Hot Work Tool Steels

The significance of tool steels extends far beyond what is generally perceived as commonplace. Nearly all the objects we are surrounded by and encounter on a daily basis are manufactured with the help of tool steels. The application spectrum for hot-work tool steels is extensive and the tools manufactured are used in the most diverse areas.

These steels enable the hot-forming of work-pieces made of iron and non-ferrous metals as well as alloy derivatives at high temperatures. They are utilized in processes such as pressure die casting, extrusion and drop forging as well as in tube and glass manufacturing.

Tools made from hot-work tool steels are not only subject to constantly high temperatures when employed, but also to fluctuating thermic loads occurring where the tool surfaces come into contact with the materials to be processed.

Combined with the wear caused by abrasion or impact, these thermic loads constitute very specific requirements on the hot-work tool steels. Key demands are high tempering resistance, temperature strength, thermal shock resistance, high-temperature toughness and wear resistance.

The use of high-quality hot-work tool steels is imperative to ensure an optimal degree of operational efficiency and high productivity.

A hot-work tool steel's functionality is defined by its chemical composition, the technology applied during production and by the ensuing heat treatment.

In general, hot-work steels are of the medium and high-alloy type and most of them has relatively low carbon content (0.25 to 0.6%). Hot work steels should have the following physical characteristics:
1. Resistance to deformation at the working temperature.
2. Resistance to shock.
3. Resistance to wear at the working temperature.
4. Resistance to heat treating deformation.
5. Resistance to heat checking.
6. Good machinability in the annealed condition.

While there are various processes for shaping and forming of hot metals, such as stamping, piercing, drawing, extruding, upsetting and swaging, to perform successfully at elevated temperatures, the tools must possess a combination of strength, wear resistance and toughness. The operating temperature is taken as the maximum temperature which the face of tool reaches in service. This is always higher than the average temperature attained by the tool. For example, in tools such as extrusion and compression dies, the metal is in continuous contact with the die for appreciable times and therefore local heating becomes quite intense. In hot stamping dies, on the other hand, contact is intermittent and therefore heat dissipation tends to equalize the temperature.

To withstand such service conditions, red hardness and high resistance to softening by tempering or drawing action are essential criteria of hot work steels. The amount of thermal shock in service is of extreme importance in this whole family of hot work tooling. In operations such as encountered in piercing, the temperature are high and contact time long; therefore, to prevent softening by annealing, the tool face must be cooled between each piercing operation. Since the most intense heating is on the extreme surface, this cyclic heating and cooling has the effect of expanding and contracting the surface layer. To withstand such service, the steel must be very ductile through the entire temperature range encountered in service; otherwise the surface of the tool develops heat checks. This condition, illustrated in Figure 1 is possibly one of the most prevalent and troublesome faults encountered in hot work tool service.

Figure 1: Hot work die steel showing severe heat checking.

In the conventional hot work steels, the groups are divided into three, depending on the principal elements utilized for red hardness: thus the steels are classed as chromium, tungsten or molybdenum types. Table 1 shows the nominal composition being listed under AISI H series.

Chromium Hot Work Tool Steels

This group of the H10 to H19 steels contains chromium with, in certain cases, additions of tungsten, molybdenum, vanadium and cobalt. The carbon in this group is held relatively low, around 0.35-0.40 per cent, and this, together with the relatively low total alloy content, promotes toughness at the normal working hardness of between 400 and 600 HV.

The high chromium in this group, coupled with low carbon, ensures depth hardening, therefore these steels may be air hardened to full working hardness in sections up to 30 cm. The higher tungsten and molybdenum contents of the H10 and H14 steels increase the red hardness and hot strength, but tend to slightly reduce toughness.

In this group the H11, H12 and H13 steels possibly represent the greatest tonnage used in all hot work die steels. The air hardening qualities and balanced alloy content are responsible for low distortion in hardening. These grades are especially adapted to hot die work of all kinds, particularly white metal extrusion dies and die casting dies, forging dies, mandrels and hot shears. The chief advantage of this group is ability to resist continued exposure to temperature up to 540°C, and at the same time, provide a tough and ductile tool with tensile strength levels of ~5 MPa at this temperature.

The H10 is relative newcomer to this family of steels, being introduced first in the USA, and now also used in Europe. This grade provides a steel with improved toughness, and so important has been its application that many steel suppliers are marketing modifications within the H10 nominal composition, to give a range of selected properties.

Tungsten Hot Work Tool Steels

The principal alloying elements of these steels are carbon, tungsten and chromium and in certain cases vanadium. The high alloy content increases resistance to high temperature softening when compared to the straight chromium steels, but the steels in this group are more brittle. The normal working hardness is in the range 450 to 600HV.

In contrast to the steels in the hot work chromium group, the high tungsten content makes the group unsuitable for water cooling in service. If we examine the composition of steels in this group, we will note that they resemble the high speed steels and, in fact, type H26 is a low carbon version of the T1 high speed steel. In the hot work tool steels in this group, toughness and thermal shock resistance are generally obtained by reducing the carbon content. In doing so, however, it is necessary also to adjust the tungsten and vanadium, since these both reduce hardenability by holding too much carbon in complex carbides and therefore allowing insufficient carbon in the austenite matrix. The adjusted composition, therefore, represents the best combination of hardness and red hardness, with a considerable degree of toughness and resistance to thermal shock.

Molybdenum Hot Work Tool Steels

The principal alloying elements in this group are molybdenum, chromium and vanadium, together with tungsten and varying amounts of carbon. Like the high speed steels, the molybdenum grades of hot work steels have almost identical properties and uses to the corresponding tungsten type. The principal advantage of these steels compared to the former group is more resistance to heat checking. In common with all high molybdenum steels, greater care in heat treatment is necessary in order to avoid decarburization.