Hardening and Tempering of Tool Steels

Reckoned on a tonnage basis, tool steel represents only a few percent of the total quantity of steel produced but its importance to the industry as a whole is immense. Regrettably this fact is seldom sufficiently appreciated. Perhaps in greatest measure this applies to the heat treatment of tool steel.

The cost of the steel and its heat treatment amounts generally to less than a quarter of the total cost of the whole tool. A wrong choice of steel or faulty heat treatment may give rise to serious disruption of production and higher costs.

In this text, an example is tool steel W1, designated only by the type letter and numeral as used in the USA and the UK for standardized tool steels. This designation system is so well known by steel consumers all over the world that no qualifying institutional designations are necessary.

Carbon steels and vanadium-alloyed steels

The hardening of these steels, which are made with carbon contents between 0,80% and 1,20%, is quite straightforward: Since the rate of carbide dissolution proceeds rapidly, the holding time, as a consequence, is short and therefore the heating of small tools can often take place without any extra precautions against atmospheric oxidation.

The hardening temperature is about 780°C. Quenching is carried out direct into brine with tempering following immediately. The quenching operation is the most critical part of the heat treatment since too slow a rate of cooling might give rise to either soft spots or quenching cracks.

If the tool is designed to contain hardened areas around holes or reentrant angles the cooling effect must be very intensive at these areas. Manual stirring will often suffice but in many cases the coolant must be sprayed on to the tool. For sections heavier than 20 mm the depth of hardening, i.e. the distance from the surface to the 550 HV level, is about 4 mm. Sections less than about 8 mm in thickness will harden through.

For awkward tools, hardenability may be a crucial factor and under such circumstances the composition of the steel must be adjusted in accordance herewith, in particular as regards the alloying elements Mn and Cr, which have a powerful influence on hardenability.

The diagram in Figure 1 shows how the hardening temperature affects the depth of hardening and fracture number on Wl-type steel of conventional composition. The V-content is only 0,04%, which implies that the steel starts to be coarse-grained when the hardening temperature exceeds 815°C.

In Figure 2 are shown the results of corresponding trials with steel containing somewhat larger amounts of alloying elements. The depth of hardening is considerably greater. Owing to the high content of V the steel remains fine-grained even when hardened from exceptionally high temperatures.

The very considerable toughness inherent in plain-carbon steel, due to its shallow-hardening properties, is forfeited if the tool through-hardens locally at some sections because the cross-sectional area there is too small. For shearing tools or small tools generally, such as scissors, knives or letter die punches, which are not subjected to heavy impact blows, this drawback is of less importance. Tools operating under heavy blows, e.g. upsetting dies for cold-heading of bolts, must not be through-hardened.

Coining and striking punches are other examples of carbon tool steels that require high wear resistance. Such tools may also be subjected to bending stresses and should therefore not be through-hardened. The tempering temperature normally used for tools belonging to this group lies in the range 170°C, the hardness being generally about 60-64 HRC. Representative examples of tools made from grade W1 are shown in Figure 3.

Figure 3. Punches made from steel W1