Size of the grains in a metal, rated with the ASTM E112 number, where each step up roughly doubles the grain count per unit area. For hardening, what counts is prior austenite grain size: fine grain (ASTM 5 or finer) gives better toughness, while overheating or long holds cause grain growth and coarse, brittle martensite. Aluminum-killed fine-grain steels resist growth during long carburizing cycles.
Fracture face is coarse and sparkly, that grain is ASTM 3 or coarser, somebody overheated that batch.
Also heard as
- prior austenite grain size
- ASTM grain size
Related terms
Austenite
The face-centered cubic form of iron that dissolves carbon and must form before steel can be hardened.
Cooked
Shop slang for parts that were overheated or held too long, ending up with coarse grain or burned structure.
Normalizing
Heating steel above its upper critical temperature and cooling it in still air to refine and even out grain.
Carburizing
Diffusing carbon into the surface of low-carbon steel at high heat, then quenching for a hard case.