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Small punch test evaluation of intergranular embrittlement of an alloy steel
Scripta METALLURGICA
Vol. 17, pp. 1443-1447, 1983 Printed in the U.S.A.
Pergamon Press Ltd.
SMALL PUNCHTEST EVALUATION OF INTERGRANULAR EMBRITTLEMENT OF AN ALLOY STEEL Jai-Man Baik, J. Kameda, and O. Buck Ames Laboratory Iowa State University Ames, Iowa 50011
(Received September 12, 1983) Introduction The d u c t i l e - b r i t t l e transition temperature in steel is commonly determined using Charpy V-notch impact specimens as specified by ASTM E23-81. In some specific cases, however, the use of this standardized test specimen may be impractical, i f not impossible. For instance, i t is well known that f e r r i t i c steels show a substantial degradation of the mechanical properties after long time exposure to an irradiation environment. Because of the increase in strength and the reduction in d u c t i l i t y due to neutron irradiation, the Charpy V-notch transition temperature is raised causing concern from a safety point of view. To study these radiation effects, a test specimen much smaller than the standard Charpy V-notch specimen would be extremely desirable for two reasons. First, to study neutron damage small specimens take less space within a reactor. Secondly, the damage achieved in simulation experiments, such as proton or electron accelerators, is limited to small penetration depths. Several efforts on the development of such a small test specimen, similar to that used to determine the d u c t i l i t y of sheet metal, as recommended by ASTM E643-78, have been described in the literature [1, 2]. The objective of the present paper is to report on the observation of correlations between small punch (SP) and Charpy V-notch (CVN) test results obtained on temper-embrittled NiCr steel. Determined was the d u c t i l e - b r i t t l e transition temperature (DBTT) with intergranular embrittlement being induced by grain boundary segregation of specific impurities. The relation between SP and CVN test results w i l l be discussed in terms of the micromechanisms of intergranular cracking. Based upon these findings, i t is tempting to suggest that in radiation embrittlement investigations, e.g., similar correlations may be obtained. Experimental Vacuum-melted and argon-cast laboratory heats of Ni-Cr steel which were doped individually with phosphorous or t i n and heat-treated to produce a variety of microstructures and intergranular embrittlement conditions were obtained from the University of Pennsylvania as broken CVN bars. The nominal base chemical composition of the tested material is shown in Table i . The material had three different grain sizes, i . e . , coarse (C), medium (M), and fine (F) and two different strength levels of HRC 20 and HRC 30. Sma11, thin plate specimens of 10x10xO.5 mm were TABLE 1.
Nominal chemical composition (wt pct) of steels Ni
Cr
Sn-doped
3.5
1.7
0.29
0.004 0.004 0.061
Res.
P-doped
3.5
1.7
0.03
0.060 ......
Res.
C
P
S
Sn
Fe
sliced, as shown in Fig. 1, from the undamaged portion of broken CVN bars. The specimens were mechanically polished up to 16 um (600 g r i t ) ; the deviation of thickness was less than 1%. The punch deformation was performed perpendicularly to the plate with a 2.4 mm (3/32") steel bail of more than HRC 60 in a specially designed specimen holder, shown in Fig. 2. All the tests have been done on an !nstron tensile testing machine in a controlled bath (liquid nitrogen cooled isopentane) at temperatures as low as -160°C. The cross-head moving speed of the Instron was
0036-9748/83
1443 $3..00 + .@0
1444
TEST OF INTERGRANULAR EHBRITTLEMENT
V o l . 17, No. I2
LoT i'::::::................
I
tO mm
PUNCHER
I -
I
CLAMPING~ SCREWS-4~ VI6" S T E E L ~
tO m m
SPECIMEN-FIG. I .
FIG. 2.
Extraction of small punch specimens from broken Charpy V-notched bars.
Loading and specimen supporting configuration of small punch tests.
fixed at 0.02 mm/sec throughout the tests. From the load-deflection curves obtained at various temperatures, the fracture energy was calculated. Results and Discussion Four typical load vs. deflection curves of the small punch specimens tested at different temperatures are shown in Fig. 3. At the lower test temperatures, the macroscopically detectable cracking occurred at the maximum load and at the higher temperatures after the maximum load was achieved. We defined the energy necessary for detectable cracking by the area under the loaddeflection curves. Fig. 4 shows an example of the fracture energy vs. test temperature and the corresponding cracking patterns at the various test temperatures. As can be seen, SP test results exhibit a clear d u c t i l e - b r i t t l e fracture transition behavior. Below the transition temperature range, b r i t t l e intergranular cracking i n i t i a t e d at the center part of the small punch specimens propagating r a d i a l l y along the different directions. As the testing temperature increases, the cracking mode changes. At f i r s t , the crescent shape of fibrous crack i n i t i a t e d along the circular edge of the bulged specimen eventually changing to intergranular cracking. The ductile fibrous crack tended to i n i t i a t e and b r i t t l e crack propagated along the region where a large amount of plastic deformation was accumulated during the punching.
-52°C
='2 r~
-91oc
-195"C .Smm DEFLECTION
FIG. 3.
Load vs. deflection curves of small punch specimens at various testing temperatures.
Vol.
17, No.
12
TEST OF I N T E R G R A N U L A R
I
EMBRITTLEMENT
~
1445
I
I
3
---2
>.n.w z w
aO3 I
\ 0
I
-
FIG. 4.
\
200
I
-I00 0 TEMPERATURE (°C)
I
I00
Example of fracture energy vs. test temperature and corresponding fracture patterns.
Fig. 5 shows a comparison of fracture energy vs. temperature between curves from the SP tests and the CVN tests.* (The l a t t e r results have been obtained from Takayama et al. [ 3 ] . ) The fracture energy transitions in the SP tests are found to occur at much lower temperature ranges than those in the CVN tests. The different degrees of intergranular embrittlement of steels can not be demonstrated by the SP tests as dramatically as by the CVN testS. However, since the fracture energy changes very precipitiously in the SP tests (compared with the more gradual change of the CVN fracture energy over a wider temperature range), the SP tests provide a clearer determination of the DBTT. I t should also be noted that in the CVN tests materia|s having lower transition temperatures show higher upper shelf energy ]eve]s; SP tests, on the other hand, show the opposite results. This observation can be explained in the following terms. The notch toughness of ductile materials (in the upper shelf region of the CVN tests) is related to the resistance to ductile crack growth ahead of the notch which generally decreases as the yield strength increases. In contrast, the SP tests (without notch) characterize the ductile i n s t a b i l i t y resistance. The work necessary for ductile i n s t a b i l i t y becomes larger as the yield strength increases since the ]oad bearing capacity increases with increasing yield strength. *In Fig. 5, in order to demonstrate a striking contrast between the SP test and CVN test, the results of two different steels are shown; Sn-doped steel, having HRC 30, medium grain size and having been aged at 480°C for 200 hrs; and P-doped steel, having HRC 20, medium grain size and having been aged at 480°C for 100 hrs.
1446
TEST OF I N T E R G R A N U L A R E H B R I T T L E M E N T
Vol.
17, No.
12
o
300 ~&M20-'P o e M 3 0 - Sn
/•CVN
SP TEST
TEST
1"
-~3 >-
/?/";/
200~
2oo~ Z
L9 OE
I00
I00 z >
Q_ c/)
0
I
-200 -I00
FIG. 5.
0 I00 200 300 TEMPERATURE (°C)
-2 o
400
,Go
o
Tsp(°G)
Comparison of fracture energy transition behavior between CVN and SP tests in steels having two different amounts of embrittlement.
FIG. 6.
Correlation o~ DBTT between CVN and SP tests in P- and Sn-doped steels.
In order to establish a correlation between the DBTT of the present SP tests and those of the CVN tests, the results obtained from the steels with different microstructures ( i . e . , grain size and hardness) and types and quantities of segregated impurities have been plotted in Fig. 6. I t is found that there exists a unique linear relationship of the transition temperature between the SP and CVN tests for each of the Sn- and P-doped steels. This indicates that the DBTT correlation between two types of tests does not depend on the metallurgical variables (grain size and hardness) but strongly depends on the type of segregated impurities. The empirical correlation between the transition temperature on the CVN tests, TCVN, and that on the SP tests, TSp, is given by
TCVN = ~Tsp + B
(1)
where : is a mechanical correlation factor (in this case, :~2.5) and B is the offset transition temperature (400°C for the P-doped steels and 485°C for the Sn-doped steels). The mechanical correlation factor is d i r e c t l y related to the combined effects of strain rate and stress state. The dynamic loading and t r i a x i a l stress state in the CVN tests f a c i l i t a t e b r i t t l e behavior over a wider temperature range than the static loading and biaxial stress state in the SP tests. The impurity dependence of the transition temperature correlation between the two types of tests can be rationalized in terms of the variation of the fracture strength along grain boundaries. I t has been found [4, 5] that in the same alloy steel the segregated impurity concentration is not the same from one boundary to another, and that in the notched-bar experiments the macroscopically detectable fracture stage is controlled by the intergranular crack i n i t i a t i o n at the most embrittled grain boundary in the highly stressed region ahead of the notch t i p . In this case, the r e l a t i v e l y shallow stress distribution ahead of the notch tip allows the i n i t i a t e d crack to propagate through the whole specimen. Thus, the measurements of the d u c t i l e - b r i t t l e transition in the CVN tests is related to the strength of the grain boundary containing the highest impurity concentration. On the other hand, the detectable cracking process of the SP tests results from the extension of a number of grain boundary cracks since the steep local stress distribution under the loading ball extrusion prevents the f i r s t initiated b r i t t l e crack from immediately propagating to a detectable size. This is analogous to the KIC measurement using precracked specimens having steep local stress distribution [6]. The detectable cracking stage represents the average grain boundary strength rather than the grain boundary strength of the highly impurity-concentrated grain boundary. This indicates that the
Vol.
17, No.
12
TEST OF I N T E R G R A N U L A R
Et.IBRITTLEMENT
1447
correspondence between TCVN and TSp with respect to the same aging condition is related to the different mechanisms of detectable b r i t t l e cracking in these tests since the distributed impurity segregation represents the DBTT in a different manner for two types of tests. I t has also been found [5] that the dependence of the intergranular fracture strength on the impurity concentration at grain boundaries was not the same for the different impurity element-doped steels. The Sn-doped steel showed the more drastic decrease in the fracture strength than the P-doped steel when the impurity concentration at grain boundaries increased. The difference in the intergranular embrittlement mechanism explains why the correlation of the measured values of DBTT between in the i n i t i a t i o n - c o n t r o l CVN and in the propagation-control SP tests depends on the type of impurities. Conclusion The SP tests demonstrate precipitous d u c t i l e - b r i t t l e transition behavior of the steel i n d i v i d u a l l y doped with Sn and P. However, the transition temperature on the SP tests is found to be at a lower temperature than that on the CVN tests. A unique linear correlation between TCVN and TSp has been found in the Sn and P-doped steels, respectively. The dependence of the transition temperature correlation between the SP and CVN tests on different impurity elements has been discussed in terms of the mechanism for intergranular embrittlement. Acknowledgement The authors would like to express their appreciation to Professor C. J. McMahon, Jr. for supplying of materials. They extend their gratitude to Frank V. Nolfi, Jr. for stimulating discussions and encouragement. This work was performed for the U.S. Department of Energy by Iowa State University under contract No. W-7405-Eng-82. References 1. 2. 3. 4. 5. 6.
M. F. S. J. J. J.
P. Manahan, A. S. Argon and O. K. Harling, J. Nucl. Mater., 1981, 103 & 104, p. 1545. H. Huang, M. L. Hamilton and G. L. Wire, Nucl. Tech., 1982, Vol. 57, p. 234. Takayama, T. Ogura, S. Fu and C. J. McMahon, Jr., Met. Trans. A, 1980, Vol 11A, p. 1513. Kameda and C. J. McMahon, J r . , Met. Trans. A, 1980, vol. 11A, p. 91. Kameda and C. J. McMahon, J r . , Met. Trans. A, 1981, Vol. 12A, p. 31. Kameda, Met. Trans. A, 1981, Vol. 12A, p. 2039.
Vol. 17, pp. 1443-1447, 1983 Printed in the U.S.A.
Pergamon Press Ltd.
SMALL PUNCHTEST EVALUATION OF INTERGRANULAR EMBRITTLEMENT OF AN ALLOY STEEL Jai-Man Baik, J. Kameda, and O. Buck Ames Laboratory Iowa State University Ames, Iowa 50011
(Received September 12, 1983) Introduction The d u c t i l e - b r i t t l e transition temperature in steel is commonly determined using Charpy V-notch impact specimens as specified by ASTM E23-81. In some specific cases, however, the use of this standardized test specimen may be impractical, i f not impossible. For instance, i t is well known that f e r r i t i c steels show a substantial degradation of the mechanical properties after long time exposure to an irradiation environment. Because of the increase in strength and the reduction in d u c t i l i t y due to neutron irradiation, the Charpy V-notch transition temperature is raised causing concern from a safety point of view. To study these radiation effects, a test specimen much smaller than the standard Charpy V-notch specimen would be extremely desirable for two reasons. First, to study neutron damage small specimens take less space within a reactor. Secondly, the damage achieved in simulation experiments, such as proton or electron accelerators, is limited to small penetration depths. Several efforts on the development of such a small test specimen, similar to that used to determine the d u c t i l i t y of sheet metal, as recommended by ASTM E643-78, have been described in the literature [1, 2]. The objective of the present paper is to report on the observation of correlations between small punch (SP) and Charpy V-notch (CVN) test results obtained on temper-embrittled NiCr steel. Determined was the d u c t i l e - b r i t t l e transition temperature (DBTT) with intergranular embrittlement being induced by grain boundary segregation of specific impurities. The relation between SP and CVN test results w i l l be discussed in terms of the micromechanisms of intergranular cracking. Based upon these findings, i t is tempting to suggest that in radiation embrittlement investigations, e.g., similar correlations may be obtained. Experimental Vacuum-melted and argon-cast laboratory heats of Ni-Cr steel which were doped individually with phosphorous or t i n and heat-treated to produce a variety of microstructures and intergranular embrittlement conditions were obtained from the University of Pennsylvania as broken CVN bars. The nominal base chemical composition of the tested material is shown in Table i . The material had three different grain sizes, i . e . , coarse (C), medium (M), and fine (F) and two different strength levels of HRC 20 and HRC 30. Sma11, thin plate specimens of 10x10xO.5 mm were TABLE 1.
Nominal chemical composition (wt pct) of steels Ni
Cr
Sn-doped
3.5
1.7
0.29
0.004 0.004 0.061
Res.
P-doped
3.5
1.7
0.03
0.060 ......
Res.
C
P
S
Sn
Fe
sliced, as shown in Fig. 1, from the undamaged portion of broken CVN bars. The specimens were mechanically polished up to 16 um (600 g r i t ) ; the deviation of thickness was less than 1%. The punch deformation was performed perpendicularly to the plate with a 2.4 mm (3/32") steel bail of more than HRC 60 in a specially designed specimen holder, shown in Fig. 2. All the tests have been done on an !nstron tensile testing machine in a controlled bath (liquid nitrogen cooled isopentane) at temperatures as low as -160°C. The cross-head moving speed of the Instron was
0036-9748/83
1443 $3..00 + .@0
1444
TEST OF INTERGRANULAR EHBRITTLEMENT
V o l . 17, No. I2
LoT i'::::::................
I
tO mm
PUNCHER
I -
I
CLAMPING~ SCREWS-4~ VI6" S T E E L ~
tO m m
SPECIMEN-FIG. I .
FIG. 2.
Extraction of small punch specimens from broken Charpy V-notched bars.
Loading and specimen supporting configuration of small punch tests.
fixed at 0.02 mm/sec throughout the tests. From the load-deflection curves obtained at various temperatures, the fracture energy was calculated. Results and Discussion Four typical load vs. deflection curves of the small punch specimens tested at different temperatures are shown in Fig. 3. At the lower test temperatures, the macroscopically detectable cracking occurred at the maximum load and at the higher temperatures after the maximum load was achieved. We defined the energy necessary for detectable cracking by the area under the loaddeflection curves. Fig. 4 shows an example of the fracture energy vs. test temperature and the corresponding cracking patterns at the various test temperatures. As can be seen, SP test results exhibit a clear d u c t i l e - b r i t t l e fracture transition behavior. Below the transition temperature range, b r i t t l e intergranular cracking i n i t i a t e d at the center part of the small punch specimens propagating r a d i a l l y along the different directions. As the testing temperature increases, the cracking mode changes. At f i r s t , the crescent shape of fibrous crack i n i t i a t e d along the circular edge of the bulged specimen eventually changing to intergranular cracking. The ductile fibrous crack tended to i n i t i a t e and b r i t t l e crack propagated along the region where a large amount of plastic deformation was accumulated during the punching.
-52°C
='2 r~
-91oc
-195"C .Smm DEFLECTION
FIG. 3.
Load vs. deflection curves of small punch specimens at various testing temperatures.
Vol.
17, No.
12
TEST OF I N T E R G R A N U L A R
I
EMBRITTLEMENT
~
1445
I
I
3
---2
>.n.w z w
aO3 I
\ 0
I
-
FIG. 4.
\
200
I
-I00 0 TEMPERATURE (°C)
I
I00
Example of fracture energy vs. test temperature and corresponding fracture patterns.
Fig. 5 shows a comparison of fracture energy vs. temperature between curves from the SP tests and the CVN tests.* (The l a t t e r results have been obtained from Takayama et al. [ 3 ] . ) The fracture energy transitions in the SP tests are found to occur at much lower temperature ranges than those in the CVN tests. The different degrees of intergranular embrittlement of steels can not be demonstrated by the SP tests as dramatically as by the CVN testS. However, since the fracture energy changes very precipitiously in the SP tests (compared with the more gradual change of the CVN fracture energy over a wider temperature range), the SP tests provide a clearer determination of the DBTT. I t should also be noted that in the CVN tests materia|s having lower transition temperatures show higher upper shelf energy ]eve]s; SP tests, on the other hand, show the opposite results. This observation can be explained in the following terms. The notch toughness of ductile materials (in the upper shelf region of the CVN tests) is related to the resistance to ductile crack growth ahead of the notch which generally decreases as the yield strength increases. In contrast, the SP tests (without notch) characterize the ductile i n s t a b i l i t y resistance. The work necessary for ductile i n s t a b i l i t y becomes larger as the yield strength increases since the ]oad bearing capacity increases with increasing yield strength. *In Fig. 5, in order to demonstrate a striking contrast between the SP test and CVN test, the results of two different steels are shown; Sn-doped steel, having HRC 30, medium grain size and having been aged at 480°C for 200 hrs; and P-doped steel, having HRC 20, medium grain size and having been aged at 480°C for 100 hrs.
1446
TEST OF I N T E R G R A N U L A R E H B R I T T L E M E N T
Vol.
17, No.
12
o
300 ~&M20-'P o e M 3 0 - Sn
/•CVN
SP TEST
TEST
1"
-~3 >-
/?/";/
200~
2oo~ Z
L9 OE
I00
I00 z >
Q_ c/)
0
I
-200 -I00
FIG. 5.
0 I00 200 300 TEMPERATURE (°C)
-2 o
400
,Go
o
Tsp(°G)
Comparison of fracture energy transition behavior between CVN and SP tests in steels having two different amounts of embrittlement.
FIG. 6.
Correlation o~ DBTT between CVN and SP tests in P- and Sn-doped steels.
In order to establish a correlation between the DBTT of the present SP tests and those of the CVN tests, the results obtained from the steels with different microstructures ( i . e . , grain size and hardness) and types and quantities of segregated impurities have been plotted in Fig. 6. I t is found that there exists a unique linear relationship of the transition temperature between the SP and CVN tests for each of the Sn- and P-doped steels. This indicates that the DBTT correlation between two types of tests does not depend on the metallurgical variables (grain size and hardness) but strongly depends on the type of segregated impurities. The empirical correlation between the transition temperature on the CVN tests, TCVN, and that on the SP tests, TSp, is given by
TCVN = ~Tsp + B
(1)
where : is a mechanical correlation factor (in this case, :~2.5) and B is the offset transition temperature (400°C for the P-doped steels and 485°C for the Sn-doped steels). The mechanical correlation factor is d i r e c t l y related to the combined effects of strain rate and stress state. The dynamic loading and t r i a x i a l stress state in the CVN tests f a c i l i t a t e b r i t t l e behavior over a wider temperature range than the static loading and biaxial stress state in the SP tests. The impurity dependence of the transition temperature correlation between the two types of tests can be rationalized in terms of the variation of the fracture strength along grain boundaries. I t has been found [4, 5] that in the same alloy steel the segregated impurity concentration is not the same from one boundary to another, and that in the notched-bar experiments the macroscopically detectable fracture stage is controlled by the intergranular crack i n i t i a t i o n at the most embrittled grain boundary in the highly stressed region ahead of the notch t i p . In this case, the r e l a t i v e l y shallow stress distribution ahead of the notch tip allows the i n i t i a t e d crack to propagate through the whole specimen. Thus, the measurements of the d u c t i l e - b r i t t l e transition in the CVN tests is related to the strength of the grain boundary containing the highest impurity concentration. On the other hand, the detectable cracking process of the SP tests results from the extension of a number of grain boundary cracks since the steep local stress distribution under the loading ball extrusion prevents the f i r s t initiated b r i t t l e crack from immediately propagating to a detectable size. This is analogous to the KIC measurement using precracked specimens having steep local stress distribution [6]. The detectable cracking stage represents the average grain boundary strength rather than the grain boundary strength of the highly impurity-concentrated grain boundary. This indicates that the
Vol.
17, No.
12
TEST OF I N T E R G R A N U L A R
Et.IBRITTLEMENT
1447
correspondence between TCVN and TSp with respect to the same aging condition is related to the different mechanisms of detectable b r i t t l e cracking in these tests since the distributed impurity segregation represents the DBTT in a different manner for two types of tests. I t has also been found [5] that the dependence of the intergranular fracture strength on the impurity concentration at grain boundaries was not the same for the different impurity element-doped steels. The Sn-doped steel showed the more drastic decrease in the fracture strength than the P-doped steel when the impurity concentration at grain boundaries increased. The difference in the intergranular embrittlement mechanism explains why the correlation of the measured values of DBTT between in the i n i t i a t i o n - c o n t r o l CVN and in the propagation-control SP tests depends on the type of impurities. Conclusion The SP tests demonstrate precipitous d u c t i l e - b r i t t l e transition behavior of the steel i n d i v i d u a l l y doped with Sn and P. However, the transition temperature on the SP tests is found to be at a lower temperature than that on the CVN tests. A unique linear correlation between TCVN and TSp has been found in the Sn and P-doped steels, respectively. The dependence of the transition temperature correlation between the SP and CVN tests on different impurity elements has been discussed in terms of the mechanism for intergranular embrittlement. Acknowledgement The authors would like to express their appreciation to Professor C. J. McMahon, Jr. for supplying of materials. They extend their gratitude to Frank V. Nolfi, Jr. for stimulating discussions and encouragement. This work was performed for the U.S. Department of Energy by Iowa State University under contract No. W-7405-Eng-82. References 1. 2. 3. 4. 5. 6.
M. F. S. J. J. J.
P. Manahan, A. S. Argon and O. K. Harling, J. Nucl. Mater., 1981, 103 & 104, p. 1545. H. Huang, M. L. Hamilton and G. L. Wire, Nucl. Tech., 1982, Vol. 57, p. 234. Takayama, T. Ogura, S. Fu and C. J. McMahon, Jr., Met. Trans. A, 1980, Vol 11A, p. 1513. Kameda and C. J. McMahon, J r . , Met. Trans. A, 1980, vol. 11A, p. 91. Kameda and C. J. McMahon, J r . , Met. Trans. A, 1981, Vol. 12A, p. 31. Kameda, Met. Trans. A, 1981, Vol. 12A, p. 2039.