This work is part of a collaborative study between CEA-Saclay and LMT-Cachan on the numerical simulation of multi-pass GTA-Welding of thick specimens made of X10CrMoVNb9-1 (ASTM 387 or “T91”) steel. This material is considered as a candidate for some components of future Very High Temperature nuclear Reactors. Some parts of these components should be manufactured by assembling thick components (typically 200 mm) using narrow groove multi-pass GTA-Welding process. This welding process generates complex thermo-mechanical cycles in the HAZ (Heat Affected Zone) inducing complex microstructural transformations and residual stresses which should affect the integrity of the vessels behaviour. In a previous study, G.-M. Roux  developed a first version of a Thermo-Metallurgical-Mechanical "TMM" model for the X10CrMoVNb9-1 martensitic steel. This model was validated regarding residual stresses on simple mono-pass spot-welding tests. In this paper, focus is made on the modelling of the complex austenitisation process of the tempered martensitic steel as induced by the multi-pass process. Three different approaches are presented, viz. a model first proposed by Brachet et al., second a new model based on JMA approach and last, the simple differential Leblond model that is implemented in various finite element codes. These models are identified from standard dilatometry tests performed over a large range of heating rates, viz. [0.1°C/s, 100°C/s]. Finally, the response of these models, and therefore, their predicting capabilities, are compared to the experimental response of the material for different transients that have been designed to be representative of the temperature history in different points of a multi-pass welding HAZ.