A few secrets of the metallurgist simply told

by Gerald W. Hinkley

Therefore, let us place a piece of .9% carbon tool steel in the heating furnace and bring it up to and beyond the point of recalescence. Now, grasping the piece firmly in a pair of tongs with all possible speed we plunge it into a nearby pail of ice water, keeping the steel constantly in motion. Almost instantly the steel becomes black and within a few seconds is actually brought down to room temperature.

Now let us take the steel out and examine it. The act of tapping it on the anvil in order to knock off the surplus water gives us a hint that our test piece has undergone some sort of a change. For now it rings with a bell-like clearness and gives the hammer with which we strike it a quick snapping rebound which in itself indicates great hardness. Next, we test the piece with a hardened steel file with which we could easily have made a deep ridge before we attempted the heating operation and to our surprise the file has as little effect as if it had been made of wood. And to our surprise on closer examination, we actually find that our test piece has scratched the file—surely it must be very hard. We are convinced that some marked change must have taken place. What can it be? Why it must be that due to the rapid cooling in the pail of ice water we brought the temperature of the test piece down below the critical range before the abnormal condition at which it existed while at and above the critical range had found time to change back to its former condition. And we remember that if one of these allotropic changes is going to take place at all, nature says it must do so while the steel is within the critical range and therefore having forced the steel through that critical range which separates one allotropic condition from another, before it had found time to effect its desired change, we managed to entrap the abnormal condition so that we could see it and feel it and get familiar with it at room temperature.

If we so desire we can now make other hardness tests on our piece of steel at our leisure. For these scientists have invented several machines. One of the most common is called the scleroscope in which a hardened steel ball is allowed to drop from a given height on to the piece of steel to be tested. Then the rebound of the ball is carefully noted. The higher the rebound, the harder the piece. That is natural isn’t it? We know that if the ball were allowed to drop on butter, it wouldn’t rebound at all, because the butter is so soft. A piece of wood would possibly record a very tiny rebound, while a piece of hardened tool steel would effect a very material action of the scleroscope ball, thus indicating extreme hardness.

Now let us take our test piece to the grind stone and grind it down to the shape of a cutting tool. It is necessary to resort to the grind stone, in order to get the desired shape, because of course, our test piece is far too hard to cut with any other metal. After having produced a tool of the desired shape and size, let us fasten the same securely into the carriage of a lathe, and then upon applying the cutting edge to a revolving piece of cast iron, or soft steel, or even to a piece of the very same grade of steel out of which the tool was made, only while it is still in the softened or annealed condition, we find that it is capable of easily and quickly cutting out a good sized ribbon of chips from the metal which is to be machined.

However, we are soon confronted with a new difficulty, for as the cut progresses, our tool runs into a rough spot which causes it to tremble and chatter and then suddenly our tool cracks in two in the middle and is at once completely ruined.

It is evident that as we are able to increase the desirable element of hardness in a piece of tool steel, we also automatically increase the undesirable element of brittleness, and therefore some new method must be devised which will allow a sufficient degree of hardness to allow the tool to cut other metals and at the same time not cause so much brittleness that it will crack in two at the first rough spot which it encounters.

One method of assisting the toughening of a piece of hardened tool steel is accomplished by the process of "drawing". This simply means heating the piece of hardened tool steel up to some fairly warm temperature, which of course must be kept well below the critical range (at which the steel would jump at the chance to quickly change back into one of its softer allotropic forms) and then keeping the steel at this drawing temperature for a while until the unusual strains and stress caused by the rapid cooling have had an opportunity to have become somewhat relieved. Therefore, the process of "drawing" is quite as important as is the first act of hardening itself, and great care must be exercised in undertaking the same.

CHAPTER IV.

HIGH SPEED STEELS.

After the processes of hardening and drawing our sample of simple carbon tool steel have become thoroughly mastered, it might seem that all which was desired had been accomplished and that we could go on indefinitely making and using our simple carbon steel tools. However, when the extraordinary demands of modern industry required faster and faster cutting speeds, and deeper and deeper cuts, we commenced to realize that our familiar carbon tool steels would not fill the bill. This was due to the fact that as the tools became pressed with the faster speeds and deeper cuts, they could not do their work without becoming over-heated by the friction caused by the work of upsetting the chip and therefore the critical temperature was rapidly approached. Of course we know that if this temperature should be reached the steel would quickly lose its hardness and its cutting edge would therefore be completely ruined.