Method of sintering stainless steel powder

A stainless steel powder is mixed with at least a Ni--Mn and a Ni--Cr powder, and the powder mixture is formed by loose packing into a required configuration. The powder mixture is sintered in a non-oxidizing atmosphere at the melting point of the Ni--Mn powder or at a higher temperature thereby to obtain a porous body.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to method for manufacturing a porous body of 
a stainless steel powder by sintering the stainless steel powder and, more 
particularly to a method which permits sintering of the stainless steel 
powder without applying any pressure to the stainless steel powder at a 
temperature below its melting point to obtain a porous body which is 
excellent in mechanical strength, corrosion resistance and heat 
resistance. 
2. Description of the Prior Art 
Heretofore, a porous body obtained by sintering an iron powder, copper 
powder or the like has been used as an oil, water or similar liquid 
filter, but in recent years the porous body is receiving particular 
attention as a sound-absorbing material since the porous structure of the 
porous body has excellent sound-absorbing properties. Although the porous 
body is such a useful industrial material, porous bodies now placed on the 
market are sintered bodies of iron, copper and like powders and these 
sintered bodies are poor in corrosion resistance and heavy, and hence 
limited in use. To avoid the abovesaid defects, the present inventors have 
previously proposed a method for sintering a porous body of a lightweight 
aluminum powder. The porous body obtained by this method has a pore ratio 
of, for example, 40% or more and exhibits very excellent sound-absorbing 
properties but is poor in heat resistance and in mechanical strength. 
In contrast thereto, a porous body of a stainless steel powder is excellent 
mechanical properties and rich in corrosion resistance and in heat 
resistance, and hence is preferred as a sound-absorbing material. Since 
the stainless steel powder has a large hardness of, for example, H.sub.RC 
40 to 50 or so and has a high sintering temperature, however, the 
sintering method itself poses a problem. According to a prior art method 
for sintering the stainless steel powder, the powder is heat treated prior 
to sintering, to reduce its hardness to less than H.sub.RC 40, and the 
powder is formed by rolling into a compact body, which compact body is 
then sintered at such a high temperature as 1300.degree. to 1400.degree. 
C. This conventional method involves preheat treatment of the stainless 
steel powder and, moreover, the high sintering temperature requires 
expensive sintering facilities and raises the sintering cost and, 
moreover, a porous body of the stainless steel powder can not be obtained 
by the conventional method; accordingly, there is a strong demand for 
improvement of the sintering method. 
The present inventors had make a study of a sintering method which would 
permit sintering of the stainless steel powder at a relatively low 
temperature regardless of the shapes of powder particles to obtain a 
porous body of excellent mechanical strength and corrosion resistance. As 
a result of their study, they have proposed a method in which one or more 
of Cu-Mn or Ni-Mn alloy powders are mixed in the stainless steel powder 
and the powder mixture is loosely packed into a required configuration and 
sintered in a non-oxidizing atmosphere at the melting point of the alloy 
powder or at a higher temperature. With this method, the sintering 
temperature is relatively low and the pore ratio of the sintered body can 
freely be adjusted. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a 
stainless steel sintering method which permits sintering a stainless steel 
powder at a relatively low temperature to obtain a porous body of 
excellent mechanical strength and corrosion resistance. 
Briefly stated, according to the present invention, at least a Ni-Mn and a 
Ni-Cr powder are mixed in the stainless steel powder and the powder 
mixture is loosely packed into a required configuration and then sintered 
in a non-oxidizing atmosphere at the melting point of the Ni-Mn powder or 
at a higher temperature.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A detailed description will hereinafter be given of the sintering method of 
the present invention. 
At first, a Ni-Mn powder and a Cr-Mn powder are mixed in a stainless steel 
powder. In this case, the stainless steel powder used may be an ordinary 
austenite or ferrite system stainless steel powder. The stainless steel 
powder may also be preheated to reduce its hardness. 
Next, the powder mixture is loosely packed into a required configuration, 
for example, by pouring it into a vessel, or in the case where the 
stainless steel powder is heat-treated, the powder is formed into the 
required configuration without applying any pressure to the powder. 
Thereafter, the formed body is sintered in a non-oxidizing atmosphere 
such, for instance, as a hydrogen atmosphere, at the melting point of the 
Ni-Mn powder or at a higher temperature without applying any pressure to 
the formed body. By such sintering, as the Ni-Mn powder is molten, the 
Cr-Ni powder is molten and the powder mixture is sintered, with one part 
thereof remaining in liquid phase. 
Whether it is of the ferrite or the austenite system, the stainless steel 
powder contains at least 12% or more of chromium and the powder particles 
are each covered with an chromium oxide (Cr.sub.2 O.sub.3) film as a 
result of oxidation of the chromium. This oxide film is very hard to be 
reduced and cannot be reduced by an ordinary industrial furnace but can be 
reduced in a special industrial furnace at a temperature DP=-45.degree. C. 
(1000.degree. C.) or less. But, when the powder covered with such oxide 
film is held in a non-oxidizing atmosphere, even if the oxide film is not 
reduced, film cracks due to a difference in expansion coefficient between 
the film and the internal stainless steel and the aforesaid Ni-Mn and 
Cr-Ni powders are likely to be diffused into the stainless steel through 
the crack. To perform this, it is necessary that the dew point in the 
non-oxidizing atmosphere be at least -45.degree. C. or lower, preferably, 
-50.degree. C. The reason is that when the dew point is higher than 
-45.degree. C., oxygen enters into the particle from the crack and 
combines with chromium in the ground of stainless steel. 
Further, at least a Ni-Cr powder is mixed in the stainless steel powder 
along with the Ni-Mn powder and sintering of the powder mixture is started 
at a temperature raised up to the melting point of the Ni-Mn powder and 
terminated at a temperature in the vicinity of the melting point of the 
Ni-Cr powder. As is evident from the phase diagram of the Ni-Mn powder, 
its melting point is the lowest when the Ni content is 40% and melting 
point is 1018.degree. C. Accordingly, mixing the Ni-Mn (Ni 40%, Mn 60%) 
powder in the stainless steel powder and sintering the powder mixture at 
1020.degree. to 1050.degree. C., the alloy powder with the 40% Ni content 
is molten and diffused into the stainless steel particle from the 
aforesaid crack, and as the sintering proceeds, the liquid phase powder is 
also diffused into the Ni-Cr powder particle. As a consequence, the Ni-Cr 
powder is alloyed with the Ni-Mn powder and its composition changes. When 
the composition of the Ni-Cr powder thus alloyed with the Ni-Mn powder 
becomes the eutectic composition (Ni 50%, Cr 50%) or close thereto, the 
melting point lowers; for example, at a temperature in the vicinity of the 
eutectic temperature (1343.degree. C.), the Ni-Cr powder is molten and 
sintered in liquid phase. The change in the composition of the Ni-Cr 
powder by the diffusion thereinto of the Ni-Mn powder need not always be 
made throughout the Ni-Cr powder; namely, it is sufficient that only one 
portion of the Ni-Cr powder is alloyed with the Ni-Mn powder and that the 
composition of the alloyed portion becomes close to the eutectic 
composition. The reason is that when melting of the portion of the 
eutectic composition is once started, the compositions of the other 
portions are also sequentially changed and they are molten as the 
sintering proceeds. 
The stainless steel powder may be either the ferrite system or the 
austenite system, as referred to previously. Even in the stainless steel 
powder of the ferrite system, any of chromium, nickel and manganese has a 
certain degree of solid solubility with respect to the stainless steel 
powder. In the stainless steel powder of the austenite system, the 
abovesaid elements have sufficient solid solubility and the corrosion 
resistance of the sintered body can be further increased by adding them in 
suitable amounts. 
It is also possible to add a sintering property improving component and a 
ground reinforcing component, as required, other than the Ni-Mn and Ni-Cr 
powders. For example, when copper or its alloy powder is added, it 
enhances the wetting property of the surface of the powder particle, 
during sintering, within the solid solubility limit of the copper, thus 
promoting the sintering of the powder. Further, since the melting point of 
the Cu-Mn powder is 868.degree. C. in its eutectic composition, the 
sintering starts when the sintering temperature reaches 870.degree. C. or 
so. With the copper content exceeding its solid solubility limit, however, 
copper is precipitated at the coupling portions of adjacent powder 
particles to degrade the corrosion resistance of the sintered body; 
accordingly, it is preferred that the copper content be smaller than 3% of 
the solid solubility limit. 
In the case of sintering the stainless steel powder mixed with the Ni-Mn 
powder, for example, 52 to 54% of Mn and the remainder Ni and the Ni-Cr 
powder, the sintering is usually started at about 1000.degree. C. and the 
sintering temperature is gradually raised and then the sintering is 
finished at 1200.degree. to 1350.degree. C. In such a case, the Ni-Mn 
powder is molten first and this liquid phase portion is diffused into the 
stainless steel powder particles and the Ni-Cr powder particles as the 
sintering proceeds, and when the sintering temperature reaches 
1350.degree. C. or so, the Ni-Cr powder starts to be molten and the liquid 
phase sintering proceeds, providing a porous body. For enhancement of the 
pore ratio of the porous body, it is preferred that the powder mixture is 
loosely packed into a required shape prior to sintering; but when the pore 
ratio need not be raised so high, the powder mixture may also be formed 
under a predetermined pressure prior to sintering. In such a case, since 
the powder mixture is pressed during the press forming, a dense sintered 
body is obtained. Also it is possible to fill voids of the sintered body 
with a lubricant and a material for increasing the bearing performance, 
such as a sulfide, oxide, metal, inorganic substance or organic substance; 
namely, the sintered body can also be used as a bearing and some other 
parts. 
In the case of mixing the Ni-Mn or Ni-Cr powder in the stainless steel 
powder, it is preferred that the particle of the Ni-Mn or Ni-Cr powder is 
smaller than the stainless steel powder particle which frames the porous 
sintered body. To this end, the particle size of the stainless steel 
powder is usually adjusted to range from 20 to 100 meshes and, in this 
case, it is desirable that the particle size of the Ni-Mn or Ni-Cr powder 
is adjusted to be less than 100 meshes. Furthermore, the mixing ratios of 
the Ni-Mn and the Ni-Cr powder to the stainless steel powder can be 
determined in accordance with the pore ratio and the alloy composition of 
the porous body desired to obtain, but it is usually preferred that the 
Ni-Mn powder in the range of 5 to 10% and the Ni-Cr powder in the range of 
5 to 20%. 
Although in the foregoing the stainless steel powder is sintered without 
any pretreatment, it is desirable to subject the stainless steel powder to 
preheat treatment to lower its hardness before being mixed with the Ni-Cr 
or Ni-Mn powder. In the case of sintering the powder mixture into a porous 
body after press-forming the powder mixture into a required configuration, 
a very large pressure is required for the press-forming of the powder 
mixture since the hardness of the stainless steel powder is very large; 
but when the hardness of the stainless steel powder is reduced by the 
preheat treatment, the pressure for the press-forming may be very small, 
allowing much ease the manufacture of porous bodies. 
The present invention will be further described in connection with its 
examples. 
EXAMPLE 1 
A ferrite system stainless steel powder (with a mean particle size of 70 
meshes) consisting of 0.2 wt % of CO, 0.9 wt % of Si. 0.1 wt % of Mn, 17.5 
wt % of Cr, 1 wt % of Mo and the balance Fe was mixed with a Ni-Mn powder 
(with a mean particle size of 150 meshes) consisting of 60 wt % of Mn and 
40 wt % of Ni and a Ni-Cr powder (with a mean particle size of 150 meshes) 
consisting of 40 wt % of Ni and 60 wt % of Cr in the ratio of 80 to 10 to 
10 by weight. Then, the powder mixture was loosely packed in a heat-proof 
vessel, which was placed in a furnace under non-pressure condition. In the 
furnace the dew point of the atmosphere was -45.degree. C. and the powder 
mixture packed in the vessel was sintered at 1200.degree. C. for 60 
minutes, gradually raising the sintering temperature from 1050.degree. C. 
As a result of this, a porous body of stainless steel was obtained and its 
pore ratio was about 50%. It was found that, during the sintering, Ni, Cr 
and Mn were diffused into the ground of the stainless steel powder 
particle. 
EXAMPLE 2 
An austenite system stainless steel powder (with a mean particle size of 70 
meshes) consisting of 0.2 wt % of CO, 0.9 wt % of Si, 0.2 wt % of Mn, 10.5 
wt % of Ni, 19 wt % of Cr and the balance Fe was mixed with a Ni-Cr powder 
(with a mean particle size of 150 meshes) consisting of 50 wt % of Ni and 
50 wt % of Cr, a Ni-Mn powder (with a mean particle size of 150 meshes) 
consisting of 60 wt % of Ni and 40 wt % of Mn and a Mn-Cu powder 
consisting of 35 wt % of Mn and 65 wt % of Cu in the ratio of 90 to 5 to 
2.5 to 2.5 by weight. Then, the powder mixture was packed in a heat-proof 
vessel, which was placed in a furnace under non-pressure condition. In the 
furnace the dew point of the atmosphere was -45.degree. C. and the powder 
mixture packed in the vessel was sintered at 980.degree. C. for 30 minutes 
first and then further sintered at 1350.degree. C. for an hour. In this 
example, at 980.degree. C., the Cu-Mn powder was molten and precipitated 
on the stainless steel powder particle; at about 1020.degree. C., the 
Ni-Mn powder was molten; at 1340.degree. C., the Ni-Cr powder was molten; 
and at 1350.degree. C., the sintering was completely finished. A porous 
body (with a pore ratio of 30 wt %) was obtained. 
EXAMPLE 3 
The same powder mixtures as used in Examples 1 and 2 were prepared. During 
the preparation about 1 wt % of zinc stearate was added to each of them. 
Prior to the preparation of each powder mixture, the stainless steel 
powder was heat treated to reduce its hardness to H.sub.RC 40 or so. The 
both powder mixtures was sintered after being pressed under a pressure of 
7 tons/cm.sup.2. The sintering conditions were the same as those employed 
in Examples 1 and 2, respectively. In this Example low melting alloy was 
disappeared and porous bodies were obtained. 
As has been described in detail in the foregoing, according to the present 
invention, a stainless steel powder is mixed with at least a Ni-Mn and a 
Ni-Cr powders and the powder mixture is sintered under non-pressure 
condition, so that the sintering may be performed at a relatively low 
temperature and in a short time, and in addition, a porous body of 
excellent mechanical properties, corrosion resistance and heat resistance 
can be obtained. It is also possible to sinter the powder mixture after 
pressing it into a required shape; in this case, a porous body which 
disappeared low melting alloy can easily be obtained. 
Although the foregoing description has been given on the assumption that 
particle of the stainless steel powder is spherical, the particle need not 
always be spherical but may also be irregular-shaped one such as a 
rod-like one and similar configurations. 
It will be apparent that many modifications and variations may be effected 
without departing from the scope of the novel concepts of this invention.