US2340854A - Malleable cast iron - Google Patents

Description

Feb. 8,4 1944. n N, A, zlEGLER ErAL n 2,340,854

MALLEABLE GAST IRON Filed Nov. v5, 1942 LEAD ADD/770W.'-

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M//v/MUM FOR l//ELD Pol/v7? Plantea res. s, 1944 massamcAs'r mon Nicholas A. Zcieger, Chicago, Ill., andllomer W.

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ttanoon, y to Crane Co., Chicago, lll., a corporation of Illi- Tenn.,

application November s, 1942, serial No. 464,684

' v4 claims. (ci. 14s-21.8)

This invention relates to improvements in metals and more particularly to a group of alloys suitable for making better malleable iron castings. 'I'he latter after being subjected to a suitable malleableizing heat treatment will possess improved tensile strength and yield point in accordance with the highest commercial standards, and with ductility, expressed in terms of percentage elongation, superior to that found in conventional material not treated in accordance with our invention.

At the outseciitsnoum be understood that malleable iron` is first produced as a white iron with practically all of its carbon in the combined form. Itis subsequently rendered soft and ductile by means of annealing for prolonged periods at temperatures customarily in the range of 1560 to 1850 F. thereby causing the undissolved combined carbon or iron carbide tochange to graphite; then follows cooling to a temperature in the vicinity of the critical range. 'I'he final product sought is a microstructure consisting of rounded particles of temper carbon (graphite) embedded in a soft iron matrix.

The present invention contemplates the production of a high-quality malleable ca'st iron by the addition of small amounts of lead, the purpose of such addition being to obstruct the formation of flake graphite in the white iron, and also incombination therewith by employing an -annealing cycle less time-consuming )than the priorart processes.

It is well known to those skilled in the art that silicon (as well as other graphite formers like copper or nickel) promotes the .formation of graphite. This is equally true regarding the formation of (l) ilakegraphite during the solidific-ation period, resulting in a product known 'as gray iron, and (2) temper carbon during the annealing" or malleableizing of originally white iron, resulting inthe product known as .malleable iron. It is equally well accepted that `manganese (as well as other carbide formera" such as chromium. molybdenum. tungsten. and the like) acts in exactly opposite direction. i. e. the formation of graphite is-retarded and the formation of "combined carbon" or carbide is promoted.

In the art off-manufacturing malleable castings it thus becomes necessary to balance silicon and manganese so that upon solldiiicatlonno free or flake graphitewould V:be formed and all carbon would be present in combined form as carbides. At the same time these carbides shouldbe sumciently unstable so thatupon reheating to the malleableizing temperatures A'they would break up or graphitize into temper carbon in reasonably short time periods. Thus it is always desirable toV have in the malleable iron as much siliconv as is permissible. without developing flake graphite in the 'original (white) `castings during the solidication period. Itfhas been shown by various investigators that silicon greatly accelerates the malleableizing process (the formation of temper carbon) byincreasing the number of temper-graphite particlesper 4unit volume, thereby reducing thedistance the carbon has to diffuse before precipitating as graphite. It also has been shown that the malleableizing rate is accelerated not only by the ease of graphite nucleus formation and the mobility of the carbon atom, but is also influencedjby the relative stability of the original combined carbon (carbides) In other words, it is desirable to add to the molten white iron some element which would stabilire and promote the formation of combined carbon (carbides) upon so1idiiication,-but, at the same time, during the malleablizing heat treatment. would not interfere with (and preferably L. would promote) the breaking up of these carbides and formation of temper carbon.

We have discovered that by the introduction of up to 1.0% of lead into the molten metal, the formation of temper carbon is facilitated, or, in other words, the tendency of the iron to solidify white is enhanced.

Lead is cheaper than many ofthe prior elements used as -a whitener such as tellurium, for example. and may therefore be used in much larger quantities at the same or, perhaps, even lower cost.

The experimental irons investigated were all made of standard cupola malleable scrap.

Fig. lshows comparative elongation test resuits.

Fig. 2 shows comparative tensile strength and yield point test results. s In the ilrst of our series of tests 0.5%' silicon was melted with the charge .in an amount which would inoculatethe metal and cause it to turn to the gray state. Then `each heit was treated with a different amount of lead, ranging from 0.01% (heat 2491) to 0.2% (heat 2498).

A study of the appearances of fractures of the fracture bars and of the test bars as shown in Table 1 indicates that with progressively larger amounts of lead added, the fractures become more and more white until, with the addition of 0.2% lead, mottling completely disappears. The iron resulting from the treatment of our invention preferably has the following typical analysis:

Silicon .5 to 2.5% Manganese .3 to 1.0% Sulphur .2% maximum Phosphorus .4% maximum Carbon 1.5 to 3.5% Lead Trace to 1.0% Iron The remainder In the accompanying drawing Fig. 1 shows a superposed chart arrangement of the minimum specifications of the American Society for Testing Materials (A. S. T. M.) relative to elongation, while Fig. 2 shows the yield point and tensile strength in comparison with the physical properties of the novel composition of our invention based upon the tests referred to.

The charts comprising the drawing are graphic representations of the physical properties of test specimens presented in tabular form in Table 2.

Table 2 Tensile strength Yield point in lbs. per in lbs. per Emrgtgftcm sq. in. alter sq. in after puulin r Heat No. annealing time annealing time time mf 52 17 52 l' 52 17 hours hours hours hours hours hours The physical properties obtained in the specimens resulted from annealing times of 52 and 17 hours respectively, both periods being substantially shorter than the time required in general foundry practice. The production of a highquality malleable iron after a comparatively short-time annealing process is another indication of the favorable effect of our lead treatment.

It is well known that in malleable iron the desired ductility, expressed as elongation, is a more diillcult property to develop than the tensile strength. The elongation percentages shown in Table 2 and represented graphically in Fig. l of the drawing indicate that the addition of lead (in the amounts specied in the space in the upper the numbers 2520, 2521 and 2522 were treated in this way. The results are tabulated in Table 3.

Table 3 Appearance oi fractures Lcad Fmuyf 1-5 in- 'rest bar 0.5 in. diam.

Ver slightly mottled. Very slightly mottled. Bligdtly mottled Wh It may therefore be concluded from the test results presented that a malleable iron of the conventional composition, when treated with small amounts of lead (up to 1.0%) will possess desirable physical properties and a white fracture appearance, after annealing for a period of time substantially shorter than that required in general foundry practice.

While we have described our invention by referring to certain practical embodiments in order that a clear disclosure may be made to those skilled in the art, it will be understood that the scope of our invention is of broad application and not to be limited except as may be determined by the following claims read in light of the prior art.

We claim:

1. A white cast iron comprising the following elements as essential constituents in approximately the proportions given: silicon 0.5-2.5%, manganese 0.31.0%, sulphur 0.2% maximum, phosphorus 0.4% maximum, carbon 1.5-3.5%, to which is added lead in amounts from a trace up to 1.0%, the remainder being iron.

2. A white cast iron to be malleableized comprising the following elements as essential constituents in approximately the proportions given: silicon 0.5-2.5%, manganese 0.3-1.0%, sulphur 0.2% maximum, phosphorus 0.4% maximum, carbon 1.5-3.5%, to which is added lead in amounts from a trace up to 1.0%, the remainder being ron.

3. The process of making a malleable cast iron article comprising casting a mixture containing 1.5-3.5% carbon, 0.5-2.5% silicon and not less than about 94% iron, in the presence of a small but effective amount of lead, up to 1.0% of the mixture, the content of the latter element being sutcient to obstruct the formation of carbon in flake form during solidication, thereby producing a white cast iron article and then annealing the article under conditions to cause separation of at least a portion of the carbon as temper carbon.

4.As an article of manufacture, a malleableized cast iron having temper carbon and containing 1.5 to 3.5% carbon, a graphitizing agent equivalent in effect to 0.5 to 2.5% silicon and lead in small but effective amounts not to exceed 1.0%.

NICHOLAS A. ZIEGLER. HOMER .W. NORTHRUP.

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