Research Papers: Materials and Fabrication

Study of Life Prediction and Damage Mechanism for Modified 9Cr-1Mo Steel Under Creep-Fatigue Interaction

[+] Author and Article Information
Guodong Zhang

e-mail: zhanggdln@163.com

Yanfen Zhao

Suzhou Nuclear Power Research Institute,
Xihuan Road,
Suzhou 215004, China

Fei Xue

Department of Material Science and
Tsinghua University,
Haidian District,
Beijing 10084, China

Changyu Zhou

School of Mechanical and Power Engineering,
Nanjing University of Technology,
Xinmofan Road,
Nanjing 210009, China

1Corresponding author.

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the Journal of Pressure Vessel Technology. Manuscript received April 3, 2012; final manuscript received December 13, 2012; published online June 11, 2013. Assoc. Editor: Osamu Watanabe.

J. Pressure Vessel Technol 135(4), 041402 (Jun 11, 2013) (8 pages) Paper No: PVT-12-1039; doi: 10.1115/1.4023424 History: Received April 03, 2012; Revised December 13, 2012

Creep-fatigue interaction is a principal cause of failures of many engineering components under high temperature and cyclic loading. In this work, stress controlled creep-fatigue interaction tests are carried out for modified 9Cr-1Mo (P91) steel. In order to study the damage mechanism of P91 steel under creep-fatigue interaction, Scanning Electron Microscopy (SEM) of specimen fracture morphology and in-situ observation experiments were conducted. Based on the ductility exhaustion theory and creep-fatigue interaction tests data, the modified ductility exhaustion life prediction model was developed. The predicted results are in a good agreement with the experiment. By comparison with frequency separation model, the life predicted by ductility exhaustion model is better than frequency separation model obviously. The results show that different stress amplitude and mean stress have great effect on the fracture damage mechanism when the hold time is invariable. By the SEM analysis of fracture morphology, the damage characters of creep, creep-fatigue interaction and fatigue can be partitioned. The specimen crack initiation source is the modified 9Cr-1Mo steel inclusion. Therefore, this work can provide a reference of life prediction and design for high temperature materials and components.

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Fig. 1

Geometry of the specimen in fatigue-creep interaction tests; dimensions in mm

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Fig. 2

Trapezium waveform used in fatigue-creep interaction loading conditions

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Fig. 3

Specimen fixed on the in-situ observation equipment

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Fig. 4

Hysteresis loops of 0–320 MPa

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Fig. 5

The relationship of stress-strain in one hysteresis loop

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Fig. 6

Relationship between stress amplitude, mean stress, and life

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Fig. 7

The P91 steel plastic strain range in creep-fatigue interaction, (a) 1#–5# specimens, (b) 6#–10# specimens, and (c) 11#–15# specimens

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Fig. 8

Best-fit curve of the DE and FS model with experimental results, (a) DE model and (b) FS model

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Fig. 9

Life prediction results comparison between DE model and FS model

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Fig. 10

Fracture morphology of the specimen under maximum stress 340 MPa, (a) −200–340 MPa, (b) 0–340 MPa, and (c) 200–340 MPa

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Fig. 11

Damage process of the P91 steel under creep-fatigue interaction observed by in-situ test, (a) inclusion; (b) crack initiation in inclusion; (c) crack propagation; (d) fracture; and (e) the inclusion chemical composition analyzed by EDS



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