The technological processes in the aerospace industry are characterised by strict requirements for product quality and reliability. In the gear manufacturing process, the appropriate development of the phase composition and microstructure of the surface layer, which is responsible for the wear resistance and fatigue strength of the teeth, is of particular importance. Thermo-chemical treatment processes are widely used in the manufacturing of aerospace gears due to their ability to influence the properties and the required surface layer in relation to their functional usability. Carburisation is the basic thermo-chemical treatment process. The final properties of the carburised layer are formed during hardening, cryogenic treatment and low temperature tempering. The final material properties are significantly influenced by the evolution of the microstructure resulting from the phase transformations that occur during the heat treatment. Advances in technology, new steel grades such as Pyrowear 53 and vacuum carburising allow gas quenching to be carried out in a single furnace chamber immediately after the carburising cycle without the need for cooling and reheating. However, this makes the numerical simulation process even more challenging. The aim of the presentation is to report progress in the development of numerical modelling of temperatures during the high pressure gas quenching process coupled with a phase transformation model. The presentation will introduce the idea and application of the quenching model based on a CFD/Thermal solver using Abaqus CFD combined with an in-house developed phase transformation model. Two types of models have been considered, based on the JMAK and Leblond approaches. Due to the complexity of the phenomena involved, the JMAK model was chosen for the fully coupled simulations. The main focus of the presentation is the modelling of phase transformations in Pyrowear 53 steels related to the carbon concentration in the carburised layer. Preliminary experimental verification of the developed model will be also presented. This study explores possibility of coupling thermal, fluid dynamics and metallurgical problems with a constitutive model for prediction of residual stresses in gear wheels. Acknowledgements Financial support of the NCBiR, project no. TECHMATSTRATEG2/406725/NCBR/2020, is acknowledged