Tecnologías para la recuperación por soldadura de álabes de turbinas hidráulicas construidas de acero inoxidable martensítico ASTM A743 CA-6NM (UNS J91540).

Abstract

The main objective of this technological proposal is to develop the welding and inspection procedures for the recovery of blades of hydraulic turbines constructed of martensitic stainless steel ASTM A743 CA-6NM. The method of recovery is by welding, as it allows filling large volumes, joining elements separated by fracture or incorporating new replacement elements. This research studies the control of the severe changes produced in the microstructures, and therefore, in the mechanical properties of the material, due to the important variations of temperature. For the recovery, three welding processes were employed, including combinations SMAW-GMAW-GTAW. Austenitic materials were used (ER309LMo) for the first passes and martensitic (E410NiMo-15 or ER410NiMo) for the forthcoming passes. Preheating and PWHT were carried out for the relief of residual stresses, in addition to temperature control among passes, with an energy input below 1 KJ/mm. The maximum width of the HAZ was 3 mm, similar to the target of 2.94

mm of the equations derived from Rosenthal. Nondestructive tests (NDT) of VT, PT, MT and UT type were performed on all coupons. Subsequently, tensile and lateral guide bend tests were performed, complying with the minimum requirements of the material specification, with minimum tensile strength values of 829 MPa. Hardness and microhardness were evaluated, showing a maximum reduction of 23.5% after PWHT. Finally, the microstructure was evaluated, finding a refinement in the size of the martensite, slight presence of islands of delta ferrite and retained austenite; in addition to the formation of carbides in a lower proportion in the grain boundaries. The Welding Procedure Specifications were scored with the mechanical tests obtaining its respective PQR, validating the quality of the welding procedures. It is recommended to continue the study of the thermal cycle for the dissolution of carbides, fractographic analyses in the tensile specimens to better understand the mechanisms of produced failure and the influence of the austenitic contribution in the resistance to cycle load and energy absorption (impact) of welding coupons.