GB2088414A - Sintering Stainless Steel Powder - Google Patents

Abstract

A method of sintering stainless steel powder comprises mixing the stainless steel powder with a powder containing Ni-Mn alloy powder and Ni-Cr alloy powder, forming the powder mixture by loose packing or under pressure into a required shape and then sintering the mixture in a non-oxidising atmosphere at a temperature at or above the melting point of the Ni-Mn powder but below the melting point of the stainless steel powder to obtain a porous body. Copper or copper alloy powder may be included in the composition. A lubricant such as zinc stearate may also be included.

Description

SPECIFICATION Method of Sintering Stainless Steel Powder This invention relates to a method of sintering stainless steel powder which allows the sintering to take place at a temperature below the the melting point of the stainless steel powder to obtain a porous body which is excellent in mechanical strength, corrosion resistance and heat resistance.

Previously porous bodies have been obtained by sintering iron powder or copper powder and these bodies have been used as oil, water or other liquid filters. However in recent years sintered porous bodies have been receiving particular attention as sound-absorbing material since the porous structure of such bodies has excellent sound-absorbing properties. Although sintered porous material is such a useful industrial material, sintered porous bodies at present available are of iron, copper or other metals which are poor in corrosion resistance and heavy, and hence limited in use. To avoid these difficulties, a method has previously been proposed for sintering a porous body of a lightweight aluminium powder.The porous body obtained by this method has a pore ratio of, for example, 40% or more and exhibits excellent sound-absorbing properties but is poor in heat resistance and in mechanical strength.

In contrast thereto, a porous body of sintered stainless steel powder has excellent mechanical properties and high corrosion resistance and heat resistance, and hence is preferred as a soundabsorbing material. However, as the stainless steel powder has a high hardness of, for example, ARC40 to 50 or so and has a high sintering temperature, the sintering itself poses a problem.

In one prior art method for sintering stainless steel powder, the powder is heat treated prior to sintering, to reduce its hardness to less than ARC40 and to obtain irregular-shaped powder particles. The powder is then formed by rolling into a body having a predetermined pore ratio, and the body is then sintered at a temperature as high as 13000 to 1 4000C. This conventional method involves preheat treatment of the stainless steel powder and preforming of the powder particles into particular shapes and, moreover, the high sintering temperature requires expensive sintering facilities and raises the sintering costs. Accordingly, there is a strong demand for improvement of the sintering method.

The inventors of the present invention made a study to discover a sintering method which would permit sintering of stainless steel powder at a relatively low temperature regardless of the shapes of the powder particles in order to obtain a porous body of good mechanical strength and corrosion resistance.

As a result of their study the inventors proposed a method in which the stainless steel powder is mixed with a powder containing Cu-Mn or Ni-Mn alloy powder after which the powder mixture is formed to a required shape and is then sintered in a non-oxidising atmosphere at the melting point of the alloy powder or at a higher temperature. With this method, the sintering temperature may be relatively low and the pore ratio of the sintered body can freely be adjusted, but the copper content remaining in the sintered body is large and the copper is concentrated on the grain boundaries resulting in the corrosion resistance being degraded.

To overcome this disadvantage, and after further experiments, the inventors discovered a better method, which, in accordance with the present invention, comprises mixing the stainless steel powder with a powder containing Ni-Mn alloy powder and Ni-Cr alloy powder, forming the powder mixture into a required shape, and sintering the formed powder mixture in a nonoxidizing atmosphere at a temperature at or above the melting point of the Ni-Mn alloy powder but below the melting point of the stainless steel powder.

This method permits sintering the stainless steel powder at a relatively low temperature to produce a porous body of excellent mechanical strength and corrosion resistance.

In more detail, firstly a Ni-Mn alloy powder and a Cr-Mn alloy powder are mixed with stainless steel powder. The stainless steel powder used may be an ordinary austenitic orferritic stainless steel powder. The stainless steel powder may also be preheated to reduce its hardness.

Next, the powder mixture is loosely formed into a required shape for example, by pouring it into a mould or other vessel, or in the case where the stainless steel powder is heat-treated, the powder is formed into the required configuration under pressure. 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. By such sintering, 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 ferritic or the austenitic type, the stainless steel powder contains at least 12% of chromium and the stainless steel powder particles are each covered with a chromium oxide (Cr203) film as a result of atmospheric oxidation of the chromium. This oxide film is very hard to reduce and cannot be reduced by an ordinary industrial furnace but can be reduced in a special industrial furnace at a temperature DP=-450C (1 0000 C) or less. But, when the powder covered with such an oxide film is held in a non-oxidizing atmosphere even if the oxide film is not reduced, the film cracks due to a difference in expansion coefficient between the film and the internal stainless steel and the aforesaid Ni-Mn and Ni-Cr powders tend to be diffused into the stainless steel through the cracks.To perform this, it is necessary that the dew point in the non-oxidizing atmosphere be -45 OC or lower, preferably, 50 C. The reason is that when the dew point is higher than -450C, oxygen enters into the particle through the crack and combines with chromium in the stainless steel to produce further Cur203.

Further, Ni-Cr alloy powder is mixed with the stainless steel powder together with the Ni-Mn powder and sintering of the powder mixture is started at a temperature which is raised to the melting point of the Ni-Mn powder but not above 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 then 101 80C. Accordingly, upon mixing the Ni-Mn (Ni 40%, Mn 60%) alloy powder in the stainless steel powder and sintering the powder mixture at temperatures of from 10200 to 1 0500C, the alloy powder with the 40% Ni content becomes molten and is diffused into the stainless steel particles through the aforesaid cracks.As the sintering proceeds, the liquid phase Ni-Mn alloy is also diffused into the Ni-Cr powder particles. 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 is lowered; for example, at a temperature in the vicinity of the eutectic temperature (1 343 OC), the Ni-Cr powder is molten and sintered in the liquid phase. The change in the composition of the Ni-Cr powder by the diffusion thereinto of the Ni-Mn powder need not always occur throughout the Ni Cr powder. It is sufficient that only part of the Ni Cr powder is alloyed with the Ni-Mn powder and that the composition of the alloyed part becomes close to the eutectic composition.The reason for this is that when melting of the part of the alloy of eutectic composition is once started, the compositions of the other parts are also sequentially changed and they become molten as the sintering proceeds.

The stainless steel powder may be of either the ferritic or of the austenitic type, as referred to previously. Even with stainless steel powder of the ferritic type, any chromium, nickel and manganese has a certain degree of solid solubility with respect to the stainless steel powder. With stainless steel powder of the austenitic type, the above elements have sufficient solid solubility and the corrosion resistance of the sintered body is further increased by adding them in suitable amounts.

It is also possible to add to the mixture a sintering property-improving component and a ground reinforcing component, as required, other than the Ni-Mn and Ni-Cr alloy powders. For example, if copper or copper alloy powder is added, it enhances the wetting property of the surface of the stainless steel powder particles during sintering, within the solid solubility limit of the copper, thus promoting the sintering of the powder. Further, since the melting point of a Cu Mn alloy powder is 8680C in its eutectic composition, the sintering starts when the sintering temperature reaches 8700C or thereabouts.With the copper content exceeding its solid solubility iimit, however, copper is precipitated at the junctions of adjacent powder particles and thus degrades the corrosion resistance of the sintered body; accordingly, it is preferred that the copper content be smaller than 3% of the solid solubility limit.

If the stainless steel powder is 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 1 0000C. The sintering temperature is then gradually raised and the sintering is finished at a temperature of from 12000 to 1 3500C. In such a case, Ni-Mn powder becomes molten first and this liquid phase is diffused into the stainless steel powder particles and the Ni-Cr powder particles as the sintering proceeds, and when the sintering temperature reaches 1 3500C or so, the Ni-Cr powder starts to become molten and the liquid phase sintering proceeds, providing a porous body.To increase the pore ratio of the porous body, it is preferred that the powder mixture is loosely formed into the required shape prior to sintering; but when the pore ratio does not have to be so high, the powder mixture may be formed under a predetermined pressure prior to sintering. In such a case, since the powder mixture is pressed during forming, a denser sintered body is obtained. Also it is possible to fill voids of the sintered body with a lubricant and a material for increasing bearing performance, if the sintered body is used as a bearing material. These materials may be, for example, a sulphide, oxide, metal, inorganic substance or organic substance.

In the mixing of the Ni-Mn or Ni-Cr powder with the stainless steel powder, it is preferred that the particle size of the Ni-Mn or Ni-Cr powder or both is smaller than the stainless steel powder particle size. The particle size of the stainless steel powder is preferably adjusted to the range of from 20 to 100 mesh and, in this case, it is desirable that the particle size of the Ni-Mn or Ni-Cr powder is less than 100 mesh. Furthermore, the mixing ratios of the Ni-Mn and the Ni-Cr powder to the stain less steel powder can be determined in accordance with the pore ratio and the alloy composition of the porous body which it is desired to obtain, but it is usually preferred that the Ni-Mn powder be in the range of from 5% to 10% and the Ni-Cr powder be 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 it is mixed with the Ni-Cr or Ni-Mn powder. When the powder mixture is sintered into a porous body after press-forming the powder mixture into a required shape, 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 has been reduced by the preheat treatment, the pressure for the press-forming may be very small, allowing easier manufacture of porous bodies.

The invention will be further described with reference to the following examples:- Example 1 A ferritic type stainless steel powder (with a mean particle size of 70 mesh), consisting by weight of 0.2% Co; 0.9% Si; 0.1% Mn, 17.5% Cr, 1% Mo and the balance Fe, was mixed with a Ni Mn alloy powder (with a mean particle size of 150 mesh) consisting by weight of 60% Mn and 40% Ni, and a Ni-Cr alloy powder (with a mean particle size of 1 50 mesh) consisting by weight of 40% of Ni and 60% of Cr, the mixture being in the ratio of 80 stainless steel to 10 Ni-Mn to 10 Ni-Cr by weight. Then, the powder mixture was loosely packed in a heat-proof vessel, which was placed in a furnace under non-pressure conditions.In the furnace the dew point of the atmosphere was -450C and the powder mixture packed in the vessel was sintered at 1 2000C for 60 minutes, the sintering temperature being gradually raised from 10500C.

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 particles.

Example 2 An austenitic type stainless steel powder (with a mean particle size of'70 mesh) consisting by weight of 0.2% Co; 0.9% Si; 0.2% Mn; 10.5% Ni; 19% Cr and the balance Fe was mixed with a Ni Cr alloy powder (with a mean particle size of 1 50 mesh) consisting by weight of 50% Ni and 50% Cr, a Ni-Mn alloy powder consisting by weight of 60% Ni and 40% Mn and a Mn-Cu powder consisting by weight of 35% Mn and 65% 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 nonpressure conditions. In the furnace the dew point of the atmosphere was -450C and the powder mixture packed in the vessel was first sintered at 980cm for 30 minutes and then further sintered at 1350"C for an hour.In this example, at 9800C, the Cu-Mn powder was molten and precipitated on the stainless steel powder particles; at about 1 0200C, the Ni-Mn powder was molten; at 1 3400C, the Ni-Cr powder was molten; and at 1 3500C, the sintering was completely finished. A porous body with a pore ratio of 30% was obtained.

Examples 3 and 4 The same powder mixtures as used in Examples 1 and 2 were prepared. During the preparation about 1% by weight of zinc stearate was added to each mixture, the stainless steel powder was heat treated to reduce its hardness to HRC40 or thereabouts. Each powder mixture was sintered after being pressed under a pressure of 7 tons/cm2. The sintering conditions were the same as those employed in Examples 1 and 2, respectively. In this Example the low melting alloys 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 powder and a Ni-Cr powder 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 in which the low melting alloy has disappeared can easily be obtained.

Although the foregoing description has been given on the assumption that the particles of the stainless steel powder are substantially spherical, the particles need not necessarily be spherical but may also be irregular-shaped, such as being rodlike or of other shapes.

Claims (8)

Claims

1. A method of sintering stainless steel powder comprising mixing the stainless steel powder with a powder containing Ni-Mn alloy powder and Ni Cr alloy powder, forming the powder mixture into a required shape, and sintering the formed powder mixture in a non-oxidising atmosphere at a temperature at or above the melting point of the Ni-Mn alloy powder but below the melting point of the stainless steel powder.

2. A method according to Claim 1, wherein the stainless steel powder is heat treated at a temperature of from 3000C to 6000Cto reduce its hardness prior to the mixing.

3. A method according to Claim 1 or Claim 2, wherein the stainless steel powder is mixed with copper powder or copper alloy powder in addition to the Ni-Mn alloy powder and the Ni-Cr alloy powder, and the powder mixture is sintered at a temperature higher than the melting point of the copper powder or copper alloy powder.

4. A method according to any one of Claims 1 to 3, wherein a sintering promoting component and/or a matrix reinforcing component are added to the powder mixture.

5. A method according to any one of the preceding Claims, wherein the non-oxidizing atmosphere is a hydrogen gas atmosphere and the dew point of the atmosphere is from -300 to -600C.

6. A method according to any one of the preceding Claims, wherein the powder mixture is formed to the required shape under a pressure of from 2 to 10 t/cm2.

7. A method according to any one of Claims 1 to 5, wherein the powder mixture is formed to the required shape by loose packing.

8. A method according to Claim 1, substantially as described with reference to any one of Examples 1 to 4 herein.