Treating Low-Alloy Cold-Work and Hot-Work Tool Steels
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.
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. 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.
A5 = 8%
Z = 33%
Rp0,2 = 1630
HRC = 54
Rm = 2000 MPa
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
If higher working
temperatures are involved it is recommended to use the special hot-work
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.
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
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.
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.