Welding and heat treatment are a modern, high efficient production technologies. During last few years requirements for quality of the welded joints have been constantly increasing in all production areas. Unfortunately, this approach increases the cost of production due to demand of intense experimental or prototype work prior the use of technology to make a final product. Preliminary experiments have to take into account proper chose of welding technology, materials, welding parameters, clamping and final optimization the welding conditions. All of these activities can be supported or even replaced by numerical simulations based on finite elements method. Tremendous advance in field of numerical simulation, facilitates very high correlation of simulation and experimental results bringing this new approach to common use. This paper highlight to usefulness of numerical simulation in heat treatment of bulk materials in various production stages. It was shown that it is possible to predict formation of metallurgical phases, hardness distribution, strains and stresses during and after quenching process. Simulations of different heating conditions and cooling media makes it possible to simulate processes such as heating, quenching, carburizing and nitriding.