Hot runner grain growth is generally irreversible; once it occurs, it cannot be reversed naturally. However, it can be refined to some extent through heat treatment processes, provided it does not reach the level of "overheating."
1. Analysis of the reversibility of grain growth
Normal grain growth: Occurs when the heating temperature is too high or the holding time is too long, resulting in uniform grain coarsening. In this case, the grains can be refined again through normalizing or annealing, which is reversible damage.
Abnormal grain growth (secondary recrystallization): A few grains suddenly coarsen, engulfing surrounding smaller grains. This process is difficult to reverse through conventional heat treatment and requires deformation treatment (such as forging) to introduce recrystallization nuclei.
Overheating or grain boundary oxidation: If oxidation or localized melting occurs at the grain boundaries, the material has failed and cannot be recovered; it must be scrapped.
References indicate that for molds with overheated structures, reheating to above the austenite recrystallization temperature followed by rapid cooling can refine the grains.
2. Feasible Restoration Treatment Methods
|
Method |
Principle |
Applicable Conditions |
|
Normalizing |
Heating to 30–50°C above Ac₃, air cooling, promotes uniform nucleation. |
Only suitable for mild grain coarsening without overheating. |
|
Complete Annealing |
Slow heating and prolonged holding followed by slow cooling to obtain uniform fine grains. |
Suitable for moderate grain growth. |
|
Deformation Heat Treatment |
Applying a certain amount of deformation, followed by heating and quenching, utilizing deformation energy to induce recrystallization. |
Suitable for the repair of high-value molds. |
|
Cyclic Heat Treatment |
Multiple heating-cooling cycles to disrupt grain growth equilibrium. |
Used for grain control in difficult-to-machine steels. |
Note: Commonly used mold steels such as H13 and P20 are sensitive to heating temperature; improper treatment may exacerbate grain inhomogeneity.
3. Practical Operation Recommendations
Inspect Before Processing: Confirm the grain size grade (GB/T 6394) through metallographic analysis to determine if repair is feasible.
Control Heating Parameters: Avoid repeated high-temperature holding; the heating rate should not be too fast.
Prefer Microalloyed Steel: Steels containing Ti, Nb, and V are more likely to pin grain boundaries through precipitates, inhibiting further growth.
If a stony fracture or grain boundary oxidation has appeared, the material has been overheated and must be scrapped; it must not be used further.
