The Working of Steel Part 21

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 coa.r.s.er structure, depending on the extent of overheating.

To determine the proper quenching temperature of any particular grade of steel it is only necessary to heat pieces to various temperatures not more than 20C. (36F.) apart, quench in water, break them, and examine the fractures. The temperature producing the finest grain should be used for annealing and hardening.

Similarly, to determine tempering temperatures, several pieces should be hardened, then tempered to various degrees, and cooled in air. Samples, say six, reheated to temperatures varying by 100 from 300 to 800C. will show a considerable range of properties, and the drawing temperature of the piece giving the desired results can be used.

For drawing tempers up to 500F. oil baths of fresh cotton seed oil can be safely and satisfactorily used. For higher temperature a bath of some kind of fused salt is recommended.

HINTS FOR TOOL STEEL USERS

Do not hesitate to ask for information from the maker as to the best steel to use for a given purpose, mentioning in as much detail as possible the use for which it is intended.

Do not heat the steel to a higher degree than that fixed in the description of each cla.s.s. Never heat the steel to more than a cherry red without forging it or giving it a definite heat treatment.

Heating steel at even moderate temperature is liable to coa.r.s.en the grain which can only be restored by forging or by heat treating.

Let the forging begin as soon as the steel is hot enough and never let tool steel soak in the fire. Continue the hammering vigorously and constantly, using lighter blows as it cools off, and stopping when the heat becomes a very dull red or a faint brown.

Should welding be necessary care should be taken not to overheat in order to make an easy weld. Keep it below the sparkling point as this indicates that the steel is burnt.

Begin to forge as soon as the welds are put together, taking care to use gentle strokes at first increasing them as the higher heat falls, but not overdoing the hammering when the steel cools. The hammering should be extended beyond the welding point and should continue until the dull red or brown heat is reached.

PREVENTING CRACKS IN HARDENING

The blacksmith in the small shop, where equipment is usually very limited, often consisting of a forge, a small open hard-coal furnace, a barrel of water and a can of oil must have skill and experience.

With this equipment the smith is expected to, and usually can, produce good results if proper care is taken.

In hardening carbon tool steel in water, too much cannot be said in favor of slow, careful heating, nor against overheating if cracks are to be avoided.

It is not wise to take the work from the hardening bath and leave it exposed to the air if there is any heat left in it, because it is more liable to crack than if left in the bath until cold.

In heating, plenty of time is taken for the work to heat evenly clear through, thus avoiding strains caused by quick and improper heating, In quenching in water, contraction is much more rapid than was the expansion while heating, and strains begin the moment the work touches the water. If the piece has any considerable size and is taken from the bath before it is cold and allowed to come to the air, expansion starts again from the inside so rapidly that the chilled hardened surface cracks before the strains can be relieved.

Many are most successful with the hardening bath about blood warm.

When the work that is being hardened is nearly cold, it is taken from the water and instantly put into a can of oil, where it is allowed to finish cooling. The heat in the body of the tool will come to the surface more slowly, thus relieving the strain and overcoming much of the danger of cracking.

Some contend that the temper should be drawn as soon as possible after hardening: but that if this cannot be done for some hours, the work should be left in the oil until the tempering can be done. It is claimed that forming dies and punch-press dies that are difficult to harden will seldom crack if treated in this way.

Small tools or pieces that are very troublesome because of peculiar shape should be made of steel which has been thoroughly annealed.

It is often well to mill or turn off the outer skin of the bar, to remove metal which has been cold-worked. Then heat slowly just through the critical range and cool in the furnace, in order to produce a very fine grain. Tools machined from such stock, and hardened with the utmost care, will have the best chance to survive without warping, growth or cracking.

SHRINKING AND ENLARGING WORK

Steel can be shrunk or enlarged by proper heating and cooling.

Pins for forced fits can be enlarged several thousandths of an inch by rapid heating to a dull red and quenching in water. The theory is that the metal is expanded in heating and that the sudden cooling sets the outer portion before the core can contract. In dipping the piece is not held under water till cold but is dipped, held a moment and removed. Then dipped again and again until cold.

Rings and drawing dies are also shrunk in a similar way. The rings are slowly heated to a cherry red, slipped on a rod and rolled in a shallow pan of water which cools only the outer edge. This holds the outside while the inner heated portion is forced inward, reducing the hole. This operation can be repeated a number of times with considerable success.

TEMPERING ROUND DIES

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. s.p.a.ce 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 3/4 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.

TABLE 24.--CHANGES DUE TO QUENCHING ---------------------------------------------------------------- | After |Tempering temperature, degrees Centigrade Thread |quenching|----------------------------------------- | | 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 220C. Practically no lapping would be required if they were tempered at 340C. 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 a.s.sumes 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.

TABLE 25.--COLORS, TEMPERATURES, DEGREES FAHRENHEIT ---------------------------------------------------------- | _A_ | _B_ | _C_ | _D_ | _E_ | Average ------------------|-----|-----|-----|-----|-----|--------- Faint yellow | 430 | 430 | 430 | 430 | 430 | 430 Light straw | 475 | 460 | 450 | ... | 450 | 458 Dark straw | 500 | 500 | 470 | 450 | 470 | 478 Purple (reddish) | 525 | 530 | 520 | 530 | 510 | 523 Purple (bluish) | ... | 555 | 550 | 550 | 550 | 551 Blue | 575 | 585 | 560 | 580 | 560 | 572 Gray blue | ... | 600 | ... | 600 | 610 | 603 Greenish blue | ... | 625 | ... | ... | 630 | 627 ----------------------------------------------------------

TABLE 26.--ANOTHER COLOR TABLE ---------------------------------------------------------- Degrees | Fahrenheit | High temperatures judged by color ------------|--------------------------------------------- 430 | Very pale yellow 460 | Straw-yellow | 480 | Dark yellow | 500 | Brown-yellow > Visible in full daylight 520 | Brown-purple | 540 | Full purple | 560 | Full blue | 600 | Very dark blue / 752 | Red heat, visible in the dark 885 | Red heat, visible in the twilight 975 | Red heat, visible in the daylight 1,292 | Dark red 1,652 | Cherry-red 1,832 | Bright cherry-red 2,012 | Orange-red 2,192 | Orange-yellow 2,372 | Yellow-white 2,552 | White welding heat 2,732 | Brilliant white 2,912 | Dazzling white (bluish-white) ----------------------------------------------------------

These differences might easily be due to the difference in the light at the time the colors were observed. It must also be remembered that even a thin coating of oil will make quite a difference and cause confusion. It is these possible sources of error, coupled with the ever present chance of human error, that makes it advisable to draw the temper of tools in an oil bath heated to the proper temperature as shown by an accurate high-temperature thermometer.

Another table, by Gilbert and Barker, runs to much higher temperatures.

Beyond 2,200, however, the eye is very uncertain.

TABLE 26.--COLORS FOR TEMPERING TOOLS ----------------------------------------------------------------------- Approximate | color and | Kind of tool temperature | --------------|-------------------------------------------------------- Yellow | Thread chasers, hollow mills (solid type) twist drills 430 to 450F.| centering tools, forming tools, cut-off tools, profile | cutters, milling cutters, reamers, dies, etc.

--------------|-------------------------------------------------------- Straw-yellow | Thread rolling dies, counterbores, countersinks. Shear 460F. | blades, boring tools, engraving tools, etc.

--------------|-------------------------------------------------------- Brown-yellow | Taps, Thread dies, cutters, reamers, etc.

500F. | --------------|-------------------------------------------------------- Light purple | Taps, dies, rock drills, knives, punches, gages, etc.

530F. | --------------|-------------------------------------------------------- Dark purple | Circular saws for metal, augers, dental and surgical 550F. | instruments, cold chisels, axes.

--------------|-------------------------------------------------------- Pale blue | Bone saws, chisels, needles, cutters, etc.

580F. | --------------|-------------------------------------------------------- Blue | Hack saws, wood saws, springs, etc.

600F. | -----------------------------------------------------------------------