Ausferritic Wear-Resistant Steel Castings
A method for preparing an austempered, work-hardening steel casting. A melt with certain chemical ranges including relatively high carbon and silicon content is poured, heat treated, cooled, austenitized, quenched and austempered, before final cooling to room temperature. This process provides a steel casting with increased wear life, characterized by a duplex microstructure containing ferrite plus carbon in solution in austenite known as ausferrite. The austenite will transform during abrasive service to a hard, wear-resistant martensite on the surface.
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This application claims priority to and the benefit of provisional patent application No. 61/235,592, filed Aug. 20, 2009, and entitled Ausferritic Wear-Resistant Steel Castings.
FIELD OF INVENTIONThe present invention is in the technical field of material science. More particularly, the present invention is in the technical field of wear resistant cast or forged steel.
BACKGROUNDDuring the manufacturing of wear resistant steels produced using the steel casting sand molding process, limitations exist that affect the overall wear life of the final product. These limitations are due to the hardness that can be achieved using conventional processing and heat treating methods. With low alloy steels, a relationship exists between carbon and hardness such that increasing the carbon content of the steel produces higher final hardness and greater wear resistance. However, as the carbon content increases, the steel becomes more brittle, and the risk of cracking during the manufacturing processor during use increases. These factors limit the hardness, toughness, and wear life of the product.
Conventional heat treatment of carbon low alloy material to achieve maximum hardness consists of converting austenite to martensite by rapid water quenching. See
The present invention satisfies these needs by providing a method for production of a cast or forged steel product with increased wear life, characterized by a duplex microstructure containing ferrite plus carbon in solution in austenite known as ausferrite. The austenite will transform during abrasive service to a hard, wear-resistant martensite on the surface.
SUMMARYOne embodiment of the present invention comprises a method of preparing an austempered, work-hardening steel casting, comprising melting an alloy mixture containing (i) from about 0.6 to about 1.0 percent by weight carbon; (ii) from about 0.5 to about 1.0 percent by weight manganese; (iii) from about 1.5 to about 2.5 percent by weight silicon; and (iv) the remainder iron. If the casting is greater than about one-inch thick, then it is advantageous to add at least one hardening agent to the melt, selected from the group consisting of from about 0.4 to about 2.5 percent by weight chrome; from about 0.0 to about 0.75 percent by weight nickel; or from about 0.0 to about 0.5 percent by weight molybdenum. After the alloy is melted, it is poured into a mold of desired shape at a temperature of about 2700°-2800° F. and cooled in the mold until it reaches a temperature of less than about 1000° F., followed by shake-out and cooling to ambient temperature. Then, the casting is austenitized at a temperature of about 1600°-1800° F. until an austenitic matrix is achieved. Next, the casting is quenched in a medium capable of providing a quench rate sufficient to transform the entire casting to the desired ausferric matrix to a temperature of about 550°-725° F., and that temperature is maintained for about 1-6 hours. The resulting ausferric matrix casting is then cooled to ambient temperature. It is preferable, between the melting and pouring steps to deoxidize the alloy mixture by adding aluminum in the range of 0.02-0.04 percent by weight. It is also preferable between the first cooling step and the austenitizing step to homogenize the casting by heat treating at a temperature of about at least 1800° F. for a sufficient length of time to reduce the number and size primary carbides. This time usually is a minimum of about 30 minutes per inch of section thickness, followed by air cooling to ambient temperature.
The present invention will be explained, by way of example only, with reference to certain embodiments and the attached Figures, in which:
In one embodiment of the present invention, steel castings are produced within the chemical ranges shown in Table 1 below. Specifically, a high carbon and silicon content are used to produce the desired work hardening results. In heavier sections, the formation of upper transformation ferrite and carbides is prevented by enhancing the hardenability of the chemistry with the addition of hardenability agents, such as chromium, molybdenum, nickel or combinations of these, in combination with the specialized austempering heat treatment.
Depending on casting size, it may be necessary to add hardenability agents, such as molybdenum, chrome, or nickel, either singly or in any combination thereof, for aiding austemperability. For example, a small casting up to an inch thick usually does not require alloying with the above mentioned hardenability agent(s) because the part is so small that sufficient quenching severity can be experienced throughout the bulk of the casting without the hardening agent(s). On the other hand, heavier castings (generally greater than about one-inch thick) usually require the addition of such agents to allow through quenching of the thicker casting components to achieve through hardening of the desired severity. Ranges of the hardenability agents change with casting size, but ranges of molybdenum 0-0.5% by weight, chrome 0.4-2.5% by weight, and nickel 0-3.5% by weight have been found particularly effective for larger castings having thicknesses greater that one inch. The amount of these elements to be incorporated is dependent upon the quenching equipment and the quenching mediums being used, as will be understood by those skilled in the art.
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In one preferred embodiment, for a casting having a three-inch thickness, the following specific chemistry was found to produce a wear-resistant steel in accordance with the present invention: carbon 0.9% by weight, manganese 0.6% by weight, silicon 2.0% by weight, chrome 0.6% by weight, nickel 0.25% by weight, and molybdenum 0.25% by weight. For this chemistry, forty to fifty-percent improvements in wear resistance, as compared to a conventional wear-resistant alloy, have been achieved. The specific chemical elements vary from the foregoing specifications within the ranges provided of Table 1, depending on casting size, thickness and effectiveness of the quenching medium; as will be understood by those skilled in the art.
Another embodiment comprises using the principles the present invention with the forging process, as follows: (a) melting the cast steel mixture as described by Table 1 to form a melt; (b) pouring the melt into a mold to form a billet or forging blank, at a temperature of about 2600°-2800° F.; (c) after shakeout, allowing the billet to cool to ambient temperature for normal cleaning operations; (d) re-heating the billet to the desired forging temperature as required by the forging process normally in the range of 2000°-2400° F.; (e) forging to near a desired net shape; (f) austempering as shown in
Therefore, utilizing the combination of the steel casting sand molding or forging processes, a chemistry as defined by Table 1 consisting of high carbon, high silicon chromium molybdenum steel along with the austempered heat treatment as shown in
Although the present invention has been described and shown with reference to certain preferred embodiments thereof, other embodiments are possible. The foregoing description is therefore considered in all respects to be illustrative and not restrictive. Therefore, the present invention should be defined with reference to the claims and their equivalents, and the spirit and scope of the claims should not be limited to the description of the preferred embodiments contained herein.
Claims
1. A method of preparing an austempered, work-hardening steel casting, comprising:
- (a) melting an alloy mixture containing (i) from about 0.6 to about 1.0 percent by weight carbon; (ii) from about 0.5 to about 1.0 percent by weight manganese; (iii) from about 1.5 to about 2.5 percent by weight silicon; and (iv) the remainder iron;
- (b) pouring the melt into a mold of desired shape at a temperature of about 2700°-2800° F.;
- (c) allowing the casting to cool in the mold until it reaches a temperature of less than about 1000° F., then shaking out said casting and allowing it to cool to ambient temperature;
- (d) austenitizing the casting at a temperature of about 1600°-1800° F. until an austenitic matrix is achieved;
- (e) quenching the austenitic casting to a temperature of about 550°-725° F. and maintain that temperature between about 1-6 hours in a medium capable of providing a quench rate sufficient to transform the entire casting to the desired ausferric matrix; and
- (f) cooling said ausferric matrix casting to ambient temperature.
2. The method of claim 1, wherein said casting is greater than about one-inch thick, and wherein said alloy mixture melted in step (a) further comprises at least one hardening agent selected from the group consisting of:
- (v) from about 0.4 to about 2.5 percent by weight chrome;
- (vi) from about 0.0 to about 0.75 percent by weight nickel; or
- (vii) from about 0.0 to about 0.5 percent by weight molybdenum.
3. The method of claim 1, further comprising, between said melting step (a) and said pouring step (b), deoxidizing said mixture by adding aluminum in the range of 0.02-0.04 percent by weight.
4. The method of claim 1, further comprising, between said cooling step (c) and said austenitizing step (d), homogenizing said casting by heat treating at a temperature of about at least 1800° F. for a sufficient length of time to reduce the number and size primary carbides.
5. The method of claim 4, wherein said homogenizing comprises holding the castings at said temperature for a minimum of about 30 minutes per inch of section thickness, followed by air cooling to ambient temperature.
6. The method of claim 5, wherein said temperature is about 1850°-1900° F.
7. The method of claim 1, wherein in said austenitizing step (d), said temperature is about 1650°-1750° F.
8. The method of claim 1, wherein the quenching medium in said quenching step (e) is molten salt.
9. The method of claim 1, wherein in said quenching step, said temperature is about 650°-725° F.
10. The method of claim 1, wherein in said quenching step, said casting is held at said temperature for about 2 to 4 hours.
11. The method of claim 1, wherein said casting is about three inches thick, and wherein in said melting step (a), said alloy mixture contains 0.9 percent carbon by weight, 0.6 percent manganese by weight, 2.0 percent silicon by weight, and further comprises 0.6 percent chrome by weight, 0.25 percent nickel by weight, and 0.25 percent molybdenum by weight.
12. A method of preparing an austempered, work-hardening steel forging, comprising:
- (a) melting an alloy mixture containing (i) from about 0.6 to about 1.0 percent by weight carbon; (ii) from about 0.5 to about 1.0 percent by weight manganese; (iii) from about 1.5 to about 2.5 percent by weight silicon; and (iv) the remainder iron;
- (b) pouring the melt into a mold to form a billet at a temperature of about 2600°-2800° F.;
- (c) allowing the billet to cool in the mold until it reaches a temperature of less than about 1000° F., then shaking out said casting and allowing it to cool to room temperature;
- (d) heating the billet to the desired forging temperature as required by the forging process;
- (e) forging the billet to near a desired shape;
- (f) austenitizing the casting at a temperature of about 1600°-1800° F. and holding the casting at said temperature until an austenitic matrix is achieved;
- (g) quenching the austenitic casting to a temperature of about 550°-725° F. and maintaining that temperature for about 1-6 hours in a medium capable of providing a quench rate sufficient to transform the entire casting to a desired ausferric matrix; and
- (h) cooling said ausferric matrix casting to ambient temperature.
13. The method of claim 12, wherein said forging is greater than about one-inch thick, and wherein said alloy mixture melted in step (a) further comprises at least one hardening agent selected from the group consisting of:
- (v) from about 0.4 to about 2.5 percent by weight chrome;
- (vi) from about 0.0 to about 0.75 percent by weight nickel; or
- (vii) from about 0.0 to about 0.5 percent by weight molybdenum.
14. The method of claim 12, wherein in said heating step (d), said forging temperature is about 2000°-2400° F.
15. An article made of a steel alloy cast in accordance with the method of claim 1.
16. An article made of a steel alloy forged in accordance with the method of claim 12.
Type: Application
Filed: Aug 17, 2010
Publication Date: May 19, 2011
Applicant: Southern Cast Products, Inc. (Meridian, MS)
Inventor: Rick Perry (Meridian, MS)
Application Number: 12/858,178
International Classification: C21D 6/00 (20060101); C21D 8/02 (20060101); C22C 38/04 (20060101);