PREDICTION OF STRENGTH REDUCTION FACTOR FOR WELDED JOINTS OF REDICTION OF STRENGTH REDUCTION FACTOR FOR WELDED JOINTS OF TURBINES WITH ULTRA SUPERCRITICAL STEAM PARAMETERS URBINES WITH ULTRA SUPERCRITICAL STEAM PARAMETERS

During long-term operation heat-resistant ferrite and martensitic steel welds have type IV cracking in a so-called ‘soft layer’, which contains part of fine grain region and inter-critical region of heat-aff ected zone (HAZ). The rupture of dissimilar welds occurs in the carbonless soft layer near fusion line. Therefore the strength reduction factor of welds decreases significantly with increasing lifetime and operating temperature.

Based on experimental investigations the design model is constructed to predict the welds strength reduction coefficient at the design stage, taking into account different structural and technological factors, also changes creep and fracture mechanisms. Creep strength tests of martensitic steel P91 welds and dissimilar welds P91 + Cr-Mo-V steel were performed at temperature of 620°С. The HAZ metal investigation was performed after the thermal welding cycle simulation at the «IMETCKTI» and «Gleeble-3800» units. In addition, numerical modeling was carried out, using the Kachanov-Rabotnov constitutive equations, taking into account the three stages of creep, the influence of complex stress state, changes creep and fracture mechanisms.

Based on the calculation results, the life time of P91 welds depends from the soft layer relative width and the creep rate ratio the base metal vs. the soft layer metal. A quantitative evaluation of the dissimilar welds creep strength is obtained using a sample model with two soft layers. Both have the same properties corresponding to the weakened region HAZ metal of the similar weld. According to the calculation results, the dissimilar welded samples rupture location moves to carbonless zone at lower stress level. Calculation results have good agreement with the experimental data. The obtained dependences are in good agreement with the theoretical and experimental studies of L. M. Kachanov, D. R. Hayhurst, V. N. Zemzin, R. Z. Schron et al. 

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