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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.   

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
TemperatureColor of Heated Carbon Steel
(oF)(oC)
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
TemperatureColor of Heated Carbon Steel 
(oF)(oC)
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

MaterialMelting Point
(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
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