Effect Of Hard Inclusions Plasticizing On Toughness Enhancement And Slow Cracking Characteristics Of Wheel Steel

As an important part of railway vehicle to carry and transfer power,railway wheels play a vital role in the safe operation of train.According to the statistics of failed railway wheels in recent years,the main source of failure about railway wheels is inclusions in wheel steel.Because inclusions cannot be completely removed,increase the sulfur content in wheel steel to make the soft inclusion Mn S wrap hard inclusions such as Al₂O₃ by using the technology of hard inclusions plasticizing,which is targeted at improving the mechanical properties of wheel steel.Aiming at the above requirements,the mechanism of toughness enhancement and slow cracking about wheel steel by hard inclusions plasticizing was studied.Firstly,the characteristics of inclusions in two wheel steels with different sulfur contents were analyzed.The distribution of inclusions from different depths under the tread,size and shape of inclusions and categories of inclusions were compared.It is found that Mn S and hard inclusions are the main inclusions in low sulfur wheel steels,while Mn S and composite ductile inclusions are the main inclusions in high sulfur wheel steels,which provides data support for explaining the mechanism of toughness enhancement and slow cracking about wheel steels with high sulfur content.The uniaxial tensile and fracture toughness tests were carried out for two kinds of wheel steel samples.The strain energy density consumed from beginning to fracture were obtained from the uniaxial tensile test data.Then the crack growth rate of two kinds of wheel steels was tested,and the crack growth rate curve was obtained by experimental data fitting with Paris formula.In order to explain the mechanism of slow cracking about wheel steels with high sulfur content,based on the tessellated stress theory,a finite element model of compact tensile specimen with inclusions was established to analyze the interference of different inclusions on crack propagation in wheel steels,including two different working conditions:The crack penetrates the inclusion and the crack doesn’t penetrate the inclusion When the crack penetrates the inclusion,the stress intensity factor at the crack front of high sulfur wheel steels is lower than that of low sulfur wheel steels due to the coordinated deformation of the inclusions and the matrix.The stress intensity factor of the crack tip in high sulfur wheel steels is lower than that in low sulfur wheel steels when the crack does not penetrate the inclusion,and the degree of crack deflection towards inclusions is also smaller than that of low sulfur wheel steels.Therefore,the crack growth rate of high sulfur wheel steels is lower than that of low sulfur wheel steels at the initial stage of crack growth.When crack length increases,the dominant effect of test load on crack growth is enhanced and the effect of inclusion property decreases,leading to the crack growth rate of the two materials gradually tends to be consistent.In order to explain the mechanism of toughness enhancement about wheel steels with high sulfur content,the internal factors affecting the fracture toughness of wheel steels were analyzed.The microstructure of the two wheel steels is the same.The grain size and pearlite lamellar spacing are at the same level.In terms of inclusion defects,the results calculated by Goodier formula are in agreement with those calculated by finite element model,which shows that the radial stress concentration at the interface between the hard inclusion and the matrix is greater than that at the interface between the composite ductile inclusion and the matrix.The composite ductile inclusion can be separated from the matrix only under a larger applied load,so it is more difficult to initiate microcracks from the inclusions in high sulfur wheel steels,which is the reason why the fracture toughness of high sulfur wheel steels is higher than that of low sulfur wheel steels.