US2196084A - Method of producing heat-treated cast iron alloys - Google Patents

Description

April 2, 1940 w. J. SPARLING mama 0F rnonucme HEAT-TREATED CAST mom ALLOYS Filed March 22, 1959 FERRITE FERRITE GRAIN o v 0 0 fl wi b 0 M w m w O M6 00 60 0 W A0 0 m o o 9 D a my GRAPHITE BOUNDARIES FERRITE GRAIN BOUNDARIES CEMENTITE WHICH HAS NOT MIG-RATED MATRIX ERLO CKING FERRITE GRAIN BOUNDARY AREAS rm mm: ms

AND FORMED AN INT UNIFORMLY DISTRIBUTED THROUGHOUT THE SPHEROIDIZED CEM'ENIITE SUBSTANTIALLY FERR ITE SPHEROIDIZED CEMENT MIGRATED T0 NETWORK IN TEE k LQEQQEER ZOO . INVENTOR.

TIME HOURS Patented Apr. 2, 1940 UNITED STATES METHOD OF PRODUCING HEAT-TREATED C ST IRON ALLOYS ,William J. Sparling,

Milwaukee, Wis., assignor to Chain Belt Company, corporation of Wisconsin Milwaukee, Wis., a;

Application March 22, 1939, Serial No. 263,445 8 Claims. (oi. mil-21.8)

This invention relates to heat-treated white cast iron alloys, and more particularly to improvements in the process of producing heattreated white cast iron alloy castings of the type disclosed in my Patent No. 2,119,833, granted June 7, 1938.

It is among the objects of the invention to provide a process for producing heat-treated white cast iron alloy products of the type disclosed in my aforesaid patent which affords substantial operating advantages and economies as compared with the process disclosed and claimed in that patent while producing products of similar structure and utility.

The invention will be described-with reference to the accompanying drawing in which Fig. l is a diagrammatic representation of the microstructure of a casting at one stage of the process; Fig. 2 a diagrammatic representation similar to Fig. 1 showing the microstructure of the finished product; and Fig. 3 a graph representative of the two-stage heat treatment provided by the invention.

The term "white cast iron is applied in the industry to a cast ferrous metal containing appreciable amounts of carbon and silicon which are so proportioned to one another that in a casting made from the alloy the carbon does not separate as flake graphite but is largely or wholly in combined form, i. e., as cementite, and the casting exhibits a silver-white appearance when fractured, which characteristics distinguish it from gray cast iron which contains substantial amounts of carbon separated from the alloy during solidification and cooling. White iron castings are usually subjected to a malleableizing heat treatment to produce phase changes I that develop desirable mechanical properties in the heat-treated products.

Although in white cast iron the contents of carbon and silicon, as well as those of elements incidentally present such, for instance, as manganese, sulfur and phosphorus, vary somewhat,

white cast iron as commonly made for mal- 0 leableizing may be said to have an average composition approximately within the following limits:

Per cent Carbon 1.90' to 3.00 Manganese 0.20 to 0.40 Phosphorus 0.08 to 0.16. Silicon 1.30 to 0.60- Sulfur 0.05 to 0.12

I The invention of my aforesaid Patent No. 2,119,833 is predicated upon the application of a novel heat treatment to castings of white cast iron alloys containing manganese and copper in excess of the amounts used in the art prior to that invention, which treatment and composition cooperate to provide articles of novel structure' and improved properties. In accordance with that invention castings are made from an alloy cast iron adapted to produce white iron castings and containing at least about 0.6 per cent of manganese together with at least about 0.5 per cent of copper. In the preferred practice of that invention the White cast iron is preferably of the composition range stated above, containing carbon and silicon in amounts such as to produce a white iron casting, and it contains from about 0.6 to 1.1 per cent of manganese, and about 0.5 to 1.5 percent of copper.

Prior to the invention of my patent the white cast iron industry had preferred low manganese irons, containing less than 0.6 per cent of manganese, because of the tendency of manganese to interfere with malleableizing. Copper was also avoided because of its so-called softening effect, or where it was used chromium was added also to counteract the undesired effect of copper. In the practice of my' patented invention the copper and manganese so cooperate under the influence of the heat treatment of the invention that" their undesirable effects are not observed, and the use of chromium is unnecessary.

Such white cast iron alloy castings are heat treated, in accordance with my patented invention, by heating them to a temperature above the critical point (A1), e. g., above 1500 F. and most suitably at about 1600 to 1800 F., for a period of time sufiicient to decompose all of the massive cementite with production of an austenitic matrix containing finely divided graphitic carbon. After decomposition of the cementite has been completed the casting is cooled to a temperature well below the critical point (A1), as by quenching it in air, water, oil, or other suitable medium to a temperature of, for example, about1200 F. The casting is then reheated to a temperature approaching but somewhat below the critical point, say 1250 to 1300 F., where it is held for a suflicient length of time to cause spheroidizing of the pearlitic cementite. ,The structure of the spheroid-ized product is shown diagrammatically in Fig. 1. It comprises a ferrite matrix through which spheroidized cementite is substantially uniformly distributed, together with some proportion of finely divided graphitic carbon produced in the decomposition of the massive cementite.

After thegcasting has been thus spheroidized the temperature is raised to within the critical range, say between 1325 and 1360 F. and held at that temperature for a period of time such that in consequence of the use of copper and manganese, the spheroidized cementite undergoes a rearrangement in which a substantial proportion of it appears in the ferrite grain boundaries in the form of an interlocked structure. The article is then cooled to preserve that structure which is a novel feature of the patented invention and the cause of the remarkable properties of the castings. The structure and properties are obtained, as far as I am aware, only by the use of copper and manganese in the amounts stated conjointly with final heating of the spheroidized article in the critical range. The structure of the product is shown diagrammtically in Fig. 2.

Articles produced in accordance with my patented invention possess various properties which render them substantially superior to ordinary malleableized white iron castings of normal composition. For example, malleableized white iron castings of the composition stated hereinabove may be expected to exhibit mechanical properties approximately within the following limits:

Tensile strength, psi "45,000 to 60,000 Elastic limit, psi 32,000 to 38,000 Elongation in 2" per cent 25 to 10 Hardness, Brinell .115 to 140 Impact value, ft. lbs 15 to '7.

In contrast, a casting made in accordance with my patented invention from a cast iron alloy containing 2.29 per cent of carbon,,1.00 per cent of copper, 0.85 per cent of manganese, and 0.83 per cent of silicon which had been heated to 1700 F. for thirty hours, quenched below the critical temperature, reheated to 1270 F. for thirty hours, and finally heated to 1340 F. for five hours, followed by quenching, possessed the following mechanical properties: Tensile strength, s1 "90,700 Elastic limit, psi ...56,300 Elongation in 2" per cent..- 13.5

' In general, products made in accordance with the process of my patent will possess mechanical properties about as follows:

Tensile strength, psi --67,500 to 110,000 Elastic limit, psi 47,500 to 05,000 Elongation in 2". per cent 18 to 12 Hardness, Brinell 170 to 220 Impact value, ft. lbs -30 to 15 Although the products provided by the patented invention possess substantiallyimproved strength, one of the major features of that in-' notching the surface of a malleable casting, or

of a casting made from my special alloy, more appreciably decreases the impact value than is the case with steel test bars.' However, the specific figures given herein for impact values are representative of the impact strengths and are comparable to standard Charpy values although not to the same scale because of the differences in size and section of the specimens. The figures given for impact are, however, comparable between themselves.

In addition, the products of my patented invention furthermore possess other improved properties, such as improved resistance to corrosion, which are set forth in detail in the said patent to which reference may be made for further details and further data concerning the chemical, mechanical and physical properties.

Extensive commercial practice of my patented invention has demonstrated admirably its ability to produce heat-treated white cast iron alloy products of the type described in the patent and summarized hereinabove, and the products are recognized as being novel and possessing properties superior to those attainable theretofore in malleableized products.

The heat treatment disclosed in my aforesaid patent differs from that of ordinary malleable iron practice in that accurate control of temperature is necessary inasmuch as the process involves heating close to but below the critical range followed by raising the temperature relatively slightly to within the critical range. The factor of temperature control is therefore more critical in the practice of the patented process than is the case with ordinary malleableizing treatments. Additionally, it is necessary to conduct the process in a non-oxidizing atmosphere to minimize surface decarburization. In consequence of such factors the process of my patent No. 2,119,833 requires performance in electric or similar accurately temperature-controlled furnaces which are considerably more expensive to build, operate and maintain than are the furnaces commonly used in ordinary malleable iron practice. Also, to obtain the best results through the practice of the patented invention it is desirable to heat treat at one time castings of relatively the same section, rather than to heat treat together castings of widely varying section. Experience has shown that the best results are obtained by segregating for heat treatmentcastings up to inch section, castings from about to inch section, and, castings having a sectiongreater than inch. Consequently, although my patented invention performs satisfactorily to provide desirable products as described in the patent, the heat-treating costs are high not only because of the overhead and op- J crating costs inherent in the use of electric furnaces, or similarly accurately temperature-controlled furnaces, but also in consequence of the necessity for either holding castings of a given section group until enough are accumulated for an entire heat, or of operating the furnace with less than a full load.

I have discovered, and it is upon this that my invention is predicated, that all of the benefits of the patented invention may be obtained while substantially reducing production costs by subjecting the castings to a modified heat treatment in two stages. The first stage consists of an ordinary malleableizing heat treatment conducted in the manner and with equipment standard in the malleable iron art. In the second stage the malleableized articles are heated to a temperature to cause carbon to be redissolved, or recombined, after which they are cooled to a temperature below the critical range for a time to effect spheroidizingyand finally the temperature is increased to within the critical range to cause spheroidized cementite to become rearranged and interlocked along the grain boundaries. The castings are then cooled. Hence in the second stage of the heat treatment of the present invention the castings are spheroidized substantially uniformly and then caused to develop a structure in which a substantial proportion of the spheroidized cementite becomes rearranged and interlocked along the grain boundaries.

The products produced by the two-stage heat treatment of the present invention thus possess a composition and microstructure which provide mechanical, physical and chemical properties the same as those of products produced in accordance with the patented invention. However, material and valuable economies result from the practice of the present invention because a large portion of the expensive heat treatment cycle of the patented invention is replaced by the much less cost- 1y ordinary malleableizing treatment, with major benefits that will be detailed more fully hereinafter.

As indicated hereinabove, in the practice of the present invention castings are made from white cast iron alloy as described in my aforesaid patent. the alloy containing at least about 0.6 per cent of manganese and at least about 0.5 per cent of copper, most suitably from about 0.65 to 1.1 per cent of manganese and from about 0.5 to 1.5 per cent of copper. The best results are obtained with castings in which all of the carbon is in combined form, i. e., in which there is no graphitic carbon, or substantially none, in the casting at the start of the malleableizing treatment.

Such castings are then heat treated in accordance with the present invention. The first stage of the heat treatment is conducted in general accordance with standard malleableizing practice. That is, the castings may be, and preferably are, packed in iron pots with sand, cinders or other packing material in accordance with practice Which is quite well standardized. The pots are then heated in a furnace of the type used for making ordinary malleableized castings such, for example. as periodic furnaces fired with powdered coal. Such furnaces are much cheaper to build and maintain, and the operating expense is much lower, than electric furnaces. this malleableizing stage is to cause decomposition of all of the massive cementite followed by more or less complete decomposition of pearlite. although for the purposes of the present invention complete decomposition of the pearlite is unnecessary and is much less critical than in ordinary malleable iron practice. At the end of this stage the castings have a structure containing ferrite and graphite, with or without pearlite depending on the rate of cooling after decomposition of massive cementite.

At any time after the end of the malleableizing cycle the castings are ready for the second stage of the heat treatment. In this stage the malleableized castings are placed in a furnace of the type used in the practice of the patented invention and heated to a temperature such as to cause carbon to be recombined. as by heating them to, for example, 1400 to 1700" F., or higher. higher the temperature in this step, the more rapidly will the maximum amount of carbon be recombined. In this step carbon is dissolved up to the eutectoid composition in the austenite, which contains the excess carbon distributed thereth.ough in the form of graphite. For most purposes heating for a period of about two hours will condition the casting for the subsequent steps of this stage of the heat treatment.

The function of The After the maximum amount of carbon has been recombined the treatment is the same as in the patented process after decomposition of the massive cementite. That is, the castings are then quenched in any suitable medium, such as air, water, or oil, to bring them to a temperature Well below the critical range, for instance to about 1200 F. They are then reheated to a temperature somewhat below the critical range, suitably 1250 to 1300 F., preferablyto approximately 1270 F., and held there for a sufiicient time to spheroidize the cementite, which is now in pearlitic form. This results in the production of a structure like that shown in Fig. 1, in which the cementite is substantially completely spheroidized and substantially uniformly distributed throughout the entire matrix. During this step there is no substantial decomposition of the pearlite into ferrite and graphite, apparently because this is prevented by the action of the relatively high content of manganese.

After spheroidization, the time period of which A will vary with the section of the articles and the exact composition of the alloy, but which usually ranges between ten and thirty-five hours, the temperature is raised to within the critical range, suitably between 1325 and 1360 F., to cause a substantial proportion of the spheroidized cementite particles to rearrange into an interlocked structure along the ferrite grain boundaries. This structure, shown in Fig. 2, characterizes the final product of both the patented and the present invention.

The article is then cooled at a rate such as to preserve the structure, as by quenching it in air, water, oil, or other medium with suificient rapidity to retain the structure produced through heating within the critical range.

'A heat treatment typical of the present invention is illustrated by the graphs of Fig. 3. The first stage represents a standard, or conventional, malleableizing treatment conducted in an ordinary periodic malleableizing furnace. The castings are heated during, for example, forty hours to a temperature of about 1600 F. which is maintained long enough to cause decomposition of all of the massive cementite, say during about fifty hours. The articles are then cooled to room temperature. In the embodiment shown the cooling is slow, requiring about eighty hours to reach room temperature. t permit decomposition of pearlite.

The actual times and temperatures used in the first stage may vary in practice from those shown in Fig. 3. Thus, the castings may be heated above the critical range more rapidly or more slowly than shown, and they need be held at temperature only long enough to effect breakdown of all of the massive cementite. Also, the temperature of l600 F. for this part of the first stage is not critical, it being possible to effect breakdown of cementite at lower or higher tempera tures, say l500 to 1800 F. The decomposition wilLbe accelerated by increase of temperature, but 1600 F. is chosen because it represents an average value productive of economically rapid decomposition.

Similarly, although the first stage is represented as being in accordance with standard malleable practice, it is not essential for the purpcses of the invention that the articles be fully malleableized. That is, it is not essential that the articles to held below 1400 F. or cooled slowly to break up pearlite, nor is it necessary that they consist essentially of ferrite and graphite. Provided the massive cementite be completely decomposed it is entirely feasible in the practice of this invention for the articles to contain a greater or lesser proportion of pearlite at the end of the first stage. Hence the articles may be cooled more rapidly from above the critical range than is indicated in the first stage of Fig. 3. Thus, they might be cooled as shown in the broken line of the first stage graph, say by allowing them to cool without refiring the furnace, or by hauling the pots from the furnace for air cooling, thus reducing the time involved in this stage.

As thus malleableized the articles are ready for the second stage of the treatment in accordance with the present invention. This stage of the heat treatment may, of course, be applied at any time after malleableizing. In the embodiment of the second stage graph of Fig. 3, the articles are heated to a temperature and for a period of time to effect recombination of carbon. They are then cooled, suitably by quenching, to a temperature well below the critical range, shown in Fig. 3 as being 1200 F., followed by reheating to a temperature close to but below the critical range, 1275" F. in the embodiment shown, and held at that temperature to effect spheroidization as described hereinabove. Finally, the articles are heated to within the critical range, say 1340 F., followed by cooling at a rate such as to preserve the structure produced in the last step. As in the case of the first stage and as explained in my aforesaid patent, the exact times and temperatures applied in the second stage may vary from those shown in Fig. 3 which is included purely by way of illustration and not by way of limitation.

In the practice of the process of the patented invention a complete cycle in the electric furnace may require between ninety and one hundred hours. In the present invention only the second stage of the heat treatment is performed in the electric furnace, and for most sizes of casting this may be completed in about forty to sixty hours. Thus the electric furnace cycle is reduced to about one-half or two-thirds of that necessary heretofore. This results in major economies, and this is so even though the malleableizing cycle of the first stage be of considerable duration because experience has shown that the malleableizing stage can be performed much more cheaply than by decomposing the cementite in an electric furnace cycle.

Other economies result from the fact that in the malleableizing stage castings of all sizes and sections may be treated at once so that it is possible always to run the malleableizing furnace at full load. The malleableized castings may then be segregated into groups of appropriate sections for treatment in the second, or electric furnace, stage. In this manner it is possible to accumulate the various section sizes, through operation of the malleableizing furnace at full load on all sections, so that full loads are always available for the electric furnace cycles.

In this manner also the capacity of the electric furnaces is substantially doubled because the electric furnaces are freed from the first half of the cycle used in the patented process so that they can be continuously operated for the second stage only, thus effectively doubling the production capacity of a given electric furnace installation. This further reduces operating costs because if the entire operation be conducted in electric furnaces twice the capacity would be required that is necessary in practicing the present invention, with doubled first, upkeep and operating costs.

- A major advantage of the present invention is that any warpage which a casting is likely to undergo will be substantially completed in the first stage. In consequence of its malleableized condition the casting may then be straightened easily prior to being subjected to the second stage. The importance of this feature arises from the fact that it is not possible satisfactorily to straighten articles which may have warped during the heat treatment of the patented invention because their high tensile strength and hardness after heat treatment are comparable in certain respects, especially as to straightening of warped castings, to high carbon steel. This ability to straighten the castings in the present invention results in greater output through reduction in the proportion of castings which may have to be rejected because of warpage and allied defects.

According to the provisions of the patent statutes, I have explained the principle and method of practicing my invention and have illustrated and described what I now consider to represent its best embodiment. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.

I claim:

1. That method of treating castings of white cast iron alloy containing in excess of about 0.5 per cent of copper and in excess of about 0.6 per cent of manganese, which comprises subjecting said casting to a malleableizing treatment to effect decomposition of massive cementite, then reheating the malleableized castings to temperatures successively above and below the critical range for times to effect, respectively, solution of carbon and formation of spheroidized, cementite, and then heating within the critical range for a time to cause rearrangement of the spheroidized cementite to produce at the grain boundaries an interlocked structure of spheroidized cementite.

2. That method of treating castings of white cast iron alloy containing in excess of about 0.5 per cent of copper and in excess of about 0.6 per cent of manganese, which comprises subjecting said castings to a malleableizing treatment involving heating the castings to a temperature and for a time to effect decomposition of massive cementite, and cooling the castings to room temperature; then reheating the malleableized castings to a temperature above the critical point for a time to effect solution ofcarbon, then at a temperature below the critical range to effect formation of substantially uniformly distributed spheroidized cementite, and then heating within the critical range for a time to produce at the grain boundaries an interlocked structure of spheroidized cementite, and cooling the castings at a rate to preserve said structure.

3. A two-stage method of treating castings of white cast iron alloy containing in excess of about 0.5 per cent of copper and in excess of about 0.6 per cent of manganese, which comprises, as a first stage, heating said castings to a temperature between about 1500 and 1800 F. for a time to effect decomposition of substantiallyall of the massive cementite, and cooling the castings to room temperature at a rate such as to effect decomposition of pearlite with formation of ferrite and graphite; then, as the second stage, reheating the malleableized castings to a temperature above about 1400" F. for a time to effect soto a malleableizing treatment, then reheating the malleablized castings to a temperature and for a time to effect solution of carbon, cooling the castings to a temperature below the critical range and heating for a time and at a temperature adjacent but below the critical range to produce substantially uniformly distributed spheroidized cementite, and then raising the temperature into the critical range for a time to cause spheroidized cementite to form an interlocked structure along the grain boundaries, and cooling at a rate to preserve said grain boundary structure.

5. That method of heat treating white cast iron alloy castings containing about 0.5 to 1.5 per cent of copper and about 0.6 to 1.1 per cent of manganese, which comprises subjecting said castings to a malleableizing treatment, to decompose substantially all of the massive cementite, cooling the castings to room temperature, then heating the malleableized castings to a temperature of about 1300 to 1700 F. for a time to effect solution of carbon, cooling the castings to a temperature below the critical range and heating for a time and at a temperature of about 1200 to 1300 F. to produce substantially uniformly distributed spheroidized cementite, and then raising the temperature into the critical range for a time to cause spheroidized cementite to form an interlocked structure along the grain boundaries, and cooling at a rate to preserve said grain boundary structure.

6. That method of heat treating white cast iron alloy castings containing about 0.5 to 1.5 per cent of copper and about 0.6 to 1.1 per cent of manganese, which comprises subjecting said castings to a malleableizing treatment to decompose substantially all of the massive cementite, cooling the castings to room temperature, then heating the malleableized castings to a temperature above about 1400 F. for a time to efiect solution of carbon, then cooling the castings to a temperature below the-critical range and heating them for a time and at a temperature between about 1200 and 1300 F. to produce substantially uniformly distributed spheroidized cementite, and then raising the temperature to between about 1325" to 1360 F. for a time to cause spheroidized cementite to form any interlocked structure along the grain boundaries, and cooling at a rate to preserve said grain boundary structure.

7. That method of heat treating white cast iron alloy castings containing about 0.5 to 1.5 per cent of copper and about 0.6 to 1.1 per cent of manganese, which comprises heating said castings to a temperature for a time to decompose substanstantially all of the massive cementite, cooling the castings, then heating the castings to a temperature above about 1400 F. and below about 1800 F. for a time to efi'ect solution of carbon, then cooling the castings to a temperature below about 1300 F. and heating for a time and at a temperature of about 1250 to 1300 F. to produce substantially uniformly distributed spheroidized cementite, then raising the temperature to between about 1325 to 1360 F. for a time to cause spheroidized cementite to form an interlocked net work structure along the grain boundaries, and cooling at a rate to preserve said grain boundary structure.

8. That method of treating castings of white cast iron alloy containing in excess of about 0.5 per cent of copper and in excess of about 0.6 per cent of manganese, which comprises subjecting said casting to a malleableizing treatment involving heating the castings to a temperature and for a time to effect decomposition of massive cementite, and cooling the castings; straightening warped castings after such malleableizing treatment; then reheating the malleableized castings to temperatures above and below the critical temperature for times to form substantially uniformly distributed spheroidized cementite, and then heating within the critical range for a time to produce at the grain boundaries an interlocked structure of spheroidized cementite.

WILLIAM J. SPARLING.

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