Ferrous sintered alloy vane and rotary compressor

A vane used in a rotary compressor, and rotary compressor including the vane are made from a ferrous sintered alloy. The method comprises the steps of preparing metal powder mixture primarily containing iron, compacting the powder mixture to obtain a powder compact, sintering the powder compact to obtain a sintered body, subjecting sub-zero treatment to the sintered body, and tempering the sintered body. The sintered alloy product is used as a vane slidably disposed in a vane groove of the rotary compressor whose cooling medium is maintainable without deterioration of its property.

BACKGROUND OF THE INVENTION 
The present invention relates to a method for producing a ferrous sintered 
alloy, and to a ferrous sintered alloy product applied to a vane used in a 
rotary compressor available for an air conditioner and an air cooling 
device. 
A structure of an ordinary rotary compressor provided with an eccentric 
rotor is shown in FIG. 1. In FIG. 1, a rotor housing 2 is disposed in a 
casing 1, and the rotor housing 2 is formed with a vane groove 3 in the 
radial direction thereof. A vane 4 is disposed slidable with respect to 
the vane groove 3. In the rotor housing 2, a rotor 5 is rotatably 
disposed. The rotor 5 is fitted with a crankshaft 6 whose rotation shaft 
6a is provided coaxial with the rotor housing 2, and whose crank portion 
6b is disposed eccentrical with respect to the rotation shaft 6a. A 
radially inner end of the vane 4 is in sliding contact with the outer 
peripheral surface of the rotor 5, and a radially outer end of the vane 4 
is connected to a coil spring 9 disposed in a recess 10 of the rotor 
housing 2. Therefore, the vane 4 is urged radially inwardly by the spring 
9, so that the inner end of the vane is in continuous contact with the 
rotor 5. Upon rotation of the rotor 5, the vane 4 is reciprocally movable 
along the vane groove 3, and fluid intake and discharge operation is 
performed. The vane 4 fluid-tightly divides a cavity of the rotor housing 
2 into two chambers as shown. 
In this connection, the vane 4 must provide sufficient fluid tightness to 
positively partition the two pressure chambers. Further, the vane 4 must 
provide high wear resistivity due to sliding contact with the rotating 
rotor 5. 
Recently, there has been produced a vane for use in the rotary compressor 
made of a sintered alloy formed primarily of ferrous powders so as to 
obtain a resultant vane having high wear resistance and fluid-tightness. 
In such a sintered alloy, the alloy generally employed is one in which 
carbide and other alloy particles are dispersed in a pearlitic matrix or 
martensitic matrix. 
However, a rotary compressor vane formed of the above-described sintered 
alloy may contain retained austenite in its metal structure upon 
production thereof. If the retained austenite exists in the sintered alloy 
vane, the retained austenite is transformed into martensite due to ambient 
temperature change provided by the frictional sliding motion of the vane 
relative to the vane groove upon operation of the compressor. This 
transformation causes a deformation with the passing of time together with 
expansion of the vane. 
This change with time is disadvantagous for the vane assembled in the 
compressor shown in FIG. 1, since such vane requires extremely high 
dimensional accuracy and stability. 
In order to remove the retained austenite, the sintered alloy is subjected 
to oil hardening or oil tempering to obtain martensitic structure. 
However, since the sintered product contains pores or voids, oil 
accumulated therein may ooze out of the sintered product. If such a 
sintered product is used as a vane of the rotary compressor, the oil may 
deteriorate the property of flon gas used as a cooling medium. This oil 
tempering is disclosed for example, Japanese Patent Application 
Publication (KoKai) No. 56-5955. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to overcome the 
above-described drawbacks and disadvantages, and to provide an improved 
method for producing ferrous sintered alloy and to provide the sintered 
alloy product available for a vane in a rotary compressor. 
Another object of the present invention is to provide a method which can 
produce a ferrous sintered alloy product having excellent wear resistivity 
and fluid-tightness. 
Still another object of this invention is to provide a ferrous sintered 
alloy product produced at low cost with high productivity. 
Still another object of this invention is to provide a ferrous sintered 
alloy product free from oil oozing therefrom when it is used as a vane of 
a rotary compressor. 
These and other objects of the present invention will be attained by 
performing sub-zero treatment to a sintered body and then tempering the 
sintered body. Briefly, and in accordance with a method of the present 
invention, metal powder mixture mainly containing iron is initially 
compacted, and the powder compact is sintered. Thereafter, the sintered 
body is subjected to sub-zero treatment, and then subjected to tempering. 
By the sub-zero treatment, retained austenite in the sintered body can be 
transformed into martensite. As a result, deformation of the product with 
time can be eliminated. Further, since no oil is employed for hardening 
during production steps, property of the flon gas used in the compressor 
can be maintained without any affect from the oil.

DETAILED DESCRIPTION OF THE INVENTION 
Refering now to an embodiment of the present invention, powders having the 
following compositions were prepared (the percentages are all percent by 
weight): 
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C: 0.8-1.5% 
Ni: 0.5-2.0% 
Cr: 5.0-10.0% 
Mo: 0.8-2.0% 
Fe: balance 
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The above described compositions are prepared by mixing together atomized 
SUS system powders (SUS is stainless steel defined by Japanese Industrial 
Standard, JIS G4301), low alloy steel powders, Ni powders, Mo powders, and 
C powders. SUS system is, for example, martensitic system SUS 403 or SUS 
410, and low alloy steel powders include components other than Fe such as, 
for example, not more than 3% of Cr, not more than 3% of Mo, not more than 
3% of Ni, and the balance Fe. The term "low" implies relatively small 
amounts of metals other than Fe such as Cr, Mo and Ni. 
More specifically, the powder mixture contains 1.3% by weight of C, 0.8 wt% 
of Ni, 7.0 wt% of Cr, 1.2 wt% of Mo, and the balance Fe and impurities. 
Zinc stearate is added as a lubricant into the powder mixture, and the 
mixture is compacted at a compacting pressure of 6 ton/cm.sup.2. Then the 
powder compact is sintered at a temperature ranging from 1100.degree. to 
1200.degree. C. in ammonia decomposed gas. Thereafter, the sintered body 
is subjected to sub-zero treatment at a temperature of not more than 
-100.degree. C., and then the product is tempered at a temperature of not 
less than 200.degree. C. Resultant product is subjected to final machining 
to obtain a ferrous sintered alloy product. 
Generally, sub-zero treatment is performed by dipping a steel product into 
liquid nitrogen or dry ice immediately after hardening of the steel 
product. Inventive feature of this invention resides in sub-zero treatment 
to the sintered body so as to eliminate austenitic structure in the alloy 
structure. 
Further, the above-described compositions per se have been described in 
Japanese Patent Application Publication (Kokai) No. 56-5955. Here, the 
most ideal way is to find out optimum compositions which do not provide 
retained austenite after sintering. However, it would be rather difficult 
and time consuming to investigate such compositions. Rather, in the 
present invention, known compositions are used, which inherently provide 
some technical advantages as described in the Publication, and drawbacks 
attendant thereto, i.e., exsistence of retained austenite in the sintered 
alloy, have been overcome by the application of sub-zero treatment to the 
sintered body. Condition of the sub-zero treatment is dependent on the 
shape and dimension of the sintered body. However, the sub-zero treatment 
should be conducted at a temperature not more than -80.degree. C. so as to 
transform the retained austenite into martensite. As is apparent from FIG. 
4 (400 magnifications), in the ferrous sintered alloy subjected to the 
sub-zero treatment, minute carbides (composite carbide comprising Fe-C-Cr 
system) are primarily dispersed in tempered martensitic matrix without 
retained austenite. In FIG. 4, white portions A and black portions B 
designate carbide and martensite, respectively. On the other hand, the 
retained austenite C remains in the sintered alloy body subjected to no 
sub-zero treatment as shown in FIG. 5 (400 magnifications), wherein small 
white areas A designate carbide, black portions B designate martensite and 
grey portions D designate bainite. 
The tempering performed at the final step of this invention serves to 
absorb any deformation or strain in the sintered product, which 
deformation being generated at the sub-zero treatment step. 
In order to investigate superiority of the present invention, two kinds of 
test pieces were prepared, one being a sintered product subjected to 
sub-zero treatment and tempering, and the other being a sintered product 
subjected to no sub-zero treatment. Compositions of the sintered bodies 
were the same as those described above, and structure of the sintered 
bodies contained bainite, martensite and retained austenite (see FIG. 5). 
For testing wear resistivity in the ferrous sintered product according to 
the present invention and that of the comparative test piece, these test 
pieces 7 (corresponding to the vane member) were stationarily mounted on a 
rotary piece 8 (corresponding to the rotor) formed of Ni-Cr-Mo cast iron. 
The stationary piece 7 was urged toward the rotary piece 8 with supplying 
lubricant therebetween for testing wear amount. Testing conditions were as 
follows: 
Load applied to the test piece: 40 kg 
Peripheral speed of the rotary piece: 1.5 m/sec. 
Lubricant: freezing machine oil (equivallent to ISO 56) 
Oil amount: 0.3 liters/min. 
Temperature: room temperature 
Testing period: 3 hours 
Test results are shown in FIG. 3. As is apparent from FIG. 3, the sintered 
alloy product produced by the method of the present invention provides 
excellent wear resistivity with reduced wear amount in comparison with the 
comparative piece wherein no sub-zero treatment was performed. 
Further, the comparative test piece was expanded by not less than 5 micron 
meters due to deformation with time when the piece was assembled and used 
in the rotary compressor shown in FIG. 1. Therefore, the comparative piece 
is not available as the vane member which requires high dimensional 
accuracy and stability, as generally not more than 5 .mu.m tolerable 
clearance between the vane and the vane groove is required. 
The ferrous sintered alloy product produced in accordance with the method 
of this invention is particularly available as vanes for use in the rotary 
compressor installed in an air conditioner and an air cooling device. 
However, the alloy product is also available for various sintered 
mechanical parts which required high wear resistance, fluid-tightness and 
dimensional accuracy. 
As described above, according to the present invention, the resultant 
sintered product provides excellent wear resistivity and high dimension 
accuracy and stability as well as high productivity. 
While the invention has been described in detail and with reference to 
specific embodiment thereof, it will be apparent for those skilled in the 
art that various changes and modifications can be made therein without 
departing from the spirit and scope of the invention.