US1925116A - Differential graphitization of cast articles - Google Patents

Description

Sept. 5, 1933. H. L. SPENCE ET AL 1,925,116

DIFFERENTIAL GRAPHITIZATION OF CAST ARTICLES Filed May 15, 1929 mmmmzmmm A 0 fl' 7mm I I p i? g .D E g T4 g 44 p n y INVENTORS Hl/Bf/FT L. SPE/W'E BY HARRY A. SCHWARTZ ATToRNEa Patented Sept. 5, 1933 DIFFERENTIAL GRAPHITIZATION OF CAST ARTICLES Hubcrt *L; ,Sp ence, East Cleveland, and Harry A. Schwarti, Cleveland Heights, Ohio, assignors to National Malleable and Steel Castings Company, Cleveland, Ohio, a corporation of Ohio Application May 15, 1929. Serial No. 363,160 2 Claims. (Cl. 148-4) another portion, the head, must be extremely 1 hard and wear-resistant. An object of our invention is to prepare a cast article, one portion of which is of hard or substantially ungraphitized iron and another portion of which is of graphitized iron, and readily machinable. We accomplish this by subjecting the cast article to our novel procedure of controlling the degree of graphitization in each such portion. Another object is to produce an article having diiTerent degrees of machinability or hardness, which hardness may vary gradually throughout the length of the article. A further object is to decrease the time required to produce such articles. Other objects will become apparent.

In describing our invention reference will be made particularly to its application to the production of valve tappets having heads which are of hard or substantially white iron and having stems of iron which is readily machinable. It is not intended, however, to restrict the use of our invention to the production of these articles, it being applicable wherever a differential hardness or machinability is desired.

In describing our invention, reference will be made to the diagrammatical drawing, in which:

Figure 1 represents a sectional view of a partition supporting a number of valve tappets.

Figure 2 represents a cross-sectional view of 40 the electric furnace, and

Figure 3 represents a cross-sectional view of a quenching tank.

To illustrate an application of our process, the valve tappets (A), of iron containing carbon, silicon, phosphorous and sulphur in the proportions normally found in white iron used for making malleable iron castings, are removed from their molds and allowed to cool to a temperature below the critical temperature of the cast iron. They are then supported upon a plate (B) of asbestos or other poor heat conducting material, with their stems extending through the plate. The stems of the valve tappets are then subjected to heat by placing the plate (B) in a suitable heating furnace such as the electric furnace (C) The the. coils (H) plate (B) may be supported by brackets (D) and (E) and the lower portion of the furnace may be heated by the heating element (F). The temperature in the furnace (C) is so controlled that the stems of the tappets may be uniformly heat- 0 ed to a temperature well above the critical (A1) temperature (for example to 900-1000" 0.). The asbestos plate (B) will protect the heads of the tappets which should not be heated to a temperature substantially above 750 C.

When thestems of the tappets have been uniformly heated to the desired temperature, the plate (B) is removed from the furnace (C) and the stems only are subjected to a quenching or cooling action such as that described in Patent 7 No. 1,688,438 granted to Harry A. Schwartz October 23, 1928. This may be accomplished by placing the plate (B) in a quenching tank (G),

'filled to the bottom surface of the plate (B) with a suitable quenching liquid, suchas oil or water. The temperature of the quenching liquid may be controlled by circulating a cooling liquid through The temperature of this bath should be maintained at such a value that the tappet stems will cool rapidly. The rate of cool- 30 ing of the stems may be approximately 5 C. per second or faster, while the heads of the tappets, being insulated from the cooling liquid, will cool much more slowly.

After the tappet stems have been rapidly cooled to substantially below the critical (A1) temperature, the plate (B) may be inserted in an an nealing furnace in which the stems are subjected to a suitable graphitizing temperature, preferably above the critical temperature. This elevated temperature should be maintained from two to four hours, or a suificient time to decompose cementite and graphitize the iron to produce the desired degree of machinability.

The plate is then removed and the tappets may be emptied into quicklime, sil-o-cell (powdered infusorial diatom'aceous earth), sand, or other material wherein cooling at a moderate rate may be effected. The rate of cooling may be appropriately varied to produce the degree of hardness which is desired.

If still greater softness is desired, the stems of the tappets may be subjected to further heat, as in an electric furnace or a lead bath, the temperature being maintained as high as possible but below A1 critical temperature, or substantially at 700 C. The tappets may be allowed to re main at this elevated temperature for a sufficient time to produce the degree of softening which is required. This may vary from a few hours to 0 obtain a slight increase in softening, to about fifteen hours for the maximum softening. The tappets are then removed and allowed to cool.

In following the process described, the time required to graphitize the portion subjected to prequenching will be substantially shortened, with the result that the stems of the tappets will be suitably graphitized during the shortened period in the annealing furnace. The heads of the tappets, having been insulated during the heating and cooling periods, will remain hard or substantially ungraphitized, while the stems will be readily machinable. Even though there may be some conduction of heat from the stems to the heads of the valve tappets, this should not be sumcient to heat the heads above the critical temperature before quenching or to cool them through the critical temperature at a rate sufficiently high to substantially shorten the period required for graphitizing. By varying the rate of cooling in the pre-quenching bath the time required to graphitize the stems may be varied, i. e., by increasing the rate of cooling, the period required for graphitizing will be shortened.

Although we have described the process as applied to an apparatus in which the heads are insulated throughout the entire process from the heat and rapid cooling to which the stems are subjected, it is apparent that many modifications of this procedure may be used. For instance, the stems only of the hot cast tappets may be subjected, while both stems and heads are still above the critical temperature, to the pre-quenching bath, and the entire tappet may be subsequently subjected to the graphitizing and subsequent steps. By following this modiflcation the acceleration of the graphitizing step will not be applied to the head of the tappet since the latter is not subjected to pre-quenching. In the subsequent graphitizing step the elevated temperature will not be maintained a suflicient length of time to accomplish the graphitize.- tion of that portion of the casting which was not subjected to the rapid precooling.

By another modification the cast tappets may be allowed to cool to below the critical temperature and the stems only may then be heated to a temperature above the critical temperature. The entire tappets may then be subjected to the rapid precooling and the subsequent graphitizing steps. In this modification, since the heads of the tappets were not rapidly cooled through the critical temperature, there will be no substantial acceleration of the decomposition of the cementite in the heads of the tappets and no resultant shortening of the period required for graphitization. The time required to graphitize the stem portions of the tappets, which have been rapidly cooled through the critical temperature,

will not be suflicient to accomplish a like result in the head portions.

It is apparent that, due to the conduction of heat through the stems of the tappets, there will be some graduation of the differential graphitization. By varying conditions, governing this conduction of heat, such as the rate of temperature change or the insulating medium used, this graduation may be varied to meet the desired conditions.

In addition to quenching before the graphitizing cycle, the castings may, if desired, be rapidly cooled by quenching at the end of the high temperature graphitization. Such a quenching has the'beneficial result of increasing the rate of decomposition of the remaining combined carbon when the castings so quenched are again heated to a temperature distinctly below the critical point and are h 'e1d-for a time at that tem-.

perature.

The graphitizing step may also be carried on entirely at an elevated temperature below the critical point, and the pre-quenching to which the article has been subjected will shorten the time required for such graphitizing. The product of such an operation is superior in ductility to the usual malleable iron castings.

By the reference to graphitization in the description and claims, we refer to the decomposition of the cementite in the iron and the result ant deposition of carbon. This is a process often called annealing in the art of making malleable iron and which, as is well known, is entirely different in chemical principle from the annealing of articles made of other ferrous materials.

An example of another application of our invention is in the manufacture of pug mill blades, where it is necessary to keep the blade itself hard and resistant to abrasive action andwhere the bolt (which must be machined) must necessarily be relatively soft and shock-resistant. Many other applications will be apparent to one skilled in the art and it is not intended to hereby limit the invention to the particular applications described.

Now having described our invention, we claim:

1. An integrally cast valve tappet having a head of white iron and a stem of malleable cast iron.

2. An improvement in the art of making valve tappets, comprising cooling the, stem of a valve tappet containing the elements normally present in malleable iron castings, from a temperature above its critical temperature to a temperature below said critical temperature and subsequently graphitizing the stem of the valve tappet.

HUBERT L. SPENCE. HARRY A. SCHWARTZ.

Images