Masaharu Ueda

Effects of carbon content on wear property in pearlitic steels

To clarify the effects of carbon content on the rolling contact wear in steels, the authors conducted a two-cylinder rolling contact wear test using pearlitic steels with a carbon content in a range from 0.8 to 1.0 mass% and studied the relationship between the carbon content and the rolling contact wear. In addition, the authors examined the dominating factor in the rolling contact wear in pearlitic steels and the work-hardening rate of the rolling contact surface. The main findings obtained are as follows: (1) The wear resistance of pearlitic steels improve as carbon content increases. (2) The dominating factor in the rolling contact wear of pearlitic steels is the rolling contact surface hardness (RCSH). (3) The improved wear resistance of pearlitic steels is attributable to an increase in RCSH due to raising the work-hardening rate of the rolling contact surface as carbon content increases. (4) The reason why the work-hardening rate of the rolling contact surface of pearlitic steel rises as carbon content increases is considered to be as follows: an increase in the cementite density increases the amount of dislocation in the matrix ferrite and promotes the grain refinement of the matrix ferrite. As a result, the matrix ferrite is strengthened through the promotion of dislocation hardening and grain refinement.

Nanoscale characterisation of rolling contact wear surface of pearlitic steel

Abstract Nanoscale characterisation of a rolling–sliding wear surface layer of pearlitic steel was performed with transmission electron microscopy and atom probe tomography to reveal microstructural changes in the pearlite structure. Plastically deformed fine pearlitic lamellae with interlamellar spacing of ∼10 nm were observed just beneath the contact surface after the rolling–sliding wear test, where the hardness of the surface reached >800 HV, twice the initial bulk hardness of 400 HV. Lamellar cementite was slightly decomposed, but most lamellar cementite was retained as thinned lamellae in the deformed pearlitic structure. The large increment in hardness was mostly explained by the reduction in interlamellar spacing. The formation mechanism of the microstructure of the worn surface was compared with that of the white etching layer on the pearlitic rail surface.

Proposal for an engineering definition of a fatigue crack initiation unit for evaluating the fatigue limit on the basis of crystallographic analysis of pearlitic steel

In this study, in order to define a fatigue crack initiation unit, the relationship between the fatigue crack initiation process and the crystal structure in the pearlitic steel used for railroad rails was examined and fatigue tests, focusing on crack initiation, were performed. The fracture surfaces were analyzed using a scanning electron microscope (SEM) and electron backscatter diffraction (EBSD). The crystal structure of the pearlitic steel is composed of “pearlitic colonies” that have the same lamellar structure direction and “pearlitic blocks” that have the same ferrite crystal direction. The SEM and EBSD results revealed that the crack initiation depends on the pearlitic colonies. Therefore, we defined the characteristic dimension for fatigue crack initiation as the pearlitic colony size. However, for safety purposes, the pearlitic block size should be considered the engineering definition of the fatigue crack initiation unit, since decreasing the pearlitic block size should cause an improvement in the fatigue limit of pearlitic steel.