Metallurgical Investigation of Cold-formed Fillet Pieces Made of Metastable Austenitic Stainless Steel, for the Turbine Exhaust Casing of a Heavy-Duty Gas Turbine Engine

With a novel class of heavy-duty gas turbine engines, turbine exhaust temperatures are expected to further increase. There are concerns that the propensity to cracking in service of the welded turbine exhaust casing (TEC) liners might also increase. Cold-formed “collars”, featuring large stress-reducing fillet radii, were designed to decrease thermal stresses in the welds, where the struts of the turbine bearing casing are aerodynamically clad by liner plates. Such collars were received from a supplier in the course of a product and process qualification (PPQ). The results of the metallurgical investigation ordered by an internal client are the subject of this contribution. It was found that deformation martensite was present in the subject cold-formed fillet pieces, despite a pre-heat of 150 °C and a post-fabrication 920 °C annealing process. However, the amount of deformation martensite found, at below 5 % volume fraction, was considered not to be detrimental for engine operation.