How to Ensure Qualified Grain Size in Stainless Steel Production？

According to ASTM A213 / ASME SA213, Grade have H such 304H 321H 316H 347H 309H 310H have Grain Size test requirements. So how to ensure qualified grain size in stainless steel production？

Ensuring grain size qualification requires strict control over the Thermomechanical Process (heat + mechanical work). As a rule, stainless steel does not undergo phase transformation during heating/cooling (like carbon steel does); therefore, recrystallization is the primary method to refine grains .

A. Control of Deformation (Rolling/Forging)

Principle: Deformation stores energy in the grains, creating nucleation sites for new, strain-free grains during subsequent heat treatment.

Sufficient Reduction: Ensure a minimum deformation reduction (e.g., >15-20% in single-pass rolling) to break up coarse structures. Insufficient reduction (critical strain zone of ~7-13%) leads to mixed grains .
High Deformation Amount: Larger deformation ratios (e.g., 80% extrusion) result in finer grains. Higher deformation rates generally refine grains, especially if followed by rapid cooling .

B. Control of Heat Treatment (Solution Annealing)

Principle: Heat treatment relieves stresses and allows recrystallization. However, if too hot or too long, grains grow excessively.

Temperature: Solution treatment temperature must be controlled precisely. For 316H, staying near 1050°C helps homogenize grains, while exceeding this temperature causes rapid grain growth .
Time: Hold times should be optimized (e.g., ~40 minutes for pipes). Excess time coarsens grains without additional benefit .
Cooling Rate:
- Water quenching produces the finest grain structure.
- Air cooling results in moderately larger grains.
- Slow cooling (furnace cooling or delayed quenching) yields the coarsest grains .

C. Microstructure Homogenization

Specifically for austenitic stainless steels prone to “mixed grains” (bands of coarse and fine grains):

Thermomechanical Treatment: Applying a second rolling pass (e.g., 15-20% reduction at 800°C) before the final solution treatment increases in-grain nucleation sites. This eliminates the “millimeter-grade elongated grains” (MEGs) that cause inhomogeneity .
Avoid Critical Strain: Never perform small finishing deformations (less than 10-15%) just before annealing, as this triggers abnormal grain growth .

D. Chemical Composition and Refiners

Inoculants: Adding small amounts of rare earth elements, titanium (Ti), or boron (B) can act as grain refiners by pinning grain boundaries and preventing excessive growth .
Carbon Content: Lower carbon grades (e.g., 316L vs. 316) have a higher risk of “critical strain effects” and require more careful management of forging reductions .

Summary Checklist for Production

Process Parameter	Action to Ensure Fine/Uniform Grains
Hot Working	Apply >15% reduction in final passes. Avoid “light reduction” zones.
Solution Temp	Use lower end of allowed range (e.g., ~1050°C) to prevent growth.
Cooling	Use water quenching whenever possible (fastest cooling rate).
Inspection	Test per ASTM E112. Verify no “mixed grain” structure exists.

By mastering the balance between deformation energy and heat treatment temperature, manufacturers can reliably produce stainless steel with the required ASTM grain size number, ensuring optimal mechanical performance and corrosion resistance.