The development of highspeed and of high-grade carbon steel has resulted in the development of scientific heat treatment. Where the process is applied, those cases in which operations are conducted in an accurate and intelligent manner apparently are in by far the smaller proportion. There seems to be less attention paid to heat treatment than to the composition of the steel; but the heat treatment is far more important than a small variation in the chemical composition of the steel.
The physical changes which take place when a piece of steel is heated and quenched or slowly cooled are controlled by definite laws, and, with steels containing a high per cent of carbon with other alloying elements, the operation of heat-treating is a very delicate one, such that a slight variation even with the best of steel would give very poor results.
The proper or most practical heat treatments for the various steels have been determined by their manufacturers before placing them on the market, and the directions given should be carried out. To make one heat treatment which is satisfactory for all steels is impossible, so the manufacturers' recommendations should be followed.
Before going into the details of heat treatment, it is necessary to know some of the properties which enter into the steel, so as to understand the treatment more fully.
Steel, as we obtain it from the market at the present time, is a combination of iron and carbon and some impurities such as sulphur and phosphorus, though to produce particular results, these latter sometimes are added together with many other elements such as manganese, silicon, nickel, chromium, tungsten, molybdenum, vanadium, etc., either accidentally or purposely included for their effects.
The principal element determining the properties of crucible tool steel is carbon. Pure iron is very nearly as soft as copper, while, with an increasing amount of carbon, steel becomes harder and stronger, but less ductile, until a carbon content of about 0.90 per cent is reached. Steel of this carbon content is the one mostly used for common machine-shop tools. Above this point other additions of carbon result in not only increasing hardness but also increasing brittleness.
Steels most generally useful in the hardened state have carbon from 0.90 per cent to 1.20 per cent, between which points the hardness is sufficient for almost every purpose. Steel containing carbon up to about 0.15 per cent carbon generally is called ingot iron; that from 0.15 per cent to 0.35 per cent is machinery steel; that from 0.35 per cent to 0.60 per cent is crucible machinery steel; and that from 0.60 per cent up is tool steel. Steels containing only carbon with small quantities of silicon, manganese, and the always present sulphur and phosphorus, generally are called straight carbon steels. Those owing their peculiar properties to some ingredients other than carbon are called alloy steels.
For every grade of steel there is a so-called critical temperature or point of transformation. This temperature Varies according to the various compositions and the carbon content of the steels. The phenomenon probably is more marked in the alloy steels than in straight carbon steels. On heating through the critical temperature or critical range the steel takes on hardness when cooled quickly, the hardness being according to its carbon content and the quickness of cooling.
When working a new steel for the first time, a blacksmith should acquaint himself with the particular hardening temperature of the material, and in practice he should be very sure not to vary or exceed it much.
To harden, steel must be heated to, or a little above, this critical temperature and be cooled suddenly, the percentage of hardness which the steel is capable of assuming being proportional to the speed of cooling - the quicker the cooling, the harder the steel. If heated below the critical temperature, even though quenched, steel does not take on hardness, but remains soft or partly annealed. On heating through the critical temperature, the previously existing structure is obliterated or tends to become obliterated, and up to this temperature, or just above it, is when steel possesses the finest structure which it is capable of assuming. The rate of obliteration of the old structure in a piece of steel depends upon the temperature reached and upon the time the steel is maintained at that temperature, the change proceeding more rapidly with increase of temperature. So, with the obliteration of the old structure, a new structure begins to grow, and its size increases with time and temperature, but more rapidly with increase of temperature than with increase of time at a lower temperature.
Once heated above the critical point and having reached the heat of hardening, steel may be cooled slowly to a certain degree and not lose its power of hardening if cooled suddenly. But to so let steel cool is very bad practice, because, during slow cooling, the micro-constituents of the steel tend to separate, as the coarser appearance shows on breaking the steel for the purpose of examining the grain. Below the critical temperature, slowness of cooling has no effect on the size of grain, but leaves the steel soft.
A bad practice sometimes followed by users of steel is to break off a bar of steel as it comes from the storehouse, examine the fracture, and if the steel shows a coarse grain, to condemn the steel. The appearance of the grain in the steel depends on its heat when it left the hammer or the rolling mill; if the heat were high, it would show a coarse grain; if the heat were low, it would show a closer fin€ grain. If the steel is heated to the proper hardening point afterward, the grain is the same, and there is no difference in its physical properties.