Patent Publication Number: US-2003233910-A1

Title: Sintered alloy having wear resistance for valve seat and method for manufacturing the same

Description:
FIELD OF THE INVENTION  
       [0001] The present invention relates to sintered alloy having an excellent wear resistance for a valve seat and a method for manufacturing the same. More particularly, the present invention relates to a sintered lead (Pb) impregnated alloy having an excellent wear resistance for an valve seat, which is produced by a sub-zero treatment for a metal power containing iron (Fe) as a main component, carbon (C), optionally silicon (Si), chromium (Cr), molybdenum (Mo), cobalt (Co), maganese (Mn), vanadium (V) and tungsten (W) so that amount of metallic particles separated from a base matrix decreases and a size of the separated metallic particle becomes small when an abrasion of the sintered alloy is in process, whereupon a wear resistance and impact resistance are improved and a self-lubricity and a workability is enhanced, and method of manufacturing the same.  
       BACKGROUND OF THE INVENTION  
       [0002] A conventional sintered alloy with wear resistance used for a valve seat contains Fe as a main component, C of from 0.4 to 1.0 wt %, Si of 0.1 to 1.0 wt %, Cr of 0.5 to 2.0 wt %, Mo of from 6.0 to 10.0 wt %, Co of from 6.0 to 15.0 wt %, and lead (Pb) of from 6.0 to 18.0 wt %.  
       [0003] The processes as follows manufacture such a sintered alloy having an excellent wear resistance used for a valve seat. First of all, the metal powders as above except Pb are mixed and then a surface pressure of from 4 to 8 ton/ cm 3  is applied to the mixture of metal power. Under a reducing atmosphere, a preliminary sinter process is performed at a temperature of from 750-800° C. for 40 minutes and a forging process is performed at a surface pressure of 7 to 10 ton/cm 3 .  
       [0004] Thereafter, a main sinter process is performed at a temperature from 1,100 to 1,1400° C. for 30-50 minutes under hydrogen atmosphere and then Pb is impregnated at a temperature from 400 to 450° C. for 10-30 minutes. Then a barrel process is performed at same temperature for 80-110 minutes. The sintered alloy, having an excellent wear resistance when used for an automobile engine valve seat, is thereby produced.  
       [0005] However, the sintered alloy having components as described above has a microstructure characteristic in which giant metal particles are dispersed in the base matrix. Such giant metal particles cause local weaknesses, and a crack can form when an external impact is applied. Therefore, the impact resistance of the alloy is poor. In turn, the wear resistance will be deteriorated because the metal particles fall away from the abrasive cracked surface. Further, there is problem that compressed gas in a cylinder is leaked via these cracks.  
       [0006] The cost and productivity for producing a conventional sintered alloy with wear resistance for a valve seat is decreased due to a great deal of process steps involved, for example the separate impregnation with lead, and the impact resistance and wear resistance are deteriorated by giant particles dispersed within a base matrix. Additionally, a leakage of compressed air can occur. There is a recognized need in the industry for improved alloys for valve seats.  
       SUMMARY OF THE INVENTION  
       [0007] The present invention is contrived to solve the foregoing problems. It is an object of the present invention to provide a sintered alloy for a valve seat, and also the valve seat made from the sintered alloy, having an improved wear resistance and a workability. The sintered alloy contains iron (Fe) as a main component, carbon (C), optionally silicon (Si), chromium (Cr), molybdenum (Mo), cobalt (Co), maganese (Mn), lead (Pb), vanadium (V) and tungsten (W). It is also an object of this invention to provide a method of manufacturing the sintered alloy and also the valves made from the sintered alloy.  
       [0008] The present invention provides a sintered alloy with excellent wear resistance used for a valve seat. As used herein, the compositions of metals except lead are given as the composition in the sintered alloy, and the composition of lead is given as the amount in the lead-impregnated sintered alloy composition. The impregnated sintered alloy comprises:  
       [0009] A) between about 85 parts by weight and a 90 parts by weight, for example 88 parts by weight, based on 100 parts by weight of the lead-impregnated sintered alloy composition, of a sinterable metal powder comprising:  
       [0010] Fe as a main component, comprising for example 63.9% to 85.8% of the sintered powder;  
       [0011] C of 1.2 to 1.7 wt %, for example about 1.4%;  
       [0012] Cr of 3.5 to 5.0 wt %, for example about 4%;  
       [0013] Mo of 2.0 to 4.0 wt %, for example about 3%;  
       [0014] V of 3.0 to 5.0 wt %, for example about 3.5% to about 4%;  
       [0015] W of 7.0 to 10.0 wt %, for example about 8.5% to about 9%;  
       [0016] Co of 2.0 to 3.5 wt %, for example about 2.5% to about 3%;  
       [0017] boron nitride of 0.1 to 1.0 wt %, for example about 0.4% to about 0.6%;  
       [0018] optionally Si of 0.2 to 0.6 wt %, in one embodiment of 0.2 to 0.4 wt %, for example about 0.3%;  
       [0019] S of 0.2 to 0.4 wt %, for example about 0.3%; and  
       [0020] Mn of 0.2 to 0.5 wt %, for example about 0.3%; and  
       [0021] B) Pb of 10.0 parts by weight to 15.0 parts by weight, for example about 12 parts by weight, wherein the PB is impregnated into the sintered metal powder.  
       [0022] A method for manufacturing the sintered alloy having an excellent wear resistance for a valve comprises;  
       [0023] mixing 85 parts to about 90 parts by weight of powders of: Fe as a main component, having for example 63.9% to 85.8%, C of 1.2 to 1.7 wt %, Cr of 3.5 to 5.0 wt %, Mo of 2.0 to 4.0 wt %, V of 3.0 to 5.0 wt %, W of 7.0 to 10.0 wt %, Co of 2.0 to 3.5 wt %, boron nitride of 0.1 to 1.0 wt %, S of 0.2 to 0.4 wt %, and Mn of 0.2 to 0.5 wt %, and applying a surface pressure of 5 to 8 ton/cm 3  to obtain a mixed metal powder;  
       [0024] sintering the mixed metal powder, advantageously at a temperature from 1,140 to 1,180° C., and then cooling, for example by air or under a gas atmosphere, to form a sintered alloy;  
       [0025] further cooling the sintered alloy and performing a sub-zero treatment described herein at a temperature from −160 to −200° C.; and  
       [0026] impregnating at a temperature from 450 to 550° C. the sub-zero-treated sintered alloy with Pb so that the lead content of the impregnated sintered alloy is from 10.0 to 15.0 wt %; and  
       [0027] performing a barrel process on the sintered alloy impregnated with the Pb at a temperature from 450 to 550° C.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0028] Hereinafter, the sintered alloy according to the preferred embodiment of the present invention will be explained in more detail. A method for manufacturing a sintered alloy according to the present invention is described in detail by steps hereunder.  
       [0029] In the alloy of the present invention, Fe is a main component and the content of each alloy steel component is specified in order to improve a wear resistance and workability.  
       [0030] In a first step, Fe as a main component, C of 1.2 to 1.7 wt %, Cr of 3.5 to 5.0 wt %, Mo of 2.0 to 4.0 wt %, V of 3.0 to 5.0 wt %, W of 7.0 to 10.0 wt %, Co of 2.0 to 3.5 wt %, boron nitride of 0.1 to 1.0 wt %, S of 0.2 to 0.4 wt %, Mn of 0.2 to 0.5 wt %, and optionally S of 0.2 to 0.4% are mixed and then a surface pressure from 5 to 8 ton/cm 3  is applied.  
       [0031] The mechanical property of the sintered alloy varies significantly with a content of C. A content of C is used in the range from 1.2 to 1.7 wt % against the total weight of the composition of the sintered alloy. When the content of C is less than 1.2 wt %, the strength and hardness are insufficient. Additionally, when the content of C is more than 1.7 wt %, the tensile strength and a hardness of the sintered alloy again decrease.  
       [0032] Cr is added in order to increase a wear resistance and a cutting ability. Cr is used in the range from 3.5 to 5.0 wt % against the total weight of the composition of the sintered alloy. When the content of Cr is less than 3.5 wt %, the desired wear resistance and corrosion resistance cannot be obtained. On the other hand, when the content of Cr is more than 5.0 wt %, the processability is reduced.  
       [0033] Mo is added in order to increase a cutting ability, the tensile strength at a high temperature, and the hardness. Mo is used in the range from 2.0 to 4.0 wt % against the total weight of the composition of the sintered alloy. When the content of Mo is less or more than the above range, strength and hardness are not increased.  
       [0034] V is added in order to adjust a grain. V is used in the range from 3.0 to 5.0 wt % against the total weight of the composition of the sintered alloy. When the content of V is less than 3.0 wt %, wear resistance effect is reduced. When the content of V is more than 5.0 wt %, increased performance is not economical.  
       [0035] W is added in order to increase a tensile strength at a high temperature and hardness. W according to the present invention is used in the range from 7.0 to 10.0 wt % against the total weight of the composition of the sintered alloy. When the content of W is less than 7.0 wt %, a small quantity of carbide is formed so that a wear resistance becomes lowered. When the content of W is more than 10.0 wt %, the desired physical property is not increased any more.  
       [0036] Co used in the present invention is added in order to increase the heat resistance and the hardness at a high temperature. Co is used in the range from 2.0 to 3.5 wt % against the total weight of the composition of the sintered alloy. When the content of Co is less than 2.0 wt %, a heat resistance effect is reduced. When the content of Co is more than 3.5 wt %, increased performance is not economical.  
       [0037] Boron nitride used in the present invention is added in order to increase a wear resistance. The boron nitride is used in the range from 0.1 to 1.0 wt % against the total weight of the composition of the sintered alloy. When the content of the boron nitride is less than 0.1 wt %, a wear resistance effect is reduced. When the content of the boron nitride is more than 1.0 wt %, the composition becomes weak.  
       [0038] Si used in the present invention is added as a de-oxidizer. Si prevents grain carbides from precipitating from grain boundaries during manufacturing process. Si plays a role for decreasing a grain oxide layer. On the other hand, there is a trend that Si makes segregation in the alloy, and also becomes a silicon oxide which exists in the steel and forms a grain oxide layer so that a content of Si has to be limited. In the present invention, Si is used in the range from 0.2 to 0.6 wt %, for example from 0.2 to 0.4%, against the total weight of the composition of the sintered alloy. When the content of Si is less than 0.2 %, by weight, an effect of the de-oxidizer cannot be obtained enough. When the content of Si is more than 0.6 wt %, it is not desirable since a great deal of segregation is formed in the alloy.  
       [0039] Mn used in the present invention is added in order to combine with a very small amount S(surfur) which may be present or be added to obtain MnS. Mn is used in the range from 0.2 to 0.5 wt % against the total weight of the composition of the sintered alloy. When the content of Mn is less than 0.2 wt %, it is not desirable as it is difficult to have a self-lubricity.  
       [0040] In a second step, the mixed metal powder is sintered. Advantageously, sintering is done at a temperature from about 1,140 to about 1,180° C., preferably from 1,140 to 1,180° C., for example 1,160° C., for about 30 to about 50 minutes to form the sintered alloy. Then the sintered alloy product is cooled by air. When the sintering process is performed at lower than 1,140° C., the powder particles is not dispersed uniformly so that the base matrix becomes weaken. When the sintering process is performed at higher than 1,180° C., it is not desirable since the grain increases in size so that a mechanical property is deteriorated.  
       [0041] In a third step, the sub-zero treatment for the sintered alloy is performed at a temperature from about −200 to about −160° C. for about 5 to about 20 minutes. This sub-zero treatment favors a transformation of the alloy to reduce a residual austenite so that a mechanical property of the alloy can be improved. The inventors surprisingly found that this sub-zero treatment provides not only shortening of the prior preliminary and main sintering processes for the sintered alloy, and but also the sintered alloy has superior physical property.  
       [0042] In fourth and fifth steps, An impregnating, lubricating metal, for example Pb, is impregnated into the sub-zero treated sintered alloy at a temperature from about 450° C. to about 550° C. for about 30 to about 50 minutes and then the barrel process is performed at the same temperature for about 80 to about 100 minutes.  
       [0043] Pb provides the self-lubricity to the lead-impregnated sintered alloy composition and thus, it is possible to use for dried atmosphere fuel. Pb of the present invention is used in the range from 10.0 to 15.0 wt % against the total weight of the composition of the sintered alloy. When the content of Pb is less than 10.0 wt %, a great deal of unfilled pores remain. If the content of Pb is more than 15.0 wt %, it is not desirable since surplus Pb is precipitated on the surface after impregnating.  
       [0044] The sintered alloy has an excellent wear resistance for the automotive valve manufactured through the processes described. This is the result of a surface characteristic of the lead-impregnated sintered alloy in which micro spherical particles are dispersed uniformly in a base matrix of what can be characterized as a “high speed steel” metallic powder. A metal powder of a high speed steel containing C, Cr, Mo, V and W increases a cutting ability (and wear resistance) and improve a surface property. The excellent abrasion resistance when an abrasion of alloy is in process is in part due to the very small size of the separated carbide and other hard particles within the sintered alloy.  
       [0045] Further, the alloy of the present invention has a continuation property since dispersed metallic powders are extremely fine and the powders having a lower hardness are mixed so that the workability can be enhanced during manufacturing process. The powders can be admixed in a continuous process. Also, the alloy can renew itself as it is worn, as new hard particles are subsequently exposed.  
       [0046] Advantageously the majority of the grains of the sinterable alloy powder comprise each of the metals in the alloy, most preferably within the prescribed concentration limits. In a less preferred embodiment, some or all grains do not comprise each of the metals in the alloy, but an aliquot of the well-mixed powder having many grains comprise each of the metals in the alloy, most preferably within the prescribed concentration limits.  
       [0047] Additionally, valves and other items made from the lead-impregnated sintered alloy can be used for dried atmosphere fuel burning due to the self-lubricity obtained by the impregnation of Pb.  
       [0048] The present invention will be described in more detail taken in conjunction with the examples, however, the present invention is not limited by the examples. 
     
    
    
     EXAMPLES 1-2 AND COMPARATIVE EXAMPLES 1-2  
     [0049] The sintered alloy for the valve seat is manufactured by mixing each component and each content thereof are shown in Table 1. In each case, the mixed powder was formed under a pressure of 7 ton/cm 3  and then was sintered at 1,170° C. for 40 minutes.  
     [0050] Then, for Examples 1 and 2, the sub-zero treatment was performed at a temperature of −180° C. for 10 minutes.  
     [0051] A tempering operation was then carried out for both the Examples and the Comparative Examples at a temperature of 500° C. for 40 minutes after impregnating Pb. Then, the barrel process is performed at a temperature of 430° C. for 90 minutes to manufacture the desired lead-impregnated sintered alloy. The lead-impregnated sintered alloy of Comparative example was obtained by the same procedure of Example except skipping the sub-zero treatment.  
     [0052] [TESTING METHODS] 
     [0053] An amount abrasion from a pre-determined object made of the sintered alloy and having each component and each content as shown in Table 1, and manufactured by the process of the Example and comparative example, respectively, is measured and a result is shown in Table 1.  
     [0054] The Amount of abrasion test was performed by a simulation Rig tester wherein the CAM rotated at 2,500 rpm, the temperature was 400° C., the elapsed time was 10 hours, and the used fuel was LPG.  
     [0055] The result is the amount of the object that was abraded away during the 10 hour simulation Rig tester run. Compared with the comparative example, an amount of abrasion of the sintered alloy according the example of the present invention is about 25 to 28% less, as shown in Table 1.  
     [0056] As mentioned above, according to the present invention, a sintered alloy for an valve seat is modified by a sub-zero treatment for a metal power containing iron (Fe) as a main component, carbon (C), chromium (Cr), molybdenum (Mo), cobalt (Co), maganese (Mn), lead (Pb), vanadium (V), tungsten (W), optionally silicon (Si), and optionally sulfur (S), so that amount of metallic particles separated from a base matrix decreases and a size of the separated metallic particle becomes small when an abrasion of the sintered alloy is in process, thereupon a wear resistance and impact resistance are improved and a self-lubricity and a workability is enhanced.  
                               TABLE 1                                   Com.   Com.       Components   Ex. 1   Ex. 2   Ex. 1   Ex. 2                                                        Composition   C   1.4   1.4   0.8   0.8       (wt %)   Cr   4   4   1   1       BN   Mo   3   3   8   10           V   3.5   4   —   —           W   8.5   9   —   —           Si   —   —   0.5   0.5           Co   2.5   3   10   10           Mn   0.3   0.3   —   —           Pb   12   12   12   12           S   0.3   0.3   —   —           Boron   0.4   0.6   —   —           nitride           Fe   Remainder   Remainder   Remainder   Remainder                                 Abrasion amount(mm)   0.263   0.251   0.357   0.345                  
 
     [0057] Since corrosion resistance of the sintered alloy for a valve seat is improved according to the present invention by a sub-zero process, it is used for harsh conditions, for example in engines using smokeless gasoline and/or LPG under dried atmosphere.  
     [0058] While the present invention has been particularly shown and described with reference to a particular embodiment thereof, it will be understood by those skilled in the art that various changes in form and detail may be effected therein without departing from the spirit and scope of the invention as defined by the appended claims.