Effect Of Chromium On Rust Layer Structure,Microstructure And Properties Of Weathering Bridge Steel

As a lot of bridges need to be built in our country,a large number of weathering bridge steels with excellent atmospheric corrosion resistance,mechanical properties and welding properties need to be used.The formation of a protective rust layer in the service environment can significantly improve the atmospheric corrosion resistance of weathering bridge steel.Due to the importance of the rust layer,the structure of the rust layer has been extensively studied.However,to date,there are still some confusions and controversies on the structure of the rust layer for people.One of the most obvious confusions is the presence of the amorphous phase in the rust layer,and a considerable controversy is about the structure of the rust layer in the marine atmosphere,such as the location of akaganeite and the existence of nanograins.In addition,as one of the most common alloying elements added to weathering bridge steels,the effect of Cr on the atmospheric corrosion resistance of weathering bridge steels is controversial.In this study,three weathering bridge steels with Cr content of 0.295 wt.%,0.680wt.%,and 0.990 wt.%were designed,which were named 29Cr steel,68Cr steel,and 99Cr steel,respectively and used after thermo-mechanical controlled process.The effect of Cr on the atmospheric corrosion resistance,microstructure,mechanical properties and welding properties of weathering bridge steel was studied.The TEM samples of the rust layer were prepared by focused ion beam cutting technology,and the structure and chemical composition of the rust layer were analyzed by HRTEM and STEM on a nano-scale,and then the formation mechanism of the rust layer was clarified.The results show that in the simulated marine atmosphere,the rust layer was composed of the inner rust layer,outer rust layer and outermost rust layer.The inner rust layer was composed of densely packedα-Fe OOH nanograins with a size of approximately10-20 nm and the outer rust layer was a mixture of spindle-and rod-likeα-Fe OOH,and rod-and fiber-likeβ-Fe OOH.In addition,the outermost rust layer was composed of laminar-and feather-likeγ-Fe OOH.In the simulated industrial atmosphere,the rust layer was comprised of the inner rust layer and outer rust layer.The outer rust layer was composed of rod-likeγ-Fe OOH while the structure of the inner rust layer was different due to the influence of different Cr content.Cr was only distributed in the inner rust layer and the concentration of Cr was uneven.Low concentration of Cr promoted the formation ofα-(Fe_1-x,Cr_x)OOH nanograins and higher concentration of Cr promoted the formation ofα-(Fe_1-x,Cr_x)OOH nanograins with poor crystallinity,while high concentration of Cr led to the formation of the amorphous phase.Cu was enriched in the innermost rust layer near the steel-rust interface together with Cr to promote the formation of the amorphous phase.The atmospheric corrosion resistance of weathering bridge steels in the simulated marine and industrial atmospheres was increased with the increase of Cr content.When the content of Cr increased from 0.295 wt.%to 0.680 wt.%,the corrosion resistance increased obviously.When the content of Cr increased from 0.680 wt.%to 0.990 wt.%,the increase of the corrosion resistance was not obvious.With the increase of Cr content,the transformation temperature of undercooled austenite of weathering bridge steel decreased,and the content of low-temperature products increased,and the structure became refined.29Cr steel was mainly composed of polygonal ferrite（PF）,a small amount of granular bainite（GB）and degenerated pearlite.68Cr steel was mainly composed of GB and a small amount of PF,while 99Cr steel was mainly composed of GB.With the increase of Cr content,the proportion of high angle grain boundaries（HAGBs）of weathering bridge steel decreased,and the content of MA components increased,especially the proportion of massive MA components increased.Moreover,with the increase of Cr content,the yield strength and tensile strength of weathering bridge steel increased,while the total elongation and the impact absorption energy at-40°C decreased.The coarse-grained heat-affected zone（CGHAZ）of the three weathering bridge steels was composed of GB and lath bainite（LB）.Under the same welding heat input,with the increase of Cr content,the content of LB in CGHAZ increased,and the content of GB decreased,and the content of MA components increased,and the proportion of HAGBs decreased,and the hardness of CGHAZ increased.For the same weathering bridge steel,with the increase of welding heat input,the prior austenitic grain size in CGHAZ increased significantly,and the proportion of HAGBs decreased,and the content of MA components decreased,and the proportion of massive MA components increased significantly,the hardness of CGHAZ decreased.Under the heat input of 20 k J/cm,with the increase of Cr content,the impact absorption energy of CGHAZ of weathering bridge steel at-40°C decreased.The CGHAZ of 29Cr steel and 68Cr steel had good low-temperature impact toughness,while the low-temperature impact toughness of CGHAZ of 99Cr steel was poor.However,under the heat input of 35 k J/cm,the low-temperature impact toughness of CGHAZ of the three weathering bridge steels was poor.