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Internal strains after recovery of hardness in tempered martensitic steels for fusion reactors
Journal of Nuclear North-Holland
Materials
179-181
675
(1991) 675-678
Internal strains after recovery of hardness in tempered martensitic steels for fusion reactors L. Brunelli, P. Gondi, R. Montanari and R. Coppola * Mechanical Engineering Department, 2nd University of Rome, Tor Vergata Via 0. Raimondo, 00173 Rome, Ita@
After tempering, with recovery of hardness, MANET steels present internal strains; these residual strains increase with quenching rate prior to tempering, and they remain after prolonged tempering times. On account of their persistence, after thermal treatments which lead to low dislocation and sub-boundary densities, the possibility has been considered that the high swelling resistance of MANET is connected with these centres of strain, probably connected with the formation, in ferrite, of Cr-emiched and contiguous Cr-depleted zones which may act as sinks for interstitials. Comparative observations on the internal strain behaviour of cold worked 316L stainless steel appear consistent with this possibility.
1. Introduction According to theories of swelling [l-4] the behaviour of bubbles is determined by the number, ns, of helium gas atoms contained in each bubble. Below a critical number, nz, larger bubbles (below a superior limit) tend to shrink towards an inferior limit radius; for ns > n: bubbles grow to voids, with a rate depending on the net vacancy flux, i.e. impinging vacancies minus impinging interstitials and emitted vacancies. Under irradiation, helium is continuously generated in the matrix, and the atoms, ns, collected in the bubble depend on the partitioning among the various sink centres, i.e. bubbles, dislocations, sub-boundaries etc. Also, the concentrations and related fluxes of the other point defects depend on the various sink centres. For example, in 316 stainless steel , the preferential sink of interstitials is the Frank sessile loop, as well as dislocations. This contributes to the presence of excess vacancies which act as traps for the helium atoms. The present research deals with the examination of the defect structures, which may influence the sink processes in martensitic and ferritic steels. For comparison in austenitic steels, results of X-ray diffraction (XRD) analyses are also considered.
2. Experimental The observations were made on samples of MANET (DIN 1.4914) and on AISI 316L stainless steels. The composition of MANET is Fe-lO.SCr-0,17C0.50Mo-0.85Ni-0.20Nb-0.25V-0.32Si-0.04A1-0.003N -0.005P by wt%, and the AISI 316L steel had the composition Fe-17.44Cr-12.32Ni-0.024C-0.06N -1.82Mn-2.3Mo-0.46Si-O.l7Co-0.2OCu-O.OOlSO.O26P-O.OlTa by wt%. The MANET specimens were homogeneized for 30 ruin at 1348 K and then quenched with different cooling
* ENEA, TIB, TIR, Casaccia, ~22-3115/91/$03.50
Rome, Italy.
0 I991 - Elsevier Science Publishers
rates. Cooling rates will be expressed by F = (T,, &)/t, (T,, is the homogeneization temperature, M, and f, are the martensite start temperature and time to reach it). One group of the samples was work-hardened by cold rolling with reductions of 0.5% per each passage, down to final thickness. After the various treatments (quenching, rolling,) the samples underwent subsequent heating steps at 373, 473, 573, 673, 773, 873 and 973 K. In this paper reference is made to procedures of characte~~tion, after the various treatments, by means of XRD analyses and of microhardness measurements (1 kg load). 3. Results As mentioned in the introduction, the primary emphasis of this study is on the internal stresses remaining in MANET after tempering. The internal stresses are evaluated by analysis of the XRD line widths; for comparison, measurements were also made of the microhardnesses, which refer to thicknesses comparable to those involved in XRD. Fig. la shows the effects of subsequent heat treatments on samples quenched with a cooling rate ? = 150 K/min. Both hardnesses and XRD line widths decrease abruptly, beginning at = 800 K. However, the final line widths are larger than those expected of a well-annealed material, thus indicating that small subgrains and/or internal strains remain after the treatment at the higher temperature (973 K). Subgrain boundaries as well as centres of internal strain may be effective as sites of bubble nucleation; hence, various treatments have been explored for the purpose of controlling these structural characteristics. Larger effects were found in connection with the quenching rate from the homogeneization temperature. The phenomenon is illustrated in fig. lb. With the larger quenching rate (f = 1100 K/nun ), the behaviour of hardness remains practically unchanged, with similar softening beginning at -700 K. This may be taken as an
B.V. (North-Holland)
676
L. Brunellt et ui. / Internal strains in martensttic steels 0.15
0.10 ol -
,” 0 cl
0.05 0.15
g 5 c 3
0.10
E 2 9
0.05
250 1000
500
ANNEALING
TEMPERATURE
(K )
Fig. 1. Quenching rate effects on MANET steel behaviour. Microhardness and half height width of (110) XRD line for subsequent heat treatments of 60 min at the temperatures indicated in abscissas quenching rate f = 150 K/min in a) and ?= 1100 K/min in b)). Half height width is not corrected for
instrumental line broadening (0.025 o ).
that after the annealing the dislocation structures giving rise to hardening are independent of the quenching rate. The XRD line widths present instead much smaller decreases and large widths remain after the final heating at 973 K. Only small variations of the XRD line widths have been experienced by prolongation of the heating up to 600 min at 973 K. Size and strain contributions to line broadening may be resolved by means of Fourier analysis of the profiles according to the Warren-Averbach method. As considered in detail in another paper [5], the internal strains contribute approximately half of the XRD line widths, indicating that both small subgrains and relevant internal stresses remain after tempering in the MANET samples submitted to quenching. On account of the important role of dislocations, the evolution of the microstructure, introduced by cold work in tempered steels with different residual XRD line widths, has also been considered. Results are shown in fig. 2; they refer to two batchs of specimens distinguished by the different quenching rates, f= 150 K/mm and F= 3300 K/mm. After quenching, all of the specimens were tempered at 973 K, cold rolled with 50% reduction and then submitted to subsequent steps of heating at the temperatures indicated in the figure. Internal strains and subgrains, which cause larger line widths, present a stabilizing effect on the dislocaindication
tion structure, except for the step from 700 to 800 K. Following the faster quenching rate, the XRD line widths always remain larger. No relevant XRD line shift is observed after the various treatments. For comparison, observations were made on cold-worked 316L stainless steel. The annealing done after cold rolling to 90% of reduction was followed after subsequent steps of heating, as was done for MANET. The (111) XRD line widths for 316L (fig. 3) are smaller than those observed for MANET. More relevant decreases of the widths are presented by the (220) XRD lines, starting from = 700 K. The microhardnesses reach a maximum at 800 K. Considering the corresponding line width decreases, this maximum seems hardly attributable to increases of the dislocation densities but rather to the occurrence of obstacles to dislocation motion, such as sessile loops. With this alloy, shifts of the XRD lines are observed with a stage around = 600 K, which corresponds to a maximum of the angular gap between the (111) and (220) lines. As known from previous research [6], this gap is influenced by the presence of stacking faults. 4. Discussion The results are discussed with reference to the characteristics of resistance to swelling which are specific to the MANET steel. As recalled in the introduction, bubbles grow to the stage where they become voids, i.e. significant swelling occurs when the number of He
1 10.05
_
_ 1 10.00
_I m
,”
0
ci5 5
0 :?i
ANNEALING
TEMPERATURE
9.95 10.05 -p”
0 a-
10.00
(K )
Fig. 2. Annealing of work hardening structures (50% cold rolling) introduced in MANET steel tempered after quenching with f = 150 K/mm in (a) and F = 3300 K/mm in (b). Microhardness, half height width and peak position 8 of { 110) lines for subsequent heat treatments of 120 mm at the temperatures in the abscissas.
L. Brunellr et al. / Internal strains in marfensitic steels
- 0.15 g 2 :
E 380
I
I
I
I
I
I
I
10.10
980916.10 1000
500 ANNEALING
TEMPERATURE
(K )
Fig. 3. Annealing of work hardening structures (90% cold rolling) introduced in AISI 316 L steel. Microhardness, half height widths (a) and peak position 8 (b) of 1111) and (220) XRD lines for subsequent heat treatments of 120 min at the temperatures in abscissas.
per bubble attains the critical value, nz. Conversely, the restrained swelling of MANET results from limited numbers of He atoms per bubble, ns < ni. Not considering helium escape from the material, this may occur only In the presence of very high bubble numbers and/or of very large concentrations of other He sink centres. The peak swelling reductions in a-Fe resulting from Cr additions, especially below 5% [7], are thus consistent with Cr atoms acting as trapping centres for He and associated vacancies. On the other hand, the lower swelling resistance, observed in 316L stainless steel is considered to be Indicative of smaller concentrations of nucleation or He sink centres, coming about especially above 800 K. Our observations are consistent with recovery of the dislocations introduced by cold work at the higher temperatures examined both in MANET and In 316L steels. The sessile Frank loops are specific to 316 stainless steel; they act as sites of preferential sink for the interstitials [8]. These loops do not appear above = 800 K, consistent with the assumption that the larger swelling, above = 800 K, is due to reduced He sink densities associated with the absence of loops. While growing, these loops act as vacancy emitters, and as a consequence, their growth may be accompanied by bubble growth. In the absence of or by the disappearance of the loops, the vacancy-interstitial reatoms
611
combination will increase, and as a result, the number of free vacancies which can react with He is strongly reduced, consistent with the expected reduction of the He sink centres. An interpretation of this type has been considered by Ono et al. [9] to explain the behaviour of Al under irradiation at intermediate temperatures. The hardness maximum and the XRD line shift variations observed by us in 316 L steel at = 700 K may be related to the corresponding disappearance of loops introduced during cold-work. With MANET steel, preferential sinks for interstitials are the dislocations and the sub-boundaries associated with the lath structure of martensite. However, after tempering at = 1000 K, to which these materials are submitted before irradiation, the volume density of both dislocations and subboundaries is rather low [lo], and it seems unprobable that they play the dominant role with respect to the required high densities of He sinks. In a previous paper [5], the hypothesis was put forward that the XRD line broadening remaining after tempering is due to residual internal strains, depending on inhomogeneities in the Cr distribution associated with the formation of the (Y’ Cr rich ferrite phase. Formation of small globular particles of a’ has been observed in irradiated Cr-Fe alloys [7]; these particles have a finer and more uniform distribution than the precipitates observed [5] in our material, and they correspond to the phases observed, in the absence of irradiation, with higher Cr concentrations in ferrite [ll]. Contiguous to the Cr rich phase, zones with Cr depletion come about, which may well act as interstitial sinks with an action similar to that of the loops in 316L but persisting at higher temperatures. Evidence of vacancy emission from these zones with lower Cr concentration has been found by Little and Stow [7]. 5. Conclusions From the comparison with the behaviour of deformed 316L steel, lattice defects connected with the internal strains, remaining after prolonged tempering, appear specific to MANET steels. Experimental observations are consistent with the annealing out at the temperature of tempering, of other lattice defects such as dislocations and sub-boundaries. The hypothesis has been discussed that these residual strains are related to the occurrence of Cr enriched and contiguous Cr poor zones, which may act as sink centres, with high densities in the grain interior, consistent with the occurrence of the small stable bubbles with large densities within the grains. They may explain the high swelling resistance of MANET steel. Acknowledgements
The authors are grateful to Dr. K. Ehrlich of KfK, Karlsruhe for the supply of material, reports and infor-
678
L. Brunelli et nl. / Internal strains m mariens~t~~ steels
mation. assistance.
Thanks are due to Mr. P. Plini For technical The work has beep carried out on fulfilment
of a Research
Contract
with
ENEA.
References R.E. Staller and G.R. Odette, J. Nucl. Mater. 103 81 104 (1981) 1361. R.E. Staller and G.R. Odette, in: Effects of Radiation on Materiais - 11, ASTM-STP 782 (1982) 275. L.K. Mansur and W.A. Coghlan, J. Nuci. Mater 119 (1983) 1. G.R. Odette, J. Nucl. Mater. 155-157 (198X) 921.
[5] L. Brunelli, [6] [7] [X] [9] [lo]
[ll]
P. Gondi and R. Montanari. Metall. Sci. Technol. to be published. M.S. Paterson, J. Appl. Phys. 23 (1952) 805. E.A. Little and D.A. Stow, Metal Sci. (1980) X9. P.J. Maziasz and C.J. McHargue, Int. Mater. Rev. 32 (1987) 190. K. Ono, M. Inoue, T. Kino, S. Furuno and K. Izui, J. Nucl. Mater. 133 & 134 (1985) 477. C. Wassilew, K. Herschbach, E. Materna-Morris and K. Ehrlich, in: Proc. Topical Conf. on Ferritic Alloys for Use in Nuclear Energy Technologies, Snowbird, 1983, p. 607614. P.J. Grobner. Metall. Trans. A4 (1983) 251.
Materials
179-181
675
(1991) 675-678
Internal strains after recovery of hardness in tempered martensitic steels for fusion reactors L. Brunelli, P. Gondi, R. Montanari and R. Coppola * Mechanical Engineering Department, 2nd University of Rome, Tor Vergata Via 0. Raimondo, 00173 Rome, Ita@
After tempering, with recovery of hardness, MANET steels present internal strains; these residual strains increase with quenching rate prior to tempering, and they remain after prolonged tempering times. On account of their persistence, after thermal treatments which lead to low dislocation and sub-boundary densities, the possibility has been considered that the high swelling resistance of MANET is connected with these centres of strain, probably connected with the formation, in ferrite, of Cr-emiched and contiguous Cr-depleted zones which may act as sinks for interstitials. Comparative observations on the internal strain behaviour of cold worked 316L stainless steel appear consistent with this possibility.
1. Introduction According to theories of swelling [l-4] the behaviour of bubbles is determined by the number, ns, of helium gas atoms contained in each bubble. Below a critical number, nz, larger bubbles (below a superior limit) tend to shrink towards an inferior limit radius; for ns > n: bubbles grow to voids, with a rate depending on the net vacancy flux, i.e. impinging vacancies minus impinging interstitials and emitted vacancies. Under irradiation, helium is continuously generated in the matrix, and the atoms, ns, collected in the bubble depend on the partitioning among the various sink centres, i.e. bubbles, dislocations, sub-boundaries etc. Also, the concentrations and related fluxes of the other point defects depend on the various sink centres. For example, in 316 stainless steel , the preferential sink of interstitials is the Frank sessile loop, as well as dislocations. This contributes to the presence of excess vacancies which act as traps for the helium atoms. The present research deals with the examination of the defect structures, which may influence the sink processes in martensitic and ferritic steels. For comparison in austenitic steels, results of X-ray diffraction (XRD) analyses are also considered.
2. Experimental The observations were made on samples of MANET (DIN 1.4914) and on AISI 316L stainless steels. The composition of MANET is Fe-lO.SCr-0,17C0.50Mo-0.85Ni-0.20Nb-0.25V-0.32Si-0.04A1-0.003N -0.005P by wt%, and the AISI 316L steel had the composition Fe-17.44Cr-12.32Ni-0.024C-0.06N -1.82Mn-2.3Mo-0.46Si-O.l7Co-0.2OCu-O.OOlSO.O26P-O.OlTa by wt%. The MANET specimens were homogeneized for 30 ruin at 1348 K and then quenched with different cooling
* ENEA, TIB, TIR, Casaccia, ~22-3115/91/$03.50
Rome, Italy.
0 I991 - Elsevier Science Publishers
rates. Cooling rates will be expressed by F = (T,, &)/t, (T,, is the homogeneization temperature, M, and f, are the martensite start temperature and time to reach it). One group of the samples was work-hardened by cold rolling with reductions of 0.5% per each passage, down to final thickness. After the various treatments (quenching, rolling,) the samples underwent subsequent heating steps at 373, 473, 573, 673, 773, 873 and 973 K. In this paper reference is made to procedures of characte~~tion, after the various treatments, by means of XRD analyses and of microhardness measurements (1 kg load). 3. Results As mentioned in the introduction, the primary emphasis of this study is on the internal stresses remaining in MANET after tempering. The internal stresses are evaluated by analysis of the XRD line widths; for comparison, measurements were also made of the microhardnesses, which refer to thicknesses comparable to those involved in XRD. Fig. la shows the effects of subsequent heat treatments on samples quenched with a cooling rate ? = 150 K/min. Both hardnesses and XRD line widths decrease abruptly, beginning at = 800 K. However, the final line widths are larger than those expected of a well-annealed material, thus indicating that small subgrains and/or internal strains remain after the treatment at the higher temperature (973 K). Subgrain boundaries as well as centres of internal strain may be effective as sites of bubble nucleation; hence, various treatments have been explored for the purpose of controlling these structural characteristics. Larger effects were found in connection with the quenching rate from the homogeneization temperature. The phenomenon is illustrated in fig. lb. With the larger quenching rate (f = 1100 K/nun ), the behaviour of hardness remains practically unchanged, with similar softening beginning at -700 K. This may be taken as an
B.V. (North-Holland)
676
L. Brunellt et ui. / Internal strains in martensttic steels 0.15
0.10 ol -
,” 0 cl
0.05 0.15
g 5 c 3
0.10
E 2 9
0.05
250 1000
500
ANNEALING
TEMPERATURE
(K )
Fig. 1. Quenching rate effects on MANET steel behaviour. Microhardness and half height width of (110) XRD line for subsequent heat treatments of 60 min at the temperatures indicated in abscissas quenching rate f = 150 K/min in a) and ?= 1100 K/min in b)). Half height width is not corrected for
instrumental line broadening (0.025 o ).
that after the annealing the dislocation structures giving rise to hardening are independent of the quenching rate. The XRD line widths present instead much smaller decreases and large widths remain after the final heating at 973 K. Only small variations of the XRD line widths have been experienced by prolongation of the heating up to 600 min at 973 K. Size and strain contributions to line broadening may be resolved by means of Fourier analysis of the profiles according to the Warren-Averbach method. As considered in detail in another paper [5], the internal strains contribute approximately half of the XRD line widths, indicating that both small subgrains and relevant internal stresses remain after tempering in the MANET samples submitted to quenching. On account of the important role of dislocations, the evolution of the microstructure, introduced by cold work in tempered steels with different residual XRD line widths, has also been considered. Results are shown in fig. 2; they refer to two batchs of specimens distinguished by the different quenching rates, f= 150 K/mm and F= 3300 K/mm. After quenching, all of the specimens were tempered at 973 K, cold rolled with 50% reduction and then submitted to subsequent steps of heating at the temperatures indicated in the figure. Internal strains and subgrains, which cause larger line widths, present a stabilizing effect on the dislocaindication
tion structure, except for the step from 700 to 800 K. Following the faster quenching rate, the XRD line widths always remain larger. No relevant XRD line shift is observed after the various treatments. For comparison, observations were made on cold-worked 316L stainless steel. The annealing done after cold rolling to 90% of reduction was followed after subsequent steps of heating, as was done for MANET. The (111) XRD line widths for 316L (fig. 3) are smaller than those observed for MANET. More relevant decreases of the widths are presented by the (220) XRD lines, starting from = 700 K. The microhardnesses reach a maximum at 800 K. Considering the corresponding line width decreases, this maximum seems hardly attributable to increases of the dislocation densities but rather to the occurrence of obstacles to dislocation motion, such as sessile loops. With this alloy, shifts of the XRD lines are observed with a stage around = 600 K, which corresponds to a maximum of the angular gap between the (111) and (220) lines. As known from previous research [6], this gap is influenced by the presence of stacking faults. 4. Discussion The results are discussed with reference to the characteristics of resistance to swelling which are specific to the MANET steel. As recalled in the introduction, bubbles grow to the stage where they become voids, i.e. significant swelling occurs when the number of He
1 10.05
_
_ 1 10.00
_I m
,”
0
ci5 5
0 :?i
ANNEALING
TEMPERATURE
9.95 10.05 -p”
0 a-
10.00
(K )
Fig. 2. Annealing of work hardening structures (50% cold rolling) introduced in MANET steel tempered after quenching with f = 150 K/mm in (a) and F = 3300 K/mm in (b). Microhardness, half height width and peak position 8 of { 110) lines for subsequent heat treatments of 120 mm at the temperatures in the abscissas.
L. Brunellr et al. / Internal strains in marfensitic steels
- 0.15 g 2 :
E 380
I
I
I
I
I
I
I
10.10
980916.10 1000
500 ANNEALING
TEMPERATURE
(K )
Fig. 3. Annealing of work hardening structures (90% cold rolling) introduced in AISI 316 L steel. Microhardness, half height widths (a) and peak position 8 (b) of 1111) and (220) XRD lines for subsequent heat treatments of 120 min at the temperatures in abscissas.
per bubble attains the critical value, nz. Conversely, the restrained swelling of MANET results from limited numbers of He atoms per bubble, ns < ni. Not considering helium escape from the material, this may occur only In the presence of very high bubble numbers and/or of very large concentrations of other He sink centres. The peak swelling reductions in a-Fe resulting from Cr additions, especially below 5% [7], are thus consistent with Cr atoms acting as trapping centres for He and associated vacancies. On the other hand, the lower swelling resistance, observed in 316L stainless steel is considered to be Indicative of smaller concentrations of nucleation or He sink centres, coming about especially above 800 K. Our observations are consistent with recovery of the dislocations introduced by cold work at the higher temperatures examined both in MANET and In 316L steels. The sessile Frank loops are specific to 316 stainless steel; they act as sites of preferential sink for the interstitials [8]. These loops do not appear above = 800 K, consistent with the assumption that the larger swelling, above = 800 K, is due to reduced He sink densities associated with the absence of loops. While growing, these loops act as vacancy emitters, and as a consequence, their growth may be accompanied by bubble growth. In the absence of or by the disappearance of the loops, the vacancy-interstitial reatoms
611
combination will increase, and as a result, the number of free vacancies which can react with He is strongly reduced, consistent with the expected reduction of the He sink centres. An interpretation of this type has been considered by Ono et al. [9] to explain the behaviour of Al under irradiation at intermediate temperatures. The hardness maximum and the XRD line shift variations observed by us in 316 L steel at = 700 K may be related to the corresponding disappearance of loops introduced during cold-work. With MANET steel, preferential sinks for interstitials are the dislocations and the sub-boundaries associated with the lath structure of martensite. However, after tempering at = 1000 K, to which these materials are submitted before irradiation, the volume density of both dislocations and subboundaries is rather low [lo], and it seems unprobable that they play the dominant role with respect to the required high densities of He sinks. In a previous paper [5], the hypothesis was put forward that the XRD line broadening remaining after tempering is due to residual internal strains, depending on inhomogeneities in the Cr distribution associated with the formation of the (Y’ Cr rich ferrite phase. Formation of small globular particles of a’ has been observed in irradiated Cr-Fe alloys [7]; these particles have a finer and more uniform distribution than the precipitates observed [5] in our material, and they correspond to the phases observed, in the absence of irradiation, with higher Cr concentrations in ferrite [ll]. Contiguous to the Cr rich phase, zones with Cr depletion come about, which may well act as interstitial sinks with an action similar to that of the loops in 316L but persisting at higher temperatures. Evidence of vacancy emission from these zones with lower Cr concentration has been found by Little and Stow [7]. 5. Conclusions From the comparison with the behaviour of deformed 316L steel, lattice defects connected with the internal strains, remaining after prolonged tempering, appear specific to MANET steels. Experimental observations are consistent with the annealing out at the temperature of tempering, of other lattice defects such as dislocations and sub-boundaries. The hypothesis has been discussed that these residual strains are related to the occurrence of Cr enriched and contiguous Cr poor zones, which may act as sink centres, with high densities in the grain interior, consistent with the occurrence of the small stable bubbles with large densities within the grains. They may explain the high swelling resistance of MANET steel. Acknowledgements
The authors are grateful to Dr. K. Ehrlich of KfK, Karlsruhe for the supply of material, reports and infor-
678
L. Brunelli et nl. / Internal strains m mariens~t~~ steels
mation. assistance.
Thanks are due to Mr. P. Plini For technical The work has beep carried out on fulfilment
of a Research
Contract
with
ENEA.
References R.E. Staller and G.R. Odette, J. Nucl. Mater. 103 81 104 (1981) 1361. R.E. Staller and G.R. Odette, in: Effects of Radiation on Materiais - 11, ASTM-STP 782 (1982) 275. L.K. Mansur and W.A. Coghlan, J. Nuci. Mater 119 (1983) 1. G.R. Odette, J. Nucl. Mater. 155-157 (198X) 921.
[5] L. Brunelli, [6] [7] [X] [9] [lo]
[ll]
P. Gondi and R. Montanari. Metall. Sci. Technol. to be published. M.S. Paterson, J. Appl. Phys. 23 (1952) 805. E.A. Little and D.A. Stow, Metal Sci. (1980) X9. P.J. Maziasz and C.J. McHargue, Int. Mater. Rev. 32 (1987) 190. K. Ono, M. Inoue, T. Kino, S. Furuno and K. Izui, J. Nucl. Mater. 133 & 134 (1985) 477. C. Wassilew, K. Herschbach, E. Materna-Morris and K. Ehrlich, in: Proc. Topical Conf. on Ferritic Alloys for Use in Nuclear Energy Technologies, Snowbird, 1983, p. 607614. P.J. Grobner. Metall. Trans. A4 (1983) 251.