Temperature Stability of Railway Wheel Steels - Influence on Microstructure and Mechanical Properties

Licentiate thesis, 2010

Being the single component transferring load and traction from the vehicle to the rail, the selection of materials and structural properties of railway wheels are critical. Carbon steel with a pearlitic microstructure is the most commonly used material due to its high strength, low cost and good wear properties. However, due to the two‐phase microstructure, pearlite is susceptible to softening at higher temperatures and degradation of mechanical properties. A new type of wheel steels with increased alloying content of silicon and manganese has shown improved resistance to rolling contact fatigue. However, a clear correlation between increased alloying content and strength and how the softening behaviour of the material affects the fatigue properties have not been studied so far. To foster further knowledge on these issues, two wheel steels were used in this work: a high Si‐Mn alloyed steel and a widely used lower alloyed steel. In virgin states the two selected steels were found to have fairly similar mechanical characteristics in a wide range of temperatures and different strain rates. Upon annealing, however, both materials begin to soften at temperatures above 500 °C. Higher temperatures approaching the ferrite‐austenite transformation temperature (about 750 °C) accelerates the microstructural degeneration. However the high Si‐Mn‐alloyed steel showed to have a better resistance against thermal softening, resulting in better mechanical properties and increased fatigue life compared to the low alloyed steel. Investigation of the microstructure revealed that the softening was caused by spheroidisation of cemenite lamellas in pearlite. Heavy pre‐deformation before annealing emphasised this degeneration of pearlite even more. Both silicon and manganese are considered to retard the softening towards higher annealing times and temperatures, by a decrease in carbon flux, which thereby limit the spheroidisation rate. The cementite spheroidisation leads to lowering of hardness, monotonic and cyclic strength and fatigue life times. As the high alloyed steel resists microstructural degeneration better at elevated temperatures it retains the mechanical properties better after high temperature exposure. This is particularly evident for the fatigue performance. As an example it can be mentioned that a certain high temperature exposure leads to hardness decrease of about 20 % for the high alloyed steel, while the low alloyed steel suffers a hardness reduction of about 25 %. For fatigue life times, however, the difference between the two materials is as much as a factor three in favour for the higher alloyed steel.