# Microstructural Evolution Governing the Creep Resistance of Grade 92 Steel Under Wide-Temperature Heat Treatment: From Ferrite Recovery to Dynamic Precipitation Strengthening

## TL;DR

This study explores how heat treatment affects the strength and durability of Grade 92 steel, finding that 950 °C treatment optimizes its performance.

## Contribution

The study reveals the microstructural mechanisms behind the non-monotonic behavior of T92 steel under different heat treatments.

## Key findings

- Hardness and strength of T92 steel show a non-monotonic trend between 760–1000 °C.
- Creep life is lowest at 850 °C and recovers at 950 °C due to dynamic precipitation of M23C6.
- A 1000 °C treatment results in high hardness but poor creep life due to carbide dissolution.

## Abstract

Hardness and strength show a non-monotonic evolution across 760–1000 °C.Creep life hits a minimum at 850 °C and recovers to the AR level at 950 °C.Decoupling occurs as 1000 °C has highest hardness but 1/4 the creep life of 950 °C sample.Rejuvenation at 950 °C restores integrity of damaged T92 steel via dynamic precipitation.

Hardness and strength show a non-monotonic evolution across 760–1000 °C.

Creep life hits a minimum at 850 °C and recovers to the AR level at 950 °C.

Decoupling occurs as 1000 °C has highest hardness but 1/4 the creep life of 950 °C sample.

Rejuvenation at 950 °C restores integrity of damaged T92 steel via dynamic precipitation.

Thermal excursions during post-weld heat treatment (PWHT) and on-site fabrication frequently compromise the integrity of Grade 92 steel. While hardness fluctuations are documented, the correlation between initial properties and long-term creep stability remains controversial. This study aims to evaluate the relationship between thermal history and subsequent creep performance. Heat treatments of T92 steel across a wide temperature range (760–1000 °C) were performed, followed by creep tests at 600 °C/130 MPa and microstructural characterization. Results reveal a non-monotonic evolution of hardness and strength, reaching a minimum at 850 °C due to martensitic lath recovery into ferrite, but nearly doubling the as-received (AR) values above 900 °C due to fresh martensite formation. Creep life drops to a minimum at 850 °C and recovers to the AR level at 950 °C. A significant “decoupling” occurs at 1000 °C, where the sample possesses the highest hardness but only exhibits one-fourth the life of the 950 °C sample. Superior performance stems from the retained M23C6 and its dynamic precipitation, which pins dislocations to form micro-lath structures. Conversely, 1000 °C facilitates full carbide dissolution, accelerating dislocation recovery. These findings emphasize precise PWHT control and demonstrate that a 950 °C rejuvenation treatment can restore over-tempered or damaged components.