hit counter script Tempering Steel

Tempering Steel  (extracts from Machinery's Handbook)

What is Tempering

Hardened steel can be tempered or made softer and less brittle by re-heating it to a certain temperature (depending on the nature of the steel and its intended use), and then cooling.

Process

When steel is tempered by the colour method, the temper is gauged by the colours formed on the surface as the heat increases. First the surface is cleaned to brightness with emery cloth so that the oxide colours will be visible to reveal the colour changes when the steel is heated. Heating can take place either by placing it upon a piece of red-hot metal, a gas-heated plate or in any other available way. In school a gas torch is often used to heat the metal.

 

As the temper increases, various colours appear on the brightened surface. First there is a faint yellow which blends into straw, then light brown, dark brown, purple, blue and dark blue, with various intermediate shades. The temperatures corresponding to the different colours and shades are given in the table on temperatures and colours for tempering. The parts are then removed from the bath and quenched in hot water.

 

Turning and planing tools, chisels, etc., are commonly tempered by first heating the cutting end to a cherry-red, and then quenching the part to be hardened. When the tool is removed from the bath, the heat remaining in the unquenched part raises the temperature of the cooled cutting end until the desired colour (which will show on a brightened surface) is obtained, after which the entire tool is quenched. The foregoing methods are convenient, especially when only a few tools are to be treated, but the colour method of gauging temperatures is not dependable, as the colour is affected, to some extent, by the composition of the metal.

 

The modern method of tempering, especially in quantity, is to heat the hardened parts to the required temperature in a bath of molten lead, heated oil, or other liquids; The bath method makes it possible to heat the work uniformly, and to a given temperature with close limits.

 

Temperatures and colours for tempering

Degrees Centigrade

Degrees Fahrenheit

High Temperatures judged by Colour

 

Degrees Centigrade

Degrees Fahrenheit

Colours for Tempering

400

752

Red heat, visible in the dark

 

221.1

430

Very pale yellow

474

885

Red heat, visible in the twilight

 

226.7

440

Light yellow

525

975

Red heat, visible in the daylight

 

232.2

450

Pale straw-yellow

581

1077

Red heat, visible in the sunlight

 

237.8

460

Straw-yellow

700

1292

Dark red

 

243.3

470

Deep straw-yellow

800

1472

Dull cherry-red

 

248.9

480

Dark yellow

900

1652

Cherry-red

 

254.4

490

Yellow-brown

1000

1832

Bright cherry-red

 

260.0

500

Brown-yellow

1100

2012

Orange-red

 

265.6

510

Spotted red-brown

1200

2192

Orange-yellow

 

271.1

520

Brown-purple

1300

2372

Yellow-white

 

276.7

530

Light purple

1400

2552

White welding heat

 

282.2

540

Full purple

1500

2732

Brilliant white

 

287.8

550

Dark purple

1600

2912

Dazzling white (bluish-white)

 

293.3

560

Full blue

 

 

 

 

298.9

570

Dark blue

Tempering in Oil. -- Oil baths are extensively used for tempering tools (especially in quantity), the work being immersed in oil heated to the required temperature, which is indicated by a thermometer. It is important that the oil have a uniform temperature throughout and that the work be immersed long enough to acquire this temperature. Cold steel should not be plunged into a bath heated for tempering, owing to the danger of cracking it. The steel should either be pre-heated to about 300 degrees F., before placing it in the bath, or the latter should be at a comparatively low temperature before immersing the steel, and then be heated to the required degree. A temperature of from 650 degrees to 700 degrees F. can be obtained with heavy tempering oils; for higher temperatures, a lead bath is generally used. A tempering oil which has given satisfactory results in practice has the following characteristics: Composition, mineral oil, 94 per cent; saponifiable oil, 6 per cent; specific gravity 0.920; flash point, 550 degrees F.; fire test, 625 degrees F. The foregoing figures apply to new oil. When the oil has been used long enough to be rendered practically useless, an analysis shows the following changes: Composition, mineral oil, 30 per cent; saponifiable oil, 70 per cent; specific gravity, 0.950; flash point, 475 degrees F.; fire test, 550 degrees F. The great difference in the composition of new and old oil is due to the loss of mineral oil, resulting from the high heats to which tempering oil is frequently or constantly subjected; hence, the durability of the tempering bath can be increased by occasionally adding new mineral oil.

Flash Point and Fire Test. -- The distinction between the "flash point" and the "fire test" of an oil is as follows: The flash point is the temperature at which the amount of vapor given off is sufficient to form an inflammable or explosive mixture with the air over the surface of the oil, so that the gaseous mixture ignites and burns with a momentary flash when a flame is applied. As the temperature of the oil rises, more vapor is given off, and then the production of vapor is rapid enough to maintain a continuous flame, the oil takes fire and burns. The temperature at which this occurs is called the fire test, firing point or burning point of the oil.

Tempering in a Lead Bath. -- The lead bath is commonly used for heating steel preparatory to tempering, as well as for hardening. The bath is first heated to the temperature at which the steel should be tempered; the pre-heated work is then placed in the bath long enough to acquire this temperature, after which it is removed and cooled. As the melting temperature of pure lead is 618 degrees F., tin is commonly added to it to lower the temperature sufficiently for tempering. Reductions in temperature can be obtained by varying the proportions of lead and tin.