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Impurity element effects on the toughness of 9Cr-1Mo steel
Journal of Nuclear Materials 141-143 (1986) 508-512 North-Holland, Amsterdam
508
IMPURITY ELEMENT EFFECTS ON THE TOUGHNESS K.J. H A R R E L S O N ,
OF 9Cr-lMo
STEEL
S.H. R O U , a n d R.C. W I L C O X
Materials Engineering, Department of Mechanical Engineering, A uburn University, Auburn, A L 36849, USA
The effects of the trace impurity elements, phosphorus, sulfur, and silicon, on the impact toughness of a 9Cr-lMo steel were investigated. The results of Charpy impact tests showed that the additions of all three elements produced a decrease in the upper shelf energy and an increase in the ductile-brittle transition temperature (DBTT). Above 0.04% P and 0.4% Si, the upper shelf energy decreases sharply. Additions of over 0.03% P or 0.4% Si cause an increase in the DBTT. Trace amounts of sulfur were found to have a much greater detrimental effect than phosphorus. Impurity segregation and delta ferrite content are used to explain the decrease in toughness produced by the addition of P and Si, respectively. Sulfur promotes non-metallic inclusions and intergranular fracture thus producing embrittlement.
1. Introduction The resistance to irradiation swelling [1-4] and to creep [5-8] make ferritic steels candidate materials for applications in nuclear power plants. However, marginal fracture toughness at low temperatures and the transhion from ductile to brittle behavior near 0 ° C limit the applications of these alloys [9]. The 9 C r - l M o ferritic steel has competitive properties to 300 series stainless in many applications and has a lower level of alloy element content. The better high temperature strength of 9 C r - l M o , compared to other ferritic steels, makes this ~aterial attractive for superheater applications in heat exchanger systems. Prototype systems of 9 C r - l M o have been operating well in tests at Tennessee Valley Authority power plants [10]. In the production of steels where a high degree of mechanical property reproducibility is required, the knowledge of the effects of small changes in impurity concentration is important. Understanding the mechanism of how the impurity elements induce variations in the fracture toughness can supply needed information on how tightly impurity specifications must be set. This work investigates the effects of the impurity elements, sulfur, phosphorus, and silicon on the impact behavior of the 9 C r - l M o ferritic steel. The study investigated which impurity element has the greatest effect on the impact properties and to what degree the impact properties might be affected as a function of impurity concentration.
2. Experimental procedure A bar of 9 C r - l M o steel manufactured by Carpenter Technology Corportion was used as the basis of this investigation. An independent metallurgical consulting service determined the composition of each ingot by the use of a vacuum spectrometer. The composition of original bar in weight percent was 9.44 Cr, 1.03 Mo, 0.11 C, 0.017 P, 0.0007 S, 0.28 Si, 0.05 W, 0.21 V, 0.15 Ni, 0.45 Mn, < 0.08 Cu with the balance iron. The
possible existance of other elements were not reported by the spectrometric analysis. Phosphorus and sulfur were varied by adding ferrous phosphide (Fe2P) and ferrous sulfide (FeS), respectively, to the base steel. Additions of silicon were made as the pure metal. Raw materials were vacuum melted producing ingots of 300 grams. The variation in phosphorus-content was 0.017, 0.020, 0.030, 0.040, and 0.050 wt%. The sulfur-content varied as 0.0007, 0.007, 0.011, and 0.034 wt% while silicon varied as 0.28, 0.41, 0.73, and 0.78 wt%. The smallest amount in each case corresponds to the control bar composition. No variation was found from heat to heat for the elements carbon, tungsten, vanadium, nickel or copper. The content of the elements phosphorus, sulfur and silicon also remained constant except when purposely varied. The only differences between heats were found with chromium, m o l y b d e n u m and manganese. Chromium varied from 8.5-8.56 wt%, 8.65-8.67 wt% and 8.2-8.61 wt% in the heats for phosphorus, sulfur and silicon, respectively. These values are down from 9.44 wt% in the original ingot. Molybdenum was found to be 0.95-0.97 wt% in all heats down from 1.03 wt% in the control ingot. The manganese content was 0.49-0.52 wt% in the sulfur and silicon heats. In the phosphorus ingots, the 0.02 wt% phosphorus heat only contained 0.38 wt% manganese while the others were found to contain 0.50-0.53 wt% manganese. Each ingot was preheated to 760°C and hot rolled to a thickness of 12 ram. Twelve to twenty standard size V-notched Charpy specimens were machined from each ingot. Ch.arpy specimens were machined so that the notch was perpendicular to and the long dimension of the specimen was parallel to the rolling direction of the ingot. Specimens were then placed with Ta and Zr foils in sealed fused quartz capsules at 4 psia of At. The heat treatment for each specimen consisted of 1040°C/1 h / A C + 760 ° C / 1 h / A C . Impact tests were conducted at temperatures between - 1 9 6 ° C to 250°C to investigate the ductile-brittle transition behavior. Specimens were allowed to stabilize for 15 min at the test temperature (_+2°C) prior to testing. All the test procedures were performed according to ASTM E-23 standards.
0 0 2 2 - 3 1 1 5 / 8 6 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g Division)
K.J. Harrelson et al. / Impurity element effects on the toughness of 9 C r - l Mo steel
3. Results and discussion
400
Impact values plotted as a function of temperatu're yielded sigmoid curves. Such curves depict the upper shelf region, the lower shelf region, and the transition region. The average upper shelf energies were calculated by averaging data points of the upper shelf. The 50-J ductile-brittle transition temperatures (DBTT) were determined by interpolating the corresponding temperature from the sigmoid curves at this impact energy.
Fig. I shows the impact curves obtained for 9 C r - l M o steels with additions of 0.017, 0.02, 0.03, 0.04, and 0.05 wt% phosphorus. The impact curves show a general tendency to shift down and to the right as phosphorus concentrations are increased. Below 0.04 wt% phosphorus (with the exception of 0.02%), there is little change in the average upper shelf energy (fig. 2). For 0.02 wt% phosphorus, the upper shelf energy decreased from 358 to 305 J. This difference in behavior might be explained on the basis of compositional differences between ingots. However, the only obvious reported difference in composition is the manganese content which is 0.38 wt% in the 0.02% phosphorus ingot compared to 0.50-0.53 wt% manganese in the other ingots. As the phosphorus concentration goes above 0.04 wt%, the average upper self energy drops sharply. The phosphorus additions produce no change in value of the average lower knee energies (1-5 J), but do cause the lower knee to extend to higher temperatures. Phosphorus addition produced a rapid increase in the DBTT
400 350
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Fig. 1. Effect of phosphorus on impact energy.
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Fig. 2. Effect of impurity content on the upper shelf energies.
(fig. 3). For additions above the currently specified 0.02 wt% limit, the DBTT increased rapidly. To maintain the DBTT below 0°C, the phosphorus content must be kept below 0.03 wt% when utilizing the heat treatment used in this investiation. The basic microstructure of 9 C r - l M o steel, delta ferrite plus tempered martensite is shown in fig. 4a. The effect of 0.05 wt% phosphorus on this microstructure is illustrated in fig. 4b. Small individual ferrite islands are found in the tempered martensitic matrix. Precipitated secondary carbides were still evident at delta ferrite grain boundaries with primary carbides found within the matrix. N o obvious differences in the microstructures could be found with the use of optical microscopy. Fig. 3 shows some tendency of the DBTT to increase at a slower rate for the high phosphorus concentrations as has been reported [11]. This tendency possibly can be explained by the equilibrium segregation model [12]. Phosphorus tends to segregate to regions of high lattice misfit. Such regions can be secondary phase interfaces and grain boundaries. However, these imperfections only provide a fixed number of segregation sites. As more and more segration sites are occupied, the movement of phosphorus to these sites is progressively reduced. As the number of phosphorus atoms exceeds the available segregation sites, uniform distribution of the excess phosphorus is likely to occur. The rate at which phosphorus increases the brittleness of the steel should then decrease somewhat at high phosphorus contents, Since the segregation of phosphorus is progressively reduced with additional amounts of phosphorus concentrations, a gradual change in the rate of increase in the DBTT also should be expected.
510
KJ. Harrelson et al. / Impurity element effects on the toughness of 9Cr- IMo steel
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'Fig. 3. Effect of impurity content on the DBTT.
Impact curves for the different sulfur contents are shown in fig. 5. In general, as sulfur content was increased the uper shelf energy decreased drastically (fig. 2). The 0.011 wt% sulfur curve in fig. 5 does not seem to
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.
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.
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:25 -150 -75 0 75 155 225 TEMPERATURE (*C)
fit this trend but the data points indicate somewhat of a tendency for the curve to bend toward the horizontal in the 25 to 50°C range. Thus, the upper shelf energy for the 0.011 wt% sulfur alloy (taken at 50°C) likely is below that shown for 100°C. The lower knee region is little effected by sulfur. Also, as sulfur was increased the DBTT increased (fig. 3). To maintain a DBTT (at 50 J) below 0°C, the sulfur level must be less than 0.01 wt%. Additions of sulfur to 9 C r - l M o steel were found to cause similar effects as phosphorus, an increase in the DBTT and a decrease in the upper shelf region. However, sulfur was more effective than phosphorus in increasing the DBTT. The microstructure of the 0.034% sulfur alloy is shown in fig. 4c. The amount of free ferrite was affected by the sulfur impurity content. Delta ferrite initially was found to decrease with sulfur additions and then increased as sulfur began to appear as nonmetallic inclusions. These microstructural changes are considered to be the likely reason for the adverse influence of sulfur on the impact properties of this material.
.
3.3. Silicon effects
k.
~
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Fig. 5. Effect of sulfur on impact energy.
3.2. Sulfur effects
.
.~
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Fig. 4. Microstructure of 9Cr-lMo; (a) control, (b) with 0.05% P, (c) with 0.034% S, and (d) with 0.78% Si.
The impact energies of the four 9 C r - l M o steels with 0.28, 0.41, 0.73, and 0.78 wt% silicon are illustrated in fig. 6. The element silicon has a detrimental effect on the impact properties of 9 C r - l M o steels. Generally. the addition of silicon shifted the curves to higher temperatures and depressed the upper shelves. The changes in the upper shelf energies are plotted in fig. 2. A silicon content of less than 0.41 wt% did not have any effect on the upper shelf energy of 358 J. Most Charpy specimens
K.J. Harrelson et al. / Impurity element effects on the toughness of 9 C r - l Mo steel
400 Ld
0.28%Si 300 z
250
o 41%si ~ / .
. . . . . . .
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the equilibrium austenite region. The increase in the DBTT is felt to be directly attributed to this increase in delta ferrite. As delta ferrite increases with increasing silicon, more grain boundary regions are generated and thereby provide extra places for carbides to precipitate. The total quantity of all precipitated carbides remain unchanged in the silicon-doped alloys: Thus, fewer carbides can precipitate in the matrix. Usually, the secondary carbides forming along the delta ferrite grain boundaries are larger than those in the tempered martensite matrix. These larger secondary carbides provide favorable places for critical microcracks to nucleate and hence the DBTT is degraded.
>
4. Conclusions
50
5
-225 -150 -75 0 75 155 TEMPERATURE(°C)
225
Fig. 6. Effect of silicon on'impact energy.
from the upper shelf regions did not completely break on impact. Further additions of silicon caused a sharp decrease in the upper shelf energy. The upper shelf energies decreased to 190 J as the concentration of silicon increased to 0.78 wt%. The energies associated with the lower knee region (1-5 J) were not influenced by silicon additions. However, the lower knee region was extended to higher temperatures with increasing silicon content. Also, silicon produced an increase in the DBTT with the greatest effect being caused by additions over 0.5 wt%. To maintain the DBTT below 0°C, the silicon content must be kept below 0.5 wt% (fig. 3). If the silicon level is reduced to a maximum of 0.4%, then no measurable effect of silicon on the DBTT would be observed. The typical microstructure of the 9 C r - l M o steel with silicon additions (fig. 4d) consists of tempered martensite and elongated delta ferrite. However, as silicon was increased, the amount of delta ferrite was found to increase. This increase in delta ferrite is felt to be the cause of the decrease in impact properties with increasing silicon because delta ferrite was found to be detrimental to impact properties in a pervious investagtion [13]. Silicon is a ferrite stabilizer [14] and serves to balance the volume fraction of ferrite and austenite/ martensite in steels. The addition of silicon causes the austenite region to contract which in effect changes the fraction of ferrite and austenite/martensite during the cooling of the casting, during annealing for fabrication, and during the final heat treatment. High silicon contents produce a more stable delta ferrite, thereby, reducing the tendency of ferrite to transform to austenite in
The following conclusions resulted from the investigation of trace impurity elements on the impact properties of 9 C r - l M o steels: (1) Phosphorus additions increase the DBTT and decrease the upper shelf energy. (2) Sulfur additions result in drastic embrittlement of 9 C r - l M o steels. The effect of sulfur is more pronounced than that of phosphorus. Sulfur promotes the formation of non-metallic inclusions which reduced toughness. (3) Silicon contents greater than 0.4 wt% cause embrittlement. The DBTT is raised and the upper shelf energies are decreased. The increase in delta ferrite with silicon additions is postulated to explain the change in the DBTT. (4) The existing maximum levels of phosphorus, sulfur, and silicon in 9 C r - l M o steels can not be raised without adverse effects on the DBTT of the alloy. References [1] R.W. Powell, D.F. Peterson, M.K. Zimmershield and J.F. Bates, J. Nucl. Mater. 103 & 104 (1981) 969. [2] J. Erler, M. Maillard, G. Bru, J. Lehman and J.M. Dupouy, in: Proc. Conf. on Irradation Behavior of Metallic Materials for Fast Reactor Core Components, Ajaccio, France, 1979, p. 11. [3] D.S. Gelles, J. Nucl. Mater. 103 & 104 (1981) 975. [4] E.A. Little and D.A. Stow, J. Nucl. Mater. 87 (1979) 25. [5] M.M. Paxton, B.A. Chin and E.R. Gilbert, J. Nucl. Mater. 95 (1980) 185. [6] M.M. Paxton, B.A. Chin, E.R. Gilbert and R.E. Nygren, J. Nucl. Mater. 80 (1979) 144. [7] D.R. Harries, J. Standring, W.D. Barnes and G.L. Lloyd, in: Proc. llth Conf. on Effects of Radiation on Materials, ASTM STP 782, Eds. H.R. Brager and J.S. Perrin (ASTM, Philadelphia, 1982) p. 1197. [8] R.R. Hasiguti, J. Nucl. Mater. 103 & 104 (1981) 51. [9] F.A. Smidt, Jr., J.R. Hawthorne and V. Provenzano, in: Proc. 10th Conf. on Effects of Radiation on Materials,
512
K.J. Harrelson et al. / Impurity element effects on the toughness of 9 C r - I M o steel
ASTM sTP 725, Eds. D. Kramer, H.R. Brager and J.S. Perrin (ASTM, Philadelphia, 1981) p. 269. [10] V.K. Sikka, C.T. Ward, K.C. Thomas, in: Proc. Int. Conf. on Ferritic Steels for High Temperature Application (ASTM, Philadelphia, 1981). [11] C.J. McMahon, Jr., Mater. Sci. and Eng. 42 (1980) 215. [12] D. McLean, Grain Boundaries in Metals (Clarendon Press, Oxford, 1957).
[13] R.C. Wilcox and B.A. Chin, J. Nuci. Mater. 122 & 123 (1984) 349. [14] R.S. Fidler and D.J. Gooch, Proc. BNES Int. Conf. on Ferritic Steels for Fast Reactor Steam Generators, London, 1978; p. 128.
508
IMPURITY ELEMENT EFFECTS ON THE TOUGHNESS K.J. H A R R E L S O N ,
OF 9Cr-lMo
STEEL
S.H. R O U , a n d R.C. W I L C O X
Materials Engineering, Department of Mechanical Engineering, A uburn University, Auburn, A L 36849, USA
The effects of the trace impurity elements, phosphorus, sulfur, and silicon, on the impact toughness of a 9Cr-lMo steel were investigated. The results of Charpy impact tests showed that the additions of all three elements produced a decrease in the upper shelf energy and an increase in the ductile-brittle transition temperature (DBTT). Above 0.04% P and 0.4% Si, the upper shelf energy decreases sharply. Additions of over 0.03% P or 0.4% Si cause an increase in the DBTT. Trace amounts of sulfur were found to have a much greater detrimental effect than phosphorus. Impurity segregation and delta ferrite content are used to explain the decrease in toughness produced by the addition of P and Si, respectively. Sulfur promotes non-metallic inclusions and intergranular fracture thus producing embrittlement.
1. Introduction The resistance to irradiation swelling [1-4] and to creep [5-8] make ferritic steels candidate materials for applications in nuclear power plants. However, marginal fracture toughness at low temperatures and the transhion from ductile to brittle behavior near 0 ° C limit the applications of these alloys [9]. The 9 C r - l M o ferritic steel has competitive properties to 300 series stainless in many applications and has a lower level of alloy element content. The better high temperature strength of 9 C r - l M o , compared to other ferritic steels, makes this ~aterial attractive for superheater applications in heat exchanger systems. Prototype systems of 9 C r - l M o have been operating well in tests at Tennessee Valley Authority power plants [10]. In the production of steels where a high degree of mechanical property reproducibility is required, the knowledge of the effects of small changes in impurity concentration is important. Understanding the mechanism of how the impurity elements induce variations in the fracture toughness can supply needed information on how tightly impurity specifications must be set. This work investigates the effects of the impurity elements, sulfur, phosphorus, and silicon on the impact behavior of the 9 C r - l M o ferritic steel. The study investigated which impurity element has the greatest effect on the impact properties and to what degree the impact properties might be affected as a function of impurity concentration.
2. Experimental procedure A bar of 9 C r - l M o steel manufactured by Carpenter Technology Corportion was used as the basis of this investigation. An independent metallurgical consulting service determined the composition of each ingot by the use of a vacuum spectrometer. The composition of original bar in weight percent was 9.44 Cr, 1.03 Mo, 0.11 C, 0.017 P, 0.0007 S, 0.28 Si, 0.05 W, 0.21 V, 0.15 Ni, 0.45 Mn, < 0.08 Cu with the balance iron. The
possible existance of other elements were not reported by the spectrometric analysis. Phosphorus and sulfur were varied by adding ferrous phosphide (Fe2P) and ferrous sulfide (FeS), respectively, to the base steel. Additions of silicon were made as the pure metal. Raw materials were vacuum melted producing ingots of 300 grams. The variation in phosphorus-content was 0.017, 0.020, 0.030, 0.040, and 0.050 wt%. The sulfur-content varied as 0.0007, 0.007, 0.011, and 0.034 wt% while silicon varied as 0.28, 0.41, 0.73, and 0.78 wt%. The smallest amount in each case corresponds to the control bar composition. No variation was found from heat to heat for the elements carbon, tungsten, vanadium, nickel or copper. The content of the elements phosphorus, sulfur and silicon also remained constant except when purposely varied. The only differences between heats were found with chromium, m o l y b d e n u m and manganese. Chromium varied from 8.5-8.56 wt%, 8.65-8.67 wt% and 8.2-8.61 wt% in the heats for phosphorus, sulfur and silicon, respectively. These values are down from 9.44 wt% in the original ingot. Molybdenum was found to be 0.95-0.97 wt% in all heats down from 1.03 wt% in the control ingot. The manganese content was 0.49-0.52 wt% in the sulfur and silicon heats. In the phosphorus ingots, the 0.02 wt% phosphorus heat only contained 0.38 wt% manganese while the others were found to contain 0.50-0.53 wt% manganese. Each ingot was preheated to 760°C and hot rolled to a thickness of 12 ram. Twelve to twenty standard size V-notched Charpy specimens were machined from each ingot. Ch.arpy specimens were machined so that the notch was perpendicular to and the long dimension of the specimen was parallel to the rolling direction of the ingot. Specimens were then placed with Ta and Zr foils in sealed fused quartz capsules at 4 psia of At. The heat treatment for each specimen consisted of 1040°C/1 h / A C + 760 ° C / 1 h / A C . Impact tests were conducted at temperatures between - 1 9 6 ° C to 250°C to investigate the ductile-brittle transition behavior. Specimens were allowed to stabilize for 15 min at the test temperature (_+2°C) prior to testing. All the test procedures were performed according to ASTM E-23 standards.
0 0 2 2 - 3 1 1 5 / 8 6 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g Division)
K.J. Harrelson et al. / Impurity element effects on the toughness of 9 C r - l Mo steel
3. Results and discussion
400
Impact values plotted as a function of temperatu're yielded sigmoid curves. Such curves depict the upper shelf region, the lower shelf region, and the transition region. The average upper shelf energies were calculated by averaging data points of the upper shelf. The 50-J ductile-brittle transition temperatures (DBTT) were determined by interpolating the corresponding temperature from the sigmoid curves at this impact energy.
Fig. I shows the impact curves obtained for 9 C r - l M o steels with additions of 0.017, 0.02, 0.03, 0.04, and 0.05 wt% phosphorus. The impact curves show a general tendency to shift down and to the right as phosphorus concentrations are increased. Below 0.04 wt% phosphorus (with the exception of 0.02%), there is little change in the average upper shelf energy (fig. 2). For 0.02 wt% phosphorus, the upper shelf energy decreased from 358 to 305 J. This difference in behavior might be explained on the basis of compositional differences between ingots. However, the only obvious reported difference in composition is the manganese content which is 0.38 wt% in the 0.02% phosphorus ingot compared to 0.50-0.53 wt% manganese in the other ingots. As the phosphorus concentration goes above 0.04 wt%, the average upper self energy drops sharply. The phosphorus additions produce no change in value of the average lower knee energies (1-5 J), but do cause the lower knee to extend to higher temperatures. Phosphorus addition produced a rapid increase in the DBTT
400 350
0.017%P~ffff~ "4"e-°
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250 200
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3.1. Phosphorus effects
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TEMPERATURE (*C)
Fig. 1. Effect of phosphorus on impact energy.
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.3
.4
.5
.6
.7
.8
.9
COMPOSITIO(w/o) N
Fig. 2. Effect of impurity content on the upper shelf energies.
(fig. 3). For additions above the currently specified 0.02 wt% limit, the DBTT increased rapidly. To maintain the DBTT below 0°C, the phosphorus content must be kept below 0.03 wt% when utilizing the heat treatment used in this investiation. The basic microstructure of 9 C r - l M o steel, delta ferrite plus tempered martensite is shown in fig. 4a. The effect of 0.05 wt% phosphorus on this microstructure is illustrated in fig. 4b. Small individual ferrite islands are found in the tempered martensitic matrix. Precipitated secondary carbides were still evident at delta ferrite grain boundaries with primary carbides found within the matrix. N o obvious differences in the microstructures could be found with the use of optical microscopy. Fig. 3 shows some tendency of the DBTT to increase at a slower rate for the high phosphorus concentrations as has been reported [11]. This tendency possibly can be explained by the equilibrium segregation model [12]. Phosphorus tends to segregate to regions of high lattice misfit. Such regions can be secondary phase interfaces and grain boundaries. However, these imperfections only provide a fixed number of segregation sites. As more and more segration sites are occupied, the movement of phosphorus to these sites is progressively reduced. As the number of phosphorus atoms exceeds the available segregation sites, uniform distribution of the excess phosphorus is likely to occur. The rate at which phosphorus increases the brittleness of the steel should then decrease somewhat at high phosphorus contents, Since the segregation of phosphorus is progressively reduced with additional amounts of phosphorus concentrations, a gradual change in the rate of increase in the DBTT also should be expected.
510
KJ. Harrelson et al. / Impurity element effects on the toughness of 9Cr- IMo steel
8O CO Ld J
400
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i
'Fig. 3. Effect of impurity content on the DBTT.
Impact curves for the different sulfur contents are shown in fig. 5. In general, as sulfur content was increased the uper shelf energy decreased drastically (fig. 2). The 0.011 wt% sulfur curve in fig. 5 does not seem to
m~
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I .
.
i~
.
-
•
42
5
0
:25 -150 -75 0 75 155 225 TEMPERATURE (*C)
fit this trend but the data points indicate somewhat of a tendency for the curve to bend toward the horizontal in the 25 to 50°C range. Thus, the upper shelf energy for the 0.011 wt% sulfur alloy (taken at 50°C) likely is below that shown for 100°C. The lower knee region is little effected by sulfur. Also, as sulfur was increased the DBTT increased (fig. 3). To maintain a DBTT (at 50 J) below 0°C, the sulfur level must be less than 0.01 wt%. Additions of sulfur to 9 C r - l M o steel were found to cause similar effects as phosphorus, an increase in the DBTT and a decrease in the upper shelf region. However, sulfur was more effective than phosphorus in increasing the DBTT. The microstructure of the 0.034% sulfur alloy is shown in fig. 4c. The amount of free ferrite was affected by the sulfur impurity content. Delta ferrite initially was found to decrease with sulfur additions and then increased as sulfur began to appear as nonmetallic inclusions. These microstructural changes are considered to be the likely reason for the adverse influence of sulfur on the impact properties of this material.
.
3.3. Silicon effects
k.
~
5o
Fig. 5. Effect of sulfur on impact energy.
3.2. Sulfur effects
.
.~
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L~6.007%S OOl 1%S
1
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Fig. 4. Microstructure of 9Cr-lMo; (a) control, (b) with 0.05% P, (c) with 0.034% S, and (d) with 0.78% Si.
The impact energies of the four 9 C r - l M o steels with 0.28, 0.41, 0.73, and 0.78 wt% silicon are illustrated in fig. 6. The element silicon has a detrimental effect on the impact properties of 9 C r - l M o steels. Generally. the addition of silicon shifted the curves to higher temperatures and depressed the upper shelves. The changes in the upper shelf energies are plotted in fig. 2. A silicon content of less than 0.41 wt% did not have any effect on the upper shelf energy of 358 J. Most Charpy specimens
K.J. Harrelson et al. / Impurity element effects on the toughness of 9 C r - l Mo steel
400 Ld
0.28%Si 300 z
250
o 41%si ~ / .
. . . . . . .
200
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the equilibrium austenite region. The increase in the DBTT is felt to be directly attributed to this increase in delta ferrite. As delta ferrite increases with increasing silicon, more grain boundary regions are generated and thereby provide extra places for carbides to precipitate. The total quantity of all precipitated carbides remain unchanged in the silicon-doped alloys: Thus, fewer carbides can precipitate in the matrix. Usually, the secondary carbides forming along the delta ferrite grain boundaries are larger than those in the tempered martensite matrix. These larger secondary carbides provide favorable places for critical microcracks to nucleate and hence the DBTT is degraded.
>
4. Conclusions
50
5
-225 -150 -75 0 75 155 TEMPERATURE(°C)
225
Fig. 6. Effect of silicon on'impact energy.
from the upper shelf regions did not completely break on impact. Further additions of silicon caused a sharp decrease in the upper shelf energy. The upper shelf energies decreased to 190 J as the concentration of silicon increased to 0.78 wt%. The energies associated with the lower knee region (1-5 J) were not influenced by silicon additions. However, the lower knee region was extended to higher temperatures with increasing silicon content. Also, silicon produced an increase in the DBTT with the greatest effect being caused by additions over 0.5 wt%. To maintain the DBTT below 0°C, the silicon content must be kept below 0.5 wt% (fig. 3). If the silicon level is reduced to a maximum of 0.4%, then no measurable effect of silicon on the DBTT would be observed. The typical microstructure of the 9 C r - l M o steel with silicon additions (fig. 4d) consists of tempered martensite and elongated delta ferrite. However, as silicon was increased, the amount of delta ferrite was found to increase. This increase in delta ferrite is felt to be the cause of the decrease in impact properties with increasing silicon because delta ferrite was found to be detrimental to impact properties in a pervious investagtion [13]. Silicon is a ferrite stabilizer [14] and serves to balance the volume fraction of ferrite and austenite/ martensite in steels. The addition of silicon causes the austenite region to contract which in effect changes the fraction of ferrite and austenite/martensite during the cooling of the casting, during annealing for fabrication, and during the final heat treatment. High silicon contents produce a more stable delta ferrite, thereby, reducing the tendency of ferrite to transform to austenite in
The following conclusions resulted from the investigation of trace impurity elements on the impact properties of 9 C r - l M o steels: (1) Phosphorus additions increase the DBTT and decrease the upper shelf energy. (2) Sulfur additions result in drastic embrittlement of 9 C r - l M o steels. The effect of sulfur is more pronounced than that of phosphorus. Sulfur promotes the formation of non-metallic inclusions which reduced toughness. (3) Silicon contents greater than 0.4 wt% cause embrittlement. The DBTT is raised and the upper shelf energies are decreased. The increase in delta ferrite with silicon additions is postulated to explain the change in the DBTT. (4) The existing maximum levels of phosphorus, sulfur, and silicon in 9 C r - l M o steels can not be raised without adverse effects on the DBTT of the alloy. References [1] R.W. Powell, D.F. Peterson, M.K. Zimmershield and J.F. Bates, J. Nucl. Mater. 103 & 104 (1981) 969. [2] J. Erler, M. Maillard, G. Bru, J. Lehman and J.M. Dupouy, in: Proc. Conf. on Irradation Behavior of Metallic Materials for Fast Reactor Core Components, Ajaccio, France, 1979, p. 11. [3] D.S. Gelles, J. Nucl. Mater. 103 & 104 (1981) 975. [4] E.A. Little and D.A. Stow, J. Nucl. Mater. 87 (1979) 25. [5] M.M. Paxton, B.A. Chin and E.R. Gilbert, J. Nucl. Mater. 95 (1980) 185. [6] M.M. Paxton, B.A. Chin, E.R. Gilbert and R.E. Nygren, J. Nucl. Mater. 80 (1979) 144. [7] D.R. Harries, J. Standring, W.D. Barnes and G.L. Lloyd, in: Proc. llth Conf. on Effects of Radiation on Materials, ASTM STP 782, Eds. H.R. Brager and J.S. Perrin (ASTM, Philadelphia, 1982) p. 1197. [8] R.R. Hasiguti, J. Nucl. Mater. 103 & 104 (1981) 51. [9] F.A. Smidt, Jr., J.R. Hawthorne and V. Provenzano, in: Proc. 10th Conf. on Effects of Radiation on Materials,
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ASTM sTP 725, Eds. D. Kramer, H.R. Brager and J.S. Perrin (ASTM, Philadelphia, 1981) p. 269. [10] V.K. Sikka, C.T. Ward, K.C. Thomas, in: Proc. Int. Conf. on Ferritic Steels for High Temperature Application (ASTM, Philadelphia, 1981). [11] C.J. McMahon, Jr., Mater. Sci. and Eng. 42 (1980) 215. [12] D. McLean, Grain Boundaries in Metals (Clarendon Press, Oxford, 1957).
[13] R.C. Wilcox and B.A. Chin, J. Nuci. Mater. 122 & 123 (1984) 349. [14] R.S. Fidler and D.J. Gooch, Proc. BNES Int. Conf. on Ferritic Steels for Fast Reactor Steam Generators, London, 1978; p. 128.