Tool steel hardening and tempering — technological considerations

Tools are usually machined from tool steel in the soft-annealed condition. After machining the tool must be hardened and then stress-relieved or tempered. Several special considerations arise during the hardening and tempering of tool steels.

Hardening of tool steels

During austenitising, it must be taken into account that most tool steels are heavily alloyed with carbide-forming elements. Heavy alloying leads to poor thermal conductivity, and dissolving the alloy carbides may require a very high hardening temperature. This has two consequences:

• Because of the poor thermal conductivity, even the heating rate can cause cracking — tool steels must be heated to the hardening temperature only cautiously, possibly in steps.
• The required hardening temperature can reach 1000–1300 °C. This cannot be achieved in ordinary heat-treatment furnaces, so the heat treatment of tool steels may require specialised equipment.

Because of the heavy alloying, the cooling rate required for hardening is low. Therefore, besides oil quenching, gas-jet cooling is also a viable option — including in vacuum furnaces.

Surface protection (scaling, decarburisation) is usually very important for tool steels, especially for tools that are not ground after hardening (e.g. files). For high-carbon tool steels, reaching the prescribed hardness is not a problem. However, hitting the target hardness alone does not guarantee quality. A tool is better the more toughness comes with the required hardness, because that gives a higher safety margin against fracture. Toughness is greater the finer the grain size of the steel — which can be achieved by optimising austenitising. When heating to the austenitising temperature in steps, the holding time at the last step is usually limited to minutes to avoid grain coarsening. Since austenitising can be optimised for hardness, wear resistance and toughness only through indirect tests, it is essential to follow the established, proven technology precisely.

Tempering

The hardness of unalloyed or low-alloy tool steels decreases monotonically with tempering temperature. This drop reduces the wear resistance and edge retention of the tool. To reduce tool brittleness while relaxing quenching stresses, only low-temperature tempering (stress relief) is generally applied. For these steels the hardness drops rapidly above 250 °C, so the usual stress relief is performed at 80–240 °C. Exceptions are tools where, for safety or similar reasons, compromises are accepted at the cost of wear resistance or edge life (e.g. spalling risk on hammers, bending requirements of saw teeth). Here the tempering temperature can be raised to as much as 400–450 °C, accepting the drastic hardness loss.

Heavily alloyed hot-work tool steels exhibit secondary hardening. As the tempering temperature is increased, the hardness of the quenched steel first drops due to martensite decomposition, then rises again because of retained-austenite transformation and the dispersed precipitation of alloy carbides. The maximum hardness — depending on steel grade and austenitising parameters — is reached around 540 °C. These steels must be tempered near the secondary-hardening peak. Multiple tempering cycles are beneficial for tool toughness.

Source: Basics of the heat treatment of steels (Budapest Polytechnic, Bánki Donát Faculty of Mechanical Engineering, teaching material, 2004)