Microstructure Control and Correlation to Creep Properties in Grade 91 Steel Weldment After Thermo-Mechanical Treatments and an Fe-30Cr-3Al Alloy Strengthened by Fe2Nb Laves Phase

Type IV cracking in weldments of steel pipes after creep deformation is a concern in modern fossil-fueled power plants. Two possible methods for minimizing or eliminating Type IV cracking will be discussed. The first method alters the initial microstructure of typical Grade 91 steel base metal before welding, while the second provides baseline microstructure characteristics and creep performance of a new alloy that is strengthened by the intermetallic Fe2Nb Laves phase. The initial microstructure of the Grade 91 steel can be controlled by Thermo-Mechanical Treatments, which aids in precipitation of fine (5-10 nm) MX particles in austenite before transformation to martensite on cooling. Results also reveal that MX is shown to have a reduced volume percent and size of M23C6 carbide formation upon tempering. A strain rate model has been created based on particle sizes and interparticle spacing to predict creep deformation strain rates from the microstructure. The model reveals that the minimum creep rate can be altered with a change in precipitate distribution, and may have competing creep mechanisms in the heterogeneous microstructure of weldments. The model indicates that Type IV cracking may not have a solution based only on precipitation control. Based on this idea a new alloy is being developed which precipitates a thermally stable Fe2Nb Laves phase with the average size between 80-150 nm. The alloy has a ferritic matrix and general composition of Fe-30Cr-3Al-.2Si-xNb. The initial base metal creep results indicate the new alloy has similar creep properties to the Grade 92 steel at 700 ºC. The microstructure characterization has revealed that a precipitate free zone (PFZ) forms parallel to all the grain boundaries with larger (200-300 nm) Laves phase decorating the grain boundary. Utilizing the strain rate model, a Grain Boundary Sliding type mechanism was identified as the deformation mechanism. In these alloys, it will be discussed how the areal fraction of grain boundary Laves phase and the width of the PFZ controls the cavitation nucleation and eventual grain boundary ductile failure. This reveals the competition between the intragranular creep resistance of the Laves phase and the intergranular deformation along the grain boundaries.