Ferrite stainless steel powder for a sintered body

Abstract: A powder is composed of ferrite stainless steel powder containing: C: ≤ 0.1 wt%, Si: ≤ 3.0 wt%, Mn: ≤ 0.30 wt%, Ni: ≤ 2.0 wt%, Cr: 11 to 22%, Mo: ≤ 3.0 wt%, the rest being substantially Fe. A metal compound of B, preferably CrB, is added to the powder with the amount of B being between 0.03 wt% and 0.2 wt% based on the weight of the powder.

Description: The present invention relates to a powder for a corrosion resistant sintered body. Conventionally, a sensor ring which issues pulses having frequencies in proportion to the rotation number of wheels in an anti-lock system of a vehicle breaking system, has been used. The sensor ring is shaped as a whole in a ring having many gear like concave and convex portions in the outer circumference for causing the sensor ring to issue pulse signals of a frequency in proportion to said wheel rotation number via an electromagnetic pick-up disposed in the vicinity of said gear like concave and convex portions. The sensor ring has a complicated configuration in the whole. If it is composed in an ingot, processing is expensive. As a result, conventionally, the sensor ring is composed of a powder sintered body. As materials for this kind of sensor ring, powders of ferrite stainless steels have conventionally been employed. However, the sensor ring composed of the sintered body of the ferrite stainless powders may develop cracks accompanying corrosion. The sensor ring is engaged with a shaft of an opposite material. When the shaft of the opposite material is expanded in diameter by corrosion, the elongation of the sensor ring can not follow this expansion and the possibility of cracks arises. To solve such problems, there exists a method of increasing the density of the sensor ring by sintering liquid phases. However, this idea has not been reduced to practice because the sizes vary considerably in the sensor ring thus requiring high precision. In the above description, these problems have been explained in connection with a sensor ring. However, these problems also arise when using mechanical parts in practice, particularly, parts used in conditions where they are fitted to or engaged with an opposite part. It is the object of the present invention to overcome the drawbacks and disadvantages of the prior art. This object is solved by the powder for a corrosion resistant sintered body according to independent claim 1. The invention is defined by the appended claims. The present invention generally relates to a powder for a corrosion resistant sintered body having excellent ductility, specifically, relates to powder for a corrosion resistant sintered body capable of maintaining a high elongation even after corrosion. In one aspect the present invention provides a powder capable of obtaining a corrosion resistant sintered body of less deterioration of elongation after corrosion, and suitable for a sensor ring and the other engagement parts such as parts used in anti-lock systems of vehicle breaks and the like. According to one aspect of the present invention, powder for a corrosion resistant sintered body having excellent ductility, comprises ferrite stainless steel powder containing about 11 to about 22wt% of Cr and a metal compound of B, the amount of B being from not less than about 0.03 to less than about 0.2wt% based on the weight of said powder. The above mentioned and other features and aspects of the present invention are illustrated by the following drawings, in which Fig. 1 is a view showing the elongation characteristics of sintered bodies composed of mixed powders where CrB and Fe-B were added to P444L powders respectively; Fig. 2A is a view showing the measured values of the loss in weight by corrosion of sintered bodies composed of mixed powders where CrB was added to P434L powders; Fig. 2B is a view showing the measured values of the loss in weight by corrosion of sintered bodies composed of mixed powders where CrB was added to P444L powders; Fig. 3 is a view showing the elongation characteristics of sintered bodies composed of mixed powders where CrB was added to P410L powders; Fig. 4 is a view showing the elongation characteristics of sintered bodies composed of mixed powders where CrB was added to P(25Cr-1Mo) powders; Fig. 5 is a view showing the elongation characteristics of sintered bodies composed of mixed powders where CrB was added to P (21Cr-0.5Mo) powders; Fig. 6 is a view showing measured results of air tightness of a sintered body of mixed powders where CrB was added to P444L powders in comparison with a powder sintered body without addition of CrB; Fig. 7 is an explanatory view of a test method for air tightness evaluation of a sintered body ring; Fig. 8 is an explanatory view of a measurement method for ductility evaluation of a sintered body ring; Figs. 9(A) and 9(B) are views showing the relationship between the continuous pore ratio and the elongation reduction ratio before and after the corrosion test for a sintered body ring; Figs. 10(A) to 10(F) show generation conditions of rust when CrB was added to P434L powder; Figs. 11(A) to 11(F) show generation conditions of rust when CrB was added to P444L powder. According to one embodiment of the present invention, a powder for a corrosion resistant sintered body having excellent ductility, is composed of ferrite stainless steel powder containing 11 to 22wt% of Cr and a metal compound of B, the amount of B being from not less than 0.03 to less than 0.2wt% based on the weight of the powder. In the powder, the ferrite stainless steel powders contains: C : ≤ 0.1wt%; Si: ≤ 3.0wt%; Mn: ≤ 0.30wt%; Ni: ≤ 2.0wt%; Cr: 11 to 22wt%; Mo: ≤ 3.0wt%; the rest being substantially Fe. Further, in the powder, B is mixed in the powder in the formation of Cr compound. According to one embodiment of the present invention, a metal compound of B is added and mixed with the ferrite stainless steel to compact powder for a sintered body. This way, the corrosion resistance of the sintered body is enhanced, and at the same time the elongation after corrosion is maintained at high levels. It has been confirmed by the present inventors that when the sensor ring is composed with such a sintered body, the sensor ring can be prevented from cracking after corrosion, but presently the detailed reasons why high elongation is maintained by adding and mixing a metal compound of B are not clear. However, the following points are assumed. The present inventors made studies on micro structures of the sintered bodies employing the powders. When comparing with micro strcutures of sintered bodies without addition of a B compound, the following facts were found. Namely, it is found that, in the products with addition of a B compound, the shapes of the pores were comparatively round, and each of them was small. On the other hand, in the products without addition of B, the shapes of the pores were long and narrow (at portions of maximum diameter) and each end of them was sharp. With respect to the different shapes of the respective pores it is assumed that in the products with the addition of a B compound, liquid phases are partially wand easily formed by reactions between B in the B compound and a matrix in the course of the sintering process. The sintering progresses with the existence of the liquid phase, while on the other hand, in the products with no addition of a B compound, the liquid phase is difficult to occur. For the liquid phases to usefully occur, it is necessary to add and mix B in the form of a metal compound. Actually, if B is solely contained as a powder element (alloying component of the powder itself), a good result is not obtained. This is assumed to be due to the fact that B is then too much uniformly dispersed in each of powders. Consequently, the liquid phase by reaction of B and the matrix does not effectively occur. Incidentally, the melting point of B itself is as high as 2300°C and B itself is never melted when sintering. In contrast, when B is added and mixed in the form of a metal compound, there appear parts wherein large amounts of B exist, and it is assumed that thereby the formation of the liquid phase by the reaction between the matrix and B is accelerated. When a B compound is added according to the invention, the sintered body also maintains the high elongation even after corrosion. This is assumed to be due to the fact that in view of the pores occuring in small and round shapes, it is more difficult that pores act as starting points for cracks when external forces are applied. Also the number of continuous pores and open pores (pores open to air) is reduced as mentioned later. The inventors confirmed that the sintered bodies with and without a B compound were not so much different in their sintered density. In general, when the density of the sintered body becomes high, the corrosion resistance is improved. However, although the density itself is not remarkably changed (does not become high), the corrosion resistance of the sintered body with the addition of a B compound according to the invention is improved. When the inventors observed the micro structure of the sintered body, it was confirmed that there were less open pores and the number of continuous pores was reduced in the surface layer of the sintered body. Additionally the shapes of the pores became small and round by the addition of a B compound. This fact is assumed to be a reason why the corrosion resistance is enhanced although the sintered density is not increased so much by the addition of a B compound. Namely, the following facts are assumed to be true. When a B compound is added in accordance with the invention, although the sintering does not advance to an extent that the sintered density is remarkably increased, the sintering is accelerated to an extent that the shapes of the pores are changed and to an extent that open pores are reduced in the outer layer. Further, the number of continuous pores is considerably decreased in comparison with the case of no addition of a B compound. Therefore, the corrosion resistance and the elongation after corrosion are effectively improved. The present inventors observed the condition of pores by enlarging (for example, 400 times) an optional section of the powder sintered body including the surface layer of the powder sintered body to which a B compound is added. The outer shape of the pores is round, and the ratio of continuous pores (open pores) which open at the surface layer of the sintered body is remarkably small in comparision with a sintered body to which a B compound is not added. It is assumed that this fact largely contributes to the corrosion resistance and the elongation after corrosion. In the present invention, the volume ratio of the open pores (continuous pore ratio) is preferably not more than about 20%, more preferably not more than about 14%. The ratio depends on the additional amount of the B compound and the production process after the addition. Accordingly, it is possible to further enhance the air tightness of the surface layer of the sintered body. When the sintered body is used for a sensor ring or an engagement part, it is possible to further improve the corrosion resistance. The shape of the pores is controlled to be round. Accordingly, the ductility is enhanced when the sintered body is used for a sensor ring or an engagement part. Therefore, it is possible to better resist a load from the outside. In this application an engagement part is a part used for a portion which is fitted to another part and needs good corrosion resistance (particularly, preservation), such as a metal bush, a fastener, or a chemical device part. In the invention, the content amount of Cr in the ferrite stainless steel is in the range of about 11 to about 22%. A reason for defining 11% or more of Cr arises from the following facts. If Cr is less than about 11%, the corrosion resistance of the ferrite stainless steel itself is insufficient. It is then difficult to sufficiently increase the corrosion resistance, although a B compound is added. On the other hand, if Cr is more than about 22%, the hardness of the sintered body becomes high and decreases the elongation. Therefore, it is difficult to obtain a large elongation after corrosion, although a B compound is added. Further, since the corrosion resistance is inherently high, in spite of the addition of a B compound, the corrosion resistance itself is not largely improved. Thus, the addition of a B compound does not give rise to large improvements. In the invention, the B compound is calculated in terms of the B content and it is added in an amount not less than about 0.03% to less than about 0.20%. If B is less than about 0.03%, the effect obtained by adding a B compound is scarcely provided. On the other hand, if it is contained in an amount more than about 0.20%, the elongation after corrosion is equivalent to or less than in the case with no addition of a B compound. The addition of a B compound is then meaningless. It is preferred in the invention to employ powders of said ferrite stainless steel containing in weight percent C: ≤ 0.1%, Si: ≤ 3.0%, Mn: ≤ 0.30%, Ni: ≤ 2.0%, Cr: 11 to 22%, Mo: ≤ 3.0% and the rest being substantially Fe. As addition embodiments of B, it may for example be added and mixed in the form of CrB, CrB 2 , Fe-B, NiB or mixtures thereof. In particular, it is confirmed that the addition of CrB brings about advantageous results. The limiting reasons of the chemical elements in the powder according to preferred embodiments of the invention is now discussed in detail. C: ≤0.1%, (more preferably C: ≤0.030%) If C is contained in an amount not less than 0.1%, the powders are hardened, and the Green density is lowered. Since deterioration of the corrosion resistance is remarkable, C is limited to be not more than 0.1%. A more preferable content is not more than 0.030%. Si: ≤ 3.0% (more preferably Si: ≤ 1.50%) If the Si amount is not less than 3.0%, the powders are considerably hardened, the Green density is lowered, and the compactability is worsened. Accordingly, Si is limited to be not more than 3.0%. A more preferable content of Si is not more than 1.50%. Mn: ≤0.30% (more preferably, Mn≤0.20%) If Mn is not less than 0.30%, oxygen in the powder becomes high and worsens the compactability. Accordingly, it is limited to be not more than 0.30%. It is more preferable that Mn is not more than 0.20%. Ni: ≤ 2.0% (more preferably, Ni ≤0.1%) If Ni is not less than 2.0%, the original surface is changed into martensite. As a result, the compactability is worsened and the density of the compressed powders does not rise. Therefore, it is limited to be not more than 2.0%. A more preferable content is that Ni is not more than 0.1%. Cr: 11 to 22% (preferably Cr: 15.5 to 18.5%) With less than about 11% of Cr, a sufficient corrosion resistance cannot be provided. If it is not less than about 22%, the powders are hardened. Accordingly, since the density is lowered and the elongation becomes small, the lower limit and the upper limit are set to be about 11% and about 22%, respectively. A preferable content is 15.5 to 18.5%. Mo: ≤ 3.0% (more preferably Mo: 0.01 to 3.0%, and most preferably 0.8 to 2.1%) With more than 3.0%, the powders are remarkably hardened, and the compactability is worsened. Accordingly, Mo is limited to be not more than 3.0%. A more preferred amount is 0.01 to 3.0%, and a most preferred amount is 0.8 to 2.1%. B: between not less than about 0.03% and less than about 0.2% (preferably B: 0.05 to 0.15%) With a content of less about 0.03%, the addition of B is hardly effective, and the corrosion resistance is not specially changed. On the other hand, with a content of not less than about 0.2%, the sintered body is hardened which causes the ductility to be lower. The elongation characteristic after corrosion is then equivalent to or less than in the case with no B addition, whereby the B addition is meaningless. If B is added in a large amount, the liquid phase appears, and a coefficient of contraction is made large and the size precision is worsened. In the present invention, therefore, B is set to be between not less than about 0.03% and less than about 0.2%. Preferable is 0.05 to 0.15%. For solidifying carbides, Nb can be added to not more than 1.0%. Examples Examples of the present invention will be discussed below in detail. To the powders of the chemical composition shown in Table 1, CrB powders (average particle size: 16.6 µm) shown in the same and Fe-B powders (average particle size: 14.1 µm) were added at addition amounts shown in Tables 2, 3 and Tables 4, 5, and mixed for 30 minutes together with a lubricant (zinc stearate 1%) in a blender. The addition amount of CrB and Fe-B is shown as a ratio based on the amount of P434L or P444L powder. The content amount of B is shown as a ratio based on the total amount of the mixed powder. Chemical Composition of powders (wt%) C Si Mn P S Cu Ni Cr No N O P434L 0.013 0.85 0.23 0.024 0.005 - 0.12 15.73 0.83 0.025 0.20 P444L 0.011 0.87 0.20 0.020 0.005 0.04 0.11 17.80 1.83 0.024 0.23 CrB 0.28 - - - - - - Bal. - B=16.64 Fe-B 0.025 1.13 - 0.023 0.003 - - - - B=20.71 The addition amount of CrB to 434L and the sintered density (Compacting pressure 8t/cm 2 ) CrB content (%) 0 0.25 0.50 0.75 1.00 1.25 1.50 B content (%) 0 0.041 0.083 0.124 0.165 0.205 0.246 Sintered Density (g/cm 3 ) 1150°C Sintered 6.82 6.91 8.89 6.87 6.83 6.81 6.81 1200°C Sintered 6.98 7.01 7.02 6.98 6.88 6.84 6.95 1250°C Sintered 7.11 7.12 7.13 7.11 7.10 7.08 7.08 The addition amount of CrB to 444L and the sintered density (Compacting pressure 8t/cm 2 ) CrB content (%) 0 0.25 0.50 0.75 1.00 1.25 1.50 B content (%) 0 0.041 0.083 0.124 0.165 0.205 0.246 Sintered Density (g/cm 3 ) 1150°C Sintered 6.87 6.92 6.90 6.88 6.82 6.81 6.81 1200°C Sintered 6.98 6.99 6.99 6.87 6.97 6.95 6.96 1250°C Sintered 7.11 7.11 7.10 7.11 7.09 7.08 7.08 The addition amount of Fe-B to 444L and the sintered density (Compacting pressure 8t/cm 2 ) CrB content (%) 0 0.25 0.50 0.75 1.00 1.25 1.50 B content (%) 0 0.052 0.103 0.154 0.205 0.258 0.346 Sintered Density (g/cm 3 ) 1150°C Sinered 6.82 6.88 6.87 6.85 6.81 6.79 6.79 1200°C Sinsered 6.98 6.88 6.95 6.90 6.94 6.92 6.91 1250°C Sintered 7.11 7.08 7.09 7.09 7.08 7.06 7.06 The addition amount of Fe-B to 434L and the sintered density (Compacting pressure 8t/cm 2 ) CrB content (%) 0 0.25 0.50 0.75 1.00 1.25 1.50 B concent (%) 0 0.052 0.103 0.154 0.205 0.256 0.306 Sintered Density (g/cm 3 ) 1150°C Sintered 6.87 6.90 6.90 6.85 6.85 6.83 6.81 1200°C Sintered 6.98 6.98 6.94 6.90 6.92 6.91 6.90 1250°C Sintered 7.11 7.08 7.09 7.09 7.07 7.06 7.06 The mixed powders were compacted at a pressure of 8 t/cm 2 and tensile test pieces were made. Under a condition of 400°C x 30 min in an atmospheric air, the test pieces were subjected to dewaxing (removing of zinc stearate) and sintered under the following conditions: In vacuum 1150°C x 60 min - FC (Furnace cooling) In vacuum 1200°C x 60 min - FC (Furnace cooling) In vacuum 1250°C x 60 min - FC (Furnace cooling) The densities of the sintered bodies were investigated, tensile tests were conducted, the elongations were measured before and after the corrosion resistant tests, and the loss in weight by corrosion was measured. Results are shown in tables 2 to 5, Fig.1 and Figs.2(A) and 2(B). Fig.1 shows measured values of elongation before and after the corrosion resistance tests with respect to P444L (sintering temperature: 1250°C). Fig.2(A) shows measured values of the loss in weight by corrosion with respect to P434L. Fig.2(B) shows the measured values of the loss in weight by corrosion with respect to P444L. The conditions of the corrosion resistance tests and measures of the loss in weight by corrosion were as follows. Corrosion resistance test: The above represents one cycle. 90 cycles were repeated. Measurement of the loss in weight by corrosion: The test pieces were immersed, 70°C x 24 hr, in a 30% solution of ammonium citrate, scaled by brushing, dried again, weighed, and the loss in weight was measured before and after the corrosion test. From the results of Fig.1, the following facts are understood. In each case of CrB addition and Fe-B addition, the addition of not less than 0.03% B maintains the elongation after the corrosion resistance test higher than with no addition of B. The addition effect shows one maximum and subsequently a gradual falling. When B is around 0.20%, the elongation characteristics are almost equivalent to the cases of no additions of CrB or Fe-B. In addition, the elongation characteristics go down thereafter, as B is increased. Further, with respect to the forms of B addition, CrB is superior to Fe-B. The better result of CrB than Fe-B is assumed to be due to the following reason. It is assumed that, in the case of adding B in the form of Fe-B, B combines with Cr existing in the matrix during sintering. The Cr content in the matrix is subsequently reduced which causes a reduction in the corrosion resistance. In contrast, the adding form of CrB does not cause such an effect, so that there does not appear the problem of the drop of the corrosion resistance caused by a decrease of Cr in the matrix. Also in the results of Fig.2(A) and 2(B), it is acknowledged that the loss in weight by corrosion goes down by the B addition (CrB addition). The corrosion resistance is improved, though the density of the sintered body does not notably increase by the B addition as mentioned above. The reason for this fact is assumed to be the decrease of the open pores and the continuous pores seen in the upper surface of the sintered body. Figs. 10(A) to 10(F) and 11(A) to 11(F) show the states of the upper surfaces of the sintered bodies after the corrosion resistance tests. Figs. 10(A) to 10(F) show the generation of rust when CrB was added to P434L powder. Figs. 11(A) to 11(F) show The generation of rust when CrB was added to P444L powder. It is observed from these photographs that the appearance of rusts is effectively controlled by adding B to 0.03% or more. These good results are obtained in particular at the high sintering temperature of 1250°C in Figs. 2(A) and (B). It is assumed that partial liquid phases are effectively compacted by sintering at the high temperature. To the chemical compositions of Tables 6 and 7, the same CrB powders as mentioned above were added at amounts of Tables 8 and 9, and test pieces were made with mixed powders in the same manner as described above (the sintering temperature: 1250°C). The densities of the sintered bodies were investigated, tensile tests were conducted before and after the corrosion resistance tests, and the elongations were measured. The results are shown in Figs.3 and 4. The chemical Composition of powders (wt%) C Si Mn P S Ni Cr Mo N O P410L 0.062 1.87 0.16 0.013 0.006 0.11 11.78 0.15 0.034 0.14 CrB 0.28 - - - - Bal. - B=16.64 The chemical Composition of powders (wt%) C Si Mn P S Ni Cr Mo N O P (25Cr -1Mo) 0.008 0.88 0.13 0.007 0.006 0.13 24.87 0.94 0.033 0.39 CrB 0.28 - - - - - Bal. - B=16.64 The addition amount of CrB to P410L and the sintered density (8t/cm 2 -1250°C sintering) CrB content (%) 0 0.25 0.50 0.75 1 1.25 1.5 B content (%) 0 0.041 0.083 0.124 0.165 0.205 0.246 Sintered Density (g/cm 3 ) 7.27 7.29 7.3 7.3 7.29 7.27 7.27 The addition amount of CrB to P (25Cr-1Mo) and the sintered density (8t/cm 2 -1250°C sintering) CrB content (%) 0 0.25 0.50 0.75 1 1.25 1.5 B content (%) 0 0.041 0.083 0.124 0.165 0.205 0.246 Sintered Density (g/cm 3 ) 6.5 6.52 6.54 6.52 6.53 6.53 6.5 As shown in Fig.3, when CrB was added to P410L containing Cr as little as around 12%, since the elongation before the corrosion resistance test was large, the reduction of the elongation value was large when the values before and after the corrosion resistance tests were compared. However, the absolute value of the elongation after the corrosion resistance test was kept at a constant level. The effect of the CrB addition appears anyway (comparing the corrosion resistances before and after in previous P444L, the decreasing degree of the elongation after the corrosion resistance test was small). On the other hand, in the case of the P (25Cr-1Mo) powders which contain Cr as much as around 25% (see Fig. 4), the elongation was already small before the corrosion resistance test since the Cr content is high. Therefore, although the reduction of the elongation after the corrosion resistance test by the CrB addition was small, the absolute value of the elongation after the corrosion resistance test was lowered. Accordingly, this powder is unsuitable for those purposes which demand a large elongation after the corrosion resistance test such as the above mentioned sensor rings. To the powders P (21Cr-0.5Mo) of the chemical compositions of Table 10, the same CrB powders as mentioned above were added in the amounts of Table 11. Test pieces were made with mixed powders in the same manner as above (the sintering temperature: 1250°C). The densities of the sintered bodies were investigated, tensile tests were conducted before and after the corrosion resistance tests, and the elongations were measured. The results are shown in Table 12 and Fig.5. The chemical Composition of powders (wt%) C Si Mn P S Ni Cr Mo N O P(21Cr-0.5Mo) 0.004 1.77 0.09 0.001 0.002 0.02 21.35 0.48 0.022 0.18 CrB 0.28 - - - - - Bal. - B=16.64 The addition amount of CrB to P(21Cr-0.5Mo) and the sintered density (8t/cm 2 -1250°C sintering) CrB content (%) 0 0.25 0.50 0.75 1 1.25 1.5 B content (%) 0 0.041 0.083 0.124 0.165 0.205 0.246 Sintered Density (g/cm 3 ) 7.01 7.05 7.05 7.03 7.02 7.00 6.99 Elongation (%) Powders B Content (%) Sintered Bodies After Corrosion Resistant Tests P(21Cr-0.5Mo) 0.000 17.1 7.5 0.041 18.2 13.1 0.083 18.3 14.5 0.124 18.0 13.0 0.165 17.2 11.9 0.205 15.8 10.6 0.246 14.2 9.8 As shown in Table 12 and Fig.5, also when, to the P (21Cr-0.5Mo) powders where the Cr content was near the upper limit of the invention of 22%, CrB was added in an amount not less than 0.03% to less than 0.2%, the advantageous effect of adding CrB apparently appeared. Namely, in the present invention, when a metal compound of B is added to powders of ferrite stainless steel of not more than about 22% Cr, high elongation characteristics can be obtained after the corrosion resistance test. Referring to the powders P444L of the chemical composition shown in Table 1, 1% of zinc stearate was incorporated into the powders In one 0.5% of CrB was added and mixed and in another P444L powder no CrB was added. Those mixtures were press-compacted under a pressure of 6t/cm 2 to 10 mm thickness by means of the metal ring mold of an outer diameter of Φ 34mm and an inner diameter of Φ 20 mm. The density of the compressed powders was then 6.1 g/cm 3 for both samples. Those compacted bodies were de-waxed 500°C x 30 min in a vacuum and thereafter sintered 1250°C x 60 min in a vacuum. The sintered densities then were 7.0 g/cm 3 for both samples. The air tightness of both sintered bodies was measured by applying a pressure of about 0.98 MPa, and the results shown in Fig. 6 were obtained. The measurement of air tightness was performed in the method shown in Fig. 7. Namely, opposite end surfaces of a sintered body ring 10 are closed by a rubber packing 12, and the whole structure is pressed by an air cylinder 14 to maintain air tightness (pressure force: about 20 kgf/cm 2 (19.6 MPa)). Under these conditions, N 2 gas at a pressure of 1 kgf/cm 2 (0.98 MPa) is introduced to the inside of the sintered ring 10 through a tube 16. Then, when the interior pressure of the sintered ring 10 has reached 0.98 MPa (as measured by a pressure meter 20), a valve 18 is closed and the reduction of pressure with time is measured. As seen in the results of Fig.6, the interior pressure of the sintered body of P444L powders without addition of CrB went down considerably, while the decrease in pressure of the sintered body added and mixed with 0.5% CrB was scarcely recognized. This is because in the case of the sintered body of powders with the addition of Cr, no leakage occurred because the number of continuous pores was remarkably reduced. Further, compressed powder bodies of mixture powder in which 0.25 to 1.25 wt% of CrB were added to P434L powder and P444L powder, respectively were sintered in vacuum at 1100°C to 1290°C for 60 minutes to thereby produce sintered bodies having a density of about 7 g/cm 3 . Then, the elongation, the continuous pore ratio and the air tightness of the sintered bodies were evaluated in the following manner: (1) Elongation: In order to apply the sintered body to a sensor ring, it is necessary to prevent cracks due to expansion accompanying the corrosion of an opposite material. For this purpose, it is necessary to suppress a lowering of the ductility and the strength. Here, the crack (elongation) during compressed insertion as the ductility evaluation after a corrosion test in a sensor shape which is one of corrosion resistance evaluations required for the sensor body. The test conditions are as follows. A sintered body ring 22 (sensor ring sample: see Fig. 8) having an outer diameter of 98mm, an inner diameter of Φ 92mm, and a thickness of 9 mm was used. The elongation of the sintered body ring before and after the corrosion test was measured with the method shown in Fig. 8. That is, the sintered ring 22 was compressedly inserted into a taper cone 24 having a taper degree of 1.75/100. The elongation was calculated from the inner diameter when the sintered body 22 was cracked and the inner diameter before the compressed insertion. The elongation ratio was obtained in the following manner. Elongation = {(inner diameter when cracked) / (inner diameter before compressed insertion)} X 100 (%) The corrosion resistance test was performed in the following manner. Cyclic corrosion test: The above represents one cycle (24 hours); the test was repeated for 2400 hours. The results are shown in Table 13. As seen in Table 13, the sintered body rings according to the present invention show an elongation of not less than 4% after the corrosion test. It is thus confirmed that the sintered rings are suitable for use as sensor rings. (2) Continuous pore ratio: In case of compacting the sensor ring by the sintered body, if pores which open at the surface and continue to the inside of the sintered body, water and the like from the outside enters through the pores from the surface of the sintered body to the inside of the sintered body. As a result, corrosion progresses to the inside of the sintered body, thereby lowering the ductility of the sensor ring. A sintered body ring was produced in the same manner as described in (1). The continuous pore ratio was measured in the following manner. Test condition: the sintered body ring having an inner diameter of 20mm, an outer diameter of 34mm, and a thickness of Φ10mm was molded and sintered so that the final density became 7.1 g/cm 3 . An oil content volume was measured using the thus produced sintered body ring. The continuous pore ratio was obtained from the results of the measurement. Here, the continuous pore ratio was obtained by the following formula. Continuous pore ratio = {(oil content ratio *1 /whole pore ratio) x 100 = [(oil content volume)/{1-(sintered body density *2 )/(theoretical density)}] x 100, with (*1) oil content ratio: volume ratio of continuous pores opening at the surface of the sintered body to the whole of the sintered body (*2) sintered body density: (weight of sintered body)/(volume of the sintered body) The process of the measurement was as follows: First, the sintered body was placed in vacuum, and an oil is impregnated into the sintered body. Then, the volume of the impregnated oil was calculated by the vacuum impregnation method. The volume of the content oil at this time corresponds to the volume due to the continuous pores. Separately, the volume of the sintered body and the density of the sintered body had been obtained, and they were substituted in the above formula to thereby obtain the continuous pore ratio. The theoretical density used was 7.8 g/cm 3 . The results are again shown in Table 13. As seen from the results in Table 13, in the examples according to the present invention, the continuous pore ratio was not more than 20 vol.%, and good characteristic were obtained after the corrosion test. (3) Air tightness: The sintered body ring having an outer diameter of Φ 34mm, an inner diameter of Φ 20mm and a thickness of 10mm was used to conduct the air tightness evaluation test as shown in Fig. 7. Nitrogen gas was introduced into the interior of the sintered body ring at 0.98 MPa. This initial pressure and the pressure after 180 minutes were measured to thereby obtain the reduction ratio of the pressure before and after the test. The results are also shown in Table 13. The smaller the number of continuous pores on the surface of the sintered body is, the smaller the reduction of the pressure will be. Therefore, it is possible to obtain high air tightness. In any of the examples according to the present invention in which the continuous pore ratio is not more than 20%, the pressure reduction ratio with respect to the initial pressure is not more than 5%. Particularly, when the continuous pore ratio is not more than 14%, the pressure reduction ratio is not more than 1%. Accordingly, in this case, it was ascertained that air tightness is high. Figs. 9(A) and 9(B) show the relationship between the continuous pore ratio and the elongation reduction ratio before and after the corrosion test. As shown in these graphs, there is a close relationship between the continuous pore ratio and the elongation reduction ratio. Namely, the higher the continuous pore ratio is, the higher the elongation reduction ratio is. If the continuous pore ratio is not more than 20%, the elongation reduction ratio is not more than 70%. Particularly, if the continuous pore ratio is not more than 14%, the elongation reduction ratio is not more than 60%. Accordingly the following is recognized. The continuous pore ratio is controlled to be of a small value to thereby suppress the reduction of the elongation after corrosion. Further, in case of a sintered body according to the present invention, since the continuous pore ratio is small, it has a good elongation characteristic even after corrosion. As mentioned above in detail, ferrite stainless steel powder containing about 11 to about 22wt% of Cr is mixed with a metal compound of B in a predetermined amount. It is possible to effectively enhance the corrosion resistance and the high elongation characteristic after corrosion. Therefore, when applied to a sensor ring, the ocurrence of cracks in the sensor ring can be avoided. Also, cracks in the sensor ring by corrosion in the shafts of opposite materials can be avoided. Alternatively, when the invention is applied to other engagement parts, good corrosion resistance and ductility can be achieved. The metal compound of B may be added to a powder of ferrite stainless steel according to claim 2. When the sintered body is then used itself or is used in a part comprising the sintered body, it is possible to effectively enhance the elongation characteristic thereof after corrosion. Further, B may be added in the form of CrB according to claim 3. Then the corrosion resistance and the elongation characteristic after corrosion can be further enhanced when the sintered body is used by itself or is used in a part such as the sensor ring.

Claims: Powder for a corrosion resistant sintered body having excellent ductility, wherein said powder consists of a stainless steel powder and a metal compound of B, wherein: said stainless steel powder is a ferrite stainless steel powder containing 11 wt% ≤ Cr ≤ 22 wt%, 0 < C ≤ 0.1 wt%, 0 < Si ≤ 3.0 wt%, 0 < Mn ≤ 0.30 wt%, 0 < Ni ≤ 2.0 wt%, 0 ≤ Mo ≤ 3.0 wt%, and 0 ≤ Nb ≤ 1.0 wt%, the balance being Fe and incidental impurities; and the amount of B is from not less than 0.03 to less than 0.2 wt% based on the weight of said powder. The powder according to claim 1, wherein said metal compound of B is a Cr compound. The powder according to claim 1, wherein said metal compound of B is one of CrB, CrB 2 , Fe-B, NiB, or a mixture thereof. The powder according to one of the preceding claims, wherein said ferrite stainless steel powder contains: 0 < C ≤ 0.03 wt%; 0 < Si ≤ 1.50 wt%; 15.5 wt% ≤ Cr ≤ 18.5 wt%; 0.01 wt% < Mo ≤ 3.0 wt%. The powder according to any of the preceding claims, wherein the amount of B is in the range of 0.05 to 0.15 wt% based on the weight of said powder. The powder according to any of the preceding claims, wherein the amount of Mo is in the range of 0.8 to 2.1 wt%.

Inventors: KONDOH TETSUYA, SHIMIZU TAKAYOSHI, KONDOH, TETSUYA, SHIMIZU, TAKAYOSHI
Applicants: DAIDO STEEL CO LTD, DAIDO TOKUSHUKO KABUSHIKI KAISHA
Application year: 1998
Main classification: C22C33
IPC codes: C22C33, F02B77