Temper Colors

          Tempering can be judged by the temper color which appear on the bright red surface, and experienced eyes are generally guided by those colors while heating steel materials for tempering. The following are the colors formed on steel in the process of tempering:

English Color Text	Fahrenheit	Color	Celsius	SAE 1040	SAE 1050	SAE 1070	SAE 1095
Clear - As fully hardened	100°		38°	54	59	64	67
Pale yellow 2	420°		216°	51	55	59	63
Very pale yellow 1	430°		221°
Light yellow 1, 2, Straw 3	440°		227°
Pale straw-yellow 1, Straw 3	450°		232°	50	54
Straw-yellow 1, 2	460°		238°				62
Deep straw-yellow 1, 3	470°		243°
Dark yellow 1, Light orange 3	480°		249°			58
Yellow-brown 1, Orange 3	490°		254°	49	53		61
Brown-yellow 1, 3, Bronze 2	500°		260°
Spotted red-brown 1, Dark Brown 2	510°		266°
Brown with purple spots 1, 2	520°		271°		52	57
Light purple 1, 2, Purple 3	530°		277°	48			60
Full purple 1, Purple 2	540°		282°
Dark purple 1	550°		288°
Full blue 1, 3	560°		293°	47	51	56	59
Dark blue 1, Blue 2	570°		299°
Dark blue 2	590°		310°				58
Pale blue 3				46	50	55
Light blue 2	610°		321°
Greenish blue 2, Grey 3	630°		332°	45		54	57
Light blue 1	640°		338°
Steel grey 2	650°		343°	44	47	53	56
Steel grey	700°		371°	42	46	51	54
	750°		400°	40	44	50	52
	800°		427°	38	42	47	49
	900°		482°	35	37	43	44
Lowest visible red	1000°		538°	30	32	36	39
English Colour Text	Fahrenheit	Colour	Celsius	SAE 1040	SAE 1050	SAE 1070	SAE 1095

Effects of tempering temperature on the mechanical properties of steel:

Tempering process decomposes the martensite into a ferrite-cementite mixture and thus the properties of steel are strongly affected.

1. At low tempering temperatures (upto 200 to 250)
• bending and true tensile strength are increased, and
• hardness changes to a small extent.
2. If the temperature is further increased the following result:
• hardness, true tensile strength , proportional limit and yield point are reduced.
• reduction of area and relative elongation are increased.
3. Tempering at 250 to 400 reduces the impact strength of steel. Therefore, the temperature range 250 to 400 should be avoided in assigning tempering temperature.
4. The properties after structural improvement i.e., hardening followed by high tempering are always higher than those of annealed steel. This is due to the difference in structure of the ferrite-cementitic mixture.