Tempering

          Martensitic structure formed by direct quenching of high-carbon steel are hard and strong, but unfortunately are also brittle. They cannot be plastically deformed and have very little toughness, and although strong they are unable to resist impact loads and are extremely sensitive to stress concentrations. Some of the hardness and strength must be sacrificed to obtain suitable ductility and toughness. This is done by tempering the martensitic steel.

          Tempering means giving up a certain amount of hardness but shedding a great deal of brittleness acquired in the process of hardening. It is a trade off between hardness and brittleness, so that hardened component may give useful service without failure. Tempering involves heating the carbon steel part to a temperature varying from  150°-600°C (depending upon how much trade off is required) and cooling the component in an oil or salt bath or even in air.

Thus tempering process is carried out to:

• Increase toughness,
• Decrease hardness,
• Stabilise structure,
• Relieve stress, and
• Change volume.

Method Of Tempering

          Tempering of steel may be carried out in liquid bath such as oil, salt or lead, the bath being heated to the correct temperature and steel immersed in the bath for the determined length of time after which component (steel) is removed and allowed to cool to room temperature. Air tempering furnaces are very successfully employed for tempering. These furnaces are fully automatic and the hot air (heated by gas or electric means) is circulated around the parts to be tempered.

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.