Grain growth in hot runner materials can be repaired through processes such as normalizing, annealing, or deformation heat treatment, but only if overheating has occurred; otherwise, the material must be scrapped.
1. Feasible Repair Methods
|
Method |
Applicable Conditions |
Key Points of Operation |
|
Normalizing |
Mild grain coarsening (grain size ≤ 4) |
Heat to 30–50°C above Ac₃, hold, then cool in air to promote grain recombination and refinement. |
|
Complete Annealing |
Moderate grain growth |
Slowly heat to austenitizing temperature, hold for a long time, then slowly cool in the furnace to obtain a uniform fine-grained structure. |
|
Deformation Heat Treatment |
High-value mold repair |
Apply 5–15% deformation, then heat and quench to induce recrystallization using deformation energy, refining the grains. |
|
Cyclic Heat Treatment |
Difficult-to-machine steels (e.g., H13) |
Multiple heating (slightly below Ac₁)-cooling cycles to disrupt grain growth equilibrium and inhibit abnormal growth. |
References indicate that for mold steels with overheated structures, grain refinement can be achieved by reheating to above the austenite recrystallization temperature and rapid cooling.
2. Repair Prerequisites and Limitations
Repairable Conditions: Limited to "overheated" states where grain boundary oxidation or localized melting has not occurred;
Irreversible Situations: If "overheating" has occurred, resulting in voids, oxidation, or microcracks at grain boundaries, the material will fail and cannot be repaired; the component must be replaced;
Risk Warning: Commonly used mold steels such as H13 and P20 are sensitive to heating temperatures. Improper handling may exacerbate grain inhomogeneity or introduce new defects.
3. Practical Operation Recommendations
Inspect Before Processing: Confirm the grain size grade (GB/T 6394) through metallographic examination to determine if repair conditions are met;
Control Heating Parameters: Avoid excessively rapid heating rates or prolonged holding times to prevent secondary coarsening;
Prefer Microalloyed Steels: Steels containing Ti, Nb, and V are more likely to pin grain boundaries through precipitates, inhibiting further growth.
