Abstract:
Disclosed is a roller for use in a rotary compressor, which roller comprising a sintered body consisting essentially of 0.5-2.0% by weight of C, 1.0-5.0% by weight of Cu, 0.5-3.5% by weight of Cr, 0.1-1.0% by weight of Co, 0.1-1.0% by weight of W and a balance Fe and unavoidable impurities. Hard particles of Cr-Co-W alloy are dispersed in one of pearlitic and tempering martensitic matrix, and sintered pores of the sintered body are sealed with tri-iron tetroxide. Resultant sintered body has high wear resistance and scuffing resistance capable of being used as an inverter type compressor.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a roller formed of a sintered alloy having high wear resistance and assembled in a rotary compressor having high fluid tightness. 
     In a recent trend, a rotary compressor for use with domestic electrifications becomes light in weight and more compact in size. Further, for the reduction of production cost and for high performance of the compressor, improvements have been made on materials of respective mechanical components of the compressor. 
     A rotary compressor mainly includes, as shown in FIGS. 1 and 2, an outer case 10, a cylindrical housing 11 assembled in the case 10 and formed with a vane groove 11A extending in radial direction of the housing, a roller 13 rotatable eccentrically in the housing 11, a shaft 14 integrally fixed to the roller 13 for its rotation, and a vane 12 slidably disposed in a vane groove 11A and moved in radial direction of the roller 13. A compression spring 15 is disposed in the groove 11A to urge the vane 12 radially inwardly. Therefore, a radially inner end face of the vane 12 is in slide contact with an outer peripheral surface of the roller 13. A working chamber is provided by a space defined between the housing 11 and the roler 13, and the vane 12 devides the chamber into intake and discharge chambers. The intake chamber is connected to an intake port 16 and the discharge chamber is connected to a discharge port 17. A fluid sucked in the working chamber is compressed and fed out by the eccentric rotation of the roller 13. 
     Among those components, the vane 12 and roller 13 perform relative sliding motion at high load, and therefore, these components must have high wear resistance. On this standpoint, various materials, for example, sintered alloy, have been proposed for the materials of the vane and roller. 
     However, regarding the material of the vane, SKH51 has been still a major material in an actual production. SKH 51 is defined by JIS (Japanese Industrial Standard), which is a high-speed tool steel, containing 0.80 to 0.90% of C, 3.80 to 4.50% of Cr, 4.50 to 5.50% of Mo, 5.50 to 6.70% of W, 1.60 to 2.20% of V and balance Fe. 
     Further, regarding the material of the roller, a sintered material has been employed as described above rather than a cast iron. According to the proposed sintered material, a hard metal carbide and a metal oxide formed by a steam treatment are dispersed in a matrix. Such sintered alloy is disclosed in Japanese Patent Application Kokai Nos. 60-73082 and 60-174853. 
     More specifically, according to the publication No. 60-174853, it discloses a sintered alloy consisting of 3-10% by weight of chromium, 1-5% by weight of graphite and balance iron and impurities. Metal carbide, metal oxide and free graphite are dispersed in the tempered martensitic matrix. The metal oxide is formed at interiors of sintered voids by steam treatment. This metal oxide seals the sintered voids to thereby obtain lubrication oil retainability. 
     According to the publication No. 60-73082, it discloses a rotary compressor. A rotor and/or a vane are formed of ferrous sintered alloy. Metal carbide and metal oxide are formed during tempering and are dispersed in the tempered martensitic matrix. Further, nitrogen is solid-solved in the martensitic matrix. 
     The above sintered material for the roller is intended to improve wear resistance and fluid-tightness. The metal oxide which seals sintered pores serves to enhance fluid-tightness of the compressor. However, with the use of such sintered material, a compressor roller has been burdened with much higher load because of the recent use of an inverter system. Accordingly, such sintered material may be worn out and undergoes scuffing if applied in the inverter system. In order to overcome this problem, there is a further demand for further improvement on wear resistivity and scuffing by using specific composition instead of conventional metal carbide dispersed in the matrix. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of this invention to prevent a roller for a compressor from being worn out or scuffing under a high load and to improve fluid-tightness of the compressor. 
     Another object of this invention is to provide an improved compressor roller capable of being used in an inverter system which provides extremely high load. 
     To achieve the object, according to this invention, there is provided a compressor roller formed of a sintered alloy consisting essentially of 0.5-2.0% by weight of C, 1.0-5.0% by weight of Cu, 0.5-3.5% by weight of Cr, 0.1-1.0% by weight of Co, 0.1-1.0% by weight of W and a balance Fe and unavoidable impurities. Further, hard particles of Cr-Co-W alloy are dispersed in a pearlitic or tempering martensitic matrix, and sintered pores of the sintered alloy are sealed with tri-iron tetroxide. 
     Further, according to the present invention, there is provided a roller formed of a sintered body produced by the steps of; mixing together 0.9 to 1.9% by weight of graphite powders, 2.5 to 4.0% by weight of pure copper powders, 1.0 to 5.0% by weight of alloy powders mainly consisting of Cr, Co and W and balance pure iron powders to obtain a powder mixture; compacting the powder mixture to form a powder compact; sintering the powder compact to obtain a sintered body formed with sintered pores; and, steam treating the sintered body to seal the sintered pores, a resultant sintered body consisting essentially of 0.5-2.0% by weight of C, 1.0-5.0% by weight of Cu, 0.5-3.5% by weight of Cr, 0.1-1.0% by weight of Co, 0.1-1,0% by weight of W and a balance Fe and unavoidable impurities; hard particles of Cr-Co-W alloy being dispersed in pearlitic matrix, and the sintered pores being sealed with tri-iron tetroxide. 
     If martensitic matrix is intended in the final sintered alloy, after the sintering step, there is provided the step of hardening the sintered body; steam treating the hardened sintered body to seal sintered pores, and, tempering the sintered body, a resultant sintered body consisting essentially of 0.5-2.0% by weight of C, 1.0-5.0% by weight of Cu, 0.5-3.5% by weight of Cr, 0.1-1.0% by weight of Co, 0.1-1,0% by weight of W and a balance Fe and unavoidable impurities; hard particles of Cr-Co-W alloy being dispersed in martensitic matrix, and the sintered pores being sealed with tri-iron tetroxide. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings; 
     FIG. 1 is a vertical-cross section illustrating a structure of a compressor which employs a roller embodying this invention; 
     FIG. 2 is a perspective view showing the compressor showin in FIG. 1; 
     FIG. 3 is a schematic view showing a wear-testing manner; and, 
     FIG. 4 is a microscopic photograph showing a structure of a sintered material for the roller of this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A roller of this invention is produced by adding 1.0 to 5.0% by weight of alloy particles mainly consisting of Cr, Co and W to a mixture of graphite powders, pure copper powders and pure iron powders to obtain a powder mixture, compacting the powder mixture to obtain a powder compact, sintering the powder compact to obtain a sintered product with pearlitic matrix, and then subjecting the sintered product to a steam treatment or subjecting the sintered product to a sequential steps of hardening, steam treatment and tempering in order to provide a tempered martensitic matrix. 
     The pearlitic matrix has inherently high toughness. However, the martensitic matrix has higher hardness and increases the wear-resistance of the roller. After the sintering process, the alloy particles mainly consisiting of Cr, Co and W are dispersed in the matrix as hard particles of Cr-Co-Mo alloy to significantly improve the wear-resistance and scuffing resistance of the roller. With the amount of alloy particles consisting mainly of Cr, Co and W at the time of initial adding process being less than 1.0%, sufficient wear-resistance would not be obtainable. On the other hand, if the addition amount of the alloy particles is more than 5.0%, resultant alloy has excessively high hardness to attack the opponent sliding members, such as the inner end portion of the vane and side housing plates of the compressor. Therefore, these opponent members are excessively worn out and further, such excessive amount of alloy particles is disadvantageous in terms of manufacturing cost. 
     After the sintering, there exist continuous pores or open cells in the sintered product. Such open cells may degrade fluid-tightness of the compressor. Therefore, these sintered pores are sealed with tri-iron tetroxide (Fe 3  O 4 ). This seal also contributes to the improvement of wear-resistance. 
     The following will describe the reasons for the percentagewise limitations on the respective compositions. 
     Carbon C will harden the matrix when solid-solved in the matrix. With this component being less than 0.5% by weight, generation of pearlite and martensite is insufficient, to thereby reduce strength of the matrix. When carbon amount is more than 2.0% by weight, excessive amount of cementite is generated in the matrix, thus render the resultant alloy brittle. 
     Copper will harden and stabilize the matrix. This effect is not prominant with the component being less than 1.0% by weight. On the other hand, further improved effect may not be obtainable when the component exceeds 5.0% by weight, so that such excessive amount of copper is economically disadvantageous, and further, segregation occurs to thereby lower dimensional accuracy of the final product. 
     The amounts of Cr, Co and W are within ranges of 0.5-3.5% by weight, 0.1-1.0% by weight, and 0.1-1.0% by weight, respectively in the final sintered product, by adding 1.0 to 5.0% of alloy particles mainly consisting of Cr, Co and W in the powder mixture. These alloy powders have fine and coarse particles. Upon the fine particles being solid solved in the matrix, matrix becomes strengthened. On the other hand, the coarse particles are dispersed in the matrix as hard particles of Cr-Co-W alloy to improve the wear-resistance and scuffing-resistance. Sufficient strength of the matrix may not be obtainable if the addition amount of the these alloy particles mainly consisting of Cr, Co and W is less than 1.0%. And, if the addition amount is more than 5%, the above mentioned opponent sliding members will be attacked by the sintered material and are worn out. 
     A description will now be given with regard to results of performance tests according to the present invention. 
     [METHOD FOR PRODUCING SPECIMENS] 
     The powder mixtures indicated by No. 1-11 in Table 1 were prepared as raw materials of the roller. In Table 1, Nos. 1 throu 5 belong to the present invention, and Nos. 6 thru 11 are comparative materials. Each of the powder mixtures was compacted at a pressure of 5-6 tons/cm 2  into a solid cylindrical shape having a diameter of 40 mm and an axial length of 10 mm. Then each of the powder compacts was subjected to various treatments shown in Table 2 where (steam) represents steam treatment; (heat) implies hardening and tempering, and (heat+steam) implies a combination of hardening, tempering and steam treatment. As a result, specimens were obtained which have the compositions, structures and hardness as shown in Table 2. 
     
                       TABLE 1______________________________________SpecimenNo.    Powder Mixture______________________________________1-5    Graphite Powder:         0.9-1.9%  Pure Copper Powder:      2.5-4.0%  63.5Cr--10Co--19W--2.5C--5Fe Powder:                           3.0-5.0%  Zinc Stearic Acid:       1.0%  Pure Iron Powder:        Balance6 &amp; 7  Graphite Powder:         2.3%  Pure Copper Powder:      2.6%  63.5Cr--10Co--19W--2.5C--5Fe                           10%  Alloy Powder:  Zinc Stearic Acid:       1.0%  Pure Iron Powder:        Balance8 &amp; 9  Graphite Powder:         1.3%  Ni Powder:               1.0%  Mo Powder:               1.3%  Zinc Stearic Acid:       1.0%  Cr(1%)--Fe Alloy Powder: Balance10 &amp; 11  Graphite Powder:         1.35%  Pure Copper Powder:      3.0%  Zinc Stearic Acid:       1.0%  Pure Iron Powder:        Balance______________________________________ 
    
     As specimen No. 12 (compared material), prepared was FC 30 which is a gray cast iron consisting of C: 3.2%, Si: 2.3%, Mn: 0.7%, P: 0.11%, S: 0.04%, Cu: 0.3%, Cr: 0.2%, Fe: balance, and which is conventionally widely available as a roller material. The cast iron was of solid cylindrical shape having a diameter of 40 mm and a length of 10 mm and was subjected to hardening at a temperature of about 870° C. 
     [TEST CONDITIONS] 
     The above specimens were subjected to wear test according to Amsler&#39;s basic method. Each of the columnar specimens No. 1-12 (corresponding to a roller) serving as a rotating part was assembled in a plane contact slide wear testing machine, and a SKH 51 plate (corresponding to a vane) having the size of 8 mm×7 mm×5 mm was also assembled in the machine for serving as a stationary part. As shown in FIG. 3, the stationary part 112 was in pressure  contact with an outer peripheral surface of the specimen 113, and the latter was rotated at high speed about its axis for slide contact with the stationary part while supplying a lubricant L to the slide-contact section. 
     Condition details were as follows: 
     Load: 100 Kg 
     Peripheral Velocity: 1 m/sec. 
     Lubricant: freezing machine oil (equivalent to ISO 56) 
     Oil Temperature: 75° C. 
     Sliding Period: 20 hours. 
     Under the above conditions, the amount of wearing of the fixed part 112 and rotating part 113 were measured. The test results are shown in Table 2. 
     Further, scuffing test was also conducted according to the Amsler&#39;s wear test. The specimens involved in this test were the same as those involved in the above wear test. While rotating the rotating pieces No. 1-12 at the peripheral velocity of 1.13 m/sec., load of 10 kg was initially applied to the fixed part 112, and then, the load was added by 20 kg at every 2 minutes until the load reached 50 Kg, and thereafter, added by 10 kg at every 2 minutes. The loads at which scuffing occurred were regarded as the maximum limit pressure to scuffing which is also shown in Table 2. 
     
           Wear  Amount (μ) Maximum Limit Specimen Composition (wt. %)  Fixed Rotating Pressure to No. C Cu Cr Co W Mo Ni Fe Treatment Structure Hardness Part Part Scuffing (Kg)         Cr--Co--W, Fe.sub.3 O.sub.4 are dispersed in      1 0.95 2.93 1.60 0.28 0.58 -- -- Balance (Steam) perlitic matrix HRB 97 4.2 0.65 110            Cr--Co--W, Fe.sub.3 O.sub.4 are dispersed in Present 2 0.95 2.93 1.60 0.28 0.58 -- -- Balance (Heat + Steam) martensitic matrix HRC 34 4.9 0.50 130 Material           Cr--Co--W, Fe.sub.3 O.sub.4 are dispersed in  3 1.42 3.60 2.51 0.41 0.76 -- -- Balance (Steam) perlitic matrix HRB 100 4.4 0.55 120            Cr--Co--W, Fe.sub.3 O.sub.4 are dispersed in  4 1.48 3.54 2.52 0.40 0.76 -- -- Balance (Heat + Steam) martensitic matrix HRC 40 3.8 0.41 140 Cr--Co--W, Fe.sub.3 O.sub.4 are dispersed in  5 1.83 2.98 3.10 0.47 0.93 -- -- Balance (Steam) perlitic matrix HRB 101 4.6 0.56 110 Cr--Co--W, Fe.sub.3 O.sub.4 are dispersed in  6 2.20 2.51 6.35 1.94 1.90 -- -- Balance (Steam) perlitic matrix HRC 26 6.2 0.52 120 Cr--Co--W, Fe.sub.3 O.sub.4 are dispersed in  7 2.20 2.51 6.35 1.94 1.90 -- -- Balance (Heat + Steam) martensitic matrix HRC 42 7.3 0.50 140  Fe.sub.3 C is dispersed in  8 1.02 -- 0.98 -- -- 1.34 1.04 Balance (Heat) martensitic matrix HRC 36 5.9 1.6 110 Compared           Fe.sub.3 C and Fe.sub.3 O.sub.4 are dispersed in Material 9 1.02 -- 0.98 -- -- 1.34 1.04 Balance (Heat + Steam) martensitic matrix HRC 40 5.8 1.2 110     Fe.sub.3 O.sub.4 is dispersed in  10 1.16 3.05 -- -- -- -- -- Balance (Steam) perlitic matrix HRB 95 4.8 1.7 90            Fe.sub.3 O.sub.4 is dispersed in 11 1.16 3.05 -- -- -- -- -- Balance (Heat + Steam) martensitic matrix HRC 31 Scuffing 80 12 FC30 Gray Cast Iron HRC 49 5.0 2.6 90 
    
     [TEST RESULTS] 
     As is apparent from the test results shown in Table 2, when using the present roller which contains hard particles of Cr-Co-W alloy and is formed of specific components which their specific percentages, wear amounts of both roller and vane were smaller than those when using the comparative roller specimens. Further, the maximum limit pressure to scuffing in case of the employment of the present roller was much superior to the comparative cases. Therefore, excellent wear-resistance and scuffing-resistance are obtainable with respect to both vane and roller in the present invention as compared with the comparative cases. 
     FIG. 4 is a microscopic photograph (×200 magnifications) showing a metallic structure of specimen No. 2 of Table 1. The specimen was subjected to etching treatment with Niter Corrosion liquid. According to this photograph, hard particles of Cr-Co-W alloy 2 and tri-iron tetroxides 3 are dispersed in martensitic matrix 1. 
     Incidentally, the alloy powders mainly including Cr, Co and W also contains carbon and iron as is apparent from Specimens 1-5 in Table 1. Even though this alloy particles contained C and Fe, such alloy cannot be referred to as metal carbide which has been included in the conventional sintered alloy described in the above described Japanese publication, since the amount of C and Fe is extremely small. 
     As described above, the roller of this invention has excellent wear-resistance, scuffing-resistance and fluid-tightness, and will exhibits excellent performance when used particularly in a compressor which is burdened with a high load.