Microstructure and Creep Behavior of Heat-Affected Zone in Grade 91 Steel Weldments

• Author / Creator

  Wang, Yiyu

• Grade 91 steel is one of the most important 9-12% Cr creep strength enhanced ferritic (CSEF) steels used in fossil-fuel fired power plants. A narrow heat-affected zone with heterogeneous structures is generated in the base metal, due to non-equilibrium phase transformations during the arc welding processes. This heterogeneous heat-affected zone has been reported to cause short-term creep failures of welded components, such as the infamous Type IV cracking. Two remaining questions associated with the Type IV cracking are: where is the exact fracture location and what structure feature is critically responsible for the failure. Post-weld heat treatment (PWHT) plays a significant role in improving weldments' toughness and maximizing their creep lifetime. How do these PWHT parameters (temperature, holding time) eventually affect creep behavior of the weldments needs a deeper investigation from local structural evolution point of view. In this work, heterogeneous structures in the heat-affected zone in three thermal stages, including the as-welded, post-weld heat-treated, and crept conditions, have been systematically characterized with advanced microscopy techniques. Non-equilibrium phase transformations in the heat-affected zone, including austenization and martensitic transformation, have been investigated through experimental study and thermodynamic modelling. High-temperature creep rupture mechanisms for Type IV cracking and Type I cracking, have been further analyzed and discussed. Structures of the heat-affected zone in the as-welded condition in 1-inch-thick pipe weldments and 5-inch-thick heavy section weldments are quantitatively characterized. This as-welded structure analysis provides a foundation for understanding structural evolutions of weldments in the PWHT-ed and crept conditions. A correlation between intercritical heat-affected zone and Type IV creep damaged zone has been built. The Type IV cracking mechanism is understood from a new perspective of the local chemistry at the grain-scale level. The so-called soft zone responsible for the Type IV cracking has been confidently identified in the ICHAZ in all three thermal stages (as-welded, PWHT-ed, and crept). Finite element analysis further verifies that the ICHAZ, exhibiting the highest first principal stress, the largest stress triaxialty, and the highest creep strain/strain rate, is the most creep-susceptible region when the PWHT temperature is below A1 temperature of the base metal. In high-temperature creep conditions, weak Cr-depleted ferrite grains in the ICHAZ preferentially deformed, which accelerates creep strength degradation of the ICHAZ. At the mesoscale level, nucleation of creep cavities along the grain boundaries is caused by a high stress triaxiality. A creep rupture transition from the Type IV cracking in the ICHAZ to the Type I cracking in the fusion zone was observed in the cross-weld samples when the PWHT temperature increases from 600 °C to 840 °C. Structural analysis uncovers that the PWHT-ed fusion zone, resulting in the Type I cracking, also has an intercritical structure of transformed martensite and untransformed tempered martensite. Finite element analysis indicates that the fusion zone becomes the new creep-susceptible region with the largest stress triaxiality and the highest creep strain/strain rate when the PWHT temperature is between the A1 and A3 temperatures of the filler metal. It is concluded that the intercritical structure of hard transformed martensite and soft untransformed tempered martensite/ferrite is likely to behave as the most creep-vulnerable structure.

• Subjects / Keywords

  • Micro-structure
  • Characterisation
  • Welding Metallurgy
  • Creep Behavior
  • Grade 91 Steel
  • Heat Affected Zone
  • Type IV Cracking

• Graduation date

  Spring 2018

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3XP6VJ6T

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.