JPH09202950A - Austenitic stainless steel sintered compact and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce an austenitic stainless steel sintered compact excellent in corrosion resistance and to provide a method for economically producing the same. SOLUTION: In a stainless steel sintered compact contg., by weight, 16 to 20% chromium and 7 to 15% nickel or furthermore contg. 2 to 4% molybdenum, and the balance substantial iron, a layer having 1% porosity is applied to the surface of the sintered compact by >=10μm thickness. It is sintered in the temp. range in which ferrite is present in a hydrogen stream and is thereafter held in the temp. range in which the phase is formed into an austenite single one, by which the layer having <=1% porosity is applied to the surface of the sintered compact by >=10μm thickness.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an austenitic stainless steel sintered body having excellent pitting corrosion resistance and a method for producing the austenitic stainless steel sintered body which can economically obtain the sintered body. .

[0002]

2. Description of the Related Art Sintered parts produced by powder metallurgy have a shape close to that of the final product, and the machining process can be largely omitted. In order to obtain the desired part shape by the powder metallurgy method, in addition to the conventionally used powder pressing method, a method of mixing an organic substance and a metal powder to obtain a product having a complicated shape by injection molding is also put into practical use. In recent years, the demand has been increasing. Thus, austenitic stainless steel has excellent corrosion resistance, especially pitting corrosion resistance, and is therefore widely used in the applications of these sintered parts.

However, the density of the sintered body obtained by the powder metallurgy method is 90 to 95 that of the parts obtained by the melting method.
%, And pores exist inside the sintered body. Since the pores act as a starting point of pitting corrosion generation, the austenitic stainless steel sintered body has a lower pitting corrosion generation potential and inferior corrosion resistance as compared with the ingot. If such a sintered component is used, pitting corrosion may occur depending on the use environment, and in a serious case, a leakage accident due to the through hole may occur. In order to prevent such an accident, it is necessary to raise the density of the sintered body to the same level as that of the ingot, and in order to reduce the pores in the sintered body and increase the density, it is considered to raise the sintering temperature. To be
As a method for complete densification, there is a hot isostatic pressing method and the like, which has been put to practical use in tools and super heat resistant materials.

[0004]

However, even if sintering is performed at a temperature just below the melting point, the density of the sintered body is at most 97.
%, And the corrosion resistance cannot be greatly improved. Further, this method has a problem that it causes great difficulty and danger in temperature control, and further, the hot isostatic pressing method is high in cost and economically difficult to apply to general austenitic stainless steel. There is a problem.

As described above, the corrosion resistance of sintered parts is affected by the presence of pores, that is, the density of the sintered body, but the chemical composition of the outermost surface and the pores and grain sizes contribute to the corrosion resistance. Is a microscopic structure of.

An object of the present invention is to provide an austenitic stainless steel sintered body having excellent corrosion resistance and an economical manufacturing method thereof.

[0007]

The present inventor has carried out research to solve the above problems and achieve the above objects, and as a result, formed a dense layer of a specific thickness on the surface of an austenitic stainless steel sintered body. The present invention has been completed by finding that the object can be achieved by providing it, or by performing sintering or heating and holding at a specific temperature. That is, the first embodiment of the present invention comprises 16 to 20% by weight of chromium and 7 to
In a stainless steel sintered body containing 15% by weight of nickel and the balance substantially consisting of iron, an austenitic stainless steel having a layer having a porosity of 1% or less with a thickness of 10 μm or more on the surface of the sintered body. Characterized by a sintered body, and second
Embodiments of 16-20% by weight of chromium, and 7-15
In a stainless steel sintered body containing nickel of 2% by weight and molybdenum of 2 to 4% by weight, and the balance being substantially iron, a layer having a porosity of 1% or less was formed on the surface of the sintered body in an amount of 10 μm.
The third embodiment is characterized by an austenitic stainless steel sintered body having a thickness of m or more. Further, the third embodiment becomes an austenite single phase after sintering in a temperature range where ferrite exists in a hydrogen stream. By maintaining the temperature range, a layer having a porosity of 1% or less can be formed on the surface of the sintered body.
The method is characterized by a method for producing an austenitic stainless steel sintered body having a thickness of 0 μm or more.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The corrosion resistance of a sintered part is first determined by its chemical composition. That is, when chromium is contained in iron in an amount of 16% by weight or more, a dense passivation film is formed on the surface and the corrosion resistance is greatly improved. Chromium is more excellent in corrosion resistance as the content thereof is higher, but as the cost increases, and when the chromium content exceeds 20% by weight, a chromium-rich sigma phase precipitates, so that the chromium concentration in the vicinity of the precipitation phase increases. It becomes a starting point of occurrence of pitting corrosion. Chromium is also a ferrite stabilizing element, and it becomes difficult to form an austenite phase having excellent pitting corrosion resistance together with the amount of chromium. Therefore, an austenite phase can be obtained by adding nickel in an amount of 7 to 15% by weight, but the stability and corrosion resistance of the austenite phase do not change even if nickel is added in an amount of more than 15% by weight. Molybdenum
Although it forms a solid solution in iron to further increase pitting corrosion resistance, if the addition amount is less than 2% by weight, the effect of improving pitting corrosion resistance cannot be obtained, and if it exceeds 4% by weight, ferrite phase and sigma phase are formed. To deposit.

The metal powder used as a raw material for these sintered parts can be obtained by mixing several kinds of metal powders to obtain a desired composition. In recent years, however, an atomizing method using water or an inert gas has been used. It is widespread, its composition can be freely changed, and powder having a particle size of about several tens of μm can be obtained at low cost.

The powder may be molded by any method such as powder molding using a die press, powder injection molding in which a binder is mixed and injection molding is performed, but powder molding is a three-dimensional complex shape. Molding of articles is difficult, and powder injection molding requires removal of the binder by heating or solvent extraction before sintering. The molding method is selected according to the product shape.

The austenitic stainless steel sintered body varies depending on the composition and is sintered at 1250 to 1350 ° C. A vacuum or a reducing atmosphere is used as the sintering atmosphere, but since chromium has a high vapor pressure, it is necessary to take measures to suppress the vaporization of chromium when sintering is performed in vacuum. The density of the sintered body is generally about 90 to 95%,
By performing the treatment in hydrogen at 250 ° C. or higher, a dense layer having a porosity of 1% or less (hereinafter referred to as “dense layer”) is formed on the surface. The thickness of this layer increases with sintering temperature and time,
For example, even at 1350 ° C. for 2 hours, it is about 6 μm. Since the surface of the sintered body is generally not smooth and foreign matter is attached, it is rarely used as it is as a machine part, and blasting or other polishing is performed. At this time, since the surface layer is removed by about several μm, the dense layer hardly remains with the above thickness.
Therefore, in order to obtain high corrosion resistance as a final product, the dense layer on the surface of the sintered body needs to be 10 μm or more. In order to obtain a surface dense layer of 10 μm, it is necessary to perform sintering at 1350 ° C. for 4 hours or more in a hydrogen atmosphere.
The denser the layer, the more reliable it is, but the growth rate follows the parabolic law, and therefore, the sintering time needs to be quadrupled in order to obtain the doubled thickness, so that the dense layer can be manufactured. Is not economical.

Stainless steel has different phases depending on its composition and temperature. When sintering is performed at a high temperature at which a ferrite phase appears, the formation rate of the surface dense layer increases, which is presumably because the diffusion rate in the ferrite phase is higher than the diffusion rate in the austenite phase. However, if sintering is performed at a temperature at which ferrite exists, the sigma phase remains in the sintered body due to the ferrite phase and the cooling rate. Since these phases are rich in chromium,
When chromium is present in the sintered body, the concentration of chromium around the sintered body lowers, which serves as a starting point for the occurrence of pitting corrosion and reduces corrosion resistance. Therefore, sintering is performed at a temperature in which a ferrite phase is present, a dense layer having a thickness of 10 μm or more is formed on the surface, and then the temperature is lowered to maintain the temperature in the austenite single-phase region, thereby eliminating the ferrite phase and reducing corrosion resistance. It was something that could be prevented. However, even if the dense layer is formed to have a thickness of more than 30 μm, there is no difference in the effect of preventing deterioration of corrosion resistance.

Since the formation rate of the dense layer at the temperature where the ferrite phase exists is several times higher than that in the austenite temperature range, even if the holding time in the austenite temperature range is added, it is necessary to obtain the dense layer having the same thickness. The setting time can be shortened as compared with the case where sintering is performed in the austenite temperature range alone. This is particularly advantageous when increasing the thickness of the dense layer.

Since the temperature at which the ferrite phase appears differs depending on the composition of the alloy, the sintering temperature and the holding time must be determined according to the composition of the powder. The sintering time depends on the thickness of the dense layer to be obtained. Generally, a holding time of about 1 hour is sufficient, but it is necessary to appropriately determine the holding time depending on the soaking property in the furnace and the size of the sample. The sintered body thus obtained can be used with a sufficient dense layer left after the post-treatment such as surface polishing as described above.

[0015]

EXAMPLES Examples of the present invention will be described below together with comparative examples.

Example 1 Water atomized stainless steel powder 1 having a composition as shown in Table 1 below was mixed with a wax-based binder, and a finished dimension of 8 mm × 4 mm × 30 mm was obtained by an injection molding method. A test piece was manufactured. This was degreased in nitrogen at 350 ° C. and then sintered in hydrogen at 1350 ° C. for 4 hours to obtain a sintered body. The sinter obtained was not blast-polished, the sample was inserted into a sealed container, artificial sweat was placed in the container up to 1/2 of the sample height, and the sample was immersed at 25 ° C. for 24 hours and then visually examined. The presence or absence of corrosion was examined. Also,
The dense layer thickness of the sample surface was measured by observing the sample section with an optical microscope. The results are shown in Table 2 below.

Example 2 Various tests were conducted in the same manner as in Example 1 except that a part of the sintered body obtained by the same treatment as in Example 1 was blast-polished. Table 2 shows the results.

Example 3 Example 1 except that water atomized stainless steel powder 2 having the composition shown in Table 1 was used.
The resulting sintered body was treated in the same manner as above, and various tests were conducted in the same manner as in Example 1 except that the immersion temperature in artificial sweat was 50 ° C. Table 2 shows the results.

Example 4 Example 1 except that water atomized stainless steel powder 2 was used and the sintering time was 8 hours.
Various tests were conducted in the same manner as in Example 1 except that a part of the obtained sintered body was treated by the same manner as in 1. and the temperature during immersion in artificial sweat was 50 ° C. without blasting and polishing. Table 2 shows these results.
Shown in

Example 5: Example 1 except that water atomized stainless steel powder 3 having the composition shown in Table 1 was used.
Various tests were conducted in the same manner as in Example 1 except that a part of the obtained sintered body was treated by the same manner as in 1. and the temperature during immersion in artificial sweat was 50 ° C. without blasting and polishing. Table 2 shows these results.
Shown in

Example 6 A sintered body obtained by the same process as in Example 1 except that the water atomized stainless steel powder 3 was used, the sintering temperature was 1300 ° C., and the sintering time was 12 hours. Was subjected to various tests in the same manner as in Example 1, except that the temperature during immersion in artificial sweat was set to 50 ° C. without blasting and polishing.
Table 2 shows the results.

Example 7 Example 1 except that water atomized stainless steel powder 3 was used and the sintering time was 4 hours.
In the same manner as in Example 1, various tests were performed in the same manner as in Example 1 except that the obtained sintered body was not blast-polished and the temperature during immersion in artificial sweat was 50 ° C. Table 2 shows the results.

Example 8: The same treatment as in Example 1 was carried out except that the water atomized stainless steel powder 3 was used and the sintering time was 16 hours, and a part of the obtained sintered body was blast-polished. After finishing, set the temperature during immersion in artificial sweat to 50 ° C,
Various tests were conducted in the same manner as in Example 1. Table 2 shows the results.

[0024]

[Table 1]

[0025]

[Table 2]

Comparative Example 1: Example 1 except that water atomized stainless steel powder 1 was used and the sintering time was 2 hours.
Various tests were performed in the same manner as in Example 1 except that the obtained sintered body was treated in the same manner as in 1. The results are shown in Table 3 below.

Comparative Example 2: Water atomized stainless steel powder 1 was used, treated in the same manner as in Comparative Example 1, a part of the obtained sintered body was blast-polished, and then various tests were conducted in the same manner as in Example 1. The results are shown in Table 3.

Comparative Example 3: Water atomized stainless steel powder 2 was used and the same treatment as in Comparative Example 1 was carried out except that the sintering temperature was 1280 ° C., and a part of the obtained sintered body was blast-polished. The temperature during immersion in artificial sweat at 50 ° C without
Various tests were conducted in the same manner as in Example 1, and these results are shown in Table 3.

Comparative Example 4: Water atomized stainless steel powder 2 was used, treated in the same manner as in Comparative Example 1, a part of the obtained sintered body was blast-polished, and then the temperature during immersion in artificial sweat was 50. Various tests were conducted in the same manner as in Example 1 at a temperature of ° C. Table 3 shows the results.

Comparative Example 5: A sintered body obtained by the same treatment as Comparative Example 1 except that the water atomized stainless steel powder 3 was used, the sintering temperature was 1300 ° C., and the sintering time was 4 hours. Was subjected to various tests in the same manner as in Example 1, except that the temperature during immersion in artificial sweat was set to 50 ° C. without blasting and polishing. Table 3 shows the results.

Comparative Example 6: Water atomized stainless steel powder 3 was used, treated in the same manner as in Comparative Example 1, a part of the obtained sintered body was blast-polished, and then the temperature during immersion in artificial sweat was 50. Various tests were conducted in the same manner as in Example 1 at a temperature of ° C. Table 3 shows the results.

Comparative Example 7: 18.2% by weight chromium, 8.0
With respect to the ingot material containing nickel by weight, various tests were carried out in the same manner as in Example 1 without blast-polishing a part of the obtained sintered body. Table 3 shows the results.

Comparative Example 8: 18.2% by weight chromium, 8.0
A part of the ingot material containing nickel by weight was blast-polished, and then various tests were conducted in the same manner as in Example 1. Table 3 shows the results.

[0034]

[Table 3]

From the above, none of the samples according to the present invention is corroded by immersion in artificial sweat. No corrosion is observed in the ingot. On the other hand, in the comparative example sintered body, it is understood that the dense layer is not formed on the surface due to the sintering temperature, the sintering time, and the polishing, or the thickness is less than 10 μm, and the pitting corrosion is also generated.

Example 9: Water atomized stainless steel powder 4 having the composition shown in Table 4 below was mixed with a wax-based binder, and the finished dimensions were 8 mm × 4 mm × 30 mm by injection molding. A test piece was manufactured. This was degreased in nitrogen at 350 ° C., sintered in hydrogen at 1400 ° C. for 2 hours, and then held at 1350 ° C. for 1 hour to obtain a sintered body. After blasting and polishing the obtained sintered body, insert the sample into a sealed container, put artificial sweat up to 1/2 of the sample height in the container, immerse it at 25 ° C for 24 hours, and then visually corrode it. Was checked for. The dense layer thickness on the sample surface was measured by observing the sample section with an optical microscope. The results are shown in Table 5 below.

Example 10: Using the water atomized stainless steel powder 4, the same treatment as in Example 9 was carried out except that the holding time after sintering was 2 hours. Various tests were conducted in the same manner as in Example 9. Table 5 shows the results.

Example 11: Obtained by treating in the same manner as in Example 9 except that water atomized stainless steel powder 5 having the composition shown in Table 4 was used and the holding time after sintering was 2 hours. For the sintered body, the artificial sweat immersion temperature was set to 50 ° C,
Various tests were conducted in the same manner as in Example 9. Table 5 shows the results.

Example 12: A water-atomized stainless steel powder 5 was used and the sintering time was set to 4 hours, but the sintered body obtained by the same treatment as in Example 9 was subjected to artificial sweat immersion temperature. Various tests were conducted in the same manner as in Example 9 at 50 ° C. Table 5 shows the results.

Example 13: A water-atomized stainless steel powder 5 was used and treated in the same manner as in Example 9 except that the sintering time was 1 hour. Various tests were conducted in the same manner as in Example 9 at 50 ° C. Table 5 shows the results.

Example 14: A water-atomized stainless steel powder 5 was used and the sintering temperature was 1380 ° C. The same procedure as in Example 9 was carried out. Various tests were conducted in the same manner as in Example 9 at 50 ° C. Table 5 shows the results.

Example 15: Using water atomized stainless steel powder 5, the sintering time was 4 hours, and the holding temperature was 1.
The same treatment as in Example 9 was carried out except that the temperature was changed to 300 ° C., and the various tests were carried out in the same manner as in Example 9 with the artificial sweat immersion temperature set to 50 ° C. for the obtained sintered body. Table 5 shows these results.
Shown in

[0043]

[Table 4]

[0044]

[Table 5]

Comparative Example 9: The same process as in Example 9 was carried out except that water atomized stainless steel powder 4 was used for sintering at a sintering temperature of 1350 ° C. for 2 hours and the subsequent holding treatment was not carried out. Various tests were carried out on the obtained sintered body in the same manner as in Example 9 except that the artificial sweat immersion temperature was set to 25 ° C. The results are shown in Table 6 below.

Comparative Example 10: A sintered body obtained by the same treatment as in Comparative Example 9 except that the water atomized stainless steel powder 4 was used and the sintering treatment was performed at a sintering temperature of 1400 ° C. for 4 hours. Various tests were conducted in the same manner as in Example 9 with the artificial sweat immersion temperature set to 25 ° C. Table 6 shows the results.

Comparative Example 11: A sintered body obtained by the same treatment as in Comparative Example 9 except that the water atomized stainless steel powder 5 was used and the sintering treatment was performed at a sintering temperature of 1280 ° C. for 2 hours. Various tests were conducted in the same manner as in Example 9 with the artificial sweat immersion temperature set to 50 ° C. Table 6 shows the results.

Comparative Example 12: A water-atomized stainless steel powder 5 was used and the sintering temperature was 1400 ° C., and the same treatment as in Comparative Example 9 was carried out. Various tests were conducted in the same manner as in Example 9 at 50 ° C. Table 6 shows the results.

Comparative Example 13: A water-atomized stainless steel powder 5 was used and the sintering time was 3 hours. The same treatment as in Comparative Example 9 was carried out. Various tests were conducted in the same manner as in Example 9 at 50 ° C. Table 6 shows the results.

Comparative Example 14: A water-atomized stainless steel powder 5 was used and the sintering time was 8 hours, and the same treatment as in Comparative Example 9 was carried out. Various tests were conducted in the same manner as in Example 9 at 50 ° C. Table 6 shows the results.

Comparative Example 15: A sintered body obtained by the same process as in Comparative Example 9 except that the water atomized stainless steel powder 5 was used and sintered at a sintering temperature of 1300 ° C. for 8 hours. Various tests were conducted in the same manner as in Example 9 with the artificial sweat immersion temperature set to 50 ° C. Table 6 shows the results.

Comparative Example 16: A sintered body obtained by the same process as in Comparative Example 9 except that the water atomized stainless steel powder 5 was used and the sintering process was performed at a sintering temperature of 1350 ° C. for 15 hours. Various tests were conducted in the same manner as in Example 9 with the artificial sweat immersion temperature set to 50 ° C. Table 6 shows the results.

[0053]

[Table 6]

From these results, according to the present invention, 138
By performing sintering at 0 to 1400 ° C. for 1 to 4 hours,
Forming a dense layer of 10 μm or more on the surface of the sintered body,
Sintering for 2 hours yields a dense layer of about 10 μm, and sintering for 4 hours yields a dense layer of 20 μm or more.
By holding at 0 ° C. for 1 to 2 hours, an austenite single-phase sintered body in which a ferrite phase did not remain was obtained, and in any of these samples, corrosion due to artificial sweat immersion did not occur, and therefore, about 1 hour. It can be seen that it takes 3 hours in total to obtain a dense layer of 10 μm and 5 hours in total to obtain a dense layer of 20 μm. This is a shorter time than the methods shown in Examples 1 to 8, and it can be seen that the manufacturing method of the present invention is superior. On the other hand, in Comparative Examples 9, 11, and 13 which were sintered at 1280 to 1350 ° C., which is the austenite temperature range, a dense layer of 10 μm could not be obtained by sintering for up to 3 hours, and Both of them caused pitting corrosion. Comparative Example 1 in which sintering was performed at 1400 ° C
In Nos. 0 and 12, a dense layer of 10 μm or more was formed, but the ferrite phase remained, and pitting corrosion still occurred in the artificial sweat immersion test. 10μ after sintering in austenite temperature range
In order to obtain a dense layer of m or more, a comparative example 14 (manufacturing method corresponding to Example 4 is used, and this manufacturing method shows that it takes a long time to obtain a sintered body such as the product of the present invention. As a comparative example), 4 hours or more,
As shown in Comparative Example 16, it can be seen that sintering for about 16 hours is required to obtain 20 μm or more.

As described above, according to the present invention, the sintered body is excellent in pitting corrosion resistance, and it is understood that such a sintered body can be produced in a short time.

[0056]

The present invention provides a dense layer having a specific thickness on the surface of an austenitic stainless steel sintered body,
Further, since it is a manufacturing method in which sintering and heating are held at a specific temperature, a sintered body having excellent pitting corrosion resistance can be obtained, and a single-phase austenitic stainless steel sintered body can be economically manufactured. A remarkable effect is recognized.

Claims (3)

[Claims] 1. 16-20% by weight chromium and 7-15
In a stainless steel sintered body containing nickel by weight and the balance substantially consisting of iron, a layer having a porosity of 1% or less is formed on the surface of the sintered body in a thickness of 10 μm or more. Austenitic stainless steel sintered body. 2. 16-20% by weight of chromium and 7-15
In a stainless steel sintered body containing nickel of 2% by weight and molybdenum of 2 to 4% by weight, and the balance being substantially iron, a layer having a porosity of 1% or less was formed on the surface of the sintered body in an amount of 10 μm.
An austenitic stainless steel sintered body having a thickness of m or more. 3. A layer having a porosity of 1% or less is formed on the surface of a sintered body by performing sintering in a temperature range where ferrite is present in a hydrogen stream and then maintaining the temperature range where an austenite single phase is obtained. The method for producing an austenitic stainless steel sintered body is characterized in that it is provided with a thickness of 10 μm or more.