What are the repair methods for grain growth in hot runner materials?

Hot runner materials can be repaired using normalising, annealing, or deformation heat treatment . But only if overheating has happened . If not . The material must be destroyed .

1. Repair methods in practice

Methods

Conditions that apply

Key operating points

Normalising

Grain coarsening mild (grain size < 4)

Heat to 30–50°C above Ac₃, hold and then cool in air to facilitate grain recombination and refining.

Full Annealing-

Moderate grain-growth

Heat slowly to austenitizing temperature, hold for a long period and then cool slowly in the furnace to get homogeneous fine grained structure.

Deformation Heat Treatment

Mould repair for high-value

Deform 5 - 15 % Heat and quench - recrystallise with heat Energy of deformation - recrystallise, refine grains

Heat Treatment (Cyclic

Difficult to machine steels (H13 etc.)

Multiple heating (slightly below Ac₁)-cooling cycles to break the grain growth balance and avoid aberrant development.

References show that grain refinement in overheated mould steels can be achieved by heating above the austenite recrystallisation temperature and fast cooling .

2. Repair Conditions and Limitations

Repairable Conditions: Limited to "overheated" conditions when grain boundary oxidation or localised melting has not taken place;

Irreversible Conditions: If "overheating" has caused the formation of voids, oxidation, or microcracks at the grain boundaries, the material will be broken down and will not be reparable; the part must be replaced;

Risk Warning: H13 and P20 are general mould steels and are susceptible to heating temperatures. If not properly handled, it might worsen the grain inhomogeneity or introduce new faults.

3. Operation Recommendations in Practice

Inspect Before Processing Metallographic examination to confirm the grain size grade (GB/T 6394) to see if the conditions for repair are met;

Control Heating Parameters: Avoid fast heating rates or long holding durations to prevent secondary coarsening;

Prefer Microalloyed Steels: Steels containing Ti, Nb, and V are more likely to pin grain boundaries by precipitates and hinder further growth.