The parts to be casehardened should be placed in the crucible in such a way that none of them may come in contact with it, for otherwise the result would be soft spots; there should be a space of at least 1 inch from the wall of the crucible and about 1/2 inch between the parts.
The packing material should be well tamped down so as to leave no open space. This is a very important and necessary precaution which must be observed to obtain a satisfactory result. After heating the contents of the crucible, a shrinkage takes place which amounts to about 16 per cent of the total value in the crucible. If the specimens are not packed tightly, some of them, or portions of them, are liable to be exposed and no appreciable carburization will result in the exposed surfaces. This statement simply reverts to the principles involved in carburizing with solid material, viz: There must be solid carbon in contact with the steel, and an intervening gas (CO) to effect carburization.
Having carefully packed the specimens, the crucible should be covered with a lid and luted with fire clay. If neglected, this operation would cause an almost total failure - the specimen at the bottom would have a slight case; those on top would not have taken on any carbon. If the crucible is not sealed, the gases formed inside escape, which means, eventually, the absence of one of the principal elements in carburizing, viz, the intervening gas.
To determine the length of time required for the crucible to heat through, the wire test is the safest. Steel wires 1/16 inch in diameter are inserted through suitable holes in the cover into the packing mixture. Fig. 179 shows the manner in which they were placed. These wires are drawn one by one at different intervals and the temperature noted from the color of the wire. Suppose the outside wire is drawn and found to be up to heat. The next is drawn, and if not up to heat, as indicated by the color, more tune should be given for heating. When the last wire is up to heat, count the time for carburizing.
Crucibles should not be too large. If so, the parts near the crucible wall take on more carbon due to being reached by the heat quicker. The average time required for packing the material and heating up the crucible should not be more than 1 1/2 or 2 hours.
Ordinary cast-iron boxes are used mostly in the majority of shops and give good service if the heat used is not too high. Malleable-iron boxes stand a higher heat. If a heat of about 1800 degrees is required, a graphite crucible will give the best results. General Packing. Observe the customary rules, allowing a sufficient amount of carbonizing material in proportion to the amount of steel to be carbonized. Pack uniformly for uniform results. Shake the box so that the carbonizing material will pack close to the steel. When the last layer of steel has been placed in the box, fill the latter heaping full with carbonizing material and tamp down with a wooden block. Cover the boxes and seal with fire clay.
Most of the compounds can be used over several times before being entirely exhausted, in from 50 to 75 hours, so by adding from 20 to 25 per cent of new material to the old each day there will be sufficient to provide for shrinkage and keep the carbonizer up to its normal strength.
Most carbonizing material has a carbonizing range from about 1500° to 1800° Fahrenheit. The temperature should be chosen so that the core of the carbonized steel does not crystallize excessively. By observing this precaution, good results can be obtained with a single heat treatment. A temperature of from 1600° to 1700° Fahrenheit will suit the majority of carbon steels; the depth of case and percentage of carbon acquired in various times are shown in Table V.
This is not a good practice, but is sufficient for some kinds of work. When this practice is resorted to, the heat must not be above 1600° Fahrenheit, or the steel will be coarse and brittle. A carbonizing heat of about 1500° Fahrenheit is better though it means a slower penetration.
Fig. 180. Fractures of Steel Bars Showing Various Teste.
The heat treatment following carbonizing should be done very carefully owing to the fact that the piece must have a very hard outer surface in order to resist wear and also a non-brittle core which will resist strains.
Fig. 181. Fractures of Bled Bars showing Effect of Heat Treatment after Carbonising.
For the best results, as the carbonizing temperature is a high one, the piece should be allowed to cool, then reheated to 1650° Fahrenheit and quenched, and reheated again 1400° Fahrenheit and quenched.
The reason for the double quenching is that the piece must be heated above its point of transformation to destroy the crystallization and consequent brittleness which is liable to be in the core when it is carbonized at a high temperature; but this leaves the carbonated surface layer not hard enough to resist wear; therefore it must be quenched again at 1400° Fahrenheit.
By quenching directly from the carbonizing retort a distinct line is formed between the high-carbon outer shell and the low-carbon core, and this is liable to cause the metal to crack on this line, but if the work is properly heat-treated after carbonizing, this distinct line is made to disappear, and the danger of the steel cracking is removed.
Fig. 180 shows the fractures of different samples of steel and the effect of time and temperature on the depth of case-hardening of steel with different percentages of carbon.
Fig. 181 shows the effect of heat-treatment after carbonizing. Untreated means that the steel was quenched direct from the carbonizing retort, which is too high a heat for good results. First treatment shows that steel was allowed to cool and then reheated to 1400° or 1450° Fahrenheit and quenched. This gives a good case but not a good core. Second treatment shows the results that come from proper heat-treatment. The steel was allowed to cool, reheated to 1650 degrees, quenched, reheated to 1450 degrees and quenched, which gives both a tough core and a hard fine-grain case.