If a piece of steel is heated to a certain temperature and held there, the tempering color, instead of remaining unchanged at this temperature, will advance in the tempering-color scale as it would with increasing temperature. This means that the tempering colors do not absolutely correspond to the temperatures of steels, but the variations are so slight that we can use them in actual practice. (See Table 23, page 158.)
Temperatures to Use.—As soon as the temperature of the steel reaches 100°C. (212°F.) the transformation begins, increasing in intensity as the temperature is raised, until finally when the lower critical range is reached, the steel has been all changed into the ordinary constituents of unhardened steels.
If a piece of polished steel is heated in an ordinary furnace, a thin film of oxides will form on its surface. The colors of this film change with temperature, and so, in tempering, they are generally used as an indication of the temperature of the steel. The steel should have at least one polished face so that this film of oxides may be seen.
An alternative method to the determination of temper by color is to temper by heating in an oil or salt bath. Oil baths can be used up to temperatures of 500°F.; above this, fused-salt baths are required. The article to be tempered is put into the bath, brought up to and held at the required temperature for a certain length of time, and then cooled, either rapidly or slowly. This takes longer than the color method, but with low temperatures the results are more satisfactory, because the temperature of the bath can be controlled with a pyrometer. The tempering temperatures given in the following table are taken from a handbook issued by the Midvale Steel Company.
| Temperature for 1 hr. | Color | Temperature for 8 min. | Uses | ||
|---|---|---|---|---|---|
| Deg. F. | Deg. C. | Deg. F. | Deg. C. | ||
| 370 | 188 | Faint yellow | 460 | 238 | Scrapers, brass-turning tools, reamers, taps, milling cutters, saw teeth. |
| 390 | 199 | Light straw | 510 | 265 | Twist drills, lathe tools, planer tools, finishing tools |
| 410 | 210 | Dark straw | 560 | 293 | Stone tools, hammer faces, chisels for hard work, boring cutters. |
| 430 | 221 | Brown | 610 | 321 | Trephining tools, stamps. |
| 450 | 232 | Purple | 640 | 337 | Cold chisels for ordinary work, carpenters' tools, picks, cold punches, shear blades, slicing tools, slotter tools. |
| 490 | 254 | Dark blue | 660 | 343 | Hot chisels, tools for hot work, springs. |
| 510 | 265 | Light blue | 710 | 376 | Springs, screw drivers. |
It will be noted that two sets of temperatures are shown, one being specified for a time interval of 8 min. and the other for 1 hr. For the finest work the longer time is preferable, while for ordinary rough work 8 min. is sufficient, after the steel has reached the specified temperature.
The rate of cooling after tempering seems to be immaterial, and the piece can be cooled at any rate, providing that in large pieces it is sufficiently slow to prevent strains.
Knowing What Takes Place.—How are we to know if we have given a piece of steel the very best possible treatment?
The best method is by microscopic examination of polished and etched sections, but this requires a certain expense for laboratory equipment and upkeep, which may prevent an ordinary commercial plant from attempting such a refinement. It is highly recommended that any firm that has any large amount of heat treatment to do, install such an equipment, which can be purchased for from $250 to $500. Its intelligent use will save its cost in a very short time.
The other method is by examination of fractures of small test bars. Steel heated to its correct temperatures will show the finest possible grain, whereas underheated steel has not had its grain structure refined sufficiently, and so will not be at its best. On the other hand, overheated steel will have a coarser structure, depending on the extent of overheating.