Summary This article, published in three parts, is based on a doctoral thesis undertaken at the Department of Applied Mechanics and Engine Construction of Genoa University, in collaboration with the Italian Welding Institute. The aim of this article is to study the thermological, metallurgical, economic and safety problems of preheating which arise in fusion welding of steels and to acquire a basic knowledge of these phenomena, as fully and logically as possible. On this basis, a simple, flexible and expandable tool, illustrated by experiments, has been devised for use by expert welding engineers. For this purpose, state‐of‐the‐art research, national and international standards were studied, and experimental tests conducted to correlate available data. An expert system (ES) was thus produced, using artificial intelligence techniques, in order to determine preheat temperature, evaluate costs, and make health and safety recommendations to the user. In this third part, the development of the expert shell is described, as well as a brief software user's guide.