Patent Application
20150037198
The present disclosure relates generally to a steel with increased wear resistance, and more particularly, to a wear resistant steel with a high fracture toughness.
The durability of a component subject to wear (“wear component”) is dependent on its wear resistance. Components of machines subject to high loads and operating in harsh unlubricated environments are often subject to abrasive wear. Examples of wear components include, without limitation, ground engaging tools (GET), undercarriage components of equipment, cutter rings of tunnel boring machines (TBM), rock drills, etc. During abrasive wear, sand and rock particles impinge on and abrade wear components during operation of the machine. Abrasive wear may necessitate frequent refurbishment or replacement of the wear components, and thus affect the reliability, efficiency, and operating cost of the machine.
To improve the durability of wear components, these components may be fabricated from martensitic steels having a high hardness, toughness, and temper resistance. As is known in the art, martensite is a steel microstructure usually formed as a result of rapid cooling (or quenching). When a carbon steel is heated to a high temperature (in the austenitic range of the steel) and quenched, the carbon atoms in the steel do not have sufficient time to diffuse out of the crystal structure in large enough quantities to form iron carbide or cementite. As a result, the crystalline structure of the steel transforms from an austenitic microstructure to a martensitic microstructure. The martensitic microstructure of steel results in a high hardness. However, when a component is quenched, only its surface regions get transformed into martensite and have a high hardness, unless the material is alloyed with suitable elements to increase the depth of hardening. Therefore, some wear components may be more appropriately made from deep-hardening martensitic steels. Deep-hardening steels are steels that harden deeper and do not have to be cooled as quickly as normal steels for the formation of a martensitic microstructure.
U.S. Pat. No. 5,900,077 (the '077 patent) issued to McVicker, and assigned to the current assignee, discloses a steel with a high hardenability, toughness, and temper resistance. In the '077 patent, the disclosed steel includes carbon between 0.2-0.45% by weight, chromium between 0.01-2.00% by weight, molybdenum between 0.15-1.2% by weight, and vanadium between 0.01-0.4% by weight, in addition to other constituents. The steel of the '077 patent has been found to have excellent wear resistance. However, its wear properties may nonetheless benefit from improvement.
The disclosed steel is directed at overcoming the shortcomings discussed above and/or other shortcomings in existing technology.
In one aspect, a steel article is disclosed. The steel article may include about 0.2 to 0.43 percent by weight of carbon, about 0.5 to about 3.0 percent by weight of silicon, about 0.01 to about 3.0 percent by weight of chromium, and 0.43 to about 2.5 percent by weight of vanadium.
In another aspect, a steel article is disclosed. The steel article may include about 0.35 to 0.43 percent by weight of carbon, 0.45 to about 1.3 percent by weight of vanadium, about 0.02 to about 0.06 percent by weight of titanium, and about 1.45 to about 1.8 percent by weight of silicon. The steel article may also include a martensitic microstructure.
In yet another aspect, a steel article is disclosed. The steel article may include about 0.38 to 0.43 percent by weight of carbon, about 1.45 to about 1,60 percent by weight of silicon, about 1.60 to about 1.80 percent by weight of chromium, 0.43 to about 1.0 percent by weight of vanadium, and about 0.009 to about 0.014 percent by weight of nitrogen.
For increased wear resistance, the sprockets 106 and track chains 108 (and other undercarriage 102 components) of machine 100 may be fabricated from a deep-hardening steel having high hardenability, toughness, and temper resistance. In general, the steels of the current disclosure may have a composition, by weight, as listed in Table 1. To account for experimental variations in measurements, unless expressly stated, all the weight percentages in this description are approximate values. For instance, although not expressly stated in Table I, the concentration of carbon in the embodiments of the current disclosure is from about 0.2 to about 0.43 percent by weight. The term “about” represents a possible variation of ±10% of a listed value.
The concentration of carbon between 0.20 percent and 0.43 percent by weight provides, after quenching and tempering, a steel having a fully martensitic microstructure at a. given depth below the surface where high toughness properties are desired. If the concentration of carbon is below 0.20 percent by weight, quenching and tempering treatments do not provide the required hardness for adequate wear resistance. Strength and hardness of martensitic steels are primarily a function of carbon content. As known in the art, increasing the carbon concentration increases the strength of steel. However, increasing carbon concentration also results in a rapid decrease in fracture toughness of the steel. Therefore, in order to provide sufficient strength and fracture toughness, the absolute maximum carbon concentration in the disclosed steels is limited to 0.43 percent by weight.
In addition to carbon, other constituents of the disclosed steel also contribute to the desired properties of a wear component. Manganese, silicon, chromium, and molybdenum improve the hardenability of the steel, in addition to imparting other desirable characteristics. Manganese combines with sulfur and prevents the formation of iron sulfide which detrimentally affects hot workability, toughness, and machinability of the steel. Below 0.4 percent by weight, the amount of manganese may not be sufficient to combine with all the residual sulfur. At high concentrations, manganese may cause manganese segregation of the steel. Therefore, the maximum concentration of manganese should be below 2.0 percent by weight. Silicon and chromium improve the hardness of the steel at high temperatures, and molybdenum increases the toughness of the steel by preventing the formation of undesired phases.
Under conditions of wear, vanadium in the steel combines with carbon and nitrogen to form second phases of vanadium nitride, vanadium carbide, and vanadium carbonitride which increase wear resistance. Below 0.43 percent by weight, the amount of vanadium carbonitride formed may not provide the desired improvement in wear resistance. Therefore, in order to provide sufficient wear resistance for wear components, the absolute minimum vanadium concentration in the disclosed steels is 0.43 percent by weight. At concentrations above 2.5 percent by weight, vanadium may react with carbon and reduce hardness. Nitrogen in steel may combine with titanium to form titanium nitride. Nitrogen not tied up with titanium, may be available to form aluminum nitride, and any remaining nitrogen may combine with vanadium. Though desirable in some applications of steel, aluminum nitride decreases the fracture toughness of steel. Therefore, aluminum nitride is undesirable in the steels of the current disclosure. Accordingly, the amount of aluminum, vanadium, titanium, and nitrogen in the disclosed steels are selected to prevent the formation of aluminum nitride while forming a sufficient amount vanadium carbide or carbonitride for improved wear resistance. To ensure that there is sufficient nitrogen to combine with vanadium and titanium, nitrogen concentration in the steel should be above 0.002 percent by weight.
In general, all embodiments of steel of the current disclosure have constituents with the ranges listed in Table 1. Table 2 lists the compositional ranges of four exemplary embodiments of the disclosed steel.
Although Table 2 specifically identifies four different embodiments of the disclosed steel (embodiments A, B, C, and D), it should be noted that this table is not intended as an exhaustive list of all possible embodiments of the disclosed steel. Instead, these embodiments are merely exemplary embodiments. For instance, other embodiments of the disclosed steel may include different ranges of constituents (within the range listed in Table 1) than that identified in Table 2. For instance, some embodiments of the disclosed steel may include a molybdenum concentration between, for example, 0.25 and 0.35. In some embodiments one or more constituents may have a concentration within the ranges listed under embodiment A (of Table 2), while other constituents may have a concentration within the ranges listed under another embodiment (embodiments B, C, and D). For example, in some embodiments, the concentration of carbon, manganese, and silicon may be within the ranges listed under embodiment A, the concentration of vanadium may be within the range listed under embodiment C, and the concentration of all other constituents may be within the ranges listed under embodiment B.
Any known process may be used to manufacture a steel wear component with the constituents described above. After manufacture, the steel components may be subject to quenching and tempering to achieve a desired hardness of the steel. Any suitable quenching and tempering operation that does not result in harmful decarburization, grain growth, or excessive distortion may be applied to achieve the desired properties. In some embodiments, wear components made of the disclosed steels may be heated to an austenitizing temperature of about 870° C. (1598° F.) for about an hour, and then quenched in water. In embodiments of the steel with higher molybdenum content, the components may be heated to a higher austenitizing temperature to ensure that alloy carbides in the steel are dissolved prior to quenching. After quenching, the steel components may be tempered by reheating the component for a sufficient length of time to permit temperature equalization of all sections. In some embodiments, the components may be reheated to about 200° C. (392° F.) for about an hour for tempering. After quenching and tempering, sonic embodiments of the disclosed steels may have a hardness between about 54-60 in the Rockwell scale (HRC) and a plane strain fracture toughness (measured in accordance with ASTM Test Method E) greater than or equal to about 70 MPa√m. In some embodiments, the fracture toughness may be between about 70-90 MPa√m.
To evaluate the impact of different constituents on the wear related properties of steel, several bushing 130 and sprocket 106 (see
In Table 3, sample 1 has a composition that has demonstrated good wear resistance during field testing. Therefore, sample 1 was included in the tests as a baseline case to compare the performance of other samples to the wear resistance of a known case. To study the impact of increased silicon on wear resistance, the concentration of silicon in sample 1 was increased to form sample 2. Similarly, to study the impact of chromium on wear resistance, the concentration of chromium in sample 2 was increased to form sample 3. And, to study the impact of vanadium on wear resistance, the concentration of vanadium in sample 3 was increased to form sample 4. Among samples 1-4, sample 4 is an embodiment of the current disclosure and has a composition corresponding to embodiment C of Table 2.
To replicate the wear conditions experienced by a component in an undercarriage 102 (see
As can be seen in
Hot hardness is the hardness of a material at elevated temperatures, and is a measure of the wear resistance, of the material at high temperatures. The higher the hardness of a material at high temperatures, the higher the expected wear resistance at those temperatures.
Although in the description above, components of a machine undercarriage were used to demonstrate the improved wear resistance of the disclosed steels, the wear resistant steels of the current disclosure may be used to fabricate any component or article that is subject to wear.
A wear resistant steel of the current disclosure may be beneficial for any component where improved wear resistance is desired. The steel may be especially beneficial for components that may be subject to severe abrasive wear conditions. Increased wear resistance may improve the durability of components operating in extreme wear conditions.
In contrast with common wear resistant steels known in the art, the disclosed steels of the current disclosure do not rely on chromium or nickel for their improved wear resistance. Instead, when the disclosed steels are subject to severe wear, the controlled amount of vanadium in the steel forms vanadium carbonitride particles that provide the increased wear resistance.
It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed wear resistant steel. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed wear resistant steel. It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents.
