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Effect of vanadium and titanium on mechanical properties of chromium-tungsten steels
Journal of Nuclear Materials 212-215 (1994) 569-573
ELSEVIER
Effect of vanadium and titanium on mechanical properties of chromium-tungsten steels * R.L. Klueh, D.J. Alexander Metals and Ceramics Division, Oak Ridge National Laboratory,
Oak Ridge, TN 37831-6376,
USA
Abstract
Increasing the vanadium content from 0.1 to 0.50% in a 2.25Cr-2W-O.lC steel (all concentrations are in wt%,) increased the yield stress 20% and resulted in a higher ductile-brittle transition temperature (DBTT). When vanadium was increased to OS%, a further slight increase in strength occurred with a large increase in DBTT. Thus, optimum strength and impact toughness is achieved at an intermediate vanadium concentration. With the addition of 0.02% Ti to 2.25Cr-0.25V-O.lC, 2.25Cr-2W-O.lC, and 2.25Cr-2W-0.25V-O.lC steels, yield stress decreased 10 to 30%. An increase in impact toughness accompanied the strength loss. The toughness may have been affected by a decrease in the prior austenite grain size. There was little difference in the DBTT of the Ti-modified steels tempered at 700 or 750°C. The use of a Ti-modified steel tempered at 700°C might offset the strength advantage of steels without titanium, which have to be tempered at the higher temperature.
1. Introduction
Much of the effort to develop reduced-activation or fast induced-radioactivity decay (FIRD) ferritic steels has been concentrated on Cr-W-V steels with 7-10% Cr [l-3]. Low-chromium steels have advantages as structural materials [l], and preliminary work indicated that these steels can have strengths as good or better than the higher chromium steels [4]. The strength of a 2.25Cr-2W-0.25V-O.lC (2+Cr-2WV) steel (concentrations are in wt%) exceeded that for 9Cr-2W0.25V-O.O7Ta-O.lC (9Cr-2WVTa) steel, the strongest 9Cr reduced-activation steel tested [4]. However, the Charpy impact toughness of the 2$r-2WV steel was inferior to that of 9Cr-2WVTa. Because toughness decreases during neutron irradiation, it is necessary to improve this property for a low-chromium steel before it can be considered for fusion applications. Work is in progress to develop low-Cr FIRD steels.
* Research sponsored by the Office of Fusion Energy, US Department of Energy, under contract DE-ACOS-840R21400 with Martin Marietta Energy Systems, Inc. 0022-3115/94/$07.00 0 1994 SSDI 0022-3115(94)00159-L
As part of that work, compositional variations were examined in an effort to optimize properties. In this paper, 2.25 Cr steels are examined to determine the effect of vanadium on strength and impact toughness. A second series of steels was examined to evaluate the effect of small amounts of titanium.
2. Experimental
procedure
Experimental 600 g vacuum arc-melted button heats were obtained by using material from the original low-chromium heats as starting stock [4]. The original electroslag-remelted heats were heats prepared by Combustion Engineering, Inc. (CE), Chattanooga, Tennessee. In addition to the nominal compositions of Cr, W, V, and C, concentrations of elements normally found in steels, such as Mn, P, Si, etc., were adjusted to levels typical of commercial practice. Chemical composition, microstructure, and mechanical properties data for the original heats have been published [4-61. The steels with 0.1, 0.25, and 0.5% V were obtained by adding vanadium to the nominally 2.25Cr-2W-O.lC (designated 2$r-2W) heat from CE. The 0.02% Ti additions were made to the 2$r-2W and the nomi-
Elsevier Science B.V. All rights reserved
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R.L. Klueh, D.J. Alexander /Journal
ofNuclear
nally 2.2.5Cr-0.25V-O.lC (2$rV) and 2.25Cr-2W0.25V-O.IC (2$r-2WV) heats to obtain 2$Zr-2WTi, 2$rVTi, and 2$r-2WVTi, respectively. Alloys were cast into rectangular ingots 12.7 X 25.4 X 152 mm. Half of each ingot was hot rolled to 6.4 mm and half to 0.76 mm. Tensile specimens 25.4 mm long with a reduced gage section of 20,3 x 1.52 X 0.76 mm were machined from the 0.76 mm sheet; gage lengths were parallel to the rolling direction. One-third size Charpy specimens 3.3 X 3.3 x 25.4 mm with a 0.51 mm deep 30” V-notch and a 0.05 to 0.08 mm root radius were machined from the 6.4 mm plate. Specimens were taken along the rolling direction with the notch transverse to the rolling direction. Normalized-and-tem~red specimens were tested. All but the Z$Cr-2W were normalized by austenitizing 0.5 h at lOSO”C, followed by a rapid cool in flowing helium. The 2$Cr-2W was austenitized 0.5 h at 900°C. A higher austenitizing temperature was used for the 2$rV, 2$Cr-2WV, and the steels with titanium to ensure dissolution of the V- and Ti-rich carbides. Two tempering conditions were tested: 1 h at 700°C and 1 h at 750°C. Tensile tests were made in vacuum at room temperature, 200, 300, 400, 500, and 600°C on a 44 kN capacity Instron universal testing machine at a nominal strain rate of 4.2 x 1O-4/s. Charpy tests were carried out in a pendulum-ape impact machine specially modified to accommodate subsize specimens [7]. The absorbed energy values were fit with a hyperbolic tangent function to permit the upper-shelf energy (USE) and ductile-brittle transition temperature (DBTT) to be evaluated at the energy midway between the upperand lower-shelf energies.
3. Results and discussion Because of space limitations, this paper will concentrate on mechanical properties. Microstructures will be described in more detail elsewhere. 3.1. Vanadium
uariations
Increasing vanadium concentration affected the tempered bainite microst~ct~e of the steels. First, prior austenite grain size decreased with increasing vanadium. ASTM grain size numbers were estimated at 3, 4, and 5 for the steels with 0.1, 0.25, and 0.5% V, respectively. With increasing vanadium, the number density of precipitates increased, and the size decreased. The largest change occurred between 0.1 and 0.25% V, with less change between 0.25 and 0.5% V. These fine precipitates were mainly V&a, but the steels also contained coarser M,C and M,C, precipitates [5].
Materials 212-215
(1994) 569-573
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TEMPERATURE (‘C) Fig. 1. Yield stress (top) and total elongation (bottom) as a function of test temperature for the steefs with varying vanadium concentration.
Yield stress and total elongation plotted as a function of test temperature are shown in Fig. 1. The relative behavior was similar for specimens tempered at either 700 or 75O*C, although there was a decrease in strength with increasing tempering temperature. Yield stress increased with increasing vanadium (Fig. 1). The greatest strength difference occurred between 0.1 and 0.25% V, with a lesser change between 0.25 and 0.5% V, This observation is probably associated with the larger change in precipitate number density observed between the 0.1 and 0.25% V steels than the change between 0.25 and 0.5% V. The 0.25 and 0.5% V steels were considerably stronger than the 0.1% V steel, again a reflection of the microstructure. The relative behavior of the ultimate tensile strength was similar to the yield stress. Total elongation behavior was the inverse of yield stress behavior, in that the steel with the highest strength had the lowest elongation and vice versa (Fig. 1). There was much less difference between the elonga-
RL. klueh, I3.J. Alexander /Journal of Nuclear Materials212-215 (1994) 569-573 15 I o I
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Fig. 2. ~uct~Ie-brittle transition temperature (solid symbols) and upper-shelf energy (open symbols) for the three steeh with varying vanadium concentration.
tion behavior after tempering at 700 and 750°C than for the yield stress. The total elongation data showed more scatter than the strength data. No explanation is available for the large drop in elongation for the 0.1% V steel tempered at 750°C and tested at 600°C. The relative behavior of the uniform elongation was similar to the total elongation. Charpy impact behavior was affected by vanadium (Fig. 21. For a given tempering temperature, the DBTT reflected the strength differences and increased with increasing vanadium. Relatively low values were obtained for the 0.1% V steel for both tempering conditions and the 025% V steel tempered at 750°C. The 0.5% V steel had a DBTT below 0°C only after the 750°C temper, and then it was only about 30% as low as the other two steels. Vanadium had little effect on the USE, with essentially no difference for the three steels after the 750°C temper (Fig. 2). After the 700°C temper, the USE of the 0.1% V steel was slightly larger than the USE for the 0.25 and 0.5% V steels, which had similar values. These results indicate that to optimize strength and impact toughne~ of a 2.25% Cr steel with 2% W, vanadium should be limited to about 0.25%. Increasing the vanadium concentration from 0.25 to 0.5% results in a minimal strength increase at the cost of substantially raising the DBTl’. 3.2. Titanium additions The 0.02% Ti additions were made to the 2&rV, 2iCr-2W, and 2$Cr-2WV compositions studied previously [4-61. Microst~~ures of the 0.76 mm thick tensile specimens were tempered bainite, Titanium caused a decrease in prior austenite grain size of the two steels with vanadium. ASTM grain size numbers were
511
estimated as 4 and 7 for the 2$CrV and 2@VTi, respectively, and 4 and 8 for 2$Cr-2WV and 2$Cr2WVTi, respectively. A gram size number of 10 was estimated for both 2$r-ZW and 2$Cr-2WTi. Titanium caused a decrease in the number density of precipitates relative to the steels without titanium [5]. Steels with vanadium and no titanium contained a fine distribution of vanadium-rich MC (V.,C,) in addition to M,C and M&; the 2$Zr-2W contained M,C, M,C,, and M,,C,. Although TEM analysis has not been completed on the steels with titanium, they are expected to contain some MC rich in titanium, in addition to the other carbides observed in the steels without titanium, It is believed that not all of the titanium”rich MC dissolved during the austenitization treatment. The remaining MC particles would limit grain growth, which would result in a smaller prior austenite grain size. The reduction in the number density of precipitates of the steels containing titanium could be caused by undissolved titanium-rich carbides acting as nuclei for the carbides that formed during tempering. The limited number of these pre-existing nuclei could reduce the number of precipitates that formed. TEM is required to verify this hypothesis. Addition of 0.02% Ti to the Cr-W-V steels caused a marked decrease in yield stress for all three steels, as shown in Fig. 3 for the steels tempered at 700°C. The magnitude of the decrease depended on the strength of the steel without the tjtanium. The 2$Cr-2WV steel was the strongest, and it showed the largest decrease. SmaIler changes occurred for the other two steels. These observations reflect the observations on the reduction in the precipitate number density caused by the titanium. Ultimate tensile strength behavior was similar to the yield stress behavior. A slight increase in total elongation accompanied the strength decrease caused by the titanium, as shown in Fig. 3 after the 700°C temper. Relative changes in uniform elongation were similar to those for total elongation. Tempering at 750°C reduced the strength and increased the ductility relative to these properties after the 700°C temper. However, the relative behavior after the 750°C temper was similar to that after the 700°C temper. Titanium caused a decrease in the DBTI’ of the steels that contained vanadium - the 2$CrV and 2$Cr-2WV steels (Fig. 4). It had relatively little effect on the 2$Cr-2W steel, which had the lowest DBTI prior to the titanium addition. These observations are attributed to the change in grain size and the reduction in strength caused by the titanium. The addition of titanium to 2fCr-2W did not cause a noticeable change in prior austenite grain size because the titanium addition and the use of a 1050°C austenit~ation tempera-
R.L. Kiueh, D.J. Alexander /Journal
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ture were offset by the 900°C austenitization temperature for the 2$3-2W. Note that whereas there was no effect of tempering temperature on the 2iCrV, there was a significant difference for 2$r-2WV. This is probably due to a reduction in the greater dispersion-strengthening effect in the 2$Zr-2WV by tempering at 750°C. The higher number density of smaller precipitates in 2$r-2WV makes this steel stronger than 2$rV, which contains a lower density of larger particles [5] that are affected less by tempering. When titanium is added to this steel and the particle density was decreased, the difference in DB’IT for the 700 and 750°C tempers was reduced significantly. There was relativeiy little difference in the USE of the steels with and without titanium, regardless of the tempering temperature. In fact, there was little difference between the different steels. Only the 2$Zr-2WV steel showed some variation, but it was relatively minor compared to the differences noted for the DBTI’.
~~ieri~l~ 212-215
(1994) 569-573
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Fig. 4. Ductile-brittle transition temperature (top) and upper-shelf energy (bottom) of the 2$rV, 2$r-2W, and 2$3-2WV steels with and without an addition of 0.02% Ti after normalizing and then tempering 1 h at 700°C and 1 h at 750°C.
200
300
TEMPERATURE
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(‘C)
Fig. 3. Yield stress (top) and total elongation (bottom) as a function of test temperature of the Z$CrV, 2iCr-2W, and 2$Cr-2WV steels with and without an addition of 0.02% Ti after normalizing and then tempering 1 h at 700°C.
These results indicate that the 0.02% Ti improved impact toughness. Improvement came at the expense of strength, but the magnitude of the strength decrease with the titanium addition might be alleviated by austenitizing above the 1050°C used in this experiment, More titanium carbide would then dissolve, although it would also increase the prior austenite grain size. The best austenitization temperature would have to be determined, because if the temperature was too high, the large prior austenite grain size could negate the positive effect of titanium. Another alternative is a 2$Cr2WVTi normalized at 1050°C and tempered at 7OO”C, compared to 2$Cr-2WV steel (0.25% V) that has to be tempered at 750°C to obtain adequate toughness. The 700°C temper would provide strength and toughness for 2$r-2WVTi similar to 2$Cr-2WV tempered at 750°C (Fig. 4).
RL. Klueh, D.J. Altxander/Journal
of Nuclear Materials 212-215
(1994) 569-573
573
4. Summary and conclusion
Acknowledgements
The effect of adding 0.1, 0.25, and 0.5% V to a 2$Cr-2W steel was examined. Increasing vanadium from 0.1 to 0.25% caused a large strength increase. Little further increase occurred when vanadium was increased from 0.25 to 0.5%. The DB’IT in the Charpy impact tests increased with vanadium, with the largest increase for steels tempered at 7OO”C,the DBTT of the 0.5% V steel tempered at 750°C was above room temperature. After tempering at 7SO”C, there was less difference between the steels with 0.1 and 0.25% V with a greater difference between the 0.25 and 0.5% V steels. Based on these observations, it was concluded that for optimum strength and impact toughness, a 2$Cr-2WV steel should contain about 0.25% V. A 0.02% Ti addition to Z$rV, 2$r-2W, and 2$Cr-2WV steels caused a strength decrease, but it improved the Charpy impact behavior by lowering the DBTI’. It may be possible to maintain the improved DBTT and improve the strength by increasing the austenitizing temperature, or by tempering a 2$r2WVTi steel at 700°C to get improved strength and toughness over 2$r-2WV, which must be tempered at 750°C.
Experimental work was carried out by N.H. Rouse (heat treatment and tensile tests) and J.J. Henry (Charpy tests). The manuscript was reviewed by R.K. Nanstad and J.J. Kai.
References [l] R.L. Klueh and E.E. Bloom, Nucl. Eng. Design/Fusion 2 (1985) 383. [2] M. Tamura, H. Hayakawa, M. Tanimura, A. Hishinuma and T. Kondo, J. Nucl. Mater. 141-143 (1986) 1067. [3] D. Dulieu, K.W. Tupholme and G.J. Butterworth, J. Nucl. Mater. 141-143 (1986) 1097. [4] R.L. Klueh, Metal]. Trans. 20A (1989) 463. [S] R.L. Khteh and P.J. Maziasz, Metall. Trans. 20A (1989) 373. [6] R.L. Klueh and W.R. Corwin, J. Mater. Eng. 11 (1989) 169. [7] D.J. Alexander, R.K. Nanstad, W.R. Corwin and J.T. Hutton, in Applications of Automation Technology to Fatigue and Fracture Testing, ASTM-STP 1092, eds. A.A. Braun, NE. Ashbaugh and F.M. Smith (American Society for Testing and Materials, Philadelphia, 1990) p. 83.
ELSEVIER
Effect of vanadium and titanium on mechanical properties of chromium-tungsten steels * R.L. Klueh, D.J. Alexander Metals and Ceramics Division, Oak Ridge National Laboratory,
Oak Ridge, TN 37831-6376,
USA
Abstract
Increasing the vanadium content from 0.1 to 0.50% in a 2.25Cr-2W-O.lC steel (all concentrations are in wt%,) increased the yield stress 20% and resulted in a higher ductile-brittle transition temperature (DBTT). When vanadium was increased to OS%, a further slight increase in strength occurred with a large increase in DBTT. Thus, optimum strength and impact toughness is achieved at an intermediate vanadium concentration. With the addition of 0.02% Ti to 2.25Cr-0.25V-O.lC, 2.25Cr-2W-O.lC, and 2.25Cr-2W-0.25V-O.lC steels, yield stress decreased 10 to 30%. An increase in impact toughness accompanied the strength loss. The toughness may have been affected by a decrease in the prior austenite grain size. There was little difference in the DBTT of the Ti-modified steels tempered at 700 or 750°C. The use of a Ti-modified steel tempered at 700°C might offset the strength advantage of steels without titanium, which have to be tempered at the higher temperature.
1. Introduction
Much of the effort to develop reduced-activation or fast induced-radioactivity decay (FIRD) ferritic steels has been concentrated on Cr-W-V steels with 7-10% Cr [l-3]. Low-chromium steels have advantages as structural materials [l], and preliminary work indicated that these steels can have strengths as good or better than the higher chromium steels [4]. The strength of a 2.25Cr-2W-0.25V-O.lC (2+Cr-2WV) steel (concentrations are in wt%) exceeded that for 9Cr-2W0.25V-O.O7Ta-O.lC (9Cr-2WVTa) steel, the strongest 9Cr reduced-activation steel tested [4]. However, the Charpy impact toughness of the 2$r-2WV steel was inferior to that of 9Cr-2WVTa. Because toughness decreases during neutron irradiation, it is necessary to improve this property for a low-chromium steel before it can be considered for fusion applications. Work is in progress to develop low-Cr FIRD steels.
* Research sponsored by the Office of Fusion Energy, US Department of Energy, under contract DE-ACOS-840R21400 with Martin Marietta Energy Systems, Inc. 0022-3115/94/$07.00 0 1994 SSDI 0022-3115(94)00159-L
As part of that work, compositional variations were examined in an effort to optimize properties. In this paper, 2.25 Cr steels are examined to determine the effect of vanadium on strength and impact toughness. A second series of steels was examined to evaluate the effect of small amounts of titanium.
2. Experimental
procedure
Experimental 600 g vacuum arc-melted button heats were obtained by using material from the original low-chromium heats as starting stock [4]. The original electroslag-remelted heats were heats prepared by Combustion Engineering, Inc. (CE), Chattanooga, Tennessee. In addition to the nominal compositions of Cr, W, V, and C, concentrations of elements normally found in steels, such as Mn, P, Si, etc., were adjusted to levels typical of commercial practice. Chemical composition, microstructure, and mechanical properties data for the original heats have been published [4-61. The steels with 0.1, 0.25, and 0.5% V were obtained by adding vanadium to the nominally 2.25Cr-2W-O.lC (designated 2$r-2W) heat from CE. The 0.02% Ti additions were made to the 2$r-2W and the nomi-
Elsevier Science B.V. All rights reserved
570
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ofNuclear
nally 2.2.5Cr-0.25V-O.lC (2$rV) and 2.25Cr-2W0.25V-O.IC (2$r-2WV) heats to obtain 2$Zr-2WTi, 2$rVTi, and 2$r-2WVTi, respectively. Alloys were cast into rectangular ingots 12.7 X 25.4 X 152 mm. Half of each ingot was hot rolled to 6.4 mm and half to 0.76 mm. Tensile specimens 25.4 mm long with a reduced gage section of 20,3 x 1.52 X 0.76 mm were machined from the 0.76 mm sheet; gage lengths were parallel to the rolling direction. One-third size Charpy specimens 3.3 X 3.3 x 25.4 mm with a 0.51 mm deep 30” V-notch and a 0.05 to 0.08 mm root radius were machined from the 6.4 mm plate. Specimens were taken along the rolling direction with the notch transverse to the rolling direction. Normalized-and-tem~red specimens were tested. All but the Z$Cr-2W were normalized by austenitizing 0.5 h at lOSO”C, followed by a rapid cool in flowing helium. The 2$Cr-2W was austenitized 0.5 h at 900°C. A higher austenitizing temperature was used for the 2$rV, 2$Cr-2WV, and the steels with titanium to ensure dissolution of the V- and Ti-rich carbides. Two tempering conditions were tested: 1 h at 700°C and 1 h at 750°C. Tensile tests were made in vacuum at room temperature, 200, 300, 400, 500, and 600°C on a 44 kN capacity Instron universal testing machine at a nominal strain rate of 4.2 x 1O-4/s. Charpy tests were carried out in a pendulum-ape impact machine specially modified to accommodate subsize specimens [7]. The absorbed energy values were fit with a hyperbolic tangent function to permit the upper-shelf energy (USE) and ductile-brittle transition temperature (DBTT) to be evaluated at the energy midway between the upperand lower-shelf energies.
3. Results and discussion Because of space limitations, this paper will concentrate on mechanical properties. Microstructures will be described in more detail elsewhere. 3.1. Vanadium
uariations
Increasing vanadium concentration affected the tempered bainite microst~ct~e of the steels. First, prior austenite grain size decreased with increasing vanadium. ASTM grain size numbers were estimated at 3, 4, and 5 for the steels with 0.1, 0.25, and 0.5% V, respectively. With increasing vanadium, the number density of precipitates increased, and the size decreased. The largest change occurred between 0.1 and 0.25% V, with less change between 0.25 and 0.5% V. These fine precipitates were mainly V&a, but the steels also contained coarser M,C and M,C, precipitates [5].
Materials 212-215
(1994) 569-573
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TEMPERATURE (‘C) Fig. 1. Yield stress (top) and total elongation (bottom) as a function of test temperature for the steefs with varying vanadium concentration.
Yield stress and total elongation plotted as a function of test temperature are shown in Fig. 1. The relative behavior was similar for specimens tempered at either 700 or 75O*C, although there was a decrease in strength with increasing tempering temperature. Yield stress increased with increasing vanadium (Fig. 1). The greatest strength difference occurred between 0.1 and 0.25% V, with a lesser change between 0.25 and 0.5% V, This observation is probably associated with the larger change in precipitate number density observed between the 0.1 and 0.25% V steels than the change between 0.25 and 0.5% V. The 0.25 and 0.5% V steels were considerably stronger than the 0.1% V steel, again a reflection of the microstructure. The relative behavior of the ultimate tensile strength was similar to the yield stress. Total elongation behavior was the inverse of yield stress behavior, in that the steel with the highest strength had the lowest elongation and vice versa (Fig. 1). There was much less difference between the elonga-
RL. klueh, I3.J. Alexander /Journal of Nuclear Materials212-215 (1994) 569-573 15 I o I
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Fig. 2. ~uct~Ie-brittle transition temperature (solid symbols) and upper-shelf energy (open symbols) for the three steeh with varying vanadium concentration.
tion behavior after tempering at 700 and 750°C than for the yield stress. The total elongation data showed more scatter than the strength data. No explanation is available for the large drop in elongation for the 0.1% V steel tempered at 750°C and tested at 600°C. The relative behavior of the uniform elongation was similar to the total elongation. Charpy impact behavior was affected by vanadium (Fig. 21. For a given tempering temperature, the DBTT reflected the strength differences and increased with increasing vanadium. Relatively low values were obtained for the 0.1% V steel for both tempering conditions and the 025% V steel tempered at 750°C. The 0.5% V steel had a DBTT below 0°C only after the 750°C temper, and then it was only about 30% as low as the other two steels. Vanadium had little effect on the USE, with essentially no difference for the three steels after the 750°C temper (Fig. 2). After the 700°C temper, the USE of the 0.1% V steel was slightly larger than the USE for the 0.25 and 0.5% V steels, which had similar values. These results indicate that to optimize strength and impact toughne~ of a 2.25% Cr steel with 2% W, vanadium should be limited to about 0.25%. Increasing the vanadium concentration from 0.25 to 0.5% results in a minimal strength increase at the cost of substantially raising the DBTl’. 3.2. Titanium additions The 0.02% Ti additions were made to the 2&rV, 2iCr-2W, and 2$Cr-2WV compositions studied previously [4-61. Microst~~ures of the 0.76 mm thick tensile specimens were tempered bainite, Titanium caused a decrease in prior austenite grain size of the two steels with vanadium. ASTM grain size numbers were
511
estimated as 4 and 7 for the 2$CrV and 2@VTi, respectively, and 4 and 8 for 2$Cr-2WV and 2$Cr2WVTi, respectively. A gram size number of 10 was estimated for both 2$r-ZW and 2$Cr-2WTi. Titanium caused a decrease in the number density of precipitates relative to the steels without titanium [5]. Steels with vanadium and no titanium contained a fine distribution of vanadium-rich MC (V.,C,) in addition to M,C and M&; the 2$Zr-2W contained M,C, M,C,, and M,,C,. Although TEM analysis has not been completed on the steels with titanium, they are expected to contain some MC rich in titanium, in addition to the other carbides observed in the steels without titanium, It is believed that not all of the titanium”rich MC dissolved during the austenitization treatment. The remaining MC particles would limit grain growth, which would result in a smaller prior austenite grain size. The reduction in the number density of precipitates of the steels containing titanium could be caused by undissolved titanium-rich carbides acting as nuclei for the carbides that formed during tempering. The limited number of these pre-existing nuclei could reduce the number of precipitates that formed. TEM is required to verify this hypothesis. Addition of 0.02% Ti to the Cr-W-V steels caused a marked decrease in yield stress for all three steels, as shown in Fig. 3 for the steels tempered at 700°C. The magnitude of the decrease depended on the strength of the steel without the tjtanium. The 2$Cr-2WV steel was the strongest, and it showed the largest decrease. SmaIler changes occurred for the other two steels. These observations reflect the observations on the reduction in the precipitate number density caused by the titanium. Ultimate tensile strength behavior was similar to the yield stress behavior. A slight increase in total elongation accompanied the strength decrease caused by the titanium, as shown in Fig. 3 after the 700°C temper. Relative changes in uniform elongation were similar to those for total elongation. Tempering at 750°C reduced the strength and increased the ductility relative to these properties after the 700°C temper. However, the relative behavior after the 750°C temper was similar to that after the 700°C temper. Titanium caused a decrease in the DBTI’ of the steels that contained vanadium - the 2$CrV and 2$Cr-2WV steels (Fig. 4). It had relatively little effect on the 2$Cr-2W steel, which had the lowest DBTI prior to the titanium addition. These observations are attributed to the change in grain size and the reduction in strength caused by the titanium. The addition of titanium to 2fCr-2W did not cause a noticeable change in prior austenite grain size because the titanium addition and the use of a 1050°C austenit~ation tempera-
R.L. Kiueh, D.J. Alexander /Journal
572
ofNuclear
ture were offset by the 900°C austenitization temperature for the 2$3-2W. Note that whereas there was no effect of tempering temperature on the 2iCrV, there was a significant difference for 2$r-2WV. This is probably due to a reduction in the greater dispersion-strengthening effect in the 2$Zr-2WV by tempering at 750°C. The higher number density of smaller precipitates in 2$r-2WV makes this steel stronger than 2$rV, which contains a lower density of larger particles [5] that are affected less by tempering. When titanium is added to this steel and the particle density was decreased, the difference in DB’IT for the 700 and 750°C tempers was reduced significantly. There was relativeiy little difference in the USE of the steels with and without titanium, regardless of the tempering temperature. In fact, there was little difference between the different steels. Only the 2$Zr-2WV steel showed some variation, but it was relatively minor compared to the differences noted for the DBTI’.
~~ieri~l~ 212-215
(1994) 569-573
Tempered
Tempered 75O’C
7OO’C WIthout With
TI Ti
q q
-60
-80
Tqgzd
Tempered 75O’C Without With
Ti Ti
r-l
800 700 2.5 600 0 500
400
1 0
1
100
200
300
I
400
500
600
TEMPERATURE (‘Ct
r
0 2 1140VTI A 2 l/40-2WTi
v
o 2
2
lf4cr-2WV
100
14Cr-2W
2
4Cr-2WV
-
l/40-2WVTi
7OO’C
0
2
I
m 2 lf4CrV A 2 lf4CP2W Tempered
d
2 lf4CrV
Fig. 4. Ductile-brittle transition temperature (top) and upper-shelf energy (bottom) of the 2$rV, 2$r-2W, and 2$3-2WV steels with and without an addition of 0.02% Ti after normalizing and then tempering 1 h at 700°C and 1 h at 750°C.
200
300
TEMPERATURE
400
500
600
(‘C)
Fig. 3. Yield stress (top) and total elongation (bottom) as a function of test temperature of the Z$CrV, 2iCr-2W, and 2$Cr-2WV steels with and without an addition of 0.02% Ti after normalizing and then tempering 1 h at 700°C.
These results indicate that the 0.02% Ti improved impact toughness. Improvement came at the expense of strength, but the magnitude of the strength decrease with the titanium addition might be alleviated by austenitizing above the 1050°C used in this experiment, More titanium carbide would then dissolve, although it would also increase the prior austenite grain size. The best austenitization temperature would have to be determined, because if the temperature was too high, the large prior austenite grain size could negate the positive effect of titanium. Another alternative is a 2$Cr2WVTi normalized at 1050°C and tempered at 7OO”C, compared to 2$Cr-2WV steel (0.25% V) that has to be tempered at 750°C to obtain adequate toughness. The 700°C temper would provide strength and toughness for 2$r-2WVTi similar to 2$Cr-2WV tempered at 750°C (Fig. 4).
RL. Klueh, D.J. Altxander/Journal
of Nuclear Materials 212-215
(1994) 569-573
573
4. Summary and conclusion
Acknowledgements
The effect of adding 0.1, 0.25, and 0.5% V to a 2$Cr-2W steel was examined. Increasing vanadium from 0.1 to 0.25% caused a large strength increase. Little further increase occurred when vanadium was increased from 0.25 to 0.5%. The DB’IT in the Charpy impact tests increased with vanadium, with the largest increase for steels tempered at 7OO”C,the DBTT of the 0.5% V steel tempered at 750°C was above room temperature. After tempering at 7SO”C, there was less difference between the steels with 0.1 and 0.25% V with a greater difference between the 0.25 and 0.5% V steels. Based on these observations, it was concluded that for optimum strength and impact toughness, a 2$Cr-2WV steel should contain about 0.25% V. A 0.02% Ti addition to Z$rV, 2$r-2W, and 2$Cr-2WV steels caused a strength decrease, but it improved the Charpy impact behavior by lowering the DBTI’. It may be possible to maintain the improved DBTT and improve the strength by increasing the austenitizing temperature, or by tempering a 2$r2WVTi steel at 700°C to get improved strength and toughness over 2$r-2WV, which must be tempered at 750°C.
Experimental work was carried out by N.H. Rouse (heat treatment and tensile tests) and J.J. Henry (Charpy tests). The manuscript was reviewed by R.K. Nanstad and J.J. Kai.
References [l] R.L. Klueh and E.E. Bloom, Nucl. Eng. Design/Fusion 2 (1985) 383. [2] M. Tamura, H. Hayakawa, M. Tanimura, A. Hishinuma and T. Kondo, J. Nucl. Mater. 141-143 (1986) 1067. [3] D. Dulieu, K.W. Tupholme and G.J. Butterworth, J. Nucl. Mater. 141-143 (1986) 1097. [4] R.L. Klueh, Metal]. Trans. 20A (1989) 463. [S] R.L. Khteh and P.J. Maziasz, Metall. Trans. 20A (1989) 373. [6] R.L. Klueh and W.R. Corwin, J. Mater. Eng. 11 (1989) 169. [7] D.J. Alexander, R.K. Nanstad, W.R. Corwin and J.T. Hutton, in Applications of Automation Technology to Fatigue and Fracture Testing, ASTM-STP 1092, eds. A.A. Braun, NE. Ashbaugh and F.M. Smith (American Society for Testing and Materials, Philadelphia, 1990) p. 83.