Corrosion behavior of high-strength spring steel for high-speed railway

The corrosion resistance and evolution of corrosion products in medium-carbon high-strength spring steels were investigated in a neutral salt spray(5 wt% Na Cl solution). A formation model of γ-Fe OOH and a transformation model describing the conversion of γ-Fe OOH to α-Fe OOH were constructed. The results indicated that, at the initial corrosion stage, the corrosion resistance was gradually improved with the addition of Cr; however, with the addition of alloying element V, the corrosion resistance decreased. These results were attributed mainly to the initial corrosion stage being closely related to the matrix microstructure parameters such as grain-boundary character and dislocation density. After the rust layer was formed at a later corrosion stage, the corrosion resistance was reinforced with the addition of Cr and V because Cr strongly influenced the composition, structure, and morphology of the corrosion products. The results presented herein show that Cr was conducive to the transformation of γ-Fe OOH into α-Fe OOH. Moreover, V and Cr exhibited obvious synergy and were enriched in the inner layer of the corrosion products.

Precipitation behaviors of X80 acicular ferrite pipeline steel

The precipitation behaviors of X80 acicular ferrite pipeline steel were investigated by using transmission electron microscopy （TEM） and energy dispersive spectroscopy （EDS）. The results show that dendritic precipitates in the as-cast steel slabs precipitate mainly in grain boundaries, and these dendritic precipitates dissolve and re-precipitate to two kinds of carbonitrides： Ti- and Nb-rich （Ti, Nb）（C, N） carbonitrides during reheating. Four types of precipitates mainly exist in the hot rolled plate： Ti-rich carbonitrides resulted from the dendritic carbonitrides undissolved during the reheating process; Ti-rich carbonitrides re-precipitated along austenite grain boundaries during the re-heating process; NbC carbides mainly heterogeneously nucleated on the small pre-existing Nb-rich carbonitrides in the hot rolling process; and NbC carbides precipitated on dislocations during hot rolling.

Effect of Cr/Mn segregation on pearlite–martensite banded structure of high carbon bearing steel

The effect of Cr/Mn segregation on the abnormal banded structure of high carbon bearing steel was studied by reheating and hot rolling.With the use of an optical microscope, scanning electron microscope, transmission electron microscope, and electron probe microanalyzer, the segregation characteristics of alloying elements in cast billet and their relationship with hot-rolled plate banded structure were revealed.The formation causes of an abnormal banded structure and the elimination methods were analyzed.Results indicate the serious positive segregation of C, Cr, and Mn alloy elements in the billet.Even distribution of Cr/Mn elements could not be achieved after 10 h of heat preservation at 1200℃, and the spacing of the element aggregation area increased, but the segregation index of alloy elements decreased.Obvious alloying element segregation characteristics are present in the banded structure of the hot-rolled plate.This distinct white band is composed of martensitic phases.The formation of this abnormal pearlite–martensite banded structure is due to the interaction between the undercooled austenite transformation behavior of hot-rolled metal and the segregation of its alloying elements.Under the air cooling after rolling, controlling the segregation index of alloy elements can reduce or eliminate the abnormal banded structure.

Effect of simulated thermomechanical processing on transformation behavior and microstructure of 82B steel

The effects of loop-laying temperature and austenite deformation on the phase(ransformation behavior during continuous cooling,microstructure,and pearlite interlaminar spaci ng in 82B steels were investigated.Static and dynamic conti nu ous cooling transformation(CCT)diagrams were measured with a Gleeble-3500 thermal simulator,and the mechanisms governing changes in the initial temperature,initial time,and duration of the phase transformation zone were also analyzed and discussed.The results show that CCT diagram shifted to the bottom right,the initial temperature of the phase transition decreased,the initial time of the phase transition increased,the duration of the phase transition increased,and the lamellar spacing of pearlite was finer as the loop-layi ng temperature in creased.The initial phase transition time decreased,and the phase transition duration first reduced,then increased,and finally decreased in the static condition and in the dynamic condition at 850℃as the cooling rate increased.Meanwhile,the phase transition duration continuously decreased in the dynamic condition at 900℃.At a given loop-laying temperature,the lamellar spacing in pearlite was finer due to austenite deformation compared with the undeformed case.Compared with the results shown in the dynamic CCT diagram,the corresponding phase diagrams of the static CCT diagram slightly shifted to the bottom right.Moreover,there was a clear lin ear relationship between the reciprocal of the lamellar spaci ng in pearlite and the average undercooling degree in the phase transformation zone.

Effects of Cr content on corrosion behaviour and corrosion products of spring steels

Neutral salt spray corrosion experiments of spring steels with different Cr contents were carried out for different corrosion periods. The optical microscope was used to observe the macroscopic corrosion morphology of the steel surface. The corrosion pit morphology of steel surface was observed by laser scanning confocal microscopy and scanning electron microscopy, and three-dimensional simulation was carried out. At the same time, the corrosion products (rust layer) were qualitatively and quantitatively analysed by X-ray diffraction. As the results show, Cr is beneficial to improving corrosion resistance of the experimental steel matrix, and the higher the content of Cr, the stronger the corrosion resistance will be. With increase in Cr content in steel, the development of corrosion process will be more effectively suppressed. With the increase in Cr content, the denser the corrosion products, the stronger the bond with the metal matrix is. The corrosion products have obvious stratification;the outer layer is mainly composed of c-FeOOH, which is relatively loose and not firmly integrated with the matrix, while the inner layer contains a-FeOOH and Fe3O4, which are relatively dense and closely integrated with the matrix. The types of corrosion are constantly changing during different phases of corrosion.

Effects of Chromium,Vanadium and Austenite Deformation on Transformation Behaviors of High-strength Spring Steels

The phase transformation behavior during continuous cooling of high-strength spring steels containing different amounts of Cr was studied. Furthermore, the effects of combining Cr with V as well as austenite deformation on the transformation kinetics were investigated in the method of dilatometry and metallography hardness. The re suits showed that, with the increase of Cr, the pearlite transformation field was enlarged, the ferrite transformation field was narrowed, and the entire phase field shifted to the right. With the addition of V, the start transformation temperature of undercooling austenite （Ar3） was gradually increased, but the ferrite and pearlite transformation fields were not affected. Besides, the minimum critical cooling rate of martensilic transformation was also reduced. In addition, the dynamic continuous cooling transformation （CCT） curve moves to the top left compared with the static CCT curve. The transformed microstructures showed that the addition of V and the deformation not only refined the overall transformed microstructures but also reduced the lamellar spacing of pearlite. The alloying elements Cr and V promoted the Vickers hardness. However, the effect of Cr on the Vickers hardness of martensite was stronger and the influence of V on that of pearlite was stronger. Moreover, the Vickers hardness affected by the austenite deformation was more complex and strongly depended on the transformed microstructures.