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. The final objective of this paper is to exhibit the prediction capabilities of an improved version of the Thermo-Metallurgical-Mechanical "TMM" model for X10CrMoVNb9-1 martensitic steel (initially developed by G.-M. Roux). In this paper, focus is made on the modelling of the martensite tempering due to the complex thermal loadings induced by the multi-pass process. Herein, it has been chosen to study the tempering kinetics via the evolution of the free carbon content or, conversely, the carbides precipitation overall fraction, growth and dissolution. Thermo-Electric Power (TEP) measurements as well as hardening measurements have been used to investigate the tempering phenomenon. Measurements are fast to perform and are not very sensitive to the geometry of the material (in opposite to resistivity measurements). A phenomenological tempering model was developed and identified from several tests at constant tempering temperatures ranging from 550°C up to 750°C. The improved TMM model including this tempering model was used to perform the 2D finite element analysis of the 16-pass GTA welding process of a narrow groove butt-weld. The predictions are favourably compared with the “real” microstructure.