# Microstructure Evolution of Super304H Austenitic Steel During Long-Term Creep at 700 °C

## TL;DR

This paper studies how Super304H steel changes under high heat and stress, showing how tiny structures form and affect the material's strength.

## Contribution

The study reveals how different stress levels influence the formation of precipitates and their impact on grain boundary strength during long-term creep.

## Key findings

- High-temperature creep leads to the formation of M23C6, MX carbide, σ phase, Cu-rich phase, and Z phase precipitates.
- Rapid σ phase precipitation at high stress weakens grain boundaries, while MX precipitates under low stress improve stability.
- Grain boundary strength decline is linked to low-angle boundary changes, even without cavity formation after 20,000 hours.

## Abstract

Creep tests of Super304H austenitic steel were carried out at 700 °C under different stresses. The samples were characterized by an optical microscope (OM), scanning electron microscope (SEM) and a transmission electron microscope (TEM). The results show that high-temperature creep promotes the precipitation of the M23C6, secondary MX carbide, σ phase, Cu-rich phase and Z phase. These fine precipitates improve both the matrix and grain boundary strength. Furthermore, the precipitation sequence of these second phases relates to the stress level during elevated temperature testing. The rapid precipitation of the σ phase is also observed at high stress levels, whereby fast growth at triangle boundaries notably deteriorates grain boundary strength. Conversely, the presence of dispersed fine MX precipitates under low-stress conditions during long-term creep should contribute significantly to microstructure stability and long-term creep strength. Despite the absence of homogeneous cavities observed on the grain boundary when subjected to creep for over 20,000 h, the decrease in grain boundary strength was explained from another aspect by analyzing the change in low angle grain boundary during creep.