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Steel Making

Nickel may be considered as the toughest among the non-rare a...

Typical Oil-fired Furnaces
Several types of standard oil-fired furnaces are shown herew...

Gas Consumption For Carburizing
Although the advantages offered by the gas-fired furnace for ...

Steel For Chisels And Punches
The highest grades of carbon or tempering steels are to be re...

Quenching Tool Steel
To secure proper hardness, the cooling of quenching of steel ...

High-carbon Machinery Steel
The carbon content of this steel is above 30 points and is ha...

Instructions For Working High-speed Steel
Owing to the wide variations in the composition of high-speed...

Piston Pin
The piston pin on an aviation engine must possess maximum res...

Shrinking And Enlarging Work
Steel can be shrunk or enlarged by proper heating and cooling...

The Theory Of Tempering
Steel that has been hardened is generally harder and more br...

Flange Shields For Furnaces
Such portable flame shields as the one illustrated in Fig. 1...

Case-hardening Treatments For Various Steels
Plain water, salt water and linseed oil are the three most co...

Heating Of Manganese Steel
Another form of heat-treating furnace is that which is used ...

Effects Of Proper Annealing
Proper annealing of low-carbon steels causes a complete solu...

Crucible Steel
Crucible steel is still made by melting material in a clay or...

Phosphorus is one of the impurities in steel, and it has been...

Tungsten, as an alloy in steel, has been known and used for a...

Suggestions For Handling High-speed Steels
The following suggestions for handling high-speed steels are ...

The forgings can be hardened by cooling in still air or quen...

Care In Annealing
Not only will benefits in machining be found by careful anne...

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.

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

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 deg., however, the eye is very uncertain.

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

Next: High Speed Steel

Previous: The Effect Of Tempering On Water-quenched Gages

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