Influence of precipitation on dislocation substructure and creep properties of P91 steel weld joints Dagmar Jandova ´ * and Josef Kasl S ˇ KODA VY ´ ZKUM s.r.o., Tylova 1y57, 316 00, Plzen ˇ , Czech Republic *E-mail: dagmar.jandova@skodavyzkum.cz ABSTRACT Two trial weld joints were prepared using the GTAW and SMAW methods and they underwent creep testing at temperatures between 525 and 625  C. The longest time to rupture was 45,811 h. Two main processes occurred during creep exposures: recovery and precipitation of secondary phases. Slight coarsenings of the M 23 C 6 carbide, precipitation of Laves phase and Z-phase were observed after long tests at high temperatures. Some differences in microstructure and creep failure were found in the individual zones of weldments. After long exposure at temperatures up to 600  C, fractures occurred in the fine-grain heat-affected zone as a result of a low density of fine vanadium nitride and a high density of coarse particles at grain and subgrain boundaries. At 625  C, growth of Laves phase caused a softening of the ferritic matrix and crack propagation in the weld metal. Keywords: creep resistant steels, weld joints, microstructure, precipitation 1. INTRODUCTION Grade P91 is the main representative of creep resistant modified 9Cr – 1Mo steels. It is currently used for manufac- turing boiler and turbine components of fossil fuel power plants, especially steam piping and rotors, but also turbine casings, outlets and so on. As these components have to operate at severe conditions for years, high long-term structural stability is required. Modified 9Cr – 1Mo steels are generally used in conditions after quenching and tempering. Two main processes take place during tempering. First, recovery causes a reduction in the high dislocation density and the formation of sub-grains. Second, precipitation of carbides, nitrides or carbonitrides occurs. The M 3 C carbides are dissolved and the more stable phases arise. Chromium rich M 23 C 6 carbides precipitate at prior austenite grain boundaries, ferrite lath and subgrain boundaries, while vanadium and niobium rich MX nitrides or carbonitrides are spread mainly within subgrains [1,2]. The high creep rupture strength of P91 steel relies on the martensitic transformation hardening and additionally: (i) substructural strengthening of coarse M 23 C 6 carbides, which pin the grain and subgrain boundaries; (ii) precipita- tion strengthening of fine MX particles; and (iii) strength- ening of the solid solution by molybdenum atoms. Changes in size and distribution of secondary phases taking place at creep conditions cause a decrease in the creep rupture strength and are strongly dependent on the temperature and duration of the creep exposure. Both M 23 C 6 carbides and MX nitrides are considered to be stable during long-term creep exposures at temperatures up to 600  C [3]. Significant changes in substructure can occur during loading at 550  C and higher temperatures as a result of precipitation of two undesirable phases: (i) the Fe 2 Mo Laves phase that causes depletion of ferritic matrix by molybdenum; and (ii) the modified Z-phase that causes dissolution of MX particles [4]. These microstructural processes taking place in the creep- exposed weld joints can induce an unexpected decrease in creep strength. The kinetics of precipitation depends on the local chemical composition and the initial structure of the individual zones of the weld joint. Thus, the final structures and also the creep properties of the weld metal (WM), heat affected zones (HAZ) and the base material unaffected by welding (BM) can be different. Consequently, creep failure is concentrated in specific regions of the weld joints and different types of cracking occur [5,6]. The study investigates trial weld joints which underwent long-term creep testing. As received and crept specimens were compared and microstructural changes in WM, HAZ and BM were evaluated. The goal of the study was to elucidate the microstructural processes taking place in different zones of the weld joints which result in specific types of fractures. Partial results have already been presented at several international conferences [7 – 10]. 2. EXPERIMENTAL MATERIAL AND PROCEDURES Two trial weld joints were fabricated from wrought or cast P91 steel using GTAW and SMAW methods. The weld assigned as ‘C’ was produced by joining cast plates 5006150625 mm in size, and the C1 weld was made by MATERIALS AT HIGH TEMPERATURES 27(2) 1–000 1 # 2010 Science Reviews 2000 Ltd doi: 10.3184/096034010X12710730545552 MHT090297 FIRST PROOF