WEAR RESISTANT HIGH TOUGHNESS STEEL

Abstract

A steel article is provided for improved wear resistance due to optimized hardness, toughness and temper resistance. In one exemplary embodiment, the steel article may have a composition including 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.

Description

TECHNICAL FIELD

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.

BACKGROUND

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.

SUMMARY OF THE INVENTION

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.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a tracked undercarriage of a machine;

FIG. 2 is a comparison of the measured wear rates of different steels used to fabricate the undercarriage of FIGS. 1; and

FIG. 3 is a comparison of the measured hot hardness of different steels used to fabricate the undercarriage of FIG. 1.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary machine 100 with a tracked undercarriage 102. The tracked undercarriage 102 may include a track chain 108 arranged around sprockets 106. A power source (electric motor, hydraulic motor, engine, etc.) may rotate the sprockets 106 and move the track chain 108 to propel the machine 100 at a worksite. As the machine 100 operates at the worksite, components in its undercarriage 102 may experience abrasive wear. For example, when the sprockets 106 turn the track chain 108, accumulated sand and dust may abrade the sprockets 106 and portions of the track chain 108 (such as, bushings 130) that engage with these sprockets 106.

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.

TABLE 1
Composition of disclosed steel in percent by weight.
Constituents                     Concentration by weight (%)
Carbon                           0.20-0.43
Manganese                        0.20-2.0
Silicon                          0.50-3.0
Chromium                         0.01-3.0
Molybdenum                       0.15-0.5
Vanadium                         0.43-2.5
Titanium                         0.01-0.25
Aluminum                         ≦0.02
Boron                            0.0001-0.010
Oxygen                           ≦0.005
Nitrogen                         0.002-0.025
Phosphorous                      ≦0.04
Sulphur                          ≦0.045
Iron and other residual elements Balance

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.

TABLE 2
Composition of exemplary embodiments of the disclosed
steel in percent by weight.
	Concentration by weight (%)
	Embodiment	Embodiment	Embodiment	Embodiment
Constituents	A	B	C	D
Carbon	0.25-0.43	0.35-0.43	0.38-0.43	0.41
Manganese	0.40-2.0	0.50-1.0	0.70-0.85	0.77
Silicon	0.50-2.0	1.45-1.8	1.45-1.60	1.52
Chromium	0.05-3.0	1.50-2.0	1.60-1.80	1.73
Molybdenum	0.20-0.50	0.20-0.40	0.30-0.40	0.34
Vanadium	0.45-1.5	0.45-1.3	0.45-1.0	0.46
Titanium	0.02-0.1	0.02-0.06	0.03-0.05	0.042
Aluminum	0.005-0.025	0.0075-0.02	0.01-0.02	0.015
Boron	0.0008-0.005	0.001-0.004	0.001-0.004	0.0027
Oxygen	≦0.005	≦0.003	≦0.003	≦0.003
Nitrogen	0.008-0.015	0.008-0.015	0.009-0.014	0.011
Phosphorous	≦0.04	≦0.04	≦0.01	≦0.01
Sulphur	≦0.045	≦0.045	≦0.01	≦0.005
Iron and others	Balance	Balance	Balance	Balance

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 FIG. 1) samples having different compositions were prepared and subject to wear tests. Table 3 compares the composition of these samples in percent by weight.

TABLE 3
Composition of samples.
				Embodiment C
				of Table 2
		Sample 2	Sample 3	Sample 4
		(Sample	(Sample	(Sample
	Sample 1	1 + Si)	1 + Si, Cr)	1 + Si, Cr, V)
Carbon	0.38-0.43	0.38-0.43	0.38-0.43	0.38-0.43
Manganese	0.7-0.85	0.7-0.85	0.7-0.85	0.7-0.85
Phosphorous	0.001-0.01	0.001-0.01	0.001-0.01	0.001-0.01
Sulphur	0.002-0.005	0.002-0.005	0.002-0.005	0.002-0.005
Silicon	0.59	1.50-1.60	1.50-1.60	1.50-1.60
Chromium	0.01-0.13	0.01-0.13	1.60-1.80	1.60-1.80
Molybdenum	0.30-0.40	0.30-0.40	0.30-0.40	0.30-0.40
Vanadium	0.07-0.15	0.07-0.15	0.07-0.15	0.45-1.0
Boron	0.001-0.004	0.001-0.004	0.001-0.004	0.001-0.004
Aluminum	0.01-0.02	0.01-0.02	0.01-0.02	0.01-0.02
Nitrogen	0.009-0.014	0.009-0.014	0.009-0.014	0.009-0.014
Titanium	0.03-0.05	0.03-0.05	0.03-0.05	0.03-0.05

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 FIG. 1), a custom wear test was conducted on the bushings 130 and sprockets 106 made of the different samples listed in Table 3. In this custom test, a cyclic sliding motion was applied between a bushing 130 and a sprocket 106 under a contact force of about 75.6 kN. During, the test, a mixture of sand and water was injected between the sliding surfaces, To measure the weight loss due to wear, the bushing weight loss was measured every 1000 cycles, and the mating sprocket weight loss was measured at the completion of each test.

FIG. 2 is a bar graph that compares the observed wear rate of the different bushing and sprocket samples. As previously indicated, it is known that the steel of sample 1 possess good wear resistance properties. Therefore, to indicate the relative change in the observed wear rates of samples 2-4 compared to sample 1. FIG. 2 shows the observed wear rates for the different samples normalized with the observed wear rate of sample 1. That is, the observed wear rates of the different sprocket samples were divided by the observed wear rate of the sprocket of sample 1, and the observed wear rates of the different bushing samples were divided by the observed wear rate of the bushing of sample 1. A decrease in the observed wear rate indicates an improvement in wear resistance. Therefore, in FIG. 2, a decrease in the wear rate of a sample compared to sample 1 indicates that the wear resistance of that sample is better than. that of sample 1.

As can be seen in FIG. 2, the observed wear rates of samples 2-4 are lower than that of sample 1. That is, the addition of silicon, chromium, and vanadium decreases the wear rate and increases the wear resistance of the components. The greatest reduction in wear rate is seen in the case of sample 4. Therefore, the addition of vanadium provides the most reduction in wear rate. Compared to sample 1, the observed decrease in wear rate for a sprocket of sample 4 is about 48%, and for a bushing run against a sprocket of sample 4 is about 24%. The difference in the observed decrease in wear rates between the bushing and the sprocket may be because of the differences in wear conditions (local temperature, etc.) between the two components. Compared to the effect of vanadium, the addition of silicon provides a smaller benefit to the sprocket wear rate, and the addition of chromium provides a smaller benefit to the bushing wear rate. However, as discussed below, silicon and chromium provide significant benefits to other wear related properties, such as hot hardness, of the steel.

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. FIG. 3 compares the measured hot hardness of the four samples listed in Table 3. The Y-axis of FIG. 3 is the observed hardness of the samples using a Vickers hardness tester at a 10 kg load (as per ASTM E92-82 (2003)). As evident from FIG. 3, although the hardness of all the samples decreases with increasing temperature, the hot hardness of samples 2-4 is higher than that of sample 1 at elevated temperatures. That is, the addition of silicon, chromium, and vanadium increases the high temperature wear resistance of the disclosed steels at high temperatures. As seen in FIG. 3, the addition of silicon (see sample 2) increases the hardness of the steel at temperatures between about 100 and 500° C., and the addition of chromium (see sample 3) extends the increase in hot hardness to about 700° C., Therefore, chromium and silicon increase the hot hardness of the steel at higher 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.

INDUSTRIAL APPLICABILITY

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.

Claims

1. A steel article, comprising:

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.

2. The steel article of claim 1, further including about 0.01 to about 0.25 percent by weight of titanium, and about 0.002 to about 0.025 percent by weight of nitrogen.

3. The steel article of claim 1, further including about 0.15 to about 0.5 percent by weight of molybdenum, and about 0,40 to about 2.0 percent by weight of manganese.

4. The steel article of claim 1, further including less than or equal to about 0.02 percent by weight of aluminum, and about 0.0001 to about 0.010 percent by weight of boron.

5. The steel article of claim 1, further including less than or equal to about 0.005 percent by weight of oxygen.

6. The steel article of claim 1, further including less than or equal to about 0.045 percent by weight of sulphur, and less than or equal to about 0.04 percent by weight of phosphorous.

7. The steel article of claim 1, wherein the steel article has a microstructure that is martensitic, a hardness of between about 54-60 HRC, and a fracture toughness of greater than or equal to about 70 Mpa√m.

8. The steel article of claim 1, wherein the concentration of vanadium is between 0.45 to about 1.0 percent by weight.

9. The steel article of claim 1, wherein the article is part of a machine undercarriage.

10. A steel article, comprising;

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;

about 1.45 to about 1.8 percent by weight of silicon; and

a martensitic microstructure.

11. The steel article of claim 10, further including about 0.5 to about 1.0 percent by weight of manganese, and about 1.5 to about 2.0 percent by weight of chromium.

12. The steel article of claim 10, further including about 0.008 to about 0.015 percent by weight of nitrogen.

13. The steel article of claim 10, further including about 0.0075 to about 0.02 percent by weight of aluminum.

14. The steel article of claim 10, Wherein the concentration of silicon is about 1.45 to about 1.6 percent by weight, and the concentration of titanium is about 0.03 to about 0.05 percent by weight.

15. The steel article of claim 10, wherein the steel article has a microstructure that is martensitic and a hardness between about 54-60 HRC.

16. A steel article, comprising:

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.

17. The steel article of claim 16, further including about 0.03 to about 0.05 percent by weight of titanium, and about 0.01 to about 0.02 percent by weight of aluminum.

18. The steel article of claim 16, further including about 0.70 to about 0.85 percent by weight of manganese, and about 0.30 to about 0.40 percent by weight of molybdenum.

19. The steel article of claim 16, further including about 0.001 to about 0.004 percent by weight of boron.

20. The steel article of claim 16, further including less than or equal to about 0.003 percent by weight of oxygen, less than or equal to about 0.01 percent by weight of sulphur, and less than or equal to about 0.01 percent by weight of phosphorous.