Heat 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).

When 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.

Should 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 thickness.

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 from 840°C

This 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

The 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

As 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.

The 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.

On 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.

If, 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.

Since 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.

Another 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

Figure 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

This observation is particularly striking when stainless steel tubes are being rolled, since there is no `galling` when SIS 2092 is being used.