Treatment of Low-Alloy Cold-Work Tool Steels
Two steels have
been chosen from this group as examples for the discussion, grade
O1 (RT 1733) and Swedish SIS 2092 (SR 1855).
carbon steel is used for punching dies or cold hobbing tools the
dimensions of the tool are bound by a ruling section that is determined
by the load on the tool. A punch or a die, made from carbon steel,
having a diameter of, say, 50 mm, will show rather poor resistance
to sinking on account of the shallow depth of hardening.
this resistance not suffice, another steel will have to be chosen,
in this case grade O1 or SIS 2092. From the point of view of heat
treatment, these two steels differ somewhat since their hardening
temperatures are different. Steel SIS 2092 requires 850-890°C
whereas grade O1 requires 800-840°C. Owing to its lower hardening
temperature, O1 has somewhat greater dimensional stability. This
property makes it a first choice for blanking dies and other tools
requiring a high degree of dimensional stability (Figure 1).
Figure 1. Blanking tool made from steel O1
In both steels the depth of hardening decreases by roughly the same
amount as the thickness of the section increases. In the diagram
in Figure 2 the hardening temperature was raised as the cross-sectional
area increased in order to increase the hardenability of the steel.
Tools having diameters greater than about 80 mm or equivalent sections
in flat dimensions are difficult to harden to full hardness if there
are re-entrant corners. For such designs it is advisable to choose
SIS 2092 since it obtains full surface hardness more readily and
gives a more regular depth of hardening in a tool with varying section
Figure 2. Curves showing depth of hardening for steel O1. Specimen
25 mm diameter oil quenched from 800°C. Specimen 50 mm diameter
oil quenched from 820°C. Specimen 100 mm diameter oil quenched
point is well illustrated in Figure 3 which shows longitudinal sections
through test specimens that have been hardened as normally prescribed
for each grade concerned, i.e. oil quenching for both, from 820°C
for grade O1 and from 870°C for SIS 2092.
Figure 3. Longitudinal section (etched) through stepped test specimens
made from: a) SIS 2092 and b) AISI O1. The diameters are 50, 75
and 100 mm
above-cited example is to be regarded as a practical assertion of
the possibility of estimating the depth of hardening from the Jominy
diagram. However, it should be emphasized once again that a `contour-hardened`
tool is tougher than a through-hardened one. Figure 4 shows a section
through a `contour-hardened` Pilger roll.
Figure 4. Transverse section through a Pilger roll made from SIS
2092. Size 50x120 mm
a rule both steels are oil quenched. For heavy sections, e.g. dimensions
greater than 100 mm in diameter, it is best to use water quenching
when dealing with SIS 2092. When the surface temperature of the
steel has fallen to between 400°C and 300°C the water quenching
is interrupted by transferring the tool to an oil bath.
tempering temperature for both steels is generally in the range
170-200°C which gives a hardness of more than 60 HRC. As can
be seen from Figure 5, SIS 2092 has a greater resistance to tempering
than grade O1.
being tempered in the range 250-350°C the steel suffers a reduction
in its impact strength, which in turn increases the risk of chipping.
For this reason tools that are subjected to impact stresses should
not be tempered in this temperature range. The higher impact strength
manifested after tempering at 170-200°C is due to the presence
of retained austenite, viz. about 10%.
Figure 5. Tempering curves for steel O1 and SIS 2092
This soft retained austenite can accommodate impact stresses better
than the harder constituents. Retained austenite is decomposed when
it is tempered at about 300°C.
during service, some areas of the tool have to support excessive
pressures, for example the shearing edge of circular slitting knives
(see Figure 6), retained austenite may be transformed to martensite,
with spalling at the edge as a result. Should this occur, tempering
at 300-400°C is recommended. After such a treatment the hardness
of SIS 2092 still remains around 60 HRC.
the wear resistance of SIS 2092 is as much as 25% greater than that
of grade O1, the former is very popular when a wear-resisting steel
is required that can give a better performance than grade O1. Compared
with this steel, SIS 2092 has been shown to have a considerably
longer service life, particularly as drawing die steel.
interesting application is as cane-slitting tools (see Figure 7).
The requirements of this type of tool are both high wear resistance
and toughness in its thin walls. Of all the steels tested the best
results were obtained with SIS 2092. In recent years SIS 2092 has
increasingly been used for so-called Pilger rolls, which are in
part made as rings and in part as dies.
Figure 6. Circular shears (slitting knives) made from SIS 2092
7. Cane-slitting tool made from SIS 2092
Figure 8 shows one of the world`s largest Pilger rolls, designed
for cold-rolling 10 inches tubes. The only steel suitable for this
tool was SIS 2092. Another field of application is for what are
known as Yoder rolls. A sketch showing the principle of operation
and tube manufacture is shown in Figure 9. For this mill unit, the
wear resistance of rolls made from SIS 2092 has shown itself to
be on a par with that of grade D2, in fact in some instances it
has outlasted this grade.
Figure 8. Pilger roll made from SIS 2092 for 10 in tube mill. Dimensions:
800 mm diameter x 400 mm,
weight 800 kg
Figure 9. Sketch showing tube-mill operating principle for welded
tubes (Yoder mill). Welding stage omitted
observation is particularly striking when stainless steel tubes
are being rolled, since there is no `galling` when SIS 2092 is being
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treating of steel