US2287850A - Gray cast iron - Google Patents

Description

June so, 1942. H. J. YOUNG 2,287,850

GREY CAST IRON Filed Nov. 22, 19s":

Fig. 1. 4-5 4 (mms. Mick/less) Fig. 3.

Fig. 2.

02030405060708090/00 (mms. Huck/265s) MICkWESQ Patented June 30, 1942 UNITED STATES PATENT OFFICE GRAY CAST IRON Horace John Young, Bradford, England,

Application November 22, 1937, Serial No. 175,904 In Great Britain February l0, 1937 13 Claims. (ems-123) important parts contain no more than 2.00% of The present invention relates to the manufacture of gray cast iron with-an all-pearlitic structure. A gray cast iron is said to be all-pearlitic when it contains the maximum possible amount of combined carbon without more than a trace of free cementite and no free ferrite. Gray cast iron of all-pearlitic structure is well-known to metallurgists and contains no free ferrite and in practice is a broader-line iron containing a trace of free cementite (for the purpose of ensuring that no free ferrite is present); Hence the fact that an iron is all-pearlitic can be provedby the microscope only. Sometimes other metals or 'metalloids are included in iron ofall-pearlitic structure which may tend to modify the formation and appearance of this structure as seen under the microscope without, however, deviat-' ing from the provision that no free ferrite and only a trace of free cementite is present. The lamallae oi the pearlite may ball-up and tend to become globular, in which case the metallurgical description or sorbite or sorbitic is used. Such an alteration in the appearance and form of the pearlite equally falls within the scope of this invention. The general term all-pearlitic is used in the following specification and is intended to include structures which ar all-pearlitlc, all sorbitic, or all-pearli'tic-sorbitic.

For gray cast iron for important parts of engines and machines which have to withstand friction, erosion, corrosion, heat, oxidation, the penetration of gases and other conditions of service, it is well-known that an all-pearlitic structure is most desirable. In ordinary foundry practice the all-pearlitic texture of the metal in castings of various weights and thicknesses and, consequently, cooling at different rates, is obtained by altering the silicon content of the iron. For thinner or lighter castings where the molten iron cools quickly more silicon would be required to keep the iron all-pearlitic otherwise it would contain too much cementite, whereas a thick or heavy casting cooling less quickly would require less silicon, otherwise the iron would contain ferrite. It is known that when the silicon content silicon and usually about 1.25 to 1.75%.

It was discovered by Diefenthaler (United States Patent No. 1,502,983) that a definite allpearlitic structure of the iron is absolutely necessary in order to produce metal of good wearing and other properties. His invention allowed irons of a lower silicon content than hitherto to be.

used and he overcame the difllculties of cementitic hardness that would ordinarily arise with such irons by raising the temperatureof the moulds by a predetermined amount into which ill is too low for the section or mass of the casting concerned there results an iron (in the casting) which is not all-pearlitic but is inclined to be cementitic and white and, consequently, too hard.

' On the other hand, when the silicon content is too soft.

In modern practice, heavy engine castings for the molten metal was poured, and thus obtained a delayed and known rate of cooling and iron of all-pearlitic structure in the castings. The

Diefenthaler process is known commercially as I the Perlit or "Lanz Perli process.

For example, in ordinary foundry practice for a casting of 10 millimetres thickness there would ordinarily be contained about 2% silicon when the carbon content of the iron was 3%. In Diefenthalers method, however, the silicon content was only 1%, and also, by altering the mould temperatures he was able to use this iron of 1% silicon for castings of 20 and 30 millimetres thickness. For irons containing less carbon, namely 2.8%, he used only 0;8% silicon with yet higher mould temperatures.

Thus, by means of Diefenthalers invention it was possible to use a range of cast irons, namely those containing a verylow silicon content, which had not hitherto been possible under ordinary foundry conditions. Diefenthalers invention, however, has the disadvantage that preheated moulds are necessary, the heating of which to predetermined temperatures being. a slow and costly procedure. On the other hand, the ordinary'founclry method hitherto used and still used necessitated the alteration of the silicon content be made, lowering the silicon content for heavy castings and raising it for lighter ones, and this silicon content had to be controlled by altering the original mixture of furnace charge. Under this method the amount of silicon it is possible to use is controlled by the thickness and mass of the casting to be made and means constant variation of the silicon content with progressively less silicon used as the mass or thickness of the casting is greater. Further both Diefenthalers method and the ordinary method prohibit the use of irons containing greater quantities of silicon and also other valuable materials, such as sulphur, chromium, manganese, and so on, which if they could be used in relatively large quantities would produce irons invaluable for use in important parts of engines and machines.

By means of the present invention 1 am able to use at least 0.25% and usually 0.50% (or even up to 4% or more) more silicon than is at present used for a casting of the same section and still obtain a casting of all-pearlitic structure which is so desirable in all gray cast. irons. By means of this invention the higher end of the silicon scale for the first time is made available to industry in the production of all-pearlitic gray cast irons for engineering castings. Also by means of this invention I am able at the same time to make all castings of all the masses and thicknesses used in ordinary engineering with one predetermined high silicon content only and all having an all-pearlitic structure. This has not been possible before.

The method according to this, invention is to increase the percentage of the silicon in the iron to an amount greater than is commonly used or specified for the particular castings and then to recover the pearlitic structure (which would have been destroyed by this high silicon content) by means of suitable additions of a metal or metalloid working'in the opposite direction to that of silicon, namely, any material which inhibits' the formation of free ferrite and tends towards the formation of pearlite or cementite. Suitable materials for use as inhibition materials are chromium, sulphur, manganese or molybdenum.

The all-pearlitic structure is really a boundary line between the cementitic structure and the ferritlc structure in cast irons; the addition of more silicon tends to shift the iron to the ferritic structure whilst the addition of the prohibit'ion material tends to shift it back again towards the pearlitic or cementitic structure. The

additions of silicon and inhibition materials are so adjusted according to this invention that the resultant gray cast iron has the all--pearlitic structure with possibly a trace of cementite but without the presence of any free ferrite whatsoever.

By means of this invention instead of lowering the amount of silicon or as in Diefenthalers method altering the mould temperature and lowering the amount of silicon to suit the heavier mass and thickness of the casting to be made, the necessary control is obtained by raising the amount of silicon, keeping it constant for all masses and thicknesses of castings, and altering the sulphur, chromium, or manganese content of the iron, and in many cases thismay be quite simply accomplished by merely making additions to the molten metal in the ladle just prior to pouring the iron into the mould.

The invention will be further described with reference to the graphs shown in Figs. 1, 2 and 3 of the accompanying drawing.

Figure 1 shows in curve A the approximate percentages of ,silicon used for obtaining allpearlitic irons by the ordinary method. Curve B shows that with the Diefenthaler method the percentage of silicon used is low (by way of example shown at 1.0%) and is kept constant for all thicknesses of castings, the all-pearlitic structure bein produced by varying the mould temperature in accordance with the section-of the casting. Curves C show examples of the percentages of silicon which it is possible to employ in the method according to this invention for obtaining all-pearlitic gray cast iron. With each of the various percentages of silicon shown the all-pearlitic structure is obtained by this invention for different thicknesses of castings by varying the percentage of chromium orother inhibition material in the iron. I

chromium for castings of 50 millimetres average thickness. Other percentages of chromium for different thicknesses can be easily ascertained. The mixture of iron used should contain a proportion of scrap iron made from a previous similar mixture in order to obtain the best result. Thus are produced irons heavily loaded with the same amount of silicon which are all-pearlitic when cast into moulds for castings of various masses and thicknesses.

The precentage of chromium used with a particular percentage of silicon in order to obtain an all-pearlitic casting is not fixed, but depends upon the thickness of the casting and the rate of cooling; where the silicon percentage is above 2.50% as illustrated in Figure 1, the chromium content should always be in excess of 0.75% for castings of commercial thickness. This relationship is substantially indicated in Figures 2 and 3 of the drawing and by the examples herein give according to which I balance the silicon and chromium with each other and use a percentage of chromium higher than hitherto normally found for the section'of the casting concerned, i. e. above 0.75%. with the same mixture castings differing slightly in thickness from the optimum thickness can be produced and will have the all-pearlitic structure. For example in a test which I carried out with an iron containing 3.4% total carbon, 2.8% silicon, 0.8% phosphorus, 0.055% sulphur, 0.85% manganese and 1.60% chromium in which 15%, of scrap iron of the same mixture was included, I cast three bars of Viv inch, 1 inch and-1% inch thickness in sand moulds. The bar of 1 inch section had a perfect all-pearlitic structure, whilst the castings of b inch section and 1% inch section although verynearly all-pearlitic were not quite so good as the 1 inch section. I estimate that by using 0.25% less chromoum the mixture would have been suitable for castings of $5 inch, whilst increasing the percentage of chromium by 0.25% would have made the mixture suitable for 1% inch castings. The foundry man can easily flnd by referrring to the drawing herein and by following the instructions of this disclosure the range of thicknesses with which all-pearlitic structures can be obtained with a certain mix- In practice, of course, castings are generally ofvarying thicknesses and the mixture would be calculated in accordance with the average thicknesses of the most important parts of the casting. The all-pear tic result is dependent primarily upon the ra of cooling of the casting which is, in turn, dependent upon the average times desirable to load up an iron for some special purpose with, say. chromium, and in this case the silicon content of an iron containing, say, 3.0% carbon, 0.15% phosphorus, 0.050% sulphur, 0.75% manganese, canbe put as high as possible, say 3.00% or more, in which case for a casting at 25 millimetres thickness; it thereby becomes'possible to raise the chromium content to 2.0%. Also, it is sometimes desirable to have as much sulphur as possible in the iron for certain parts to resist wear and this invention per.- mits the use of. a sulphur content much higher than has hithertobeen possible. For example, an iron containing 0.25% sulphur would require a silicon content of about 2.5% in castings or or 20 millimetres thickness.

, Hence my invention enables the use 01 heavily loaded iron such as have never before] been possible where the desired all-pearlitic structure was essential and had to be obtained ifor a certainty. It enables the use of higher silicon irons than hitherto controlling them by means of sulphur, manganese or chromium or, conversely, it enables the use of higher chromium, higher sulphur, or higher manganese irons than hitherto controlling them by the use of a larger proportion of silicon than hitherto.

The invention may also be applied to the Diefenthaler process with good effects. That is, larger amounts of silicon may be used than is normally possible in the Diefenthaler process for castings of the same size and cast in moulds with the same temperature, for example, 1.25% up to two or three or even up to seven or eight or more times the normal quantity of silicon). Highly important and beneficial effects are obtained because owing to the use of the heated moulds together with the very high silicon content a large quantity of chromium or other inhibitive material is possible while the structure of the cast iron remains all-pearlitic. As shown in Figure 3, with an iron of 3% carbon and 3% silicon, more than 3% chromium is used to obtain the all-pearlitic structure when a heated mould is employed. As will be noted from this figure it is possible to add still more chromium when heated moulds are used than with cold moulds, whilst still obtaining allpearlitic castings.

If it should be desired, when using heated moulds, to keep the percentage of chromium constant for various thicknesses, this maybe efiected by varying the mould temperature in accordance with the section or mass of the casting to be made.

It is a very simple matter for any foundry to prepare still other curves showing the necessary sulphur, chromium, manganese or other additions to be made for every desiredquantity of iron having a known silicon content higher than hitherto used in an all-pearlitic iron. Equally, it is simple to make curves for irons loaded with chromium, sulphur, etc. where the eifects oi the materials are particularly desirable in the casting of all-pearlitic iron.

It is known that nickel can be used in place of silicon in the production of all-pearlitic gray cast iron and nickel can be used instead of the whole or part of the silicon to which reference is made in this specification. The invention is in no way afiected if other elements or materials having properties similar to silicon or nickel or to chromium, sulphur, etc., be used, because chineable may be prepared to produce all-pearlitic gray iron.

good properties associated with irons of allpearlitic structure such as it has hitherto not been possible to make containing a heavy load of silicon.

It will be understood that the examples given are merely Particular examples of carrying out the lnvention and that various modifications may be made, depending upon particular conditions, such as the composition 0!. the iron, without departing fromthe scope of the invention.- Also the irons according to this invention may be cast in sand, iron or other moulds, either stationary or rotating, and either cold or preheated.

I claim:

1. As a. new article oi manufacture, a machineable gray cast iron having an all-pearlitic metallographic structure containing vno tree ferrite and comprising silicon in amount in excess of 2 up to 7%, and chromium in effective amount above 175% balanced with the silicon content in accordance with the dimensions of the section of thecasting to produce an all-pearlitic structure therein without superheating.

2. As a new article of manufacture, a magray cast iron having an all pearlitic metallographic structure containing no free rerrite and comprising silicon in amounts in excess of 3 up to 7% and chromium in eflective amounts balanced with the silicon content in accordance with the dimensions of the section of the casting to produce an all pearlitic structure therein.

3. As a new article of manufacture, a machineable gray cast iron having an all pearlitic metallographic structure containing no free i'errite and comprising an element tending to cause the formation of free ferrite having the free ferrite forming effect of silicon in excess of 2% and up to 7%, and an element tending to prohibit the formation of free ferrite and tending to the formation of cementite in amount equivalent to over 275% chromium balanced with the'iree i'errite forming element in accordance with the dimensions of the section of the casting to produce an all pearlitic structure without super-heating.

4. As a new article of manufacture, a machineable gray cast iron having in all pearlitic metallographic structure containing no free ferrite and the invention equally applies and suitable curves 7;

comprising an element tending to cause the formation or free ferrite having the free ferrite inexcessoi'3andupto and an element tending to prohibit the fori'orming eflect of silicon 7 'mation of free ferrite and tending to the formation oi cementite in amount equivalent to over chromium balanced with forming element in accordaiice with the dimensions of the section of the casting to produce an all pearlitic structure thereinwithout superheating.

5. The article as set forth in claim 3 in which the tree ferrite the ferrite-forming element is at least one of a class consisting of silicon, nickel and copper.

6. The article of claim 3 in which the cemen- V "l. The method of producing allpearlitic gray iron castings which comprises the steps of forming a cast iron melt containing silicon in amount from 2% to 7% to promote the formation of ferrite and an amount of chromium above 375% to promote the formation of cementite and inhibit the formation of ferrite, balancing and regulating the silicon and chromium contents with each other in accordance with the section of a desired casting to produce therein without superheating an all-pearlitic metallographic structure with possible traces of cementite but without any free ferrite whatsoever, and then casting.

8. The ,process of producing all-pearlitic gray iron castings which comprises forming a melt of a cast iron composition and incorporating therewith a ferrite-forming agent having the ferriteforming effect of from 2 to 7% of silicon, and. incorporating together with the same a ferriteinhibiting agent which promotes the formation of cementite characteristic of chromium above 115%, and-regulating and balancing the ferriteforming element with the cementite-promoting element in accordance with the section 'of casting to produce without superheating an all-pearlitic gray iron and possible traces of cementite but without any free ferrite whatsoever, and then casting. I"

9. The method as set forth in claim 8 inywhich the ferrite-forming agent is at least one of the 10 elements of the class consisting of silicongnickel and copper.

10. The method as set forth in claim, 8 in which the cementite-promoting agent is at least'one of the elements of the class consisting of chromium,

15 sulphur, manganese and molybdenum.

11. The method as set forth in claim 8 in which the casting step is centrifugally performed.

12. The method as set forth in claim 7 in which the melt is cast in a mould preheated s'uf- 20 ficiently to substantially retard the rate of cooling of the casting.

13. The method as set forth in claim 8 in which the melt is cast in a mould preheated sufficiently to substantially retard the rate of cool- 25 ing of the casting.

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