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ちんずいへい
氏名
陳瑞葎
学位の種類
博士(学術)
学位記番号
学術(環)博第 145 号
学位授与年月日
平成 2 3 年 9 月 6 日
学位授与の根拠法規
学位規則第 4 条第 1 項
研究科，専攻の名称
東北大学大学院環境科学研究科(博士課程)環境科学専攻
学位論文題目
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(改良 9Cr・ lMo 鋼の高温クリープ中の微視組織変化とその寿命評価への
適用)
指導教
員東北大学教授丸山公一
論文審査委 員主査東北大学教授丸山公一
東北大学教授長谷川泰土
東北大学教授野中勇
(工学研究科)
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In the primary research, it has been pointed out that hardness drop of grip portion of crept specimens well
accords with breakdown of creep strength during long-term creep. Based on this finding it is proposed that
static recovery of tempered martensite lath structure is the cause of the breakdown. This proposal is assessed
by studying the microstructural degradation of Gr.91 steel during aging, short-term creep and long-term
creep, respectively.
3.1.1 Static recovery during aging
Both spacings of MX and M23 C6 precipitates are kept constant in short-term aging. Spacing of MX
precipitates is still kept constant in long-term aging, whereas spacing of M23 C6 precipitates increases.
Coarsening of subgrains takes place in long-term aging and is controlled by the aggregation of M23 C6
precipitates, namely pure static recovery. The subgrain size controlled by aggregation ofM23 C6 precipitates is
represented by the following equation:
d=k2A2
where d is the subgrain size, k2 is a constant (2.0x 10-3 nm- I at 700°C, 2.3 xl 0-3 nm-I at 600°C and 650°C), and
A is the spacing of M23 C6 precipitates. The aggregation of M23 C6 precipitates during aging is represented by
the following equation:
A3 =A5+ k l t
where A and
..1.0
are the spacing of M23 C6 precipitates at the aging time t and before aging, respectively, kl
is Ostwald ripening rate ofM23 C6 precipitates during aging.
3.1.2. Strain-induced recovery during short-term creep
Both spacings of MX and M23 C6 precipitates are kept constant during interrupted short-term creep,
whereas the subgrian size increases with increasing creep time. Coarsening of subgrains during interrupted
short-term creep has nothing to do with MX and M23C6 precipitates and it is only caused by strain, namely
strain-induced recovery. The correlation between subgrain size and strain during interrupted short-term creep
is represented by the following equation:
d-do=k6t:
where d and do are the subgrain sizes during interrupted short-term creep and before creep test, respectively,
k6 is the coarsening rate of subgrains (3000nm), and t: is the strain.
3.1.3. Breakdown of creep strength during long-term creep
Aggregation of M23 C6 precipitates takes place during interrupted long-term creep and it is larger than that
due to pure static recovery, suggesting that strain accelerates the aggregation of M23 C6 precipitates. Therefore,
the aggregation M23 C6 precipitates during interrupted long-term creep is caused not only by pure static
recovery but also strain, namely strain-assisted static recovery. The aggregation of M23 C6 precipitates during
interrupted long-term creep is given by the following equation:
Acreep
where
ACreep
= k 7 (A5+ k l t )1/3 -
(k7 -1)..1.0
is the spacing of M23 C6 precipitates at the creep time t, and k7 is a constant (2.8 at 700°C and
40MPa, 5.3 at 700°C and 50MPa). The correlation between k7 and creep condition should be studied more in
future. Coarsening of sub grains during long-term creep is caused by strain-induced recovery, pure static
recovery and strain-assisted static recovery. Their contributions are given by the following equations,
-71-
respectively.
I1d) =k6G
I1d 2 = k2(A5
+ klt)2/3 -
k2A~
L1d 3 = k2[k7(A5 +k1t)1/3 - (k7 - 1)AoF - k2(A5
+ klt)2/3
where I1d), I1d2 andl1d3 are the subgrain coarsening due to strain-induced recovery, pure static recovery and
strain-assisted static recovery, respectively. The contributions of strain-induced recovery, pure static recovery
and strain-assisted static recovery to the subgrain coarsening during long-tenn are additive. Contribution of
static recovery including pure static recovery and strain-assisted recovery to the breakdown of creep strength
is significant. This contribution is added to that due to strain-induced recovery, resulting in the breakdown of
creep strength.
3.2. Assessment of creep rupture life using hardness measurement
The second objective in the present thesis is to assess creep rupture life of high Cr ferritic steels. The creep
life is assessed using hardness measurement based on microstructural degradation during creep.
The correlation between hardness and sub grain size of the steel is given by the following equation:
H=k4/d+B
where H is the hardness, k4 is a constant (=3 x 104nm), d is the subgrain size, and B is a material constant
(=141 in Gr.91 steel). The correlation between subgrain size and strain during interrupted short-tenn creep is
given by
d-do=k6G
Therefore, the correlation between hardness drop and strain during short-tenn creep is given by the following
equation:
(Ho-li)/[(H-B)(Ho-B)]=k6G/k4
where Hand Ho are the hardness during the interrupted short-tenn creep and before creep test, respectively.
The short-tenn creep rupture life of the steel can be assessed by the following equation:
t
where
and
t;
GO is
•
,
t
=------------------------~~~~=-----
I-ex {.o x[s + In(l+'t) _
k 4 (Ho -H)
]}
p
2
0
.01
k6(H - B)(Ho - B)
the instantaneous strain upon loading,
C, Q) and Q 2 are constants determining creep curve shape,
is the predicted creep rupture life. The accuracy of short-tenn life assessment increases with
increasing creep time. The accuracy of short-tenn life assessment is almost within 30% of the actual creep
rupture life in the time range larger than 1/3 of creep life. This accuracy is good, so this hardness
measurement method is applicable to the short-tenn life assessment of high Cr ferritic steels.
The sub grain size during interrupted long-tenn creep is given by the following equation:
.
d - do = k~8
.
where k 6 is dependent on creep conditions. It increases with decreasing creep stress. The value of k 6
IS
1.2x104 nm at 700°C and 40MPa, and it is 8x 103 nm at 700°C and 50MPa. Based on this equation and the
correlation between hardness and subgrain size of the steel, the correlation between hardness and strain
during long-tenn creep is given by the following equation:
-- 72--
(Ho -H) = k~ &
(H - B XHo - B) k4
The long-term creep life can be assessed based on the correlation between hardness and strain during
long-term creep and the correlation between strain and long-term creep life which can be represented by
creep curve. The accuracy of long-term life assessment is within 30% of the actual creep life in the creep
time range larger than 1/3 of the creep life. This accuracy is good, so this hardness measurement method is
also applicable to the long-term creep life assessment of high Cr ferritic steels. However, in order to use this
method to assess long-term creep life, the creep condition dependent k ~ should be studied more in future.
The long-term creep life is underestimated if the strain is assessed based on the correlation between hardness
and strain during short-term creep. In addition, the underestimation decreases with increasing creep time.
1000
-e
900
800
--= 700
e
•
~
QJ
.~
C"'-l
600
=
..
Calculated data
Pure static rerovery
Pure static recovery
+strain-induced recovery
Pure static rerovery+
strain-induced recovery
+strain-assisted
static recovery
Gr.91B
8=0.022
500
••
~
eJ)
400
~
= 300
8=0.014
Measured data
•
00.
200
0
100
102
7CXtCAging
70CtC40MPa
103
Exposure time [b]
Figure I. Subgrain coarsening of Gr. 9 1 steel due to three types of recovery process
during interrupted long-term creep at 700°C and 40MPa
- 73 -
論文審査結果の要旨
蒸気火力発電プラントの高温部には，高 Cr フェライト鋼が数多く使われている。この種の耐繋盛岡では，長時間吏用後に，
クリープ強度の急主主が起きることが多い。日本の蒸気火力発電プラントの 80% は設計寿命を越えて使われており，これら
の経年プラントでは，安全確保のために，構i館財寸の余寿命制面法の確立が求められている。このような背景のもとに本
論文では，改良 9Cr- lMo 鋼のクリープ損傷欄封書を明らかにし，この材料での強度急減の原因解明と寿命評価法の提案を
行った。論文は全 8 章で構成されている。
第 l 章は序論で，本研究の背景を述べている。
第 2 章は目的で，研究の目的を述べている。
第 3 章は実験方法で，使用した材料，クリープ試験，組織観震の方法等を述べている。
第 4 章では，サブグレイン組織の回復が高t畠クリープ損傷の主原因であることを示すとともに，時効言撤を行って，クリ
ープ試験温度に暴露したことによるサブグレインの翻句回復挙動を研究した。その結果， M 23C6 炭化物粒子からのピン止
め力の喪失がサブグレイン組織の索駒回復を支配していることを明らかにした。
第 5 章では，
サブグレイン組織の繋酌回復が起きない短時間クリープ条件で，変形に誘起されたサブグレイン組織の
回復挙動を研究した。ひずみ誘起回復によるサブグレイン幅の増加量はひずみ量に比例し，比伊j係数はクリープ誌験条
件に依存しなし、ことを明らかにした。
第 6 章では， サブグレイン組織の繋駒田復とひずみ誘起回復が同時に起きる"長時間"試験条件で，サブグレイン組
織の回復挙動を研究した。サブグレインのひずみ誘起回復しか起きない短時間クリープに対して，長時間クリープで繋駒
回復が加わることが，長時間クリープでの材料号制支低下の原因であることを定量旬こ解明した。
第7 章では，
主クリープ損傷であるサプグレイン組織の回復が硬さ測定で非破騨切こ制面できることを提案し，硬さ変
化を余寿命評価に適用する方法論を検討し九短時間クリープでは硬さ測定に基づく余寿命評価が有効なことを立証し，
長時間クリープでは鱗財 3 必要な課題を明らカミにし丸
第8 章は結論で，本研究の成果を総括している。
以上要するに，本論文は，
改良 9Cr- lMo 鋼のクリープ損傷樹帯冴究し，この材料でのクリープ強度急減の原因を解
明し，余寿命評価法を提案したもので，環境科学の発展に寄与するところが少なくない。
よって，本論文は博士(学術の学位論文として合格と認める。
7
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