A few secrets of the metallurgist simply told

by Gerald W. Hinkley

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.

Therefore, it was necessary to develop a new kind of steel to meet a new and severe condition and accordingly the mother of experiment and invention gave birth to the now famous "High Speed" Steel.

The general principles applying to the hardening and drawing of High Speed Steel are in many ways the same as described above for the simple carbon steel, except that as we begin to add various elements other than carbon to the melt, the resulting alloy becomes more and more complex in its form and reactions and therefore its heat treatment causes greater and greater study and skill in its successful undertaking.

It is generally known among tool hardeners that it is necessary to heat the tool to a higher degree of temperature in order to secure proper hardness when using High Speed Steel than it is when a simple Carbon Tool Steel is employed. We are told that the introduction of certain elements into the melt of a simple Carbon Tool Steel has the tendency to change the critical range. Of course, the formulas used in the manufacture of any high grade High Speed Steel contain very appreciable amounts of various elements other than Carbon which materially effect the property which the steel will have when hard. The effect which these elements appear to produce in the period of critical range can be seen from figure 7.

In this case an experiment was made with a piece of High Tungsten High Speed Steel similar to the experiment which was described in detail above with the test piece of simple Carbon Tool Steel. The readings of the pyrometer were carefully recorded and when plotted on the graph sheet produced the picture under discussion.

Here it will be noticed that the vivid reaction, which we might have expected would occur as the temperature indicating the first critical range was reached, was materially reduced. This might lead us to suspect that the desired allotropic change had not completely taken place at this point. In fact we noticed that the pyrometer needle did not record a vivid critical point until a very much higher temperature was reached. All of these observations serve as a possible explanation or indication of why it is necessary to employ very much higher temperatures in the hardening of High Speed Steel than it is in the hardening of a piece of simple Carbon Tool Steel.

In a later chapter of this little volume we define Carbon Steels as those which do not contain enough of any element other than carbon to materially affect the physical properties which the steel will have when hard. High Speed Steels which are one of a very important group of special alloy steels, are those steels to which some element other than carbon has been added in sufficient amount to materially effect the physical properties which the steel will have when hard.

The element which stands out alone as the most vital and important one as affecting the wonderful and highly desirable features looked for in High Speed Steels is Tungsten. We will discuss the various effects which the different elements give to the different alloy steels in a later chapter, but for the present we will confine ourselves to a brief discussion of the heat treatment of the now famous modern High Speed Steel.

High Speed Steel. Carbon .58%. Structure: Very fine pearlitic condition, with particles of free carbide. Mag. 500x

As previously suggested the pressing demand of modern industry for quicker work, greater efficiency and enormously increased out-put of product, gave rise to the necessity of producing far more remarkable tools than was possible with the old fashioned carbon tool steel. Therefore it became necessary to produce a steel which could be rendered sufficiently hard to cut deep furrows in the various metals which have to be machined and, which could be made sufficiently tough to stand the enormous cutting strains and chatter and vibration of the machine, and at the same time maintain all these characteristics when the work done by upsetting the chip of the machined member actually rendered the cutting edge of the tool red hot.

After the seemingly impossible task of producing a steel to meet these terrific conditions had been successfully accomplished, the next question which arose was to produce a machine which was sufficiently powerful to stand the work done by the tool, and so fast has been the progress made by the tool steel producer, that many of our modern manufacturing industries of today are constantly having to introduce new and heavier machinery into their various machine shop and tool rooms in order to keep pace with the possibilities of the tool made from the modern High Speed Steel.

Now, if we were to run an experiment with a test piece made from High Speed Steel similar to the one which we ran on the simple Carbon Tool Steel, we would find that many of the same phenomena previously noticed would again be recorded.

Probably the most important difference would be the fact that instead of having to quench the same in water it would be desirable to use a bath of oil. In fact, water would cause the High Speed Steel to cool off far too quickly so that it would be likely to crack and be rendered useless.

A peculiar action of the various elements in High Speed Steel is very likely to materially retard the change of one allotropic form into another. In fact, the change is so slow that after a piece of High Speed Steel has been heated above the critical temperature, it will actually retain its hardened or austenitic condition even if allowed to cool in the air, and it would only be possible to get it back into its softened condition by the lengthy process of annealing.

Annealing is the process of undoing exactly what the act of hardening accomplished. Long tubes are filled with the tool steel bars and sealed from the air and then placed into the annealing furnaces, wherein the annealing temperature is maintained for a sufficient number of hours, until the steel has had an opportunity to become thoroughly softened.

As before stated "drawing" or "tempering" means the careful re-heating of the steel to 400 degrees Fahr. to 600 degrees Fahr., thus allowing a slight "slipping" of enough of the higher allotropic solution to a lower form, which it is always eager to accomplish at temperatures near the point of recalescence. This, of course, relieves the excess brittleness of the hardened steel.

Annealing is the complete release of the higher allotropic form of the solution and the "trapped" carbon which allows of their return to the normal condition of pearlite and alpha iron. Therefore, it is necessary to heat the steel above the point of recalescence and cool more or less slowly. Different speeds of cooling give different grain, size, structure and physical property.

This explanation of hardening, which is known as the "allotropic theory" is not universally accepted, although it is difficult to find a better or more complete explanation of the remarkable phenomena involved. However, the fact remains that the great accomplishments which have been made by the men of science and understanding have caused remarkable results to have taken place in the manufacturing world of today and the fine and obscure lines which these patient and careful laborers are continually drawing upon the map of knowledge are doing much to make the world a better and safer and more wonderful place in which to live.