HEAT TREATING FURNACES Heat treatment, it is generally understood, comprises the heating of steel to a temperature slightly above the critical point ; quenching in oil or water ; re-heating to some temperature to give the desired physical properties and cooling slowly. Mr. James 11. Herron in the 'Journal of the Cleveland Engineering Society, September, 1914, says that "the importance of determining the correct temperature and exercising the greatest care in heating cannot be over emphasized. This is especially true of the higher carbon and alloy steels. If the value of the steel is not actually impaired, a resulting condition may occur which would render the treatment valueless.
"One of the most important forms of heat treatment is case carbonizing or so-called case hardening. Steel to be carbonized is packed in some carbonaceous material and heated for a given length of time at temperatures varying from 1600 to 1750 degrees F., depending upon the depth of penetration of the carbon desired.
"It has become common practice to give case carbonized parts a double heat treatment, i. e., heat for the refinement of the core. quench in oil, subsequently heat at a lower temperature for the refinement of the case and quench in water, after which the ma terial may be drawn to the extent necessary for the physical properties desired.
"In the heat treatment of steel castings, proper annealing is of the greatest importance. Unfortunately commercial annealing is not what it should be, and if much is expected from the material it should be properly annealed or heat treated. By heat treating large steel castings with the carbon range of 0.20 to 0.60 percent elastic limit can be increased about 50 percent with little de crease in the ductility." Mr. E. J. Janitzky, Metallurgical Engineer, Illinois Steel Company, in the Journal of the American Steel Treaters Society, December, 1918, gives the following discussion of the theories of heat treatment : "Although not going too deeply into the his tory of the theories that have been developed:in regard to hard ening, it might be interesting to describe in non-metallurgical phraseology their contents. There are several theories for the hardening of steel, the more important one being the stress theory, the carbon theory and solution theory. The stress theory basis its contention on the high stressing of the outer shell of the steel when shrinking onto the interior and the stress set up in the crystal change from the hot to the cold metal. The fact that cold working hardens steel is offered in support of this theory. The carbon theory contends that the hardness resulting from quenching steel is due to the condition the carbon exists in in the steel, it being recognized that carbon can easily exist in several allotropic forms. The solution theory contends that carbon is in
solid solution with the iron. This seems to be the most logical and all phenomena can be explained by it. It will likewise be obvious that no theory so far presented fully satisfies for an acceptable explanation of the phenomena involved and that new avenues of approach must be found to obtain a correct answer to this apparent enigma. The most progress in heat treatment has been attained with the advent of alloy steel. With few ex ceptions all alloy steels are heat treated for use, the treatment deVeloping in them physical properties they are capable of pos sessing. No general laws regarding the effects of treatment of alloy steels can be laid down. Some steels when quenched from a high heat are hardened and others are softened, the latter being generally those with the higher contents of certain of the alloying elements. In respect to the effects of heat treatment, each steel is considered by itself. Developments in the manufacture of alloys steel and in the heat treatment of steel have occurred somewhat simultaneously during the past thirty years. The highest merit is obtained from the adoption of both developments together, that is, the use of heat treated alloy steels. Usually heat treatment has contributed more to the superior properties of the metal than has the use of alloys. The effect of alloying elements in alloy steels are various, thus nickel increases the elastic limit compared to tensility, chromium increases hardness of quenched steel, and manganese destroys magnetic susceptibility effects, all of which are valuable for certain purposes." Most of the advantages of fuel oil under boilers are retained in its use for furnaces. Oil is especially desirable in furnaces because it gives a clean heat and one which is very readily kept uniform. Forging and heating furnaces of all kinds can be started and shut down instantly with fuel oil and an'early attain ment of the maximum temperature is reached with accurate and easy regulation. In enameling and japanning work, especially, where dust must be avoided, fuel oil is being used more and more. Fuel oil is in common use in all heat treating furnaces, especially in those for large and small annealing, tool dressing, bolt heading, drop forging, heavy forging, rivet rod, nut punching, continuous rod, plate and flanging, flue welding, pipe bending, pack harden ing, case hardening and tempering. Figs. 76, 77, 78, 79 and 80 show oil burners applied to various types of furnaces.