Alloy 800H is an austenitic stainless steel containing a high content of chromium and nickel together with Al and Ti additions. Its composition makes it particularly resistant to stress corrosion cracking. This is why it is considered as a very good choice for heat exchangers for the nuclear industry, including diffusion welding compact plate heat exchangers [1].

However, diffusion welding of alloy 800H is difficult because of precipitates which form at the interface during the temperature rise. They form a dense plan of brittle precipitates and prevent grain boundaries to cross the interface [2]. To improve the weld mechanical properties it seems necessary to prevent, delay or limit the formation of precipitates. Attempts to dissolve them by applying a heat treatment after diffusion welding has shown limited success [3].

In this work, the precipitation is studied during thermal treatments that simulate the diffusion welding temperature rise. There is a competition between the precipitation of carbides MC and M23C6 [4]. Chromium is widely present in the matrix and can easily form Cr23C6 thanks to C diffusion, while the formation of TiC is more difficult due to lower availability of Ti which is in smaller proportion in the matrix. However, TiC is more stable than Cr23C6 at high temperature. During the temperature rise, successive carbide precipitation and dissolution may occur depending on the heating ramp and temperature. Those phenomena can be modelled using CALPHAD calculations, especially the tool Prisma developed in the Thermo-Calc Software.

The experimental study focuses on the evolution of the surface and the bulk of the steel during different thermal treatments under secondary vacuum. The surfaces are characterised by electron microscopy and Glow Discharge Optical Emission Spectroscopy (GDOES). The surface characterisation is completed by the study of diffusion-welded joints. The experimental study will allow to validate the model and check the relevance of the calculations.

References

[1] R. E. Mizia et al. Report INL/EXT-11-21817, 2011

[2] S. H. Kim et al. Metals, 2020, 10, 480

[3] I. Sah et al. Materials and Design, 2012, 47, 581-589.

[4] W.E. Pratt Master’s Thesis, University of Tennessee, 2015, 1-76.