JPS61253349A - Corrosion resistant sintered stainless steel and its manufacture - Google Patents

Abstract

PURPOSE:To manufacture a sintered austenitic stainless steel article having superior corrosion resistance and low porosity by mixing austenitic stainless steel powder with a specified amount of Fe-P powder, compacting the powdery mixture and sintering it in a reducing atmosphere or in vacuum. CONSTITUTION:Austenitic stainless steel powder is mixed with Fe-P powder contg. 25.8% P so that the powdery mixture contains 0.2-1.0wt% P. The powdery mixture is compacted to a prescribed shape and sintered in two steps in an atmosphere to a reducing gas such as hydrogen or in vacuum at 1,000-1,045 deg.C which is below the eutectic point of the Fe-P and at 1,200-1,400 deg.C which is above the eutectic point of the Fe-P. A liq. phase is produced during the sintering by the presence of the Fe-P as a low m.p. alloy, causing thermal shrinkage and a sintered austenitic stainless steel article having superior corrosion resistance and <=7% porosity is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical field to which the invention pertains] This invention relates to a sintered stainless steel with excellent corrosion resistance and a method for manufacturing the same.

[Prior art and its problems]

Austenitic stainless steel, represented by 18Cr-8Ni steel, has relatively superior corrosion resistance, acid resistance, and heat resistance compared to other steel types, so it is used in nitric acid, sulfuric acid, ammonia synthesis, urea, chemical fibers, and paper manufacturing. Not only is it widely used in various chemical industries such as food and dyeing, but also in architecture, houses, noodle kitchens, and turbine electrical applications.

It is also used in areas such as aircraft jets, engines, automobiles, IIL@, ships, and nuclear reactors.

Conventionally, ingot materials (melting → casting → plastic working → heat treatment) have been used exclusively for the above-mentioned purposes. However, since austenitic stainless steel is sticky and non-rigid compared to iron-based and steel thread metal materials, it has the disadvantage that it takes time to cut and has a short lifespan. Therefore, from the viewpoint of reducing costs and simplifying the manufacturing process of parts, it is desirable to manufacture austenitic stainless steel parts using powder metallurgy, which requires no machining or machining. However, there are many examples in which pores exist in ordinary powder metallurgy products. In general, the mechanical g1 of a part 1 member
Since the presence of these pores significantly deteriorates the corrosion resistance, it is preferable that the number of pores be as small as possible. The following two methods are considered as methods for reducing the amount of pores.

(2) Mixing and adding components with a lower melting point than the main raw material powder causes a liquid phase to appear during sintering, thereby increasing the density.

■Once sintered, it is recompressed and resintered.

Although effective additive elements have been found for the method (2) above for iron powder and carbon steel powder, the effective ingredients for austenitic stainless steel alloy powder have not yet been sufficiently investigated. The above method ■ is effective in increasing the density, but
The process becomes complicated, causing problems in economic efficiency.

[Purpose of the invention]

This invention was made in view of the above, and takes advantage of the characteristics of powder metallurgy, which is a non-cutting and cutting-saving processing method.
The purpose of the present invention is to provide a sintered stainless steel with excellent corrosion resistance and a method for manufacturing the same.

[Key points of the invention]

This invention utilizes austenitic stainless steel alloy powder (
Nis~22%, Cr16~26%, MoO~3%.

CuO~2.5%, SiQ~1.5%, M11
In order to obtain sintered stainless steel with a porosity of 7% or less using only molding and sintering as the main raw material (0 to 2%),
Add and mix 0.2 to 1% by weight of phosphorus (P) with ferroline alloy powder (Fe-P) to austenitic stainless steel alloy powder, and after molding, heat the mixed powder at 100° to 1000° in a reducing or vacuum atmosphere. The gist is to perform temporary sintering at 1045°C and then sintering at 1200°C to 1400°C.

The present inventors intended to increase the density of sintered stainless steel by liquid phase sintering. For this reason, 7eroporon (Fe-8), which has a lower melting point than the austenitic stainless steel alloy powder, is added to the austenitic stainless steel alloy powder.

Ferroaluminum (Fe-AA), 7 x C'sy (
Fe-P).

The effect of adding and mixing eutectic alloy powder such as Ferrotita 7 (Fe-Ti) on density increase was investigated. In parallel, we also determined polarization curves in a sulfuric acid solution with the aim of understanding the basic corrosion tendency of sintered stainless steel.
The relationship between corrosion resistance and porosity was discussed.

From the above study results, we have found that for every 0.1 degree of corrosion in a % sulfuric acid solution, which is empirically used as a guideline for determining the corrosion resistance of austenitic stainless steel, the porosity of sintered stainless steel is 7.
% or less, and in order to obtain a sintered body with a porosity of 7% or less by only molding and sintering, add and mix 0.2 to 1% phosphorus in the form of ferroline alloy powder, After molding, use an aqueous atmosphere with a dew point below ℃ or 10 to
In a vacuum atmosphere below rr, the eutectic point of ferroline (approximately 10
Sintered at 1000° to 1045°C lower than 50°C, 4
1200-1400″C above the eutectic point of ferroline
It was confirmed that this is possible if sintered with

The reason why phosphorus is added in the form of ferroline rather than red phosphorus in the present invention is that the latter causes no segregation of phosphorus and generates a liquid phase more uniformly. Also, regarding the amount of phosphorus added, 0.2
If the phosphorus content is less than 1%, the sintered body will not be sufficiently densified due to the generation of a liquid phase.On the other hand, if the phosphorus content is more than 1%, the porosity will decrease, but too much liquid phase will be generated and the sintered body will not be sufficiently densified. It is important that the amount of phosphorus added is in the range of 0.2 to 1% because it causes residual phases, reduces corrosion resistance, and becomes brittle, making it impractical.

In addition, the reason why the sintering process is made into two stages is that by performing sintering at a temperature of 1000° to 1045°C, which is below the eutectic point of ferroline, sintering between the stainless steel alloy powders progresses and a strong skeleton is created. This is because it has been recognized that the shrinkage of the sintered body becomes even and effective in improving dimensional accuracy. on the other hand,
The sintering temperature above the eutectic point of ferroline is 1200° to 1
The reason for setting the temperature at 400°C is that at temperatures below 1200°C, phosphorus does not diffuse sufficiently and segregates at grain boundaries, and does not contribute to densification of the sintered body.At temperatures above 1400°C, the temperature is too high and the furnace is damaged due to evaporation of Cr. damage within.

It is preferable that the amount is less than this range since disadvantages such as composition fluctuation of the sintered body and variation in shrinkage of the sintered body occur. In addition, the object of the invention can be achieved if the compacting pressure of the mixed powder in the present invention is 2-7-complex and the holding time during sintering is in the range of 0.5 to 2 hours.

[Embodiments of the invention]

The present invention will be explained below based on examples. The austenitic stainless steel alloy powder used in the present invention is
It is SUS 304 L powder manufactured by Fukuda Metal Foil Powder. 1st
The properties are shown in the table.

Table 1: Adding phosphorus to US304L alloy powder (ferroline alloy powder)
Fe-25,8%P) 0.0.1.0.15.0.2
0.0.5.0.8゜1.0, 1.2, 1.5%, mixed with a V-type mixer, and then heated at a pressure of 7 to 11' x 1 oin' to form a molded product. Obtained. Sintering is 5X1
The test was carried out under the conditions of 1000''CxQ, 5h to 1300°C for 1.5h in a vacuum of 0 torr or less. The relationship between the porosity of the sintered body obtained under the above conditions and the amount of phosphorus added is shown in FIG.

The porosity decreases as the amount of phosphorus added increases, and at 0.2% phosphorus, the porosity becomes 7% or less.

When the phosphorus content is 0.8% or 1.0%, the porosity is almost zero. However, when the phosphorus content exceeds 1%, the porosity tends to increase, confirming that the addition of phosphorus is excessive. Incidentally, even if the molding pressure and sintering conditions were slightly changed from the above conditions, the relationship between the porosity and the phosphorus addition cap was almost the same as the results shown in diagram g1.

The polarization curve of sintered stainless steel in a 20% sulfuric acid solution was measured under the conditions shown in Table 2.

Table 2 Before measuring the polarization curve, place the measurement surface of the sample on emery paper (
After dry polishing with a polishing machine (600), and then ultrasonic cleaning with acetone, it was thoroughly dried. After this, Figure 2.

As shown in Figure 3, sample IK has a sample surface of 10 x 10 x m.

A lead wire 2 was attached, and the surface of the sample other than the measurement surface was covered with silicone sealant (Shin-Etsu Chemical Exhibition KE45) 3, and the polarization curve was measured.

The polarization curves show phosphorus-added sintered stainless steel according to the present invention, sintered stainless steel with porosity changed by molding and sintering conditions using 5US304L alloy powder without adding phosphorus, and Cr18.3. %, and 808304 melt processed material (solution treated material) K containing 9.3% Ni.

An example of measuring the polarization curve is shown in FIG. From Figure 4, melt processed material 11. Sintered stainless steels 12 and 13 with 0.8% and 0.2% phosphorus addition and with a porosity of 7% or less show a clear passivation region, whereas sintered stainless steels with a porosity of 10% This shows that the sintered stainless steel (5US304L) is not passivated due to the presence of pores, which confirms that the sintered stainless steel of the present invention and its manufacturing method are effective in improving corrosion resistance.

With respect to the porosity of sintered stainless steel obtained in the study results related to non-invention, the passivation holding current density (unpassivated sample is +0.1 to 0.9 VVs SCE range plateau) fs
It will look like Figure 5. This shows that as the porosity decreases, the current density decreases and the corrosion resistance improves. When the porosity is 7% or less, the current density becomes 10 Affl or less. When the corrosion rate IfL (corrosion degree) is calculated using the method below for this current density lQ Am, the value is 0.07.
7, which satisfies the corrosion resistance of 0.1 fir years or less, which is a standard for the corrosion resistance of austenitic stainless steel. The relationship between polarization curve and corrosion rate is as follows. In other words, since corrosion is an electrochemical reaction on the metal surface,
The corrosion rate can be estimated by applying an external potential of K and measuring the exchange of electrons with an ammeter. In sulfuric acid solution, stainless steel is Fe-+Fe-)-3e...
・・・・・・・・・・・・・・・・・・・・・・・・(
As 1), the elution is taken out, and as a reduction reaction, 2H"+26-→H2...
Possible reactions include (2). Therefore, if stainless steel is passivated, i, (passivation holding current density) becomes a parameter for the corrosion rate. If stainless steel is melted by the reaction of formula (1), the corrosion rate V when i = 10 km is obtained by formula (3).

Here, M: grams of Fe, e: density of Fe.

n: Fe ionic valence, F: Faraday constant.

Therefore, V = 0.077 Based on more than a year of research, the K that makes the corrosion resistance of sintered stainless steel equivalent to that of melt-processed materials is the passivation holding current determined in a 20% sulfuric acid solution. 10 Am-'' or less, and this value can be satisfied if the porosity is 7% or less, and molding is possible.

If the process involves only sintering, it is clear that a porosity of 7% or less can be achieved by adding 0.2 to 1.0% phosphorus as ferroline alloy powder to the austenitic stainless steel alloy powder. .

〔Effect of the invention〕

As described above, the present invention adds and mixes 0.2 to 1% phosphorus with ferroline alloy powder to austenitic stainless steel alloy powder, and by devising the sintering process after forming, the process of forming and sintering is improved. It is possible to obtain a sintered body with a porosity of 7% or less by using only sintered stainless steel, and it is possible to provide sintered stainless steel whose corrosion resistance is almost the same as that of the melt-processed material. In addition, the method of the present invention can produce a member with excellent corrosion resistance only by molding and sintering, and enables non-machining and cutting machining, which also contributes to cost reduction.

[Brief explanation of drawings]

Figure 81 is a graph showing the relationship between the porosity of sintered stainless steel and the amount of phosphorus added, Figures 2 and 3 are a front view and side sectional view respectively showing the shape of the sample pole subjected to the polarization curve, and Figure 4. The figure is a graph showing an example of measuring the polarization curve of sintered stainless steel.
FIG. 5 is a graph showing the relationship between porosity of sintered stainless steel and passivation holding current. 1: Sample, 2: Lead wire, 3 Nizi-lant, 4 Niglass tube, 11: Polarization curve of 5US304 melt processed material, 1
2: Polarization curve of 0.8% phosphorus-added sintered stainless steel, 13: Polarization curve of 0.2% phosphorus-added sintered stainless steel, 14: Polarization curve of sintered stainless steel (SUS304L). Rin no Tokaro 1 [ (ju 11 z) Sai 1r ¥3 E/v y, s scε Age 4 figure

Claims (1)

[Claims] 1) Sintered stainless steel whose main raw material is austenitic stainless steel alloy powder, with a porosity of 7% after sintering.
A corrosion-resistant sintered stainless steel characterized by: 2) 0.2 to austenitic stainless steel alloy powder
1.0% by weight of phosphorus is added and mixed as ferroline alloy powder, and after molding, the mixed powder is sintered at 1000° to 1045°C in a reducing or vacuum atmosphere, and then sintered at 1000° to 1045°C.
A method for producing corrosion-resistant sintered stainless steel, characterized by sintering at a temperature of ~1400°C.