Nano and micro-indentation driven characterisation of asperity and bulk plasticity at the surface of modern premium rail steels

The plastic deformation behaviour of rail steels due to cyclic rail-wheel contacts is important to understand due to its connection to wear and rolling contact fatigue (RCF) damage initiation in service. Simulation models such as the ‘Layer’ and ‘Brick’ model have previously been developed to estimate the accumulation of plastic damage in a rail steel; however, the data available to drive these models is currently sparse, with limited applicability to modern rail steel grades. This paper presents the research examining the shear stress-strain curve relationships of rail steels derived from plastic shear strain and shear yield stress data collected from twin-disc test samples. A combination of microhardness and nanohardness testing was used to derive the shear yield stress data, whereas the plastic shear strain was acquired from optical microscopy. Six different conditions were investigated for this research for the purpose of examining how shear stress-strain curve relationships compared between the standard R260 and the premium HP335 and R350HT rail steels and how this compares to wear damage data. The influence of the maximum Hertzian contact pressure on the shear stress-strain curve relationships of R260 between 600 and 1500 MPa contact pressure was also investigated. The wear rate results derived from the mass loss in interrupted twin-disc tests showed HP335 wearing the least, followed by R350HT and then R260 for 1500 MPa, dry contact conditions. However, the highest shear yield stress achieved was for R350HT, then HP335, and R260. The results show that the shear stress-strain curve relationships by themselves are insufficient to determine rail steel wear performance in a laboratory environment. The shear stress-strain curve relationships for R260 collected under different contact pressures showed the results are near independent of the contact pressure within the range explored.