Effect of Long-Term Aging on the Microstructural Evolution in a P91 Steel

Area of Science:

• Materials Science
• Metallurgy
• Physical Chemistry

Background:

• P91 steel is a crucial material in high-temperature applications.
• Understanding precipitate evolution is vital for predicting material performance and lifetime.
• Previous studies have focused on individual precipitate types, but their interactions require further investigation.

Purpose of the Study:

• To investigate the precipitation and growth mechanisms of Laves phase and the coarsening behaviors of Laves phase, M23C6, and MX carbonitrides in P91 steel.
• To elucidate the interaction and transformation between Laves phase and M23C6 carbides during aging.
• To determine the coarsening rates of different precipitates and their adherence to established models.

Main Methods:

• Long-term aging of P91 steel samples at 625 °C under various conditions.
• Microstructural analysis using techniques such as transmission electron microscopy (TEM) to observe precipitate morphology and distribution.
• Crystallographic analysis to determine orientation relationships between phases.
• Quantitative analysis of precipitate size and spacing to determine coarsening rates.

Main Results:

• Laves phase rapidly grew and engulfed M23C6 carbides during aging at 625 °C, leading to the disappearance of M23C6.
• A new crystallographic orientation relationship between M23C6 and Laves phase was identified: {0001}Laves∥{111}M23C6, <112¯1>Laves∥ <011>M23C6.
• The coarsening behaviors of Laves phase, M23C6, and MX carbonitrides followed the existing ripening model for multicomponent alloys.
• Coarsening rates were determined as approximately 32.2 nm/h1/3 for Laves phase (≥5000 h), 5.3 nm/h1/3 for M23C6, and 0.6 nm/h1/3 for MX.

Conclusions:

• The study reveals a significant interaction where Laves phase growth leads to the dissolution of M23C6 carbides in P91 steel at 625 °C.
• The observed coarsening rates align with theoretical models, providing valuable data for predicting the long-term stability of P91 steel microstructures.
• The newly identified crystallographic relationship offers insights into the nucleation and growth mechanisms of Laves phase in the presence of M23C6.