Thermal treatment of tool steels

Unlike general-purpose steels, and even special steels for mechanical engineering, where more and more so-called economic grades are being developed for which the properties of use are obtained directly by controlled cooling after forging or rolling, the high levels of mechanical characteristics required of tool steels call for quality heat treatment, all the more so as it is the surface of these materials that is stressed in service. Moreover, apart from a few special cases, notably in the field of hot tool steels, where specifications with mechanical characteristics and minimum toughness values are required, the only mechanical control of the treatment is usually limited to checking hardness, which only concerns a shallow zone of the metal. It is interesting to note, however, that in the plastics industry in particular, low-carbon grades with good polishing and welding properties are being developed, obtained directly after hot working in the pretreated state with a predominantly bainitic structure.

For the use of tool steels, knowledge and correct execution of heat treatments are indispensable; special care must be taken in the various operations if the following points are to be taken into account:

• the tool must maintain an extremely precise shape and dimensions, since in many cases the final grinding operation must be reduced as much as possible, or even eliminated;

• the tool works on its periphery, and this part must therefore have optimum mechanical properties;

• the heat treatment response of tool steels depends to a very large extent on their chemical composition and, for the same grade, on the annealing conditions;

• the cost of materials and machining before processing represents a very significant proportion of the overall cost of tooling.

Mastering the heat treatment of tool steels means first and foremost understanding the various operations involved: annealing, quenching and tempering, from both a metallurgical and a technological standpoint. It also requires mastery of the main incidents: deformation, tapping, risk of fracture, etc.

For the properties of tool steels obtained after heat treatment, the reader is referred to the articles :

Tool steels – Standard numerical data [M 4 590] ,

Tool steels – Non-standardized numerical data