Heat Treating Low-Alloy Cold-Work and Hot-Work Tool Steels

For considering heat treating this group of material, several typical tool steels are selected as examples, designated only by the type letter and numeral as used in the USA and the UK for standardized tool steels, e.g. H13, O1. These designations are so well known by steel consumers all over the world that no qualifying institutional designations are necessary. Steels for which there are no AISI or BS specifications are designated according to Swedish (SIS) standards.

The hardenability of SIS 2550 is considerably good. SIS 2550 is air heat treated in fairly heavy sections, which is of advantage where dimensional stability is concerned. Due to the lower carbon content the toughness is greater than the most of the other cold-work steels. When used for cold-work tools, the steel is tempered at 170-250°C, the resulting hardness then being 55-58 HRC. With regard to impact strength this steel, too, is susceptible to tempering treatments around 300°C (see Figure 1).

Figure 1. Steel SIS 2550. Hardness and impact strength as functions of tempering temperature

SIS 2550, after hardening and tempering at 200-250°C possesses very high tensile strength and good impact strength. The values given below have been obtained on tensile test specimens that were oil quenched from 830°C and tempered at 250°C.

Such favourable mechanical properties make the steel suitable for tools subjected to large static and dynamic forces. Some typical applications are dies for tableware, shear blades for heavy plate and dies for plastic moulds, which requires steel possessing a high degree of dimensional stability and excellent polishability.

SIS 2550 is also used for hot-work tools working at moderate temperatures, e.g. drop-forging dies. Such tools are tempered between 400°C and 600°C, the exact temperature depending on the hardness required and the working temperature of the tool. For working temperatures above approximately 400°C the hardness of the steel falls relatively quickly.

If higher working temperatures are involved it is recommended to use the special hot-work steels.

Grade S1 has both high wear resistance and high impact strength. The hardenability is inferior to that of the Cr-Ni-Mo steel SIS 2550. This implies that for dimensions greater than 50 mm in diameter, this steel is contour-hardening which, in fact, further increases its toughness.

The normal heat treating temperature is about 900°C but it may be raised to 950°C without any risk of grain growth being incurred. If a hardness higher than 50 HRC is required in dimensions up to about 60 mm in diameter the steel should be quenched in oil. For heavier dimensions a combined water-oil quenching procedure may be necessary.

Of the many cold-work applications for tool steel, special mention should be made of the cold punching of plate having a thickness greater than about 3 mm. If a plate of increasing thickness is being punched and consequently the thickness measurement of the plate is approaching the diameter of the hole, the punches used show an increasing tendency to break if they are made from, for example, grades O1, A2 or D2. For this type of punching work, grade S1 has been shown to possess the best combination of toughness and wear resistance. A suitable hardness is 56-58 HRC.

Wear resistance is further increased if, during the course of the hardening treatment, the tools are heated for some 20 min in a cyanide bath. After this treatment no further finishing is required; at the most a very light finish grinding is permissible. Another example is the use of this steel as the impact hammer in nail guns, used for driving nails into concrete.

Owing to its high toughness in comparatively large dimensions, grade S1 can successfully be used for tableware dies, which, depending on their dimensions should either be quenched in oil or be heat treated according to the combined oil-water quenching procedure.

Another field of application is shear blades for cold shearing of heavy plate. Because of its rather good resistance to tempering, grade S1 may also be used for hot shears, a suitable hardness for this latter use being about 45 HRC.

In the field of hot-work, grade S1 has been superseded by other grades, e.g. H13. However, mention should be made of an interesting and successful field of application for grade S1 -- as chisels used in process of electrolytic reduction of Aluminium from bauxite. The function of the chisels is to break up the hard alumina-containing crust which forms on the metal bath. During their use the chisels also come into contact with the bath itself and are thus subjected to both high temperatures and impact stresses. A suitable chisel hardness is about 350 HB.