What is the purpose of intermediate annealing for AFA steel? Eliminate Stress & Restore Ductility

The primary purpose of intermediate annealing is to serve as a critical mechanical "reset" during the manufacturing of Alumina-Forming Austenitic (AFA) steel.

Performed in a high-temperature furnace (typically at 1050 °C for 1 hour), this process is specifically designed to eliminate internal stresses that accumulate during cold rolling. By relieving these stresses, the treatment restores the steel's plasticity, preventing it from cracking or breaking as it undergoes further thickness reduction.

Core Takeaway Cold rolling significantly hardens AFA steel, making it brittle and prone to fracture. Intermediate annealing reverses this "work hardening" by relaxing the material's microstructure, allowing for continued processing without structural failure.

The Mechanics of Structural Restoration

Eliminating Internal Stresses

During cold rolling, the crystal lattice of the steel is physically deformed and compressed. This deformation stores a high amount of internal energy in the form of stress.

If left unchecked, this residual stress acts as a pre-load on the material. Intermediate annealing releases this energy, returning the lattice to a lower-energy, stable state.

Restoring Plasticity

Plasticity refers to the material's ability to undergo permanent deformation without rupturing. As steel is cold-worked, it loses plasticity and becomes rigid.

The high-temperature exposure at 1050 °C softens the material. This restores the ductility required for the steel to withstand subsequent rolling passes without snapping.

Counteracting Work Hardening

The Accumulation of Brittleness

The phenomenon of the material becoming harder and stronger—but significantly more brittle—during deformation is known as work hardening.

While hardness is often desirable in a final product, it is detrimental during the processing stage. Excessive work hardening limits how thin the steel can be rolled before it fails.

Preventing Material Failure

Without intermediate annealing, the work-hardened steel would eventually exceed its fracture limit during rolling.

This would result in edge cracking or catastrophic breakage of the sheet. The annealing step essentially "unlocks" the grain structure, permitting further reduction in thickness safely.

Distinguishing Processing Stages

Intermediate vs. Homogenization

It is vital to distinguish intermediate annealing from homogenization annealing, even though both use high-temperature furnaces.

Homogenization occurs earlier, on the initial ingot, typically at higher temperatures (around 1200 °C for 3 hours). Its goal is to eliminate dendritic segregation and ensure chemical uniformity through thermal diffusion, rather than relieving mechanical stress from rolling.

Intermediate vs. Aging Treatment

Similarly, intermediate annealing is distinct from long-term aging treatments.

Aging treatments are often performed at lower temperatures (e.g., 923 K) to study second-phase precipitation (like NiAl or Laves phases). While aging simulates service conditions, intermediate annealing is strictly a processing step to facilitate fabrication.

Optimizing Your Processing Strategy

To ensure the successful production of AFA steel components, distinct thermal treatments must be applied at the correct stages.

• If your primary focus is preventing cracks during rolling: Prioritize intermediate annealing at 1050 °C to restore plasticity between cold working passes.
• If your primary focus is ingot quality: Ensure proper homogenization at 1200 °C immediately after melting to eliminate chemical segregation.
• If your primary focus is studying service life: Utilize precise isothermal treatments (e.g., 923 K) to simulate microstructural evolution and phase precipitation.

By strategically applying intermediate annealing, you transform a brittle, unworkable intermediate product into a ductile material ready for final shaping.

Summary Table:

Treatment Type	Typical Conditions	Primary Objective	Main Benefit
Intermediate Annealing	1050°C for 1 Hour	Mechanical stress relief	Restores plasticity for further rolling
Homogenization	1200°C for 3 Hours	Chemical uniformity	Eliminates dendritic segregation in ingots
Aging Treatment	~650°C (923 K)	Phase precipitation	Simulates service life & microstructure
Cold Rolling	Ambient Temp	Thickness reduction	Increases hardness but adds brittleness

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References

1. O.M. Velikodny, O.C. Tortika. STRUCTURE AND PROPERTIES OF AFA STEEL FE-NI-CR-AL WITH VARIABLE ALUMINUM CONTENT. DOI: 10.46813/2024-150-062

This article is also based on technical information from Kintek Solution Knowledge Base .