Grain growth and hardness variation occurring in high temperature Heat Affected Zone (HAZ) during the welding processes are two thermal dependant aspects of great interest for both academic and industrial research activities. This paper presents an innovative Finite Element (FE) model capable to describe the grain growth and the hardness decrease that occur during the Gas Metal Arc Welding (GMAW) of commercial AISI 441 steel. The commercial FE software SFTC DEFORM-3DTM was used to simulate the GMAW process and a user subroutine was developed including a physical based model and the Hall-Petch (H-P) equation to predict grain size variation and hardness change. The model was validated by comparison with the experimental results showing its reliability in predicting important welding characteristics temperature dependant. The study provides an accurate numerical model (i.e. user subroutine, heat source fitting, geometry,…) able to successfully predict the thermal phenomena (i.e. coarsening of the grains and hardening decrease) that occur in the HAZ during welding process of ferritic stainless steel.