Selection guide for the treatment of tool steels

Tool steels are used in many areas of material shaping, including casting, hot and cold forming, extrusion and extrusion drawing, cutting and stamping, not to mention machining. The need for increased productivity (e.g. in plastics processing and aluminum alloy die-casting), and the use of highly refractory materials (titanium, zirconium, copper, nickel and cobalt alloys) have resulted in higher mechanical and thermal stresses, as well as increased degradation due to wear, creep, fatigue and corrosion. Under these conditions, the performance of tooling is conditioned by a number of factors, including :

• drawing and designing the part ;

• the nature and quality of the chosen material;

• machining and finishing qualities (e.g. grinding);

• heat treatment and surface treatment ;

• operating conditions.

All these factors have an influence on the state of the constraints linked, on the one hand, to the implementation of the tool and, on the other hand, to the conditions of use. Mastering these factors is absolutely essential for a global approach to tool optimization. In this field, contact forces and pressures are increasingly well known, thanks to calculation codes that simulate material flows in tooling. Progress has also been made in determining boundary conditions, thanks to a better understanding of the heat flows exchanged between the tool and the material being used or machined.

Nevertheless, the choice of tooling material remains a complex issue, combining technical factors (processing and service life) with economic factors (cost of material, processing and availability on the market). That's why practical workshop experience is so important, alongside computer codes, in our approach to tooling optimization.

This cost is defined as the total cost of tool life (material, machining, heat treatment, surface treatment, mechanical finishing, maintenance) divided by the actual number of parts it can produce (service life). In this process of optimizing tool life, heat treatment and surface treatment are two decisive criteria.

This article provides practical information to help users choose their heat treatment and surface treatment range. Information is provided on the various surface treatment processes, as well as on the properties of the main coatings used. The problem of dimensional variations is also addressed, with indications on how to minimize them. In conclusion, the main incidents which can occur during tool manufacture are described, with indications likely to provide the user with data on possible causes and corresponding...