As was the case for hot-work steels, it has been possible to replace W in the T1 grade by Mo. This has led to the development of the M2 type (6-5-4-2) which, for most purposes, can replace T1.
Yet another variant is M7 which has a higher content of Mo but less W than M2. For some applications M7 is said to possess greater toughness and wear resistance than M2. Like T1, both M2 and M7 can be alloyed with Co which gives them increased hot wear-resistance. Such a variant of M7 is designated M42. For the M steels a suitable hardening temperature is 1200-1220°C. Sometimes 1230°C is given as the maximum hardening temperature and under no circumstances should this temperature be exceeded.
As may be seen from the above, the hardening temperatures for high-speed steels are higher than those for other tool steels. Temperatures of only some tens of degrees below the incipient fusion of the steel are used. The chromium carbides go into solution around 1100°C and at the normal hardening temperature for, say, grade M2 there are undissolved carbides left amounting to some 10%, mainly V carbides and double carbides of Mo and W.
The high hardening temperatures employed for high-speed steels are conducive to rapid grain growth and hence the holding time must be carefully controlled. The hardening temperature must also be accommodated to the original dimensions of the steel stock used for the tool, since as the stock dimension increases the amount of carbide segregation increases which, in turn, lowers the temperature of incipient fusion. Therefore, the hardening temperature should be kept near the lower limit of the normal hardening temperature range when the dimension of the original steel stock exceeds about 100 mm.
The hardening temperature is chosen to suit the steel in question, always keeping in mind the use to which the tool is to be put. Tools for machining, e.g. turning and planing tools, or for rough milling, should be hardened from the highest temperature in order to be certain that they obtain the best hot-hardness properties since the cutting edges may reach temperatures as high as 600°C.
Tools to be used at lower temperatures or that require good impact strength, such as cold-upsetting tools, can be hardened from temperatures as low as 1050°C. By this treatment, resistance to tempering is reduced and if the hardening temperature is low enough (below 1000°C) the secondary hardening effect disappears.
As a rule, high-speed steels have good hardenability from which follows that tools made from such steels may be quenched in a salt bath or even air cooled. High-speed steel containing 10% Co has a somewhat reduced hardenability and in order to arrive at maximum hardness by means of air cooling, light sections only (less than 30 mm in diameter) can be treated in this way.
Having been quenched from a normal hardening temperature, high-speed steels contain between 20% and 40% retained austenite. As they cool from a tempering temperature of about 575°C there is practically complete transformation to martensite while at the same time the initially formed martensite is tempered. A second tempering treatment is required to give the last-formed martensite its optimum combination of useful properties. In Figures 1a, b and c are shown the microstructures in specimens of grade M 42 after hardening, after a single and after a double tempering treatment, respectively.
Retained austenite can also be transformed by subzero treatment. A prior subzero treatment will not affect the amount of retained austenite after tempering over about 575°C since this constituent is decomposed at the conventional tempering temperature used. At tempering temperatures above 550°C the length of the holding time produces a pronounced effect on the hardness, as may be seen from the curves in Figures 2 and 3 which show the interdependence of time and temperature. By increasing the tempering time from 1 to 4 h at 600°C the hardness of grade M 2 falls from 65,5 to 63 HRC.
Cutting tools for which the highest hardness is required, are tempered at 550°C. However, the hardness and tempering temperature must be adjusted to the toughness requirements. The impact strength is highest when the steel is tempered in the range 250-450°C and lowest at the temperature that gives maximum hardness. As the steel continues to be tempered at increasing temperatures the toughness starts to increase again. Tools that in service are subjected to high pressures give the best results if they have been tempered at about 600°C, the austenite being completely transformed at this temperature.
M2 is already a pretty crazy alloy.
. I’d like to know what alloy percentage requires high temper, or what specific alloys tip the scale at what percentage. It’s fascinating. To really test this properly, custom compositions would be required, but that’s beyond any of our means.