SINTERED STEEL ALLOY FOR WEAR RESISTANCE AT HIGH TEMPERATURES AND FABRICATION METHOD OF VALVE-SEAT USING THE SAME

Abstract

Disclosed is a sintered steel alloy for wear resistance at high temperatures, which is applied to a valve seat of an internal combustion engine including an automobile. The sintered steel alloy includes: 10.0 to 14.0 parts by weight of cobalt powder; 5.0 to 9.0 parts by weight of molybdenum powder; 1.5 to 4.1 parts by weight of chromium powder; 0.7 to 1.3 parts by weight of carbon powder; 1.0 to 1.8 parts by weight of manganese powder; 0.4 to 1.2 parts by weight of silicon powder; 0.2 to 0.8 parts by weight of sulfur powder; and 0.1 to 0.7 parts by weight of vanadium powder, based on 100 parts by weight of iron powder, and thus a service life of the valve seat is extended.

Claims

1. A sintered steel alloy for wear resistance at high temperatures, comprising: 10.0 to 14.0 parts by weight of cobalt powder; 5.0 to 9.0 parts by weight of molybdenum powder; 1.5 to 4.1 parts by weight of chromium powder; 0.7 to 1.3 parts by weight of carbon powder; 1.0 to 1.8 parts by weight of manganese powder; 0.4 to 1.2 parts by weight of silicon powder; 0.2 to 0.8 parts by weight of sulfur powder; and 0.1 to 0.7 parts by weight of vanadium powder, based on 100 parts by weight of iron powder.

2. The sintered steel alloy of claim 1, wherein 10.0 to 20.0 parts by weight of copper powder based on 100 parts by weight of the iron powder is further added as an infiltrate into a composition of the sintered steel alloy.

3. A method of manufacturing a valve seat using a sintered steel alloy for wear resistance at high temperatures, the method comprising: a mixing operation of evenly mixing the sintered steel alloy described in claim 1; a pressurizing operation of pressurizing a resulting mixture formed in the mixing operation at a set pressure; a sintering operation of sintering a resulting molded body formed in the pressurizing operation along with the infiltrate described in claim 2 to infiltrate copper into the molded body; a low temperature treatment operation of treating a resulting sintered body formed in the sintering operation at low temperatures to change residual austenite into martensite; and a heat treatment operation of tempering a resulting low temperature treated body formed in the low temperature treatment operation to remove a residual stress therefrom.

4. The method of claim 3, wherein the pressurizing operation includes pressurizing the composition of the valve seat at a surface pressure of 6 to 10 tons/cm.sup.3.

5. The method of claim 3, wherein a final product after the heat treatment operation has a hardness (HRA) of 71 to 81.

6. The method of claim 3, wherein a final product after the heat treatment operation has a density (g/cm.sup.3) of 7.4 to 8.1.

7. The method of claim 3, wherein the molded body is sintered and copper-infiltrated in a temperature range of 112020 C. for 3010 minutes in the sintering operation.

8. The method of claim 3, wherein the low temperature treatment operation includes cooling a sintered body formed in the sintering operation in a temperature range of 12010 C. for 205 minutes.

9. The method of claim 3, wherein the heat treatment operation includes heating a low temperature heated body formed in the low temperature treatment operation in a temperature range of 60020 C. for 12010 minutes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The above and other objects, features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail exemplary embodiments thereof with reference to the accompanying drawings, in which:

[0020] FIG. 1 is a diagram illustrating an installation state of a valve seat according to the related art;

[0021] FIG. 2 is an optical microscope picture (500 magnification) of Example 1 according to the present invention;

[0022] FIG. 3 is an optical microscope picture (500 magnification) of Example 2 according to the present invention; and

[0023] FIG. 4 is an optical microscope picture (500 magnification) of Example 3 according to the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0024] Hereinafter, a composition according to the present invention will be described.

[0025] As shown in FIGS. 2 to 4, a sintered steel alloy according to the present invention represents a sintered alloy applied to an internal combustion engine, and particularly applied to a valve seat that maintains airtightness of intake/exhaust valves in an opening/closing process of the intake valve and the exhaust valve and also minimizes damage in a contact process with combustion products.

[0026] Although only application of the sintered steel alloy for wear resistance at high temperatures to a valve seat is described here, it can also naturally be applied to a cylinder liner, a valve guide or the like within the same technical scope.

[0027] The composition of the sintered steel alloy according to the present invention includes a sintered alloy, in which iron powder is a main component, as well as an infiltrate, which is infiltrated into the sintered alloy, and the sintered alloy includes: 10.0 to 14.0 parts by weight of cobalt powder; 5.0 to 9.0 parts by weight of molybdenum powder; 1.5 to 4.1 parts by weight of chromium powder; 0.7 to 1.3 parts by weight of carbon powder; 1.0 to 1.8 parts by weight of manganese powder; 0.4 to 1.2 parts by weight of silicon powder; 0.2 to 0.8 parts by weight of sulfur powder; and 0.1 to 0.7 parts by weight of vanadium powder, based on 100 parts by weight of the iron powder. The infiltrate is copper powder which amounts to 10.0 to 20.0 parts by weight based on 100 parts by weight of the iron powder.

[0028] At this time, the infiltrate is infiltrated into the sintered alloy so that, in the case of the composition of the sintered steel alloy, complex carbides such as a cobalt-based hard particle phase, a molybdenum-based hard particle phase or a chromium-based hard particle phase are evenly dispersed in a martensite matrix, particularly an intermetallic compound between the manganese and sulfur or the manganese and carbon serves as a lubricant, and particles are refined by means of the silicon or the vanadium.

[0029] In other words, a reason for infiltrating the infiltrate into the sintered alloy to manufacture the valve seat is to further increase heat resistance at high temperatures, wear resistance at high temperatures and corrosion resistance at contact portions with the intake/exhaust valves.

[0030] The valve seat manufactured of the composition of the sintered steel alloy (hereinafter, collectively referred to as the composition) is a material of high strength, in which its final product has a hardness (HRA) of at least 71 to 81 and maintains a density (g/cm.sup.3) of at least 7.4 to 8.1.

[0031] In the meantime, the cobalt (Co) reacts with iron, molybdenum or carbon to precipitate complex carbides, and thus it is evenly dispersed in the matrix and contributes to wear resistance, while a part of the cobalt is solid-solved in the matrix, so that heat resistance is increased. If a content of the cobalt is less than 10.0 parts by weight, the precipitated particles and the amount of solid solubility of the matrix are decreased, and thus wear resistance and heat resistance deteriorate. If a content of the cobalt is more than 14.0 parts by weight, a matrix metal becomes vulnerable due to an excess of precipitated particles, and thus machinability deteriorates.

[0032] Also, the molybdenum (Mo) is solid-solved in the matrix or forms an intermetallic compound in a complex carbide state, and thus wear resistance and heat resistance are improved. If a content of the molybdenum is less than 5.0 parts by weight, the amount of solid solubility of the matrix and intermetallic compounds are decreased, and thus wear resistance and heat resistance deteriorate. If a content of the molybdenum is more than 9.0 parts by weight, the amount of solid solubility of the matrix metal is excessive, and thus causes the matrix metal to become vulnerable.

[0033] Further, the chromium (Cr) is a component that reacts with the carbon within the matrix to form complex carbides and improve wear resistance, and is also solid-solved in the matrix to improve heat resistance. A content thereof may be 1.5 parts by weight to 4.1 parts by weight.

[0034] If a content of the chromium is less than 1.5 parts by weight, an amount of complex carbides is decreased, and thus wear resistance and heat resistance deteriorate. If a content of the chromium is more than 4.1 parts by weight, the amount of solid solubility of the matrix metal is excessive, and thus the product becomes vulnerable.

[0035] Moreover, the carbon (C) is a component that is solid-solved or dispersed in the matrix to reinforce the matrix, and that also reacts with the cobalt, chromium or molybdenum to form complex carbides. The carbon (C) performs a function of increasing the strength and hardness of the matrix and also increasing its wear resistance or heat resistance.

[0036] If a content of the carbon is less than 0.7 parts by weight, ferrite is excessively formed in the matrix metal along with pearlite, and thus the matrix is softened and strength and wear resistance deteriorate. If a content of the carbon is more than 1.3 parts by weight, a carbon residue remaining after forming pearlite forms cementite, and thus the matrix steel becomes vulnerable.

[0037] Also, the manganese (Mn) is a component that reacts with sulfur present in the iron matrix to form MnS and improves self-lubrication. If a content of the manganese is less than 1.0 part by weight, the MnS is formed, and thus a function of self-lubrication deteriorates. If a content of the manganese is more than 1.8 parts by weight, there is concern of segregation in addition to forming of the MnS.

[0038] Further, the silicon (Si) is a component that is added for the purpose of adjusting and refining a crystal grain of the iron matrix and also improving wear resistance or heat resistance. A content of the silicon may be 0.4 to 1.2 parts by weight.

[0039] Moreover, the sulfur (S) is a component that is added into the iron matrix and dispersed in a grain boundary of the matrix in the form of MnS. The MnS is not decomposed as a compound at high temperatures but maintains a stabilized state in a grain boundary of a sintered body after going through a sintering process and deteriorates a friction coefficient in a process of processing the final product, and thus machinability is increased. In particular, a content of the sulfur may be 0.2 to 0.8 parts by weight.

[0040] The manganese and the sulfur may be mixed at a ratio of approximately 6:4 so that efficiency is increased according to forming of the MnS.

[0041] If a content of the MnS (Mn+S) is less than 1.25 parts by weight, it plays an insignificant role in remaining in the matrix of the sintered body. If a content of the MnS (Mn+S) is more than 2.6 parts by weight, the strength of the matrix is weakened, thus causing damage to the valve seat.

[0042] Also, the vanadium (V) is a component that is added for the purpose of adjusting and refining a crystal grain of the iron matrix and also improving heat resistance. A content of the vanadium may be 0.1 to 0.7 parts by weight. If the vanadium exceeds the required value, the crystal grain is coarsened, thus causing destruction of the final product of the valve seat. Hereinafter, a manufacturing method according to the present invention will be described.

[0043] First of all, the present invention includes: a mixing operation of mixing the composition to manufacture a mixture; a pressurizing operation of pressurizing the mixture; a sintering operation of sintering a resulting body; a low temperature treatment operation of changing residual austenite into martensite; and a heat treatment operation of removing a residual stress therefrom.

[0044] Also, the mixing operation is an operation of evenly mixing the steel alloy powder, a high speed tool steel powder, a superalloy powder, a manganese sulfide powder, a carbon power and the like in accordance with the required amount of each in a mixer.

[0045] Further, the pressurizing operation is an operation of compressing a mixture formed in the mixing operation to mold at a density suitable for the valve seat, and is also an operation of pressurizing the mixture at a surface pressure of 6 to 10 tons/cm.sup.2 to improve precision.

[0046] Moreover, the sintering operation is an operation of sintering a molded body molded in the pressurizing operation in a temperature range of 112020 C. for 3010 minutes to form a sintered body, and includes an operation of infiltrating 10.0 to 20.0 parts by weight of the copper powder into the sintered body.

[0047] If a sintering temperature is less than 1100 C. in the sintering operation, powder particles are not smoothly dispersed and a matrix structure is weakened. If the sintering temperature is more than 1140 C., a crystal grain is coarsened and mechanical properties deteriorate.

[0048] In the sintering operation, sintering is performed in a state in which 10.0 to 20.0 parts by weight of the copper powder are inserted and copper particles are infiltrated into the pores of the matrix structure, so that the strength of the matrix is reinforced and a lubrication role is also increased.

[0049] Also, the low temperature treatment operation is an operation of changing residual austenite into martensite by cooling the sintered body formed in the sintering operation in a temperature range of 12010 C. for 205 minutes, so that the aging of the composition is prevented from being changed, a mechanical property is improved, and structural stability is induced.

[0050] Further, the heat treatment operation is an operation of tempering a low temperature treated body formed in the low temperature treatment operation to remove a residual stress therefrom, and is also an operation of heating in a temperature range of 60020 C. for 12010 minutes to give toughness to the matrix structure.

[0051] Moreover, as a post-processing operation of the heat treatment operation, an operation of removing foreign materials like burrs from the final product and performing a mechanical processing process such as forging or polishing to obtain a completed product may be included, but description thereof will be omitted herein.

[0052] The completed product of the valve seat, having gone through the operations above, has a hardness (HRA) of about 71 to 81 and a density (g/cm.sup.3) of about 7.4 to 8.1, and it can be seen that it provides appropriate hardness and density to be used with liquid fuels and solid fuels.

[0053] Hereinafter, Examples of the present invention will be described.

TABLE-US-00001 TABLE 1 Components Example 1 Example 2 Example 3 (parts by weight) (Sample 1) (Sample 2) (Sample 3) Cobalt powder 12 14 10 Molybdenum powder 7 9 5 Chromium powder 3 4.1 1.5 Carbon powder 1.0 1.3 0.7 Manganese powder 1.5 1.8 1.0 Silicon powder 1.0 1.2 0.4 Sulfur powder 0.5 0.8 0.2 Vanadium powder 0.5 0.7 0.1 Copper powder 15 15 15 Iron powder 100 100 100

[0054] First of all, a mixture was manufactured by mixing compositions having the composition ratios of Examples 1 to 3 of Table 1 in a mixer, and the mixture was pressurized at a surface pressure of 10 tons/cm.sup.3, and then sintered and infiltrated at 1120 C. for 30 minutes in a heat treatment furnace.

[0055] Then, a low temperature treated body was manufactured by quenching a sintered body that was subjected to sintering and copper infiltration in the sintering operation in a temperature range of 120 C. for 20 minutes, and then the low temperature treated body was heated in a temperature range of 600 C. for 120 minutes and tempered.

[0056] Then, a heat treated body that was subjected to the heat treatment operation was drawn out, Samples 1 to 3 were manufactured, and then an abrasion loss was measured using an abrasion tester (Rig Tester, nitrogen atmosphere; 0.2 mm Offset; SUH35+Tuff valve, speed: 3,500 rpm, temperature: 350 C., time: 2 hours). From the results, in the case of Examples 1 to 3 (Samples 1 to 3), it can be seen that an overall abrasion loss of the valve and valve seat amounts to 48 m on average, which is appropriate for a material of the valve seat.

[0057] In other words, as shown in FIGS. 2 to 4, Samples 1 to 3 showed similar density and hardness values, and particularly, it can be seen that hard particles and elements for improving processability were evenly distributed within the martensite matrix structure.

[0058] In particular, it can be seen that the wear resistance and heat resistance of the valve seat were increased when the copper alloy was filled into the pores of the matrix structure.

[0059] As stated above, the present invention is not limited to the exemplary embodiments described above, and it will be apparent to those skilled in the art that various modifications can be made without departing from the spirit and scope of the invention as defined by the appended claims, and such modifications fall within the scope of the present invention.