1. General Industrial Range
In most standard applications (e.g. furnace components, heat exchangers, chemical process equipment), the grain size is typically specified as:
ASTM Grain Size 4–8
Coarser end (4–6): better creep and stress-rupture properties at high temperatures
Finer end (6–8): better room-temperature and low-cycle fatigue properties, as well as better formability and toughness
This range is a common compromise between high-temperature strength and fabricability.
2. Special Requirements
For specific applications, different ranges may be required:
Furnace tubes, radiant tubes, reformer tubes
Often specified as ASTM 4–6 (coarser grains) to improve creep resistance and reduce thermal fatigue cracking under long-term high-temperature service.
Thin sheets for complex forming or high-fatigue applications
May be specified as ASTM 6–8 or finer to enhance ductility and fatigue performance.
Aerospace or highly critical components
Sometimes tighter control is needed, e.g. ASTM 5–7, with limits on mixed grain sizes and abnormal grain growth.
3. How Grain Size is Controlled
Solution annealing temperature: higher temperatures (e.g. 1150–1200 °C) promote grain growth; lower temperatures help keep grains finer.
Holding time: longer holding at high temperature increases grain size.
Cooling rate: faster cooling (air cooling) after solution annealing can help suppress excessive grain growth compared to very slow cooling.
Prior thermomechanical processing (forging, rolling): proper reduction and intermediate annealing can refine the grain structure before the final heat treatment.
Unless a customer drawing or standard (e.g. ASTM B168, B166) specifies otherwise, ASTM 4–8 is a safe and common target range for Inconel 601.
