Chosen to design and assemble massive components for nuclear industry, the 316 niobium enriched

austenitic stainless steel (also called 316 Nb) suits well this function thanks to its mechanical, heat and

corrosion handling properties. However, these properties might change during steel’s life due to

thermal ageing causing changes within its microstructure. Our purpose is to

understand thermal ageing effects on the material microstructure and properties and to submit a model

predicting the evolution of 316 Nb properties as a function of temperature and time. To do so, based

on Fe-Cr and 316 Nb phase diagrams, we studied the thermal ageing of 316 Nb steel alloys (1%V of

ferrite) and welds (10%V of ferrite) for various temperatures (350, 400, and 450 °C) and ageing time

(from 1 to 10.000 hours). Higher temperatures have been chose to reduce thermal treatment time by

exploiting a kinetic effect of temperature on 316 Nb ageing without modifying reaction mechanisms.

Our results show no effect on steel’s global properties due to austenite stability, but an increase of

ferrite hardness during thermal ageing has been observed. It has been shown that austenite’s crystalline

structure (fcc) grants it a thermal stability, however, ferrite crystalline structure (bcc) favours ironchromium demixion and formation of iron-rich and chromium-rich phases within ferrite.

Observations of thermal ageing effects on ferrite’s microstructure were necessary to understand the

changes caused by the thermal treatment. Analyses have been performed by using different techniques

like Atom Probe Tomography (APT), Differential Scanning Calorimetry (DSC), and Transmission

Electronic Microscopy (TEM). A demixion of alloy’s elements leading to formation of iron-rich (α phase,

bcc structure), chromium-rich (α’ phase, bcc structure), and nickel-rich (fcc structure) phases within the

ferrite have been shown by using APT analyses (cf. Figure), and associated to ferrite’s hardness

increasing.

Microstructural analyses grant information about phases’ volume fraction, composition, and crystalline

structure. So it becomes possible to associate hardness measurements to the volume fractions of the

different phases and to set up an easy way to calculate chromium-rich and nickel-rich particles’ growth

rate depending on temperature. The same methodology has been applied to DSC results, which allowed

us to measure the enthalpy of α’ phase dissolution between 500 and 600 °C.

We tried to employ these results to predict 316 Nb properties’ change during industrial process.

Calculations have been performed using ThermoCalc software to set up our material’s final state, then

by using Dictra moduli to present atoms diffusion between phases and Prisma moduli to calculate

particles’ size and volume fraction depending on time and temperature. These calculations have been

improved by using our experimental results so we could submit a realistic model for 316 Nb thermal

ageing in real conditions.

To resume, we started from mechanical and macroscopic measurements and explained the results

through microstructural changes, and used these results for models, which allow us to certify the 316

Nb can be used during a large period.