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 to Type IV failures. Current research suggests that reducing pre-weld tempering temperature from 760 °C (high temperature temper, HTT) to 650 °C (low temperature temper, 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 be made.