US2310667A - Malleable cast iron - Google Patents

Description

Patented Feb. 9, 1943 MALLEABLE CAST IRON Nicholas A. Ziegler and Homer W. Northrup, Chicago, lll., assignors to Crane Co., Chicago, Ill., a corporation of Illinois 'Application December 4, 1941, serial Np. 421,585

4 claims. (ol

' namely, good machinability, ductility, tensile This invention relates to the improvement in the group of alloys suitable 'for making malleable iron castings, which after being subjected to a suitable malleableizing heat treatment will possess a tensile strength and yield point in accordance with the highest commercial standards and ductility, expressed in terms of percentage elongation, superior to that found in conventional material not treated in accordance with our intion.

A further object is the acceleration of the annealing rate by the addition of an element which will affect the rate of annealing and at the same time improve the physical properties of the final malleable iron casting.

established malleableizing practice and method outlined herein.

Malleable iron is first produced as a white iron with practically all its carbon in the combined form. It is rendered soft and ductile by annealing for prolonged vperiods at temperatures cusomarily in the neghborhood of 1560-1850 degrees Fahrenheit to cause the undissolved combined carbon or iron carbide to change 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. l

The annealing process is by far the most time-consuming and therefore the most costly operation in the production of malleable iron castings, and consequently many attempts have been made to reduce the time required for the annealing cycle without sacrificing the tradiduction of a high-quality malleable cast iron by the addition of a small amount of beryllium to obstruct the formation of flake graphite in the white iron, and by employing an annealing cycle less time-consuming than the prior-art processes.

It is well known that silicon (aswell as other graphite 'formers" like copper or nickel) promotes the formation of graphite. This is` equally true regarding the formation of (1) flake graphite during the solidication period, resulting in a product known as "gray iron, and (2) "temper carbon during the annealing or malleableizing of originally white iron, resulting in a product known as malleable iron. It is equally well known that manganese (as well as other carbide formers like chromium, molybdenum, tungsten, etc.) acts in exactly opposite direction, i. e. retards the formation of graphite and promotes the formation of combined carbon or carbide. I n the art of manufacturing malleable castings it thus becomes necessary to balance silicon and manganese so that upon solidication no free or flake graphite would be formed and al1 carbon would be present in combined form as carbides. At the same time these carbides should be suficiently unstable so that upon rehe'ating to the malleableizing temperatures they would break up or graphitize into temper carbon in reasonably short time periods. Thus it is always desirable to have in the malleable iron as much silicon as is permissible without developing flake graphite in the original (white) castings during the solidication period. It hasbeen shown by various investigators that silicon greatly accelerates the malleableizing process (formation of temper carbon) by increasing the number of temper-graphite particles per unit volume, thereby reducing the distance the carbon has to diiuse 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 inuenced by the relative stability of the original combined carbon (carbides). In other words, it is desirable to add to the molten white iron some material which would stabilize and promote formation of combined carbon (carbides) upon solidication, but, at the same time, during the malleableizing heat treatment, would not interfere with (and preferably would promote) breaking up of these carbides and formation of temper carbon. An outstanding example of an element which functions in this manner is tellurium.

We have discovered that by the introduction of up'to 0.2% of beryllium into the molten metal,

the annealing process will be accelerated and the formation of temper carbon facilitated. As an indication of the favorable eiect upon the physical properties of the product expressed in positive terms let us consider the results. obtained with regard to per cent elongation.

It is well known that in malleable iron the desired ductility, expressed as elongation, is a more diiiicult property to develop than the tensile strength or the yield point. We have found that adding small but effective amounts of beryllium to white iron considerably improves the ductility of the metal after the malleableizing' annealing and also allows a simplified and shortened malleableizing treatment to be employed.

A series of experimental melts were prepared to which were added the specified amounts' of beryllium or a copper-base alloy containing be ryllium. Several test bars were taken from each heat and subjected to a certain malleableizing heat treating cycleyafter which the test bars were subjected to the standard tensile tests. In

, each group of test bars thus treated, two'test bars made of ordinary commercial cupola white iron were included and tested in parallel with the copper-beryllium treated irons.

Explanation is made in the upper right hand corner of the drawing of the treatment or composition of each of the samples.

More specifically, we have discovered the following composition and percentage ranges to be satisfactory:

Silicon per cent.. .5 to 2.5 Manganese do .3 to 1.0 Sulphur per cent maximum-- .2

Phosphorus do .4

Carbon per cent 1.5 to 3.5 Beryllium do Trace to .2 Iron The remainder 'Ihe beryllium is preferably added as a copperbase alloy containing beryllium in amounts up to 4%.

Several different heat treating cycles were thus tried, but-in view of the fact thatthe copperberyllium treated irons inevitably had physical properties superior to those of the ordinary cupola malleable iron, malleableized together with the former-the test data presented here are taken from only two representative cycles.

The program of the first run was: heated to 1800 F. in 6 hours; held at 1800 F. for l2 hours; cooled to 1500 F. in 2 hours; held at 1500 F. for 4 hours; cooled to 1380 F. in 2 hours; held at 1380 F. for 4 hours; cooled to 1300 F. in one hour; held at 1300 F. for 4 hours; cooled to 1250 F. in one hour; held at 1250u F. for 4 hours; cooled to 600 F.'with the furnace and air cooled to room temperature.

Table 1 gives the compositions of the test bars andthe results of thestandard tests of the bars thus` prepared and treated, together with the same data for the industrial heat of ordinary commercial cupola malleable iron. The accompanying drawing is a graphic representation of The accompanying drawing shows a superi0 the data presented in the tables.

Table 1 I Nominal chemical analysis Physical properties Composition No. Ilgeat f. Remarks Si Mn s P T C Alloyorelement Tensile Yield Elonadded 1 strength point -gation LbaJ Lba,/ Per cent Per cent Per cent Per cent Per cent sq. in. sq. in. Per cent 1 5831 0.01 0. 41 0.15 0. 11 3.14 5&% 2:3 }mdusrria1hea1 14 241s 1.12 0.41 0.14 0.13 2.08 0.5% C11-Be auoy 22% Alzded 0.1111175 orcue 8 Oy C011- taining 47 Be. 15 2421 1. as 0.35 0.12 0.13 2.05 0.5% C11-ne a110y 2,31% gg g2g o 16 2420 0.05 0.38 0.13 0.15 2.96 1.0% C11-Be auoy gg mieduroa, ortog- 1 ea oy con a ing 47 Be.` 11.; 2423 1.22 0.35 0.12 0.14 2.72 1.0% cunea11oy.{ gggg 31% 23 a y minder to'demonsnrate that it 1s the beryllium an'dnotthe copper hat is beneiai, in Table 2 the propertiesof an iron are shown to which only copperw'asiagded (compositions #18 and #19 in Table 2) in parallel with the properties of an industrial malleable iron and those of two compositions to whichcopper-beryllium master alloy has been added. All the members of thislgroup of specimens were subjected to the following malleableizing cycle: heated ,to 1880 F. in 6 hours; held at 1800 F. for 5 hours; furnace cooled to 1250 F. in 10 hours; furnace cooled to 600 F. air cooled to room temperature.

Table 2 Nominal chemical analysis Physical properties Composition No. 11%? Au i 1 t T l Y' 1d El Remarks oy ng e amen ensi e 1e anga- S Mn s P TC' added strength point tion Lba/ Lba/ Per cent Per cent Per cent Per cent Per cent Per cent aq. m. aq. in.. 1 5531 0.01 0.41 0.15 0.11 3.14 621% 23 }111dustria1het.

,400 41,100 5.5 Add (1057() B 14 271s 1.12 0.41 0.14 0.13 2.08 v 00,500 41,300 5.5 e 0 1 e .15 2721 1.33 0.35 0.12 0.13 2.05 }5% C Beanoy l01.000 40,200 5.5 gosecmtammg 452s a 1 Added 1.0% Cu-Be 2426 1.05 0.38 0.13 0.15 2.96 56 400 ,000 8,0 17 2423 1.22 0.35 0.12 0.14 2.72-}1% C B" auy-l 571700 33.100 0.0 osl'seconalmng a .2 1 C t 59.533- gm g-g C.. ...1y ...1.1.4.- 10 2305 1.30 0.31 0.13 0.12 .64 u 62,200 37,200 3 5 It is clear that while the specimens containing copper and beryllium became ductile (that is, developed over 5% elongation) in a 15-hour anneal, similar specimens containing copper without beryllium, under similar conditions, developed only 3.5% maximum elongation.

Referring again to Table l, it may be seen that each composition there represented, with the exception of #1, industrial heat, developed better than 5% elongation after a heat treatment of only about 40 hours duration.

Referring to Table 2, it may be seen that each composition there represented, with the exception of #1, industrial heat, and' #18 and #19, heats treated with copper, developed better than 5% elongation after a heat treatment of only about hours duration.

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 not to be limited except as may be required by the following claims.

We claim:

1. A white cast iron comprising the following elements as essential constituents in approximately the proportions given: si1icon0.52.5%, manganese 0.31.0%, sulphur 0.2% maximum, phosphorus 0.4% maximum,.carbon 1.5-3.5%, to

' in approximately the proportions given: silicon 0.5-2.5 manganese 0.31.0%, sulphur 0.2% maximum, phosphorus 0.4% maximum, carbon 1.53.5%, to which a copper-base alloy, containl ing beryllium in amounts up to 4.0%, is added in amounts from a trace up to 5.0%, the remainder being iron.

4. A malleableized 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 beryllium is added in amounts from a trace up to 0.2%, the remainder being iron.

NICHOLAS A. ZIEGLER. HOMER W. NORTHRUP.

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