Molybdenum and Chromium Depletion at Grain Boundary Precipitates Alloy 926

Alloy 926 is a molybdenum-bearing stainless steel and is used in various industrial fields. This material is used as a cladding material for high diameter pipes in oil and gas industry. One opportunity of production of this material is a thermo-mechanical rolling process. Recent studies have shown that an end rolling temperature of 950 °C with subsequent water cooling has comparable corrosion properties with a solution annealed material. The impact of two different quenching and tempering processes on corrosion properties is presented. The microstructure was evaluated with high resolution SEM, and grain boundary precipitates as well as depletion zones were characterized with TEM-EDX line-scans. Isothermal annealing at 760 °C results in formation of Molybdenum rich, Chromium depleted phases. Their microstructure was not clearly detected by electron diffraction, presumably Laves phase is formed. At 900 °C and long term annealing, two s phases are present. One has a similar composition to the above mentioned Laves phase, the other is Molybdenum enriched and Chromium enriched. Thermo mechanically processed materials contains c phases with similar composition. Depletion zones of these phases are quantitatively characterized with respect to width and chemical composition and their impact on corrosion properties is discussed by use of PREN concept.