A Proposal to Mechanism of Embrittlement of 9Cr-2Mo Heat Resisting Steel during Long-term Aging

Hosoi, Yuzô; Wade, Noboru; Kunimitsu, Seiji; Urita, Tatsumi

doi:10.2355/tetsutohagane1955.76.7_1116

Cited by 14 publications

(8 citation statements)

References 1 publication

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“…Therefore, it is thought that almost all the Laves phase precipitated are liquated in the matrix when the metal is subjected to HT, the same as PWHT. This is the same mechanism as reported by Hosoi et al 10 in which the toughness lost by a base metal due to thermal ageing is recovered by HT. Figure 10 shows the Laves phases observed in the vicinity of the fracture surface of a Charpy impact test specimen which had been subjected to thermal ageing for 31.5 Ms after PWHT.…”

Section: Test Results and Discussionsupporting

confidence: 63%

“…According to Asakura et al 9 , when 8.52Cr -1.55Mo -0.05Nb-0.17 V steel, which has been normalized (1323 K and 1.8 ks) and then tempered at 973 K for 3.6 ks, is thermally aged at 823 K for 360 ks, there is a decrease in toughness; but if 973 K and 3.6 ks heat-treated, the toughness is recovered. Further, according to Hosoi et al 10 , when 9.85Cr -2.31Mo -0.06Nb-0.12 V steel, which has been normalized (1323 K and 1.8 ks) and then tempered at 1048 K for 3.6 s, is thermally aged at 873 K for 3.6 Ms, there is a decrease in toughness; but if 1048 K and 3.6 ks heat-treated, the precipitated Laves phases are liquated and the toughness recovers. It is therefore possible that when weld metal, which has been subjected to thermal ageing with a decrease in toughness, experiences a recovery in toughness due to HT similar to PWHT, this is through the same mechanism as in the base metal.…”

Section: Test Results and Discussionmentioning

confidence: 99%

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Toughness recovery of modified 9Cr–1Mo steel weld metals after long-term thermal ageing and its evaluation by electrochemical measurement

Nishikawa¹,

Ohkita²,

Horii³

2010

Welding International

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Recently, thermal power plants have tended to operate under higher temperature and pressure steam conditions for CO 2 reduction. Modified 9Cr -1Mo steels are used in the ultra-super critical power plant because of their excellent creep properties. However, the toughness of base metal decreases after long-term thermal ageing at operation temperature. Moreover, toughness of weld metal is lowest in the MIG weldment. In this study, metallurgical factors of toughness of the modified 9Cr -1Mo steel weld metals by thermal ageing at operation temperature were investigated. And the evaluation method of toughness by the electrochemical measurement used with 5% sulphuric acid aqueous solution was investigated.The weld metal that received post-welding heat treatment (PWHT: 1023 K and 5.4 ks) decreased toughness by thermal ageing in 873 K and 31.5 Ms. However, toughness of the welds after thermal ageing was recovered to the same level as PWHT welds when the thermal aged welds received the same thermal history as PWHT. A lot of the large Laves phase was observed in the weld metal after the thermal ageing. However, almost all of the Laves phase dissolved in parent phase by the same thermal history as PWHT. Therefore, it seemed that the decrease and the recovery of toughness were mainly determined by the behaviour of precipitation of the Laves phase. The peak of the current density (Ip) in the electrochemical measurement appeared in the weld metals that precipitated a lot of large Laves phase. It seemed that appearance of Ip was caused by the dissolution of the Laves phase. A good correlation between toughness and Ip was observed. Therefore, it can be said that toughness of the weld metals is supposed to be predicted using an Ip of the electrochemical measurement.

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Section: Test Results and Discussionsupporting

confidence: 63%

Section: Test Results and Discussionmentioning

confidence: 99%

Toughness recovery of modified 9Cr–1Mo steel weld metals after long-term thermal ageing and its evaluation by electrochemical measurement

Nishikawa¹,

Ohkita²,

Horii³

2010

Welding International

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“…4,5) However, general features of the Laves phase precipitation behavior is not clarified. Particularly, creep stress effect on Laves phase precipitation behavior has not been clear.…”

Section: Introductionmentioning

confidence: 99%

Creep stress Effect on the Precipitation Behavior of Laves Phase in Fe-10%Cr-6%W Alloys.

et al. 2001

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“…13) However, it has been reported that precipitation of the Laves phase is delayed in the long-term due to the reduction of the Si content. 34,35) In this study, because oxidation resistance is required for FBR structural material, the content of Si was reduced to less than 0.002%.The low Si content is believed to be the reason why the Laves phase barely precipitated with creep in the steels. 24) Three types of precipitates can be observed before creep in the Nb-containing VN03, including Nb(C, N), (Fe, Cr)23C6, and Cr2(C, N).…”

Section: Change In Precipitation During Creepmentioning

confidence: 82%

Development of High Chromium Steel for Fast Breeder Reactors with High-temperature Strength, Ductility, and Microstructural Stability

2013

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The applicability of high chromium (Cr) steel as the main structural material in fast breeder reactors (FBR) has been explored to enhance the safety, the credibility and the economic competitiveness of FBR power plants. Vanadium (V) and Niobium (Nb) are believed to improve the high-temperature strength of high Cr steels by precipitating as carbides and/or nitrides, namely fine MX particles, although the longterm efficiency and stability of such precipitation strengthening mechanisms resulting from the fine MX particles have not been clarified yet. The effects of V and Nb on degradation of creep properties were investigated under FBR operating conditions, e.g., at 550°C for 500 000 h, and the relationship between the long-term creep properties and microstructural changes was investigated considering the MX particles and the Z-phase. It was found that the optimal V and Nb contents for excellent high Cr steel of FBR grade are 0.2 mass% and <0.01 mass%, respectively, under FBR operating conditions.

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A Proposal to Mechanism of Embrittlement of 9Cr-2Mo Heat Resisting Steel during Long-term Aging

Cited by 14 publications

References 1 publication

Toughness recovery of modified 9Cr–1Mo steel weld metals after long-term thermal ageing and its evaluation by electrochemical measurement

Toughness recovery of modified 9Cr–1Mo steel weld metals after long-term thermal ageing and its evaluation by electrochemical measurement

Creep stress Effect on the Precipitation Behavior of Laves Phase in Fe-10%Cr-6%W Alloys.

Development of High Chromium Steel for Fast Breeder Reactors with High-temperature Strength, Ductility, and Microstructural Stability

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