Skip to Main Content
 

Global Search Box

 
 
 
 

Files

File List


Payton_Thesis_Final.pdf (7.89 MB)

ETD Abstract Container

Abstract Header

On the Improvement of Creep-fatigue Behavior of Grade 91 Weldments

Payton, Tyler K
http://rave.ohiolink.edu/etdc/view?acc_num=osu150185172211282

Abstract Details

2017, Master of Science, Ohio State University, Welding Engineering.
Grade 91 steel is a common structural material for fossil-fuel fired power plants because of its creep strength and oxidation resistance at high temperatures. Its high temperature properties make it an ideal candidate as a structural material for new fossil fuel and nuclear power plants. Improvements in efficiency of power production are made possible by increasing the power plant operating temperature. High temperature and the possibility of reactor shutdown (e.g., for maintenance) stimulate creep and fatigue interactions which are detrimental to the longevity of grade 91 components. Fusion welding processes are most commonly used to fabricate grade 91 joints. These grade 91 welds are known to be susceptible to type IV cracking in their heat affected zone (HAZ). Type IV cracking has caused pre-mature failure in current power plants. Currently, there is a lack of understanding of the creep-fatigue interactions in grade 91 weldments. The objective of this research is to study the fundamental relationship of creep-fatigue interactions and type IV cracking as well as mitigation of type IV cracking susceptibility in grade 91 weldments. Particularly, this study compared metallurgical characteristics and load-controlled creep-fatigue lifetime of weldments fabricated with two different fusion welding processes, conventional flux cored arc welding (FCAW) and low heat-input cold metal transfer welding (CMTW). Weldment cross-sections including fusion zone, coarse grained HAZ (CGHAZ), fine grained HAZ (FGHAZ), and inter-critical HAZ (ICHAZ) and base metal in as-welded and post weld heat treated (PWHT) conditions from each type of grade 91 joint were analyzed for metallurgical differences including grain size and microstructure phase. Hardness profiles showed that the CMTW joint contained less softening in the FGHAZ and ICHAZ compared to the same region in the FCAW joint. In collaboration with another research group at OSU, load-controlled creep-fatigue tests were conducted with ASTM standard conforming creep specimens machined from FCAW and CMTW joints in the PWHT condition. The specimens had gauge sections containing fusion zone, CGHAZ, FGHAZ, ICHAZ, and base metal. The creep-fatigue testing temperature was selected based on a collection of creep data analyzed from the literature. A type IV cracking susceptibility diagram was constructed, from which a testing temperature of 650°C was selected because it was found that increasing temperature increases susceptibility to type IV cracking. Moreover, the testing temperature is comparable to one anticipated for operation of new power plants. The creep-fatigue life of the FCAW joint tested with 85 MPa maximum stress was 56 hours. The creep-fatigue life of the CMTW joint tested with the same conditions was 590 hours. Base metal life for the same testing conditions was 420 hours. It was postulated that one plausible reason for the large difference in creep-fatigue life between the different weldments was the HAZ size difference. The cross-section of a grade 91 joint is thought of as a composite of soft FGHAZ and ICHAZ surrounded by strong fusion zone/CGHAZ and BM. The sandwich structure could induce a triaxial stress state where the weak material is restrained by the strong material when the composite is deformed. Norton’s power law, an empirical formula used to predict steady state creep behavior, was used in conjunction with finite element analysis (FEA) in ABAQUSTM to predict the possible triaxial stress state difference between simulated welds with large HAZ and small HAZ. The maximum stress state factor was found to increase from 3.76 to 3.80 which was a trend seen in the literature. The FEA models described above did not consider the possibility that creep properties, those described by Norton’s power law, could be different between weldments fabricated with different heat inputs. The weld thermal cycle induced by CMTW is different compared to FCAW; the CMTW heating rate and cooling rate is more rapid. The ICHAZ from each weldment was simulated using a GleebleTM 3800 thermo-mechanical simulator. A heating rate of 800°C/s and cooling rate of 30°C/s with 900°C peak temperature was used to simulate CMTW ICHAZ. A heating rate of 100°C/s and cooling rate of 10°C/s with 900°C peak temperature was used to simulate FCAW ICHAZ. Transmission Electron Microscopy (TEM) was used to compare the microstructures of the simulated samples to those observed in the actual weldments. Creep-fatigue testing results of the Gleeble-simulated specimens show that the CMTW well outperformed FCAW, indicating a strong benefit effect of reducing welding heat input. In summary, it is found that adding fatigue to creep markedly reduces the rupture time when compared to monotonic creep for grade 91 weldment. The creep-fatigue failure is still of type IV, occurring in the ICHAZ and FGHAZ. Low welding heat input substantially improved the creep fatigue life. These results constitute an important first step toward improving the fundamental understanding of creep fatigue failure in grade 91 weldment for safe and efficient design of next generation nuclear power plants.
Wei Zhang, PhD (Advisor)
Boian Alexandrov, PhD (Committee Member)
107 p.
Engineering; Materials Science
Grade 91; creep-fatigue; welding

Recommended Citations

Citations

  • Payton, T. K. (2017). On the Improvement of Creep-fatigue Behavior of Grade 91 Weldments [Master's thesis, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu150185172211282

    APA Style (7th edition)

  • Payton, Tyler. On the Improvement of Creep-fatigue Behavior of Grade 91 Weldments. 2017. Ohio State University, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu150185172211282.

    MLA Style (8th edition)

  • Payton, Tyler. "On the Improvement of Creep-fatigue Behavior of Grade 91 Weldments." Master's thesis, Ohio State University, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=osu150185172211282

    Chicago Manual of Style (17th edition)

osu150185172211282
799
© 2017, all rights reserved.
This open access ETD is published by The Ohio State University and OhioLINK.