Technical Field

This disclosure relates generally to high wear resistant articles having a hard surface and a tough base material. Specifically, this disclosure relates to a deep carburization process applied to low and medium carbon steels.

Background

Creating wear-resistant articles that have both a hard surface for improved wear performance and tough base material - while also facilitating high volume, low cost fabrication of the article - currently involves balancing competing interests related to the article's carbon content and distribution.

Generally, steels with lower carbon content are easier to manipulate through cold working operations like cold shearing and punching. However, higher carbon steels are preferred for increasing hardness and toughness. In typical processing of wear articles, such as track shoes, low- to medium-carbon, low alloy steels are rolled, sheared, and punched into the appropriate shape. The articles are then hardened by a full austenitizing heat treatment that is followed by quenching and tempering. It is still desirable to have articles with increased wear resistance and toughness without increasing the carbon content throughout the entire article.

One known method for improving wear resistance while maintaining the low carbon base material is the carburization process.

Carburization is a heat treatment process in which a steel article's surface is exposed to source of carbon. Under this exposure, carbon atoms are implanted into the surface layers of the article. Since the steel articles are made up of atoms arranged in a crystalline lattice, the implanted carbon atoms are forced into the crystal structure of the metal atoms and either remain in solution, forming a solid solution, or they react with the iron atoms to form a carbides. Both mechanisms strengthen the surface of the steel article, either by solid solution strengthening resulting from lattice strain when the carbon atoms are forced between the steel alloy's atoms, or by precipitation strengthening resulting from the formation of very hard particles. However, conventional low-carbon, low alloy carburizing steels do not provide adequate core hardness that is necessary to provide core yield strength and wear resistance. Moreover, it is known in the art that typical carburizing methods applied to standard low carbon, low alloy steels that when carburizing takes place in an atmosphere comprising carbon at a level higher than the carbon content of the steel article, quench cracking is seen in the surface. Such quench cracking is thought to be the result of surface tension created in the crystalline lattice resulting from the introduction of carbon atoms into the lattice matrix. Such carburizing typically creates a hardened case that extends between about 0.5 μm and about 4.0 μm below the original article surface.

Another solution that has been attempted is a hardface welding applied to specific surfaces of the steel article where wear-resistance is desired. The problem with this approach is that a soft heat-affected zone is created beneath the hard surface, degrading the wear life of the base material. Also, hardfacing an article can lead to warping of the steel article in the limited area where the hard faced layer is applied.

The present invention is directed to overcome one or more of the problems as set forth above.

Summary of the Invention

In one embodiment, the present disclosure is directed to a steel article comprising a base steel alloy having between about 0.25 wt% and about 0.40 wt% carbon, an external surface having a hardness of at least about 55 HRC, and a carburized case extending from the surface of the steel article toward the base steel alloy a distance of at least about 3 mm. The carburized case has a first region with a carbon concentration greater than about 1.0 wt%, the first region extending from the surface of the steel article toward the base steel alloy a distance of at least about 1 mm, and a second region between the first region and the base steel alloy wherein the concentration of carbon decreases from an amount greater than about 1.0 wt % to an amount between about 0.25 wt% and about 0.40 wt% over a distance of less than about 3 mm. Further, at least about 20% of the surface area of the steel article includes the carburized case.

Brief Description of the Drawings

Figure 1 is a cross sectional graphic representation of a track type tractor shoe.

Figure 2 is a graphical illustration of the normalized life of a track type tractor that has been prepared according to the disclosure.

Figure 3 is a graphical illustration of the results of the three point stress test performed in Example 1.

Detailed Description

Figure 1 illustrates an exemplary steel article 10, a track type tractor track shoe. While the track shoe will be used herein for illustrative purposes, it should be understood that this disclosure may be applied to any steel article where it is desirable to have a harder surface while maintaining a tough base material, such as in applications involving high wear resistance. The steel article 10 may be formed to a desired shape by any appropriate manufacturing process, such as machining, casting, forging, etc., or combination of processes known in the art. In the example of track shoe 10, a base material 14 is generally first rolled into plate form before undergoing various cold working steps. Such cold working steps may include shearing and punching. As seen in Figure 1, the steel article may have a non-uniform or complex cross sectional shape.

The base material 14 of steel article 10 is a low- to mid-carbon steel alloy that comprises between about 0.25 wt% and about 0.40 wt% C. For example, the steel article alloy may comprise between about 0.30 wt% and about 0.35 wt% C. Other alloying elements may include manganese, molybdenum, -A-

chromium, copper, nickel, silicon, and other elements selected as carbide-forming elements, hardenability agents, or grain refining elements. In general, the composition of the starting alloy may comprise elements in the following approximate wt%:

Table 1 : Composition of base metal in weight percent.

Next, the steel article 10 is exposed to a carburizing heat treatment. Importantly, the parameters of the carburization process are controlled such that carburized case 11 that is formed is substantially free of carbides. That is, the concentration of carbides in carburized case 11 is less than about 20 mg/cm ³ , such as less than about 10 mg/cm ³ , or even less than about 1 mg/cm ³ .

Typically, the formation of carburized case 11 that is solid solution strengthened rather than precipitation hardened occurs at lower carburization temperatures. Specifically, the carburization process takes place at between about 200 ⁰ C and about 700 ⁰ C, such as between about 400 ⁰ C and about 600 ⁰ C. The carburization process takes place over a time sufficient to produce a carburized case of the desired depth. Specifically, the carburization process requires between about 1 min and about 360 min, such as between about 10 min and about 120 min.

The carburization atmosphere generally comprises more carbon than the amount desired in the carburized case at the end of the process.

Surprisingly, the level of carbon must be about 1% or higher in order to achieve the unexpected results of the hard carburized case of the disclosure. For example, although levels of carbon in the carburizing atmosphere between about 0.50% and about 0.90% are higher than the carbon levels in the base steel alloy 14, such atmospheric carbon levels actually prove to be detrimental to the structural integrity of the final steel article's carburized case. The steel article 10 may be immersed in a carbon-bearing atmosphere for one or more cycle of the disclosed carburizing cycle. The carbon-bearing atmosphere may be continuously replenished to maintain a sufficiently high carbon potential in the atmosphere. Since carburizing processes are well known in the art, only those details of the carburizing process that are relevant to the current disclosure are discussed herein.

Following the carburizing process, steel article 10 is subjected to a hardening process that includes a heat treatment to an austenizing temperature. More specifically, the carburized steel article is heated to a temperature between about 725 ⁰ C and about 900 ⁰ C and is held within that temperature range for at least about 1 minute. For example, the carburized steel article may be held at the temperature range between about 1 minute and about 360 min.

After carburizing and hardening steel article 10 according to the above process, a carburized case 11 is formed that has a hardness of at least about 50 HRC, or at least about 55 HRC, or even at least about 60 HRC.

The carburized case 11 extends from the steel article's original surface toward the base steel alloy a distance of at least about 3 mm, such as at least about 4 mm, at least about 5 mm, or even at least about 6 mm. For example, the carburized case 11 may extend from the steel article's original surface toward the base steel alloy a distance of between about 3 mm and about 10 mm, such as between about 4 mm and about 6 mm, or between about 4 mm and about 5 mm. In a relative sense, this case depth may represent a distance of up to about 35% or even up to as much as about 40% of the steel article's thickness at a given location away from the tips or ends of the steel article. Generally, carburized case 11 will not extend over the entire width of the steel article, as the toughness of the steel article's base material would be sacrificed.

The carburized case 11 may have two regions: a first region 12 where the carbon concentration after carburization is greater than about 1.0 wt% and a second region 13 wherein the carbon concentration decreases from the concentration of the first region 12 to a carbon concentration between about 0.25 wt% and about 0.40 wt%. The first region 12 extends from the surface of the steel article toward the base steel alloy 14, while the second region 13 is between the first region 12 and the base steel alloy 14. The carburized case's first region 12 extends toward the base steel alloy 14 a distance of at least about 1 mm, such as at least about 1.5 mm, at least about 2 mm, at least about 2.5 mm, or at least about 3 mm. For example, the first region 12 may extend toward the base steel alloy 14 a distance of between about 1.5 mm and about 5 mm, between about 2 mm and about 4 mm, or even between about 2 mm and about 3 mm. The second region 13 extends from the first region toward 12 the base steel alloy 14 a distance of less than about 5 mm, such as less than about 3 mm, less than about 2 mm, or less than about 1 mm.

The carburized case 11 may be selectively applied to the steel article 10 such that only specific portions of the steel article's surface has the carburized case. In most applications, at least about 20% of the steel article's surface area will comprise the carburized case. For example, at least about 35%, 50%, 65%, 80%, or even at least about 90% of the steel article's surface area will comprise the carburized case 11.

Industrial Applicability

A steel article 10 with the carburized case 11 of the current disclosure may be beneficial for any component where a hardened case is desired, such as in high wear applications. Such wear conditions may include

uncontrolled and unlubricated conditions such as those experienced by track shoes, under-carriage components, rock drills, etc. The formation of a deep case proximate the surface of the steel article 10 increases the wear resistance of the steel article 10, which may improve the durability and longevity of the steel article 10. Figure 2 shows the impact of the increased wear resistance realized by this disclosure when the wear life is normalized.

The process of formation of such a carburized case 11 as disclosed herein using low- to medium-carbon base steel is counterintuitive because carburizing such steels using known techniques may lead to excessive surface cracking. Such cracking is believed to be the result of tensile stresses generated in the transition region between the hard carburized case 11 and the base alloy 14, labeled herein as the second region 13 of the carburized case 11, near the surface. However, by using the process of the disclosure, the depth of the hard carburized case 11 creates compressive stresses at the surface and prevents cracking.

Example 1

One method of testing a material's resistance to deformation is a three point bend test. For this test, two track type tractor track shoes were prepared, Shoe A was prepared using current production materials and processes and Shoe B was prepared using the materials and processes of this disclosure. A 1000 kN force was applied to the shoes such that the force was applied downward at the midpoint of the shoe while the shoe rested on supports at both ends of the respective shoes. Shoe A displayed 5.23 mm of displacement, while Shoe B's displacement was measured at 4.67 mm, approximately a 10.7% improvement in resistance to cold work deformation. The experimental results are shown in Figure 3.

It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the scope or spirit of the invention. Additionally, other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the disclosure. It is intended that the specification and examples be considered as exemplary only.