A number of circular dies of carbon tool steel for use in tool holders of turret lathes were required. No proper tempering oven was available, so the following method was adopted and proved quite successful.
After the dies had been hardened dead hard in water, they were cleaned up bright. A pair of ordinary smiths' tongs was made with jaws of heavy material and to fit nicely all around the outside of the die, leaving a 3/32-in. space when the jaws were closed around the die. The dies being all ready, the tongs were heated red hot, and the dies were picked up and held by the tongs. This tempered them from the outside in, left the teeth the temper required and the outside slightly softer. The dies held up the work successfully and were better than when tempered in the same bath.
THE EFFECT OF TEMPERING ON WATER-QUENCHED GAGES
The following information has been supplied by Automatic and Electric Furnaces, Ltd., 6, Queenstreet, London, S. W.:
Two gages of ¾ in. diameter, 12 threads per inch, were heated in a Wild-Barfield furnace, using the pyroscopic detector, and were quenched in cold water. They were subsequently tempered in a salt bath at various increasing temperatures, the effective diameter of each thread and the scleroscope hardness being measured at each stage. The figures are in 10,000ths of an inch, and indicate the change + or - with reference to the original effective diameter of the gages. The results for the two gages have been averaged.
| Thread | After quenching | Tempering temperature, degrees Centigrade | |||||
|---|---|---|---|---|---|---|---|
| 220 | 260 | 300 | 340 | 380 | 420 | ||
| 1 | +25 | +19 | +17 | +15 | +13 | +11 | +11 |
| 2 | +18 | +12 | +11 | + 9 | + 6 | + 5 | + 5 |
| 3 | +12 | + 6 | + 5 | + 3 | 0 | 0 | 0 |
| 4 | +10 | + 4 | + 4 | + 2 | ... | 0 | - 1 |
| 5 | + 9 | + 4 | + 4 | + 2 | 0 | 0 | 0 |
| 6 | + 9 | + 4 | + 3 | + 2 | 0 | 0 | 0 |
| 7 | +10 | + 5 | + 5 | + 3 | + 2 | + 1 | +2 |
| 8 | + 8 | + 4 | + 3 | + 2 | 0 | 0 | + 1 |
| 9 | + 9 | + 4 | + 3 | + 2 | + 1 | + 1 | + 1 |
| 10 | + 9 | + 5 | + 5 | + 3 | + 2 | + 2 | + 2 |
| 11 | + 7 | + 4 | + 4 | + 2 | + 1 | + 1 | + 1 |
| 12 | + 9 | + 5 | + 5 | + 5 | + 4 | + 4 | + 3 |
| Scleroscope | 80 | 70 | 70 | 62 | 56 | 53 | 52 |
Had these gages been formed with a plain cylindrical end projecting in front of the screw, the first two threads would have been prevented from increasing more than the rest. The gages would then have been fairly easily corrected by lapping after tempering at 220°C. Practically no lapping would be required if they were tempered at 340°C. There seems to be no advantage in going to a higher temperature than this. The same degree of hardness could have been obtained with considerably less distortion by quenching directly in fused salt. It is interesting to note that when the swelling after water quenching does not exceed 0.0012 in., practically the whole of it may be recovered by tempering at a sufficiently high temperature, but when the swelling exceeds this amount the steel assumes a permanently strained condition, and at the most only 0.0014 in. can be recovered by tempering.
TEMPERING COLORS ON CARBON STEELS
Opinions differ as to the temperature which is indicated by the various colors, or oxides, which appear on steel in tempering.
The figures shown are from five different sources and while the variations are not great, it is safer to take the average temperature shown in the last column.