This section is from the book "Lathe Design, Construction And Operation, With Practical Examples Of The Lathe Work", by Oscar E. Perrigo. Also available from Amazon: Lathe Design: Construction And Operation.
"The question of brittleness is largely a question of treatment; and intelligent experience will very largely obviate the difficulty so that it will be tough enough to stand up under any proper conditions of work. Every tool-dresser knows how to handle carbon tool steels, and is guided by his knowledge of their qualities at different temperatures as indicated by their varying colors. When he gets a high-speed steel he naturally treats it much as he would carbon steel.
"This is where most of the trouble begins. The smith must learn an entirely different set of color values and methods of treatment. He thinks that if he has succeeded in getting a hardness greater than that of his file, he has done his job. That, however, has nothing to do with the fitness of the tool. I have known cases (with a certain make of steel) where the tool would do the best work while still soft enough to take a good Swiss file.
"In other steels a similar degree of softness, or even a degree of hardness much greater than that of ordinary steel, would not work, the tool 'gumming up' and rapidly burning up. The whole secret lies in getting the tool to such a heat, in the process of hardening, that the constituent molecules are mobile, and then 'drawing' it to the right point.
"When the tool-maker has mastered this secret, he can produce a tool of high-speed steel as tough as any of carbon steel. The mastering of it is largely a matter of experience. Our own experiences have been so interesting and successful that I have thought they might prove of help to others, and I submit them herewith.
"The tools should be placed in a pipe or box, well surrounded with small pieces of coke, the packing case then being sealed up with fire clay. Small holes must be left for the escape of gases, otherwise the clay will blow out. The heating furnace should have been previously heated to a white heat. The packing case is left in the heat from one to three hours, according to size. When removed from the furnace, the box should be as near the bath of fish oil as may be, so that there will be no unnecessary delay in bringing from the gases of the packing case to the bath.
"Exposure to the air not only causes scale, and therefore variation in size, but tends to affect the precision of the hardening process. Observance of this caution will prevent a variation of more than a thousandth of an inch in tools of moderate size. Carbon steel usually varies several thousandths as a result of hardening.
"The method of packing will depend somewhat upon the shape of the tool. It is important to pack in such a way that all tools packed in one case be so placed as to be handled very quickly, and at once plunged into the bath, to prevent scaling by reason of contact with the air, as explained above. In case of milling cutters and key-seat cutters, a good way is to suspend them all from a rod, each separated from its neighbors by a slight space, sufficient to allow a free circulation of oil when plunged. Neglect of this caution will be very likely to cause cracks, from the unequal contraction of the cutters, the outer edges only being brought into immediate contact with the bath, and therefore shrinking more rapidly than the interior parts.
"For taps, drills, and similar shaped tools, this hardening leaves the steel too brittle, and as soon as the tool has become a little dull it breaks off. To avoid this the tool can be drawn as can a carbon-steel tool. But here, too, a new set of color scales must be learned. The blue heat of carbon steel is not enough of the high-speed steel. The heat must be carried on until the metal reaches a greenish tinge. It is then allowed to cool in a dry place free from air drafts.
"It is now much tougher and softer than before. In case it needs still further softening, it can be done by reheating, bringing it to a faint red, dull enough to be perceptible only in a dark place (an empty nail keg is convenient for this use) and then cooled as before. We have made taps as small as 5/16 inch in diameter to be used in an automatic nut tapping machine, about the hardest work to which a tap can be put, with gratifying results.
"In the test three taps cut 92,000 nuts, an average of almost 31,000 nuts per tap; with carbon steel taps we cut 6,000 nuts per tap. No effort was made to speed up the machine, the test being one of durability only. The carbon steel taps cost about ten cents, and the high-speed taps about forty, or four times as much. The latter, however, cut about five times as many nuts. Besides this, there is also to be taken into account the more important saving in the time used formerly for stopping the machine, and removing and grinding taps, which is five times as great when carbon steel taps are used.
"This is not mentioned as a particularly demonstrative test, but merely to show that high-speed steel can be profitably used for small tools, if properly treated. Another place where we are using high-speed steel with profit and satisfaction in small tools is in drills. The saving here is very marked; but the statements and claims of several makers of such drills is not verified by our experience. We find that we can run such a drill at about double the speed of the ordinary drill, and at the same time cut more holes.
"Makers of the new steels are in the habit of making large claims as to speeds attainable. We have tried about every such steel on the market, giving each a thorough test. Our experience usually bears out the moderate statements, and sometimes the extravagant ones, put forth by some makers as to what is possible. For instance, we have cut a 1/16 chip, 1/32 feed, at a rate of 266 feet per minute peripheral speed, from a rod of machine steel.
"Such speeds are possible for short periods; but whoever buys a rapid cutting steel with the expectation of maintaining such speed will be sadly disappointed. With a fine feed, even four hundred feet per minute can be cut under very favorable circumstances. But think of the chip that comes off! In case of steel the chip is no such thing as we are accustomed to, breaking into short pieces and dropping into the box below.