Discussion-Eliminating Strains

in Iron Castings

MR. ASA W. WHITNEY.-The method of annealing for the relief of strains only, not for the radical change of structure as accomplished in annealing white iron at high temperature, is applied to soft iron castings almost exclusively after the castings have wholly cooled and strains and distortions have become very serious. For many classes of castings this method is perhaps the only feasible one and the data given by Mr. Wiltshire as to temperature, time, oil consumption and the method of cooling, are of interest as establishing records on those points.

The long time, seven to eight hours, occupied to heat to 700 degrees Fahr. is probably partly due to the inability to heat a mass of cold castings evenly in any internally fired oven, if the flame is applied too severely. Undue haste causes uneven heating and cracks or strains many castings.

But it seems to the writer wholly unnecessary, though harmless, at the low temperature used and for soft iron castings, to take as much as 20 hours to cool from 700 degrees Fahr. to about 300 degrees Fahr. Very properly there is no draft allowed through the furnace in this cooling, but the cooling from 700 degrees Fahr. could probably be done safely in four to five hours if it is worth while to construct the furnace to this end. In such case, the whole time of heating. soaking and cooling could be reduced to 33 hours. However, unless the time could be reduced to about 22 hours the same furnace could not be used for daily heats.

Whenever possible the car wheel maker's idea seems more sensible, scientific and economical. Their aim is to properly control the original cooling of the casting from as high a temperature as admits of handling the casting without mechanically straining it, say between 1553 degrees Fahr. (light cherry red)

to 1175 degrees Fahr. (dark cherry red). In this way, all strains except those caused by the chill ring in car wheel practice are largely prevented and, with modern chill rings, the latter are minimized.

However, the general method by which wheelmakers carry out their idea is not only inefficient in relieving strains for certain patterns, for wheels delivered to the pit too cold, and for certain compositions, which are the best if properly annealed, but, because of the massed construction of pits and the consequent four to seven days required to cool, though no fire is used, the common practice actually damages the structure of the metal by enlarging the grain size, increasing the graphite size slightly, coarsening the pearlite structure and producing minute spots of partially graphitized pearlite in the chilled portion. The result is far more irregularity of strength and lower mileage than can be produced by a proper technique of annealing. Moreover, proper annealing admits the use of harder and more suitable compositions and microstructures as shown by comparing Figs. 1 and 2.

In 1847, the writer's grandfather, Asa Whitney, introduced preheated 66-hour pits and in 1898 the writer developed 42hour oil-fired annealing pits designed according to the principles of proper heat treatment of castings, as applied before the casting temperature is dissipated.

As a result of long experience with the 66-hour and 42-hour pits, the author has been able to produce wheels from which extraordinary mileage has been obtained. In direct service tests under the tender of a switch engine in a very severe service where wheels had failed in 10,000 to 26,000 miles, wheels from the 42-hour pits gave over 126,000 miles, although made in a shop equipped only for making lumber and mining car wheels. They were made of coke, pig iron, steel and miscellaneous scrap, the metal not being dosed with alloys in ladle. The wheels were cast in contracting chillers, annealed in preheated pits and removed cold in 42 hours. (See Plate 13 of "The Metallography of Steel and Cast Iron." Howe, 1916.)

Before the wheels were worn out, the writer prepared an article on chilled wheels which appeared in December, 1912, in

FIG. 2-4-INCH BELOW CHILLED FACE OF OVERANNEALED CHIP OF CHILLED PART OF WHEEL MIX. BEFORE THE LABORATORY ANNEAL THE CHILL WAS AS FREE OF GRAPHITE SPOTS AS THE PIECE OF WHEEL SHOWN IN PLATE 13, "THE METALLOGRAPHY OF STEEL AND CAST IRON," BY HOWE (30.5 DIAMETERS)

the Whitney Magazine of the University of Pennsylvania, Asa Whitney long ago having endowed the chair of dynamic engineering there. The following is from the article:

"Asa Whitney had proved to his own satisfaction that any desirable pattern of car wheel or other chilled casting could be properly annealed only by placing the red hot castings in a preheated pit, and, instead of relying on the slow cooling of the mass, actually raised its temperature by furnaces whose heat could go so far as to decompose the chilled treads into a sort of malleable iron. He had proved that by proper heat regula

tion no such damage would occur, and that by proper construction of the pits the high temperature could be quickly attained, briefly maintained and rapidly reduced. His pits, 48 in number, and 18 feet deep, were emptied after 66 hours, the wheels then being no hotter than could be rolled to cleaning shop by men wearing hand leathers."

The writer's pits of 1898 were not dependent on a cooling air draft through the mass of castings, but carried out Asa Whitney's principle more thoroughly and in less time by greatly increasing the external radiation of heat, as soon as the fire was shut off and pit sealed. This is as important as the rapid heating.

In the writer's practice medium and light wheels were charged as hot and fast as possible. The temperature was raised about 400 degrees Fahr. from an average of 1100 to 1500 degrees Fahr. in 14 hours or less. The total time of application of oil fire was about 34 hours, being preheated for about two hours. In a 12-heat test of one pit used daily, the alternate pit cooling, two net tons hot wheels were heat-treated by 5.6 gallons of residium oil at 54 cents per gallon or nearly 15 cents per ton. The pits held 31⁄2 net tons, but were used with an average of only 2 tons at this time.

Cold castings, especially if hard or under strain, must be heat treated slowly, but hot castings can be and should be heat treated rapidly to avoid damage if structure is already nearly right. For the same reasons hot castings usually should be and actually can be cooled rapidly to retain structure and avoid coarsening it, because there are no unrelieved strains engendered by a previous cooling. The only requirement seems to be, as in the case of glass annealing, that the time-temperature "curve" should be a straight line, or nearly so, for any and all parts of the casting. This precludes cold air draft being used at all for rapid work. And, as usually in cast iron the finest original structure should be preserved, the time-temperature curve must be as steep as possible, that is, the time must be short.

The Electric Furnace as an Adjunct

to the Cupola

By GEORGE K. ELLIOTT, Cincinnati

This paper is a sequel to one read by the author before the American Electrochemical society last April. In that paper, entitled "Improving the Quality of Gray-Iron by the Electric Furnace," the author described a duplex process for making gray iron suitable for the demands of modern engineering for super-grades of iron castings. The duplex process described was one installed at the plant of the Lunkenheimer Co. at Cincinnati, where it has been in successful operation for a number of months. The process consists simply of using in tandem the ordinary foundry cupola and the arc electric furnace. The purpose of the present paper is to re-emphasize the significant fact that cast iron as it comes from the cupola furnace has serious limitations that are being brought to light by the more extreme demands of modern engineering progress, and that the arc electric furnace is a competent supplement to the cupola for producing cast iron of superior quality.

Cupola Noteworthy Success in its Field

Before proceeding into his subject, the author desires to make it plain beyond all possibility of misunderstanding, that his attitude toward the cupola furnace is not one of hostility nor is his criticism destructive. On the other hand, he is decidedly friendly to the cupola and his intentions are entirely constructive and calculated to be helpful and co-operative. He would not have it even suspected that the cupola is losing its long and honorable prestige, because, in truth, he knows there is no reason why it cannot continue indefinitely to be what it always has been, the most serviceable melting furnace known to foundry practice. For the ordinary run of iron castings, comprising possibly 90 per cent of the total output, there