Stategies for the optimization of the behavior of IN718 superalloys at very high temperatures

Because of its resitance to cree, corrosion and fatigue, as well as because of its great weldability, Inconel® 718 nicked-based commercial superalloy is widely used by aerspace industry to manufacture components for turbines and engines, as well as by other sectors and applications where resistance to high temperatures and significant working loads is a mandatory requirement. Nevertheless, its use is limited to 650°C, because above this range microstructural instabilities that produce a quick deterioration of its propperties happen.

With the aim of improving the behavior of the IN718 to higher temperatures, as well as to improve its weldability, an extensive experimental study has been carried out (*) to achieve a sort of critical metallographick propperties directly linked to the behavior of castings in further processing operations (welding) and/or use-life:

• Grain size. The presence of a smaller grain size is considered favorable for weldability and to achieve specific propperties under high temperatures.
• Segregations. The presence of segregations favors unwanted phases in the border of grains, producing adverse effects on weldability and mechanical propperties.
• Presence of phases in terms of quantity and morphollogy. In this case caso, the main phases studied have been those that are common to the IN718, such as Niobium carbides, ɣ”, δ y and Laves phase. Phase δ, due to its morphollogy, reduces the ductility of the alloy and the Laves phases, because of their reduced fusion point, negatively affect to the mechanical propperties and weldability at high temperatures.

SEM images of segregations. The presence of segregations generates adverse effect in weldability and mechanical propperties.

 

Through the development of new heat treatments and the adjustment of chemical composition of the alloy, the study performed has demonstrated that the cooling conditions, the application of new heat treatments developed ad-hoc and the reduction of some residual elements, such as Si, substantially improve the charateristics aimed for and favor to manufacture components that can be processed with all the guarantees trough the whole value chain of the product.

 

Grain size macrography.

New heat treatments and study of the effect of Si on the behavior of Inconel® 718 at high temperatures 

When it comes to heat treatments, it has been analyzed how the cooling rate of molds affects to heat treatments and how a pre-treatment prior to HIP affects the end result. The following fundamental effects have been observed:

• When cooling rate is low, the amount of laves phases increases and a complete disolving is not guaranteed, after conventional HIP and homogenization treatment .
• The introduction of a Pre-HIP treatment (hot isostatic pressing) desingned by AZTERLAN, permits to reduce the presence of Laves fases at the end of conventional treatment. The result is an improved behavior at high temperatures (Gleeble).
• The cooling rate has been determined for this kind of ensayos Gleeble tests (loss of ductility above certain temperatures and temperatures to recover these propperties after heating the sample at high temperatures) in such a way that a greater grain size and the presence of a higher amount of Laves phases confirms the reduction of ductility in these alloys under mechanical testing with heating and cooling cycles.

When it comes to chemical composition of the alloy, the amount of Si has been found to be key regarding mechanical propperties at high temperatures:

• The formation of a higher amount of Laves phases while silicon amount increases, not allowing their complete solubilization after heat treatments.
• The propperties of hot traction and Geeble testing have provided significantly improved results in low silicon alloys, in front of high silicon ones.

AZTERLAN Metallurgy Research Centre has significan capabilities oriented to the optimization of the characteristics of metallic alloys and components to provide a propper response in highly requiring applications. The development of new metallic materials, the optimizazion of manufacturing processes and the configuration of new products and application fields, are some of the working fields of AZTERLAN.
 
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(*) This study has been performed inside of HIPERTURB project oriented to the modification of manufacturing process of turbine components manufactured in IN718 to improve the service life of these parts and reduce potential faillures. pHIPERTURB has been funded by H2020 under Clean Sky JU program (H2020-CS2-CFP04-2016-02).