contribution of pure corrosion increased rapidly as the velocity decreased to 1.1 m/s,and pure erosion only contributed 10.6%and 39.5%weight loss for BW300TP and BMS-1,and the synergy components increased to 79.62%and 42.3%respectively.The results indicated that pure erosion component was higher in alkaline slurry than that of in sea-water slurry.So better wear resistance is more important for the improvement of erosion corrosion resistance in alkaline slurry.The synergy weight loss of erosion and corrosion increased as pure corrosion rate or pure erosion rate increased,while corrosion played a dominate role in the synergy of erosion and corrosion.So besides higher strength,corrosion should be suppressed also,in order to reduce the synergy effect of corrosion and erosion,then improved erosion corrosion resistance can be obtained.The welding technology and pipe processing of BMS-1 was studied.the power and force of three roller bending machine subjected was calculated,a method to calculate the displacement of up working roller according the spring back of steel plate was proposed.Weak collocation welding technology was applied in order to obtain better pipes.The results of comparative erosion corrosion testing in dredging process shown that the resistance of BMS-1 steel was two times as that of carbon steel,which was in good agreement with the results in laboratory. |
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