Steel Tempering Colors

The tempering process of hardening steel.

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Steels containing more than 0.3% carbon are intended for heat treatment. Lower values of carbon makes the steel more tough than hard and suitable for applications where strength are important - like springs and high tensile bolts. Steels with carbon content in excess of 0.7% can be hardened to a higher degree and are suitable for applications where hardness rather than toughness is important. Tool steels make excellent cutting and drilling equipment as they contain tungsten, molybdenum, cobalt, and vanadium to increase heat resistance and durability.

tempering steel color

Tempering steel is a two-step process where the

1. The tool is forged and hardened

tool end is heated to bright red color and forged

Carbon Steel Tempering Colors
Temperature		Color of Heated Carbon Steel
(^oF)	(^oC)	Color of Heated Carbon Steel
2192	1200
2012	1100
1922	1050
1796	980
1706	930
1598	870
1490	810
1400	760
1292	700
1202	650
1112	600

tool quenched in cold water and cooled until touchable
tool is sharpened and polished

2. The tool is tempered

tool heated to tempering temperature as indicated below

Carbon Steel Tempering Colors
Temperature		Color of Heated Carbon Steel
(^oF)	(^oC)	Color of Heated Carbon Steel
600	316		Scrapers, spokeshaves
560	293		Screwdrivers, springs, gears
540	282		Cold chisels, center punches
520	271		Taps <= 1/4 inch
500	260		Axes, wood chisels, drifts, taps >= 1/ inch, nut taps, thread dies, press tools
480	249		Twist drills, large taps, knurls
460	238		Dies, punches, bits, reamers
450	232		Twist drills for hard use
440	227		Turning tools, lathe tools, scrapers, milling, cutters, reamers
430	221		Edge tools, reamers
420	216		Knives, hammers

tool quenched in cold water

Note that oil baths and salt bats are also used extensively for tempering steel tools. Lead baths and sand baths are also used.

If steel is heated in an oxidizing atmosphere - like air - a film of oxide forms on the surface and the color changes as the temperature increases. The colors are affected to some extent by the composition of the steel and the method may not be dependable.

Melting Points of Heat-Treating Baths

Melting Points of Heat-Treating Baths
Material	Melting Point
Material	(^oF)	(^oC)
35% lead, 65% tin	358	181
50% sodium nitrate, 50% porassium nitrate	424	218
Tin	450	232
Sodium nitrate	586	308
Lead	620	327
Potassium nitrate	642	339
45% sodium chloride, 50% sodium sulfate	1154	623
Sodium chloride	1474	801
Sodium sulfate	1618	881
Barium chloride	1760	960