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Title: In-situ Phase transformation study in fine grained heat affected zone of Grade 91 steels

Creep strength-enhanced ferritic (CSEF) steels such as the 9 Cr steel [ASTM A387 Grade 91] are widely used as tubing and piping in the new generation of fossil fired power plants. Microstructures in the fine-grained heat affected zone (FGHAZ) may significantly reduce creep strength leading Type IV failures. Current research suggest that reducing pre-weld tempering temperature from 760 C (HTT) to 650 C (LTT) has the potential to double the creep life of these welds. To understand this improvement, time-resolved X-ray diffraction (TRXRD) measurement with synchrotron radiation was used to characterize the microstructure evolution during fine grained heat-affected zone (HAZ) thermal cycling of grade 91 steel. The measurements showed both M23C6 (M=Fe, Cr) and MX (M=Nb, V; X=C,N) are present in the sample after the HTT condition. Near equilibrium fraction of M23C6 was measured in high temperature tempering condition (HTT, 760 C). However, the amount of M23C6 in LTT condition was very low since the diffraction peaks are close to the background. During simulated FGHAZ thermal cycling, the M23C6 partially dissolved in HTT sample. Interestingly, MX did not dissolve in both LTT and HTT samples. Hypothesis for correlation of M23C6 carbide distribution and pre-mature creep failure in FGHAZ will bemore » made.« less
 [1] ;  [2] ;  [2] ;  [2] ;  [3] ;  [3]
  1. Ohio State University
  2. ORNL
  3. Osaka University
Publication Date:
OSTI Identifier:
DOE Contract Number:
Resource Type:
Resource Relation:
Conference: Visual Joining and Welding, Osaka, Japan, 20121128, 20121130
Research Org:
Oak Ridge National Laboratory (ORNL)
Sponsoring Org:
FE USDOE - Office of Fossil Energy (FE)
Country of Publication:
United States