Precipitation behaviour of high-alloyed austenitic steels with 6% molybdenum and its influence on the corrosion resistance

Time-temperature-precipitation diagrams have been established for two steels with 6% Mo, 21% Cr, 25% Ni, 0.14 and 0.19% N (Cronifer 1925 h Mo = 1.4529, UNS N 08925) and for one steel with 6% Mo, 20% Cr, 18% Ni and 0.21% N (UNS S 31 254). The corresponding time-temperature-sensitization diagrams (in accordance to SEP 1877/II) and time-temperature-pitting diagrams (testing in 6% FeCl3 solution) have been evaluated as well. Precipitation of intermetallics occurs rapidly especially in the range between 700 and 10000C. In case of the 18% Ni steel and the 25% Ni/0.14% N steel grain boundaries are covered to a large extent with precipitates after only 15 min at 850 or 9500C. In case of the 25% Ni/0.19% N steel precipitation is considerably slower. Therefore, when welding according to established rules and recommendations, no deterioration of the corrosion resistance in the heat-affected zone is to be expected. Additionally, the steel with 25% Ni has an increased resistance to general corrosion in acids. Notch impact strength of the materials under consideration is increased by the initial precipitation of the intermetallic phases and decreases only after longer times of annealing below the ductility of the solution annealed material. (orig./MM)