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Effect of yttrium addition on mechanical behavior of powder metallurgy TP304 at elevated temperatures
Scripta Materialia, Vol. 36, No. 3, pp. 305-310, 1997 Elsevia Science Ltd ‘htight 0 1997 Acta Metahrgica Inc. k&cd in the USA. All rightsr&erved 1359-6462197 $17.00 + .OO
Pergamon
PII S1359-6462(96)00374-0
EFFECT OF YTTRIUM ADDITION ON MECHANICAL BEHAVIOR OF POWDER METALLURGY TP304 AT ELEVATED TEMPERATURES Tatsuro Isomoto, Tadanori Kida* and Hiroshi Nagai Department of Material Science and Engineering, Osaka University, 2-1, Yamadaoka, Suita, Japan 565 *Technological Research Lab., Sanyo Special Steel Co., Ltd. 3007 Nakashima, Shikama-ku, Himeji, Japan 672 (Received June 10,1996) (Accepted August 28, 1996) Introduction It is well known that oxide dispersion strengthened (ODS) alloys, which are mechanically alloyed with yttria, possess excellent mechanical properties in comparison with conventional ingot metallurgy (IM) materials at elevated temperatures even higher than 1000°C (1,2). However, mechanical alloying may not be desired in view of production cost for the high temperature application where such high strength is not necessarily required. There is not an extensive literature dealing with mechanical properties at above 800°C of the fully dense powder metallurgy (PM) materials without mechanical alloying. In the PM TP304 consolidated by hot extrusion, it was revealed that PM materials showed shorter creep rupture life them IM materials and their creep rupture strengths decrease with increasing oxide inclusions originated mainly from oxidation of powder (3,4). It was also shown that the kinds of oxide inclusions observed in the consolidated materials were strongly dependent on the contents of strong oxide former elements such as Al, Si and Mn in the melt before atomizing. The mechanical behavior at elevated temperaturei were affected by those kinds of oxides to a greater extent in PM materials than in IM materials (5). In the present study, the melt of TP304 containing yttrium was atomized to form yttria on and in the powder. The effects of such oxide inclusions on high temperature tensile properties and creep rupture strengths are investigated, using TP304 fully dense materials consolidated by hot extrusion. Experimental Procedures TP304 materials with and without yttrium additions were melted in a 100 kg vacuum induction furnace to make ingots. These ingots were remelted for atomization by pressurized nitrogen gas to powders. The powders were sieved to less than 500 pm. They were encapsulated in mild steel cans. The capsules were vacuum ldegassed, sealed by welding and extruded to 50 mm diameter bars with an extrusion
305
306
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Vol. 36, No. 3
TABLE 1 Chemical Compositions of TP304 PM and IM Stainless Steels (mass%, 0, N:ppm)
ratio of 9 after they were heated at 1210°C for 2 h. The mild steel on the extruded material was removed by machining. The TP304 bars thus obtained were further forged to 15 mm diameter and solu-
tion treated at 1100°C for 0.6 ks for the specimens. In addition, as a reference material, a portion of the TP304 ingots was forged to 15 mm diameter bars as well and subsequently heat treated under the same heat treatment conditions. Table 1 shows the chemical compositions of the IM and PM materials after consolidation. PM1 is PM TP304 with almost no Al, Si or Mn additions in order to form only Cr oxides. Virtually, PM2 is with yttrium addition to PMI. IMl and IM2 are ingot metallurgy materials corresponding to PM1 and PM2, respectively. Tensile tests from room temperature to 950°C were performed with the specimen of the gauge diameter of 5 mm at the strain rate of 5 x lo-’ set-’ up to yield strength and 1.25 x low5set-’ after the point. Creep rupture tests at 950°C were performed by a dead weight type testing apparatus with the specimen of a gauge diameter of 6 mm. To characterize microstructures, a scanning electron microscope with energy dispersive spectroscopy (EDS) at accelerating voltage of 20 kv and a field emission type of transmission electron microscope (TEM) with EDS at accelerating voltage of 200 kv were used. Results and Discussion Characterization of Powder, PM and IM Materials Figure 1 shows SEM observation results for surface appearance of PM1 and PM2 powders. The shape of powders is nearly spherical with PMl, but irregular with PM2. These characteristics may result from higher viscosity with PM2 due to an addition of yttrium. According to sectional view of powders observed by SEM, it has been found that, within powders, PM2 shows a great deal of particles with size up to 1 (m, whose constituents were yttrium oxides, while PM1 exhibited no such particles. This indicates that yttria with less than 1 (m, which are expected to contribute to an increase in high tem-
I
PM1 (without Y)
PhQ (with Y)
Figure I. SEM microphotographsof SUS304powder used for PM materials.
I
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Vol. 36, No. 3
307
b) PM2 (with Y)
a) PM1 (without Y) c) IM2 (without Y)
d) IM2 (with Y)
Figure 2. Microstructures of SUS304 Pm and IM materials after solution treatment.
perature strength, are dispersed in the powders to some extent. These yttria particles are the reaction products of yttrium with oxygen in the melt before atomizing because they may not form during solidification during atomization due to extremely low oxygen content soluble to the melt by the existence of yttrium (6). The microsbuctures of fully consolidated PM and IM materials after solution treatment are shown in Fig.2. The grain sizes of PMI, PM2, IMl and IM2 are approximately 40, 20, 80 and 25 pm, respec_-
I
PM1
(without Y)
I
PM2 (with Y)
-l-EM
EDS
Figure 3. TEM photographs showing oxides and EDS analysis results on SUS304 PM and IM materials.
308
EFFECT OF YTTRIUM ADDITION
600
--PM,
*Iho!AY
--IM,
wlhmnY
Vol. 36, No. 3
500
0
200
400
600
600
l&JO
0
Test Temperature (“C)
200
400
660
Test Tmpetalure
600
Vi-10
(“C)
Figure 4. Resultsof high temperaturetensile tests of SUS304PM and IM materials.
tively. PM2 and IM2 containing yttrium have much smaller grain size than the materials without yttrium and show, respectively, stringer and granular inclusions. Most of these inclusions for PM2 have been identified as yttria by EDS. It is evident that these yttria of PM2 result partly from the same that existed in powders. Oxides of IM2 are also yttria formed in the melt before ingot casing. It is considered that the finer grains with PM2 result from more uniformly distributed yttria in comparison with IM2 owing to inheritance of powder metallurgy process. To compare yttria in PM2 with oxides in PM1 in terms of shape, distribution and constituents, the extraction replicas taken from these materials were investigated by TEM with EDS. The results are shown in Fig.3. It is seen that main oxides of PM1 and PM2 are identified as pure Cr and Y oxides, respectively, and both oxides are in round shape. As to finer oxides, it is also seen that yttria with the size down to 0.05 pm in PM2 are dispersed. On the other hand, Cr oxides with the size down to 0.1 in PM1 are sporadically situated and oxides finer than 0.2 pm are scarce. In addition, PM1 exhibits large oxides with 1.5 pm which is considered to originate from agglomeration of surface oxides on powders. By image analysis with the photograph of PM2, it was determined that area fraction of yttria including larger size of oxides was approximately 2%. However, the fraction of fine oxides is much less than that of commercial Ni base ODS alloy, in which 1 to 3% is estimated only for fine oxides typically from 15 to 30 nm (7).
loo
Time to rupture(h) Figure 5. Resultsof creep rupture testsof
SUS304PM and IM materialsat 950°C.
EFFECT OF YTTRIUMADDITION
Vol. 36, No. 3
High Temperature Tensile Properties Figure 4 shows the results of high temperature tensile tests for the PM and IM materials. For a comparison between PM materials, PM2 indicates slightly higher 0.2% yield strength (YS) and tensile strength (TS) lthan PM1 in the entire testing temperature range. Although this difference becomes smaller with increasing temperatures, PM2 maintains clearly higher strength even at 950°C. It is considered that the difference results from a sum of the effects of the grain size and dispersion of yttria with PM2. The values of elongation (EL) and reduction of area (RA) are remarkably higher with PM2 especially above 800°C. For a comparison of PM and IM materials, PM materials show much greater YS and TS in the low temperature range while the strengths of PM1 are equivalent to those of IM materials above 800°C. However, values of RA and EL of PM1 are much lower than those of IMl. It is noted that RA of PM2 is comparable to that of IMI, which is close to industrially produced TP304. Creep Ruptuw Tests Figure 5 shows the results of creep rupture tests at 950°C under various stress levels. It is seen that PM2 has remarkably excellent creep rupture lives as compared with the other materials. It is also surprising that such performance is successfully attained even with original fine grain size as shown in Fig.2. It is found that this rupture strength level is comparable to solid solution strengthened type of Ni base alloy such1 as Alloy X (8) by drawing Larson-Miller curves. Although IM2 shows longer rupture lives than IMl, yttrium addition itself is not significantly effective enough to explain the improvement in PM2 from PM1 with the consideration of the difference between IMl and IM2. Furthermore, it is seen that an improvement in oxidation resistance expected by the yttrium addition may not dramatically contribute to an increase in the creep rupture strength. Accordingly, it is considered that this improvement in PM2 is attributable to the fine dispersion of yttria. Cr oxides adversely affect the strength because PM1 shows the lowest rupture lives. In general, any dispersed fine and stable oxides are effective to increase rupture strength as long as they are within grains by suppression of dislocation movement. However, since Cr oxides of PM1 are relatively large and sparsely situated, they will not contribute to an increase in strength of the matrix at high temperatures. Figure 6 shows dislocation substructures of PM1 and PM2 specimens crept at 950°C. It is seen that dislocations are pinned at and bowed between the dispersed oxides in PM2. This fact indicates that dispersion of fine yttrium oxides contribute to an increase in strength of grains by Orowan strengthening mechanism. On the other hand, dislocations are less frequently seen without such pining in PMl. Presumably, Cr oxides existing on grain boundaries promote cavity formation and cracking, and degrade creep rupture life while oxides with sporadic distribution in the grains have no significant influ-
a) PM1 (without Y) b) PM2 (with Y)
CT=12 MPa, tR=149h 0 =30 MPa, t,=190 h
Figure 6. TEM photographs showiag dislocation substructures of SUS304 PM specimens crept at 950°C.
310
EFFECT OF YTTRIUM ADDITION
Vol. 36, No. 3
ence on the strength. Accordingly, it is concluded that PM2 is a kind of ODS alloy successfully produced by atomizing without mechanical alloying. Conclusions
The effect of yttrium addition of PM TP304 was investigated on tensile strength and creep rupture behavior at elevated temperatures in comparison with IM materials. The conclusions obtained are as follows: 1. Yttria particles formed by an addition of Y in the melt before atomizing are dispersed with size of 0.05 to 1 (m in the extruded materials while Cr oxides were formed with larger size for the material without Y addition. 2. PM TP304 with Y addition shows superior tensile strength and ductility to PM and IM counterparts even above 800°C. 3. The creep rupture lives of PM TP304 with Y addition are significantly prolonged by the dispersion of yttria through suppression of dislocation movement while PM TP304 without Y addition indicated inferior rupture lives to IM TP304. References 1. 2. 3. 4. 5. 6. I. 8.
L.R. Curwick, “Frontiers of High Temperature Materials”, JSBenjamin Ed., INCO, 3 (1981). G.A.J. Hack, Powder Metall., 27,73 (1984). T. Isomto and H. Nagai, J. of Japan Sot. Powder and Powder Metall., 42,618 (1995). T. Isomto and H. Nagai, J. of Japan Sot. Powder and Powder Metall. 42,135O (1995). T. Isomto and H. Nagai, J. of Japan Sot. Powder and Powder Metall., in press. F. Ishii and S. Banya, Tetsu to Hagane, 80,359 (1994). G.H. Gessinger , “Powder Metallurgy of Superalloys”, p.214, Butterworth & Co. (1984). “The Superalloys II”, CT. Sims et al. Eds., p.596, Wiley-Interscience, (1987).
Pergamon
PII S1359-6462(96)00374-0
EFFECT OF YTTRIUM ADDITION ON MECHANICAL BEHAVIOR OF POWDER METALLURGY TP304 AT ELEVATED TEMPERATURES Tatsuro Isomoto, Tadanori Kida* and Hiroshi Nagai Department of Material Science and Engineering, Osaka University, 2-1, Yamadaoka, Suita, Japan 565 *Technological Research Lab., Sanyo Special Steel Co., Ltd. 3007 Nakashima, Shikama-ku, Himeji, Japan 672 (Received June 10,1996) (Accepted August 28, 1996) Introduction It is well known that oxide dispersion strengthened (ODS) alloys, which are mechanically alloyed with yttria, possess excellent mechanical properties in comparison with conventional ingot metallurgy (IM) materials at elevated temperatures even higher than 1000°C (1,2). However, mechanical alloying may not be desired in view of production cost for the high temperature application where such high strength is not necessarily required. There is not an extensive literature dealing with mechanical properties at above 800°C of the fully dense powder metallurgy (PM) materials without mechanical alloying. In the PM TP304 consolidated by hot extrusion, it was revealed that PM materials showed shorter creep rupture life them IM materials and their creep rupture strengths decrease with increasing oxide inclusions originated mainly from oxidation of powder (3,4). It was also shown that the kinds of oxide inclusions observed in the consolidated materials were strongly dependent on the contents of strong oxide former elements such as Al, Si and Mn in the melt before atomizing. The mechanical behavior at elevated temperaturei were affected by those kinds of oxides to a greater extent in PM materials than in IM materials (5). In the present study, the melt of TP304 containing yttrium was atomized to form yttria on and in the powder. The effects of such oxide inclusions on high temperature tensile properties and creep rupture strengths are investigated, using TP304 fully dense materials consolidated by hot extrusion. Experimental Procedures TP304 materials with and without yttrium additions were melted in a 100 kg vacuum induction furnace to make ingots. These ingots were remelted for atomization by pressurized nitrogen gas to powders. The powders were sieved to less than 500 pm. They were encapsulated in mild steel cans. The capsules were vacuum ldegassed, sealed by welding and extruded to 50 mm diameter bars with an extrusion
305
306
EFFECT OF YTTRIUM ADDITION
Vol. 36, No. 3
TABLE 1 Chemical Compositions of TP304 PM and IM Stainless Steels (mass%, 0, N:ppm)
ratio of 9 after they were heated at 1210°C for 2 h. The mild steel on the extruded material was removed by machining. The TP304 bars thus obtained were further forged to 15 mm diameter and solu-
tion treated at 1100°C for 0.6 ks for the specimens. In addition, as a reference material, a portion of the TP304 ingots was forged to 15 mm diameter bars as well and subsequently heat treated under the same heat treatment conditions. Table 1 shows the chemical compositions of the IM and PM materials after consolidation. PM1 is PM TP304 with almost no Al, Si or Mn additions in order to form only Cr oxides. Virtually, PM2 is with yttrium addition to PMI. IMl and IM2 are ingot metallurgy materials corresponding to PM1 and PM2, respectively. Tensile tests from room temperature to 950°C were performed with the specimen of the gauge diameter of 5 mm at the strain rate of 5 x lo-’ set-’ up to yield strength and 1.25 x low5set-’ after the point. Creep rupture tests at 950°C were performed by a dead weight type testing apparatus with the specimen of a gauge diameter of 6 mm. To characterize microstructures, a scanning electron microscope with energy dispersive spectroscopy (EDS) at accelerating voltage of 20 kv and a field emission type of transmission electron microscope (TEM) with EDS at accelerating voltage of 200 kv were used. Results and Discussion Characterization of Powder, PM and IM Materials Figure 1 shows SEM observation results for surface appearance of PM1 and PM2 powders. The shape of powders is nearly spherical with PMl, but irregular with PM2. These characteristics may result from higher viscosity with PM2 due to an addition of yttrium. According to sectional view of powders observed by SEM, it has been found that, within powders, PM2 shows a great deal of particles with size up to 1 (m, whose constituents were yttrium oxides, while PM1 exhibited no such particles. This indicates that yttria with less than 1 (m, which are expected to contribute to an increase in high tem-
I
PM1 (without Y)
PhQ (with Y)
Figure I. SEM microphotographsof SUS304powder used for PM materials.
I
EFFECT OF YTTRIUM ADDITION
Vol. 36, No. 3
307
b) PM2 (with Y)
a) PM1 (without Y) c) IM2 (without Y)
d) IM2 (with Y)
Figure 2. Microstructures of SUS304 Pm and IM materials after solution treatment.
perature strength, are dispersed in the powders to some extent. These yttria particles are the reaction products of yttrium with oxygen in the melt before atomizing because they may not form during solidification during atomization due to extremely low oxygen content soluble to the melt by the existence of yttrium (6). The microsbuctures of fully consolidated PM and IM materials after solution treatment are shown in Fig.2. The grain sizes of PMI, PM2, IMl and IM2 are approximately 40, 20, 80 and 25 pm, respec_-
I
PM1
(without Y)
I
PM2 (with Y)
-l-EM
EDS
Figure 3. TEM photographs showing oxides and EDS analysis results on SUS304 PM and IM materials.
308
EFFECT OF YTTRIUM ADDITION
600
--PM,
*Iho!AY
--IM,
wlhmnY
Vol. 36, No. 3
500
0
200
400
600
600
l&JO
0
Test Temperature (“C)
200
400
660
Test Tmpetalure
600
Vi-10
(“C)
Figure 4. Resultsof high temperaturetensile tests of SUS304PM and IM materials.
tively. PM2 and IM2 containing yttrium have much smaller grain size than the materials without yttrium and show, respectively, stringer and granular inclusions. Most of these inclusions for PM2 have been identified as yttria by EDS. It is evident that these yttria of PM2 result partly from the same that existed in powders. Oxides of IM2 are also yttria formed in the melt before ingot casing. It is considered that the finer grains with PM2 result from more uniformly distributed yttria in comparison with IM2 owing to inheritance of powder metallurgy process. To compare yttria in PM2 with oxides in PM1 in terms of shape, distribution and constituents, the extraction replicas taken from these materials were investigated by TEM with EDS. The results are shown in Fig.3. It is seen that main oxides of PM1 and PM2 are identified as pure Cr and Y oxides, respectively, and both oxides are in round shape. As to finer oxides, it is also seen that yttria with the size down to 0.05 pm in PM2 are dispersed. On the other hand, Cr oxides with the size down to 0.1 in PM1 are sporadically situated and oxides finer than 0.2 pm are scarce. In addition, PM1 exhibits large oxides with 1.5 pm which is considered to originate from agglomeration of surface oxides on powders. By image analysis with the photograph of PM2, it was determined that area fraction of yttria including larger size of oxides was approximately 2%. However, the fraction of fine oxides is much less than that of commercial Ni base ODS alloy, in which 1 to 3% is estimated only for fine oxides typically from 15 to 30 nm (7).
loo
Time to rupture(h) Figure 5. Resultsof creep rupture testsof
SUS304PM and IM materialsat 950°C.
EFFECT OF YTTRIUMADDITION
Vol. 36, No. 3
High Temperature Tensile Properties Figure 4 shows the results of high temperature tensile tests for the PM and IM materials. For a comparison between PM materials, PM2 indicates slightly higher 0.2% yield strength (YS) and tensile strength (TS) lthan PM1 in the entire testing temperature range. Although this difference becomes smaller with increasing temperatures, PM2 maintains clearly higher strength even at 950°C. It is considered that the difference results from a sum of the effects of the grain size and dispersion of yttria with PM2. The values of elongation (EL) and reduction of area (RA) are remarkably higher with PM2 especially above 800°C. For a comparison of PM and IM materials, PM materials show much greater YS and TS in the low temperature range while the strengths of PM1 are equivalent to those of IM materials above 800°C. However, values of RA and EL of PM1 are much lower than those of IMl. It is noted that RA of PM2 is comparable to that of IMI, which is close to industrially produced TP304. Creep Ruptuw Tests Figure 5 shows the results of creep rupture tests at 950°C under various stress levels. It is seen that PM2 has remarkably excellent creep rupture lives as compared with the other materials. It is also surprising that such performance is successfully attained even with original fine grain size as shown in Fig.2. It is found that this rupture strength level is comparable to solid solution strengthened type of Ni base alloy such1 as Alloy X (8) by drawing Larson-Miller curves. Although IM2 shows longer rupture lives than IMl, yttrium addition itself is not significantly effective enough to explain the improvement in PM2 from PM1 with the consideration of the difference between IMl and IM2. Furthermore, it is seen that an improvement in oxidation resistance expected by the yttrium addition may not dramatically contribute to an increase in the creep rupture strength. Accordingly, it is considered that this improvement in PM2 is attributable to the fine dispersion of yttria. Cr oxides adversely affect the strength because PM1 shows the lowest rupture lives. In general, any dispersed fine and stable oxides are effective to increase rupture strength as long as they are within grains by suppression of dislocation movement. However, since Cr oxides of PM1 are relatively large and sparsely situated, they will not contribute to an increase in strength of the matrix at high temperatures. Figure 6 shows dislocation substructures of PM1 and PM2 specimens crept at 950°C. It is seen that dislocations are pinned at and bowed between the dispersed oxides in PM2. This fact indicates that dispersion of fine yttrium oxides contribute to an increase in strength of grains by Orowan strengthening mechanism. On the other hand, dislocations are less frequently seen without such pining in PMl. Presumably, Cr oxides existing on grain boundaries promote cavity formation and cracking, and degrade creep rupture life while oxides with sporadic distribution in the grains have no significant influ-
a) PM1 (without Y) b) PM2 (with Y)
CT=12 MPa, tR=149h 0 =30 MPa, t,=190 h
Figure 6. TEM photographs showiag dislocation substructures of SUS304 PM specimens crept at 950°C.
310
EFFECT OF YTTRIUM ADDITION
Vol. 36, No. 3
ence on the strength. Accordingly, it is concluded that PM2 is a kind of ODS alloy successfully produced by atomizing without mechanical alloying. Conclusions
The effect of yttrium addition of PM TP304 was investigated on tensile strength and creep rupture behavior at elevated temperatures in comparison with IM materials. The conclusions obtained are as follows: 1. Yttria particles formed by an addition of Y in the melt before atomizing are dispersed with size of 0.05 to 1 (m in the extruded materials while Cr oxides were formed with larger size for the material without Y addition. 2. PM TP304 with Y addition shows superior tensile strength and ductility to PM and IM counterparts even above 800°C. 3. The creep rupture lives of PM TP304 with Y addition are significantly prolonged by the dispersion of yttria through suppression of dislocation movement while PM TP304 without Y addition indicated inferior rupture lives to IM TP304. References 1. 2. 3. 4. 5. 6. I. 8.
L.R. Curwick, “Frontiers of High Temperature Materials”, JSBenjamin Ed., INCO, 3 (1981). G.A.J. Hack, Powder Metall., 27,73 (1984). T. Isomto and H. Nagai, J. of Japan Sot. Powder and Powder Metall., 42,618 (1995). T. Isomto and H. Nagai, J. of Japan Sot. Powder and Powder Metall. 42,135O (1995). T. Isomto and H. Nagai, J. of Japan Sot. Powder and Powder Metall., in press. F. Ishii and S. Banya, Tetsu to Hagane, 80,359 (1994). G.H. Gessinger , “Powder Metallurgy of Superalloys”, p.214, Butterworth & Co. (1984). “The Superalloys II”, CT. Sims et al. Eds., p.596, Wiley-Interscience, (1987).