Creep–fatigue–oxidation interactions in a 9Cr–1Mo martensitic
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International Journal of Fatigue 30 (2008) 649–662
International Journalof Fatigue
/locate/ijfatigue
Keywords: Creep; Fatigue; Oxidation; Martensitic steel; Crack initiation; 9Cr steel; High temperature; Oxide cracking
1. Introduction The martensitic steels of the 9–12%Cr family are used for many applications in the energy industry [1–3]. The modified 9Cr–1Mo steel is a candidate for several components of the generation IV nuclear reactors. Typical in-service conditions require operating temperatures between 673 and 873 K, which means that the creep behaviour of these steels is of primary interest. In addition, the repeated startand stop-operations during service lead to loadings of creep–fatigue type, with very long holding periods (typically 1 month). As complete tests with such long holding
a CEA/DEN-DANS/DMN/SRMA, Bat. 455, 91191 Gif-sur-Yvette Cedex, France ´ riaux P.-M. Fourt, UMR CNRS 7633, BP 87, 91003 Evry, France ENSMP, Cented 23 October 2006; received in revised form 17 April 2007; accepted 14 May 2007 Available online 26 May 2007
Abstract Cyclic tests with or without tensile holding periods were conducted in air at 823 K on a modified 9Cr–1Mo martensitic steel. In addition to stress–relaxation fatigue (RF) tests with a hold time at maximum load, creep–fatigue (CF) experiments were carried out. These CF tests were strain-controlled during the cyclic part of the stress–strain hysteresis loop and then load controlled when the stress was maintained at its maximum value to produce a prescribed value of the creep strain before cyclic deformation was returned under strain-controlled conditions. This unusual testing procedure enabled larger viscoplastic strains to be reached during the holding period than during usual relaxation–fatigue (RF) tests. The relationship between the number of cycles to failure of pure fatigue tests and the cyclic strain range is established for pure fatigue tests. The lifetime reduction due to holding periods is highlighted and quantified. The fatigue lifetime reduction due to holding periods is all the more pronounced as the cyclic strain amplitude is low. No creep cavitation is visible by microscopic observations, while the environment is found to play a key role in damage accumulation and interaction. Two main failure mechanisms are observed depending both on the fatigue strain range and on the duration of the holding period. An attempt is made to explain the existence of these two domains in relation with oxidation effect. Ó 2007 Elsevier Ltd. All rights reserved.
Corresponding author. Address: CEA/DEN-DANS/DMN/SRMA, Bat. 455, 91191 Gif-sur-Yvette Cedex, France. E-mail address: benjamin.fournier@cea.fr (B. Fournier). 0142-1123/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijfatigue.2007.05.007
650
B. Fournier et al. / International Journal of Fatigue 30 (2008) 649–662
article presents the results of fatigue and true creep–fatigue mechanical tests with a specific emphasis on the cracking phenomena. The first part of this work is focussed on tensile holding periods. The results of the mechanical tests are presented first in terms of fatigue lifetime and damage accumulation. The second part of this article deals with a series of microscopic observations of the surfaces, the fracture surfaces and polished cross-sections of the specimens are presented and compared. Finally cracks density measurements and crack type, in addition to the various interactions with environment lead to the distinction between two main damage domains, depending on the applied strain range and on the holding period duration. 2. Material and experimental procedures The experiments were conducted on a P91 steel produced by Usinor (Arcelor, France). The chemical composition is given in Table 1. The steel plate was austenitized at 1323 K for 30 min, quenched and tempered at 1053 K for 1 h. The usual tensile properties at 823 K are given in Table 2. The as-received very fine tempered martensite microstructure is presented in Fig. 1. Low cycle fatigue (LCF) tests were conducted in air on MAYES ESM100 servo-mechanical machines with resistance furnace heating. Temperature along the gauge length
Creep–fatigue–oxidation interactions in a 9Cr–1Mo martensitic steel. Part I: Effect of tensile holding period on fatigue lifetime
International Journal of Fatigue 30 (2008) 649–662
International Journalof Fatigue
/locate/ijfatigue
Keywords: Creep; Fatigue; Oxidation; Martensitic steel; Crack initiation; 9Cr steel; High temperature; Oxide cracking
1. Introduction The martensitic steels of the 9–12%Cr family are used for many applications in the energy industry [1–3]. The modified 9Cr–1Mo steel is a candidate for several components of the generation IV nuclear reactors. Typical in-service conditions require operating temperatures between 673 and 873 K, which means that the creep behaviour of these steels is of primary interest. In addition, the repeated startand stop-operations during service lead to loadings of creep–fatigue type, with very long holding periods (typically 1 month). As complete tests with such long holding
a CEA/DEN-DANS/DMN/SRMA, Bat. 455, 91191 Gif-sur-Yvette Cedex, France ´ riaux P.-M. Fourt, UMR CNRS 7633, BP 87, 91003 Evry, France ENSMP, Cented 23 October 2006; received in revised form 17 April 2007; accepted 14 May 2007 Available online 26 May 2007
Abstract Cyclic tests with or without tensile holding periods were conducted in air at 823 K on a modified 9Cr–1Mo martensitic steel. In addition to stress–relaxation fatigue (RF) tests with a hold time at maximum load, creep–fatigue (CF) experiments were carried out. These CF tests were strain-controlled during the cyclic part of the stress–strain hysteresis loop and then load controlled when the stress was maintained at its maximum value to produce a prescribed value of the creep strain before cyclic deformation was returned under strain-controlled conditions. This unusual testing procedure enabled larger viscoplastic strains to be reached during the holding period than during usual relaxation–fatigue (RF) tests. The relationship between the number of cycles to failure of pure fatigue tests and the cyclic strain range is established for pure fatigue tests. The lifetime reduction due to holding periods is highlighted and quantified. The fatigue lifetime reduction due to holding periods is all the more pronounced as the cyclic strain amplitude is low. No creep cavitation is visible by microscopic observations, while the environment is found to play a key role in damage accumulation and interaction. Two main failure mechanisms are observed depending both on the fatigue strain range and on the duration of the holding period. An attempt is made to explain the existence of these two domains in relation with oxidation effect. Ó 2007 Elsevier Ltd. All rights reserved.
Corresponding author. Address: CEA/DEN-DANS/DMN/SRMA, Bat. 455, 91191 Gif-sur-Yvette Cedex, France. E-mail address: benjamin.fournier@cea.fr (B. Fournier). 0142-1123/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijfatigue.2007.05.007
650
B. Fournier et al. / International Journal of Fatigue 30 (2008) 649–662
article presents the results of fatigue and true creep–fatigue mechanical tests with a specific emphasis on the cracking phenomena. The first part of this work is focussed on tensile holding periods. The results of the mechanical tests are presented first in terms of fatigue lifetime and damage accumulation. The second part of this article deals with a series of microscopic observations of the surfaces, the fracture surfaces and polished cross-sections of the specimens are presented and compared. Finally cracks density measurements and crack type, in addition to the various interactions with environment lead to the distinction between two main damage domains, depending on the applied strain range and on the holding period duration. 2. Material and experimental procedures The experiments were conducted on a P91 steel produced by Usinor (Arcelor, France). The chemical composition is given in Table 1. The steel plate was austenitized at 1323 K for 30 min, quenched and tempered at 1053 K for 1 h. The usual tensile properties at 823 K are given in Table 2. The as-received very fine tempered martensite microstructure is presented in Fig. 1. Low cycle fatigue (LCF) tests were conducted in air on MAYES ESM100 servo-mechanical machines with resistance furnace heating. Temperature along the gauge length
Creep–fatigue–oxidation interactions in a 9Cr–1Mo martensitic steel. Part I: Effect of tensile holding period on fatigue lifetime
