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.

TABLE 24.—CHANGES DUE TO QUENCHING
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.

ABCDEAverage
Faint yellow430430430430430430
Light straw475460450...450458
Dark straw500500470450470478
Purple (reddish)525530520530510523
Purple (bluish)...555550550550551
Blue575585560580560572
Gray blue...600...600610603
Greenish blue...625......630627
TABLE 26.—ANOTHER COLOR TABLE
Degrees
Fahrenheit		 High temperatures judged by color
430Very pale yellow Visible in full daylight
460Straw-yellow
480Dark yellow
500Brown-yellow
520Brown-purple
540Full purple
560Full blue
600Very 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.

Approximate
color and
temperature		Kind of tool
Yellow
430 to 450°F.		Thread chasers, hollow mills (solid type) twist drills centering tools, forming tools, cut-off tools, profile cutters, milling cutters, reamers, dies, etc.
Straw-yellow
460°F.		Thread rolling dies, counterbores, countersinks. Shear blades, boring tools, engraving tools, etc.
Brown-yellow
500°F.		Taps, Thread dies, cutters, reamers, etc.
Light purple
530°F.		Taps, dies, rock drills, knives, punches, gages, etc.
Dark purple
550°F.		Circular saws for metal, augers, dental and surgical instruments, cold chisels, axes.
Pale blue
580°F.		Bone saws, chisels, needles, cutters, etc.
Blue
600°F.		Hack saws, wood saws, springs, etc.

CHAPTER X

HIGH-SPEED STEEL

For centuries the secret art of making tool steel was handed down from father to son. The manufacture of tool steel is still an art which, by the aid of science, has lost much of its secrecy; yet tool steel is today made by practical men skilled as melters, hammer-men, and rollers, each knowing his art. These practical men willingly accept guidance from the chemist and metallurgists.

A knowledge of conditions existing today in the manufacture of high-speed steel is essential to steel treaters. It is well for the manufacturer to have steel treaters understand some of his troubles and difficulties, so that they will better comprehend the necessity of certain trade customs and practices, and, realizing the manufacturer's desire to cooperate with them, will reciprocate.

The manufacturer of high-speed steel knows and appreciates the troubles and difficulties that may sometimes arise in the heat-treating of his product. His aim is to make a uniform steel that will best meet the requirements of the average machine shop on general work, and at the same time allow the widest variation in heat treatment to give desired results.

High speed steel is one of the most complex alloys known. A representative steel contains approximately 24 per cent of alloying metals, namely, tungsten, chromium, vanadium, silicon, manganese, and in addition there is often found cobalt, molybdenum, uranium, nickel, tin, copper and arsenic.