Creep Behavior, Deformation Mechanisms, and Creep Life of Mod.9Cr-1Mo Steel

Published on Sep 21, 2015in Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science2.556
· DOI :10.1007/S11661-015-3144-5
Fujio Abe52
Estimated H-index: 52
(National Institute for Materials Science)
Sources
Abstract
The creep behavior, deformation mechanisms, and the correlation between creep deformation parameters and creep life have been investigated for Mod.9Cr-1Mo steel (Gr.91, 9Cr-1Mo-VNb) by analyzing creep strain data at 723 K to 998 K (450 °C to 725 °C), 40 to 450 MPa, and t r = 11.4 to 68,755 hours in NIMS Creep Data Sheet. The time to rupture t r is reasonably correlated with the minimum creep rate \( {\dot{\varepsilon }}_{ \hbox{min} } \) and the acceleration of creep rate by strain in the acceleration region dln\( {\dot{\varepsilon }} \) /d e, as t r = 1.5/[\( {\dot{\varepsilon }}_{ \hbox{min} } \) (dln\( {\dot{\varepsilon }} \) /d e)], where \( {\dot{\varepsilon }}_{ \hbox{min} } \) and dln\( {\dot{\varepsilon }} \) /d e reflect the creep behavior in the transient and acceleration regions, respectively. The \( {\dot{\varepsilon }}_{ \hbox{min} } \) is inversely proportional to the time to minimum creep rate t m, while it is proportional to the strain to minimum creep rate e m, as \( {\dot{\varepsilon }}_{ \hbox{min} } \) = 0.54 (e m/t m). The e m decreases with decreasing stress, suggesting that the creep deformation in the transient region becomes localized in the vicinity of prior austenite grain boundaries with decreasing stress. The duration of acceleration region is proportional to the duration of transient region, while the dln\( {\dot{\varepsilon }} \) /d e is inversely proportional to the e m. The t r is also correlated with the t m, as t r = g t m, where g is a constant. The present creep life equations reasonably predict the degradation in creep rupture strength at long times. The downward deviation takes place in the t r vs \( {\dot{\varepsilon }}_{ \hbox{min} } \) curves (Monkman–Grant plot). At the same \( {\dot{\varepsilon }}_{ \hbox{min} } \), both the e m and t m change upon the condition of t m ∝ e m. The decrease in e m with decreasing stress, corresponding to decreasing \( {\dot{\varepsilon }}_{ \hbox{min} } \), causes a decrease in t m, indicating the downward deviation of the t r vs \( {\dot{\varepsilon }}_{ \hbox{min} } \) curves.
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References40
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#1Yuan Liu (National Institute for Materials Science)H-Index: 4
#2Susumu Tsukamoto (National Institute for Materials Science)H-Index: 16
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Microstructure evolution of newly developed 9Cr-3W-3Co-V, Nb steel with boron addition (B steel) has been analyzed during HAZ thermal cycle at the peak temperature of around Ac3 (Ac3 HAZ) and post-weld heat treatment (PWHT) to elucidate the prevention mechanism of type IV failure by boron addition. It was found that enhancement of the boundary strengthening by precipitates is the main reason for prevention of type IV failure by boron addition. In B steel HAZ, original austenite is reconstituted ...
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#1Satoru Kobayashi (National Institute for Materials Science)H-Index: 8
#2Kota Sawada (National Institute for Materials Science)H-Index: 26
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Abstract This paper investigated the relationship between creep strength and the existence of residual δ ferrite in the initial microstructure in three heats of Grade 91 steel plates and examined the origin, the transformation process and the dissolution of the δ ferrite in the steel plates. The creep strength of the steel specimens was lower when the δ ferrite was present in the initial microstructure. The δ ferrite showed a disk shape and a bumpy interface, and had MX carbonitride particle arr...
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#1Fujio Abe (National Institute for Materials Science)H-Index: 52
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#1Shigeto YamasakiH-Index: 7
#2Masatoshi Mitsuhara (Kyushu University)H-Index: 15
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#1Fujio Abe (National Institute for Materials Science)H-Index: 52
Abstract: Key issues for application of Grade 91 (Gr.91) to power plants, such as long-term creep strength of base metal and welded joints, microstructure evolution during exposure at elevated temperature, degradation in welded joints due to type IV fracture, creep–fatigue properties and steam oxidation behaviour, are comprehensively described, together with chemical, heat treatment, tensile and process requirements for Gr.91. Greater attention will be paid to technical issues on estimation of 1...
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#1Yuan Liu (National Institute for Materials Science)H-Index: 4
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The mechanism of type IV failure has been investigated by using a conventional 9Cr ferritic heat-resistant steel Gr.92. In order to clarify the main cause of type IV failure, different heat treatments were performed on the base metal in order to change the prior austenite grain (PAG) size and precipitate distribution after applying the heat-affected zone (HAZ) simulated thermal cycle at the peak temperature of around A c3 (A c3 HAZ thermal cycle) and postweld heat treatment (PWHT). The microstru...
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#1Triratna Shrestha (UIdaho: University of Idaho)H-Index: 7
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Abstract Modified 9Cr–1Mo (Grade 91) steel is currently considered as a candidate material for reactor pressure vessels (RPVs) and reactor internals for the Very High Temperature Reactor (VHTR). The tensile creep behavior of modified 9Cr–1Mo steel (Grade 91) was studied in the temperature range of 873–1023 K and stresses between 35 MPa and 350 MPa. Analysis of creep results yielded stress exponents of ∼9–11 in the higher stress regime and ∼1 in the lower stress regime. The high stress exponent i...
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Laves phases with their comparably simple crystal structure are very common intermetallic phases and can be formed from element combinations all over the periodic table resulting in a huge number of known examples. Even though this type of phases is known for almost 100 years, and although a lot of information on stability, structure, and properties has accumulated especially during the last about 20 years, systematic evaluation and rationalization of this information in particular as a function...
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