Research and development of low carbon bainitic- martensitic steels for applications in railway wheels and large gears

Abstract

Research was undertaken to determine if Low Carbon Bainitic-Martensitic (LCBM) steels can be used as railway wheels since they are expected to provide significant benefits both in longer life and improved performance compared with conventional pearlitic-ferritic steels. Railway wheel standards require that compressive stresses are formed in the tread region of as- manufactured railway wheels to retard the initiation and propagation of cracks due to fatigue. However, when LCBM railway wheels are produced in a similar fashion as pearlitic-ferritic railway wheels, unfavourable tensile residual stresses are formed in the tread region rendering this process unsuitable. Dilatometric studies were undertaken to characterise the strains associated with volumetric changes during austenite to martensite phase transformation in LCBM steels. The results were incorporated in a thermo-mechanical Finite Element (FE) model to study changes in microstructure and stress distribution in LCBM railway wheels for different quenching conditions and configurations. A novel quenching method was developed and found to give the desired compressive residual stresses in the tread region of LCBM wheels. Experiments were undertaken to verify the predictions of the FE model which relied on estimated Heat Transfer Coefficients (HTCs). A full-size experimental railway wheel quenching rig was constructed and embedded with thermocouples to measure cooling rates in various locations of a LCBM wheel for different quenching conditions. Experimental values of HTCs were determined for different quenching spray intensities and the novel quenching process was optimised by preferentially quenching selected regions of the wheel and evaluating different spray intensities. vi Residual stress measurements were undertaken to confirm the presence of compressive stresses in the tread region of LCBM railway wheels, thereby validating the novel quenching process. The risks of quench cracking in LCBM railway wheels were investigated and the results showed that adopting a low coolant spray intensity with a low carbon grade LCBM steel is expected to reduce the likelihood of quench cracking without adversely affecting standard wheel requirements for compressive stresses in the tread region. A crack growth model was also developed to investigate the fatigue performance of LCBM railway wheels produced using this novel quenching process. The level and distribution of as- manufactured rim residual stresses were shown to play an important role in retarding stress reversal due to thermal stresses from tread braking. Under similar service loadings, LCBM wheels were found to possess longer fatigue lives (by approx. 50%) compared to conventional pearlitic-ferritic wheels. There is potential for wider applications of LCBM steels if quenching processes can be developed to promote compressive stresses in fatigue prone regions. A critical review of large gear failures and mechanical requirements concluded that steels with good hardenability, core strength and toughness are beneficial for use in large gears. The FE model was used to develop a two-stage quenching process for LCBM large gears and the compressive stresses at the base of the teeth of the gear were predicted to exceed those of conventional induction hardened SAE gear steels, potentially extending the life and performance of large gears.