Mikrostrukturní stabilita a creepová odolnost svarových spojů z ocelí HR3C a Super304H pro moderní bloky fosilních elektráren s USC parametry páry

Abstract

The presented dissertation aims to verify the effect of post-weld heat treatment (PWHT) on the heat-resistant strength of homogeneous welds of austenitic steels HR3C and Super304H. The positive effect of PWHT on the creep strength RuT(W)/t/T and microstructure was experimentally demonstrated by several analytical techniques: TEM, XRD, SEM and SEM-EBSD. The degradation mechanisms and microstructural stability after creep exposure at 650 °C were studied in detail by LOM and fractographic analysis. The structural analysis extends the limited knowledge of minority phases in the welds of the studied copper-containing steels. HR3C/HR3C (Weld 1) and Super304H/HR3C (Weld 2) tubes in the as–welded (AW) condition with time to fracture up to 37 672 h and post-weld heat treatment (PWHT) up to 54 502 h were chosen as experimental material. This long time allowed parametric methods to determine a reliable extrapolation up to 100,000 h. The extrapolation of RuT(W)/t/T was carried out using a series of ruptured short-term creep tests on cross-weld (CW) samples at temperatures of 600, 650 and 700 °C. It was found that after PWHT mode, RuT(W)/105/650°C of weld 2 increased significantly to 101 MPa, which is 20 MPa higher than the creep strength of the same type of weld in the AW condition. The RuT(W)/105/650°C of weld 1 after PWHT was 109 MPa. Intergranular creep rupture always occurred along the fusion boundary (FB) on the Super304H side of the steel. The hardness in the fine–grained FB of heat–affected zone (HAZ) of Super304H steel was always the lowest in the whole weld. This region with the lowest hardness was the preferential location for the development of cavitation damage with subsequent rupture in this area. A vanadium modified Z-phase was detected in weld 2 before and after creep at 650 °C. Current thermodynamic models do not include data on the modified Z-phase and there are a limited number of studies that address this minor phase. The copper concentration in the tetragonal lattice of the modified Z-phase has been found to be as high as ~30 atomic percent in fine particles with dimensions down to 20 nm. The dimensions of the tetragonal lattice found in the modified Z-phase were verified by HR-STEM image analysis. The inter-planar spacing c = 7.593 ± 0.054 Å was larger compared to the original size c = 7.39 Å of the modified Z-phase. The measurement of the lattice parameter a = 2.801 ± 0.037 Å was in good agreement with the available literature data of a = 2.86 Å. A small concentration of copper was already present before creep in the MXI particles in weld 2 in Super304H steel before PWHT. No Cr2N nitride phase particles were found in the Super304H steel, so it is assumed that the stable nitride phase is the Z-phase. A HAZ analysis was performed using SEM-FIB thin metal foil in Super304H steel. Spherical ε-Cu phase particles were identified. These particles were also found in the vicinity of the σ-phase. The particles ε-Cu reached the size of units of nanometers, effectively hindering the movement of dislocations in the matrix and the growth of new grains. The γ' phase was identified in the Ni-alloy type 617–based weld metal. This phase enhanced the creep strength properties of the weld metal during creep tests at all investigated temperatures.