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Steel MakingSurface CarburizingCarburizing, commonly called case-hardening, is the art of pr... William Kelly's Air-boiling Process An account of Bessemer's address to the British Association w... Chrome-nickel Steel Forging heat of chrome-nickel steel depends very largely on ... Temperature Recording And Regulation Each furnace is equipped with pyrometers, but the reading an... Heating Of Manganese Steel Another form of heat-treating furnace is that which is used ... Knowing What Takes Place How are we to know if we have given a piece of steel the ver... Protective Screens For Furnaces Workmen needlessly exposed to the flames, heat and glare from... Silicon SILICON is a very widespread element (symbol Si), being an es... Silicon Silicon prevents, to a large extent, defects such as gas bubb... Pyrometers For Molten Metal Pyrometers for molten metal are connected to portable thermoc... Alloying Elements Commercial steels of even the simplest types are therefore p... Sulphur Sulphur is another impurity and high sulphur is even a greate... Robert Mushet Robert (Forester) Mushet (1811-1891), born in the Forest of D... Classifications Of Steel Among makers and sellers, carbon tool-steels are classed by g... Gas Consumption For Carburizing Although the advantages offered by the gas-fired furnace for ... The Modern Hardening Room A hardening room of today means a very different place from ... Making Steel Balls Steel balls are made from rods or coils according to size, st... Heat Treatment Of Punches And Dies Shears Taps Etc HEATING.--The degree to which tools of the above classes shou... Optical System And Electrical Circuit Of The Leeds & Northrup Optical Pyrometer For extremely high temperature, the optical pyrometer is lar... Bessemer Process The bessemer process consists of charging molten pig iron int... |
Temperatures To UseCategory: HARDENING CARBON STEEL FOR TOOLS As soon as the temperature of the steel reaches 100 deg.C. (212 deg.F.) the transformation begins, increasing in intensity as the temperature is raised, until finally when the lower critical range is reached, the steel has been all changed into the ordinary constituents of unhardened steels. If a piece of polished steel is heated in an ordinary furnace, a thin film of oxides will form on its surface. The colors of this film change with temperature, and so, in tempering, they are generally used as an indication of the temperature of the steel. The steel should have at least one polished face so that this film of oxides may be seen. An alternative method to the determination of temper by color is to temper by heating in an oil or salt bath. Oil baths can be used up to temperatures of 500 deg.F.; above this, fused-salt baths are required. The article to be tempered is put into the bath, brought up to and held at the required temperature for a certain length of time, and then cooled, either rapidly or slowly. This takes longer than the color method, but with low temperatures the results are more satisfactory, because the temperature of the bath can be controlled with a pyrometer. The tempering temperatures given in the following table are taken from a handbook issued by the Midvale Steel Company. TABLE 23.--TEMPERING TEMPERATURES FOR STEELS ---------------------------------------------------------------------------- Temperature Temperature for 1 hr. for 8 min. --------------- Color --------------- Uses Deg. F.Deg. C. Deg. F.Deg. C. ----------------------------------------------------------------------- 370 188 Faint yellow 460 238 Scrapers, brass-turning tools, reamers, taps, milling cutters, saw teeth. 390 199 Light straw 510 265 Twist drills, lathe tools, planer tools, finishing tools 410 210 Dark straw 560 293 Stone tools, hammer faces, chisels for hard work, boring cutters. 430 221 Brown 610 321 Trephining tools, stamps. 450 232 Purple 640 337 Cold chisels for ordinary work, carpenters' tools, picks, cold punches, shear blades, slicing tools, slotter tools. 490 254 Dark blue 660 343 Hot chisels, tools for hot work, springs. 510 265 Light blue 710 376 Springs, screw drivers. ---------------------------------------------------------------------------- It will be noted that two sets of temperatures are shown, one being specified for a time interval of 8 min. and the other for 1 hr. For the finest work the longer time is preferable, while for ordinary rough work 8 min. is sufficient, after the steel has reached the specified temperature. The rate of cooling after tempering seems to be immaterial, and the piece can be cooled at any rate, providing that in large pieces it is sufficiently slow to prevent strains. Next: Knowing What Takes Place Previous: The Theory Of Tempering
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