Supplementary Information accompanies the paper on Nature’s website (http://www.nature.com).
The selected area diffraction in transmission electron microscope was carried out for the identification
of precipitates, using thin films taken from the specimens. Figures S1 and S2 show the results of
identification of M23C6 carbides and MX carbonitrides, respectively, in the 0.078%C steels after heat
treatment. The M23C6 carbides were identified as face-centered cubic structure having a lattice parameter
of 1.067 nm. This is consistent with a reported result 1. The orientation relationship between MX along
grain boundary and bcc matrix of the right hand grain was found to be (110) MX // (100)bcc and [100]MX //
[100]bcc. This is consistent with a reported result on 9%Cr-1%V-0.1%C steel 2. In Figure S2, a large
number of fine precipitates, identified as MX carbonitrides, are distributed along prior austenite grain
boundaries. Combining with the results on TEM observations of replica film shown in Figure 4, one can
see that the nano-size precipitates of MX carbonitrides are linked together along prior austenite grain
boundaries. The number density of MX carbonitrides is quite high along prior austenite grain boundaries,
because of their linking together.
The MX carbonitrides were analyzed by Energy filtered transmission electron microscopy based on
EELS for the specimen with 0.002%C after heat treatment. The enrichment of V and N in the MX
particles is clearly obtained, as shown in Figure S3. Although no evidence was found for the enrichment
of carbon in the MX particles, we cannot perfectly exclude a possibility of the present of carbon. Figure
S4 shows the Thermo-calc evaluation of equilibrium phases in the present 9Cr-3W-3Co-0.2V-0.05Nb0.05N steel at a tempering temperature of 1073 K, as a function of carbon concentration. The Thermocalc evaluation predicts that the MX carbonitrides are dominant at low carbon concentration below 0.02%
and that they consist of mainly V and N in the range of 0-0.15%C. This is consistent with the results
shown in Figure S3.
The present 9%Cr steel with 0.002%C and 3% tungsten exhibited that the nano-size distributions of MX
carbonitrides were still maintained during creep at 923 K for up to 104 h, as shown in Figure S5. On the
other hand, a 9%Cr steel with 0.002%C but no addition of tungsten exhibited that the nano-size
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distributions of MX carbonitrides were observed after tempering heat treatment but that extensive
coarsening of the nano-size MX carbonitrides took place during creep, as observed to be 60 nm in size or
more after creep for 1.5x103 h at 923 K. The extensive coarsening of MX carbonitrides caused a decrease
in number density and hence a decrease in creep strength. The effect of 3% tungsten addition is
considered to result mainly from that dissolved tungsten in the matrix retards the recovery or movement
of excess dislocations during creep, which promotes diffusion of solute atoms.
The creep rupture strength at 923K and 105 h was estimated by traditional time-temperature-parameter
method using Larson-Miller parameter, which sometimes gives conservative predictions for the creep
rupture strength for 105 h at a given temperature from short-time data at higher temperature and lower
stress conditions 3. Using the creep rupture data for up to about 1x10 4 h at 923 K and those for up to
about 4x103 h at 973 K, Figure S6 shows the creep rupture stress for the present steel with 0.002%C as a
function of Larson-Miller parameter, T (C + log t), where T is the absolute temperature, t is the rupture
time and C is a constant. In this figure, the data for ASME-P92 4 are also included for comparison. In
this study, C = 36.11 5 used for ASME-P92 was also employed. The creep rupture strength at 923K and
105 h was estimated to be 100 and 71 MPa for the 0.002%C steel and ASME-P92, respectively.
1. Power Diffraction File, edited by Berry, L. G., Joint Committee on Power Diffraction Standards,
Pennsylvania USA, No. 5-0721 (1974).
2. Abe, F., Noda, T., Araki, H., Narui, H. and Kayano, H. Irradiation Hardening and Ductility Loss of A
Low-Activation 9Cr-1V Ferritic Steel at Low Temperatures, J. Nuclear Materials 166, 265-277 (1989).
3. Goldhoff, R. T. Some Observations on the extrapolation of high-temperature ferritic steel data. Trans.
ASME 82D, 848-854 (1960).
4. National Institute for Materials Science Creep Data Sheet, No.48 (2002).
5. Naoi, H., Mimura, H., Ohgami, M., Morimoto, H., Tanaka, T. and Yazaki, Y.
NF616 pipe production and properties and welding consumable development, Proc. The
EPRI/National Power Conference on New Steels for Advanced Plant Up To 620 oC, The Society of
Chemical Industry, London, UK, 8-29 (1995).
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Figure S1 A set of bright field (a) and dark field (b) of TEM micrographs of M23C6 precipitates present
in the 0.078%C steel, following a tempering treatment. The associated indexed selected area
electron diffraction pattern of M23C6 is also provided (c).
Figure S2 A set of bright field (a) and (b) of TEM micrographs, and chemical analysis by EDX (c) of
MX carbonitrides present in the 0.078%C steel, following a tempering treatment. The high
magnification micrograph (b) contains prior austenite grain boundary shown by the arrow in
(a). The associated indexed selected area electron diffraction pattern of MX is also provided
(d).
Figure S3
Energy filtered images of the specimen with 0.002%C after tempering heat treatment. (a),
bright field image; (b), mapping of V; (c) mapping of N.
Figure S4 Thermo-calc evaluation of phases appearing in the 9Cr-3W-3Co-0.2V-0.05Nb-0.05N steel
during tempering at 1073 K, as a function of carbon concentration. (a), Comparison of
M23C6 and MX; (b), Mole fraction of V, Nb, C and N in MX.
Figure S5 TEM micrographs of the 0.002%C steels after creep rupture testing at 923 K and 160 MPa.
(a), 3% W, tr = 10000h; (b), 0%W, tr = 1500h.
Figure S6 Larson-Miller plot for the 9Cr steel with 0.002%C and ASME-P92 2.
Correspondence and requests for materials should be addressed to F.A.
(e-mail: ABE.Fujio@nims.go.jp)
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