Session: 18-05 Metallurgy, Coating and Repair I

Paper Number: 124014

124014 - Rejuvenation Heat Treatment Development to Extend the Service Life of René N4 Single Crystal Gas Turbine Blades 

Single crystal superalloys are widely used for gas turbine blades exposed to high stresses, high temperature and aggressive environments. When these components reach their serviceable limit, they must be replaced with new ones, with an important impact in terms of maintenance costs. Significant savings can be achieved through the application of a rejuvenation heat treatment, able to extend the life of a serviced turbine blade beyond its original design intent.

The success of such a technology for SX-superalloys is related to the effective restoration of the original γ/γ’ microstructure, which is linked to the material mechanical properties, without impairing the original single crystal structure. One challenge in designing a rejuvenation heat treatment for single crystal superalloys is the risk of recrystallization, which must be kept under control.

This work is focused on the development of a rejuvenation heat treatment for René N4 alloy. Different rejuvenation cycles have been explored through an in-depth material characterization, using samples taken from both new material and real serviced gas turbine blades. The rejuvenation cycles investigated consist in a stress relief treatment, followed by solution annealing and aging steps. The stress relief is at a temperature lower than the solution annealing and has been introduced specifically to retard the onset of recrystallization by decreasing its driving force. Several solution annealing temperatures have been investigated in a window ranging from the theoretical γ’ solvus temperature (simulated by Thermo-Calc) and the original solution annealing temperature used during blades manufacturing.

The assessment of the candidate rejuvenation heat treatments effectiveness is based on the evaluation of microstructure γ/γ’ restoration, the presence of recrystallized grains and the effect on mechanical properties.

Microstructure assessment has been performed through SEM images comparison between reference material, heavily altered specimens taken in the hottest airfoil section and rejuvenated material.

Assessment of recrystallization due to residual stresses coming from original manufacturing processes has been done on shot-peened dovetail sections submitted to the rejuvenation cycles. Higher thickness of recrystallized layer corresponds to higher solution temperature.

To evaluate recrystallization due to service, samples with imposed plastic deformations have been used. On one set of samples these deformations have been induced by hardness indenter, to simulate the damage due to foreign object impact on airfoil, while to evaluate the relationship between bulk material deformation and recrystallization, interrupted tensile tests at different strain levels have been considered.

Next steps of the research activity will be focused towards the optimization of the rejuvenation heat treatment cycle and the material characterization in terms of mechanical properties (creep and fatigue), also leveraging interrupted creep tests on pre-crept and rejuvenated samples.

Presenting Author: Cristina Motta Politecnico di Milano

Presenting Author Biography: I have a Master degree in Materials Engineering and Nanotechnology and now I am a PhD Student in Mechanical Engineering at Politecnico di Milano, focusing on damage models and repair of Nickel based superalloys.

Authors:

Cristina Motta Politecnico di Milano
Francesco Mastromatteo Baker Hughes - Nuovo Pignone Tecnologie
Elisabetta Gariboldi Politecnico di Milano
Filippo Cappuccini Baker Hughes - Nuovo Pignone Tecnologie