Study On Microstructure Properties And Corrosion Resistance Mechanism Of Weathering Bridge Steel

The mainland coastline in our country is18,000kilometres. With the rapid development of economy in coastal regions, the construction of cross-sea bridge also develop quickly, the need and requirement of bridge steel are increasing. So it is of great meaning to study the microstructure, mechanical properties and corrosion resistance of weathering bridge steel.Based on the results of grain size and precipitated phase of the experimental steel after different heating systems, we make three kinds of heating systems and analysis microstructure, mechanical properties and corrosion mechanism of the experimental steel after three heating systems. Cyclic wet/dry corrosion test was adopted to simulate the marine atmosphere. Through the analysis of corrosion kinetics, morphology of rust surface and section, constituent of rust and distribution of alloy elements on rust section characterize the corrosion resistance and corrosion mechanism of the experimental steel after three heating systems. The conclusions are as follows:(1) With the higher heating temperature and longer holding time, the grain size of austenite increases. According to the experimental results, the heating temperature must be control from1000℃to1150℃and the holding time should be30minites.(2) With the higher heating temperature and longer holding time, the grain size of the precipitated phase increases gradually and the quantity of the precipitated phase decreases. The precipitated phase in the experimental steel is mainly compound phase of Ti and Nb. The shapes of the precipitated phase are different. Some are big and present square or rectangle. based on the energy spectrum analysis, these precipitated phase are mainly contain Ti; Some precipitated phase are small and present irregular shapes or circular and ellipse. these precipitated phase are mainly contain Nb.(3) The microstructural of three experimental steel plates which go through different heating systems are mainly composed of quasi-polygonal ferrite, granular bainite and little pearlite. The yield stress, tensile stress, elongation and icontraction of area are325MPa～437MPa,543MPa～623MPa,24.76%～30.68%and55.16%～60.83%respectively, and with the increasing of heating temperature, the intensity is increased but the toughness is decreased slightly.(4) With the higher heating temperature, corrosion resistance of three experimental steels increased slightly. After corroding a period of time, the rust can be divided into two layers apparently. The outer rust is composed of defects such as cracks and holes which make the outer rust loose. However, the inner rust with compact and dense structure is composed of few cracks and holes. The distribution of alloy elements are different in the rust section. Cr enriches in inner rust and the alloy element of Cu also enriches in the local rust. The alloy element of Mo distributes uniformly in the rust section without apparent enrichment. In the later stage, the crystalline constituents of the rust are almost same, and only the relative contents are a little different. The rust are roughly composed of α-FeOOH, Fe3O4and γ-FeOOH. With the higher heating temperature, the content of α-FeOOH and α/γ*increase, therefore, the protective capability of the rust increases.