Heat Treatment of Steel

From the 1924 edition of Machinery's Handbook
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Furnaces and Baths for Heating Steel

Steel Heating Furnaces. -- The furnaces used for the hardening or tempering of steel are heated either by gas, oil, electricity, or solid fuel. Furnaces using oil or gas are made in many different styles and sizes to suit various classes of work, but differ very little in their general arrangement. Crude oil and kerosene are commonly used in oil-heated furnaces. To insure an unvarying temperature the air and fuel pressures should be uniform. Gas furnaces use either natural, artificial, or producer gas. Some gas furnaces are equipped with an automatic apparatus which operates in conjunction with a pyrometer for controlling the temperature to within a few degrees of a given point. The air supply is generally obtained from a positive blower, although where a compressor is installed for operating pneumatic tools, the air is sometimes utilized for the furnaces by interposing reducing valves to diminish the pressure. Artificial gas is more expensive than oil, but is cleaner, and the installation of supply tanks, such as are required for oil, is avoided. Producer gas obtained from a separate plant is not economical unless there is a considerable number of furnaces. When oxidization or the formation of scale is particularly objectionable, furnaces of the muffle type are often used. These furnaces contain a refractory retort in which the steel is placed, thus excluding the products of combustion. These muffles must be replaced quite frequently, and more fuel is required than for an oven type of furnace.

Electrically-heated furnaces are generally considered very satisfactory for the heat-treatment of high-grade work, although the cost of electricity exceeds that of liquid or gaseous fuels. A type of electric furnace that is commonly used, deriving its heat from a heavy, low-voltage current which passes through electrodes to resistance elements in the heating chamber. This type of furnace gives a uniform heat and is adapted to accurate regulation. Electrically-heated furnaces are also used in conjunction with heating baths, the current being transmitted through a bath of metallic salts by two electrodes on opposite sides of the crucible. The conductivity of the salt is very small at normal temperatures, but at high temperatures, when the salt is in the molten condition, it offers a comparatively low resistance to the electric current, and therefore, when the bath is hot, it forms an electric conductor, and each part of the bath produces its own heat.

Solid fuels, such as coke, coal, charcoal, etc., are also used, in many cases, for heating steel. A common type of solid-fuel furnace is equipped with a grate upon which the fuel is burned, and an arch above the grate which reflects the heat back to the plate that holds the steel to be heated. This plate should be so located that the flames will not come into direct contact with the steel and injure the finished surfaces. To prevent this the steel is sometimes safeguarded by placing it inside of a clay or cast-iron retort, which is encircled by the flames. The solid-fuel type of furnace is inferior to other types, for most purposes, because it is almost impossible to maintain a uniform temperature, and the gases of combustion are liable to injure the steel.

Heating Steel in Liquid Baths. -- The liquid baths commonly used for heating steel tools preparatory to hardening are molten lead, cyanide of potassium, barium chloride, a mixture of barium and potassium chloride, and other metallic salts. The molten substance is retained in a crucible which is usually heated by gas or oil. The principal advantages of heating baths are as follows: No part of the work can be heated to a temperature above that of the bath; the temperature can be easily maintained at whatever degree has proved, in practice, to give the best results; the submerged steel can be heated uniformly; and the finished surfaces are protected against oxidization.

The Lead Bath. -- The lead bath is extensively used, but is not adapted to the high temperatures required for hardening high-speed steel, as it begins to vaporize at about 1190 degrees F., and if heated much above that point, rapidly volatilizes and gives off poisonous vapors; hence, lead furnaces should be equipped with hoods to carry away the fumes. Lead baths are especially adapted for heating small pieces which must be hardened in quantities. Gas is the most satisfactory fuel for heating the crucible. It is important to use pure lead that is free from sulphur. The work should be pre-heated before plunging it into the molten lead.

Cyanide of Potassium Bath. -- Many steel hardeners prefer cyanide of potassium to lead, for heating steel cutting tools, dies, etc. When cyanide is used the parts should be suspended from the side of the crucible by means of wires or wire cloth baskets, to prevent them from sinking to the bottom. Steel will not sink in a lead bath, as lead has a higher specific gravity than steel. Cyanide of potassium should be carefully used, as it is a violent poison. The fumes are very injurious, and the crucible should be enclosed with a hood connecting with a chimney or ventilating shaft. This bath is extensively used for hardening in gun shops, in order to harden parts and at the same time secure ornamental color effects.

Barium Chloride Bath. -- As a temperature of about 2400 degrees F. can be obtained with this bath, it is used to some extent for heating high-speed steel. Owing to certain disadvantages, however, barium chloride has been discarded by many manufacturers. (See "Disadvantages of Barium Chloride Bath".) When barium chloride is used for the lower temperatures required for carbon steel, it is mixed with chloride of potassium. For temperatures between 1400 and 1650 degrees F., use three parts of barium chloride and two parts chloride of potassium. For higher temperatures the amount of potassium chloride should be proportionately reduced, pure barium chloride being used for temperatures above 2000 degrees F. All steel should be pre-heated to 600 degrees or 800 degrees F. before being immersed in the bath. Temperatures below 1075 degrees F. can be obtained by using equal parts of potassium nitrate and sodium nitrate. This mixture sets at 400 degrees F., and is used as a tempering bath.

Disadvantages of Barium Chloride Bath. -- While barium chloride baths are still used to a considerable extent, both in this country and abroad, the results obtained in the heat-treatment of steel have not been as favorable as was at first expected. In fact, many former users of barium chloride have abandoned it in preference to other methods of heating. The principal difficulty has been that steel heated in barium chloride has a film of soft metal probably 0.003 to 0.006 inch deep. Tests made to determine the influence on the cutting qualities of tools showed conclusively that those heated in a barium chloride bath would not stand as high a cutting speed as steel heated in an oven furnace. When about 0.010 is ground from the cutting edges, the effect of heating in barium chloride is not apparent. The injurious effects are increased when using an electric hardening furnace with a barium-chloride bath. The objections mentioned in the foregoing apply only to the hardening of high-speed steel; the electric hardening furnace using a barium chloride bath has proved very satisfactory for ordinary carbon steel tools.

As mentioned in the preceding paragraph, the lower temperature required for carbon steel is obtained by adding a certain percentage of potassium chloride. The barium chloride bath has the advantage of protecting the heated steel, while it is been transferred to the quenching bath, by a thin coating which usually falls off when the steel is being quenched.

BaumÈ Gravity and Corresponding Specific Gravities,
Weights per Gallon and Calorific Value of Fuel Oil
Kind of Oil BaumÈ Specific Gravity Pounds per Gallon Calculated B.T.U. per Pound Calculated B.T.U. per Gallon
Mexico, California, Texas and Kansas Crude Fuel Oil 14 0.9722 8.10 18,810 152,361
15 0.9655 8.05 18,850 151,743
16 0.9589 7.99 18,890 150,931
17 0.9523 7.94 18,930 150,304
18 0.9459 7.88 18,970 149,484
19 0.9395 7.83 19,010 148,848
20 0.9333 7.78 19,050 148,209
21 0.9271 7.73 19,090 147,506
22 0.9210 7.68 19,130 146,918
23 0.9150 7.63 19,170 146,267
24 0.9090 7.58 19,210 145,612
25 0.9032 7.54 19,250 145,145
26 0.8974 7.49 19,290 144,482
27 0.8917 7.44 19,330 143,815
Kansas, Indian Territory and Illinois Crudes, Penn. Fuel, California Refined Fuel Oil 28 0.8860 7.39 19,370 143,144
29 0.8805 7.34 19,410 142,469
30 0.8750 7.29 19,450 141,790
31 0.8695 7.25 19,490 141,303
32 0.8641 7.21 19,530 140,811
33 0.8588 7.16 19,570 140,121
34 0.8536 7.12 19,610 139,623
35 0.8484 7.07 19,650 138,926
36 0.8433 7.03 19,690 138,421
Ohio, Penn. and West Virginia Crude, California and Kansas Refined37 0.8383 6.99 19,730 137,913
38 0.8333 6.95 19,770 137,402
39 0.8284 6.91 19,810 136,887
40 0.8235 6.87 19,850 136,370
41 0.8187 6.83 19,890 135,849
42 0.8139 6.80 19,930 135,524
43 0.8092 6.76 19,970 134,997
44 0.8045 6.72 20,010 134,467
45 0.8000 6.68 20,050 133,934
Kerosene and Gasoline 46 0.7954 6.64 20,090 133,398
47 0.7909 6.60 20,130 132,858
48 0.7865 6.57 20,170 132,517
49 0.7821 6.53 20,210 131,971
50 0.7777 6.49 20,250 131,423

Characteristics of Fuel Oils. -- The calorific values in B.T.U. per pound of oil, as given in the table "BaumÈ Gravity and Corresponding Specific Gravities, Weights per Gallon and Calorific Value of Fuel Oil", were determined by the formula:

B.T.U. = 16850 + 40 (No. of Degrees BaumÈ - 10).
Sixty four samples of petroleum oils, ranging from heavy crude to gasoline, and representing the products of the principal oil fields of the United States, were examined for calorific power by combustion in oxygen in the Atwater Mahler bomb calorimeter with results ranging from 18,572 to 21,120 B.T.U. per pound. In general, the decrease in calorific power with increase in specific gravity was fairly regular, so that the relation between the two may be expressed, approximately, by means of a simple formula. When the calorific powers calculated from the densities by means of this formula were compared with those actually determined, it was found that in one-ninth of the cases the difference was greater and in eight-ninths it was less than one per cent; in only one-thirtieth was it greater than two per cent; and in no case was it as great as three per cent; hence the calorific value of commercially pure petroleum oils can be determined from the density with sufficient accuracy for most practical purposes.

The heat value of oil is reduced by the presence of small percentages of water. Therfore, if the oil contains water, it should be passed through a filtering tank before going to the burners. In this filtering tank the water settles to the bottom and can be easily drawn off. The oil should be heated before going to the filtering tank, as the water in the oil is more easily separated out of hot oil than cold oil, first, because heated oil offers less resistance to freeing the water, and, second, because there is a greater expansion of oil than water due to the heat, and the water, therefore, has a relatively greater specific gravity.

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