Pellet Die Surface Pitting: Causes, Inspection and Repair
News 2026-10-30
A pellet die surface that is pitting has lost small areas of material from the working surface, and the loss is different from the even wear that a die shows at the end of its life. A severe abrasion or a corrosion can produce a pitting that appears together with the wear, and the surface inspection and the die record separate the two. The pit is a local removal, and it changes the compression at that point, the pellet quality, and the die life. The pit changes the hole geometry and the compression, and the effect appears in the pellet density, the durability, and the fines, with the die supplier documentation and the pellet test as the reference.
This article is written for plant operators and maintenance teams who have to decide why the die surface is pitting, whether the die can be used, and what to correct before a replacement is installed. It covers the pitting types, the causes, the inspection, the repair or replacement decision, and what to confirm with the die supplier.
Pitting Type and What It Points To
The pit shape and the surface around it point to the cause.
| What the surface shows | Likely cause | What to check |
|---|---|---|
| Small, scattered pits with a dark or rust-coloured base | Corrosion from moisture or a corrosive material | The moisture, the storage, and the material |
| Pits concentrated at the hole entrance | Abrasion from a hard contaminant, or a chipped hole edge | The die hole condition, the magnet, and the screen |
| Pits with a rough, torn edge | Abrasion from a foreign object, or a spalled surface | The die chamber, the roller, and the material |
| Pits with a discoloured or heat-tinted surface | Heat damage from a dry die or an overheating event | The die temperature, the feeder, and the cooling |
| Pits along the die surface, with a polished area | Wear combined with a roller slip | The roller surface, the moisture, and the die hours |
| Pits at the die edge or the flange | Handling damage or an impact | The die handling and the installation |
Small scattered pits with a dark base point to corrosion. Pits at the hole entrance point to abrasion or a chipped edge. Pits with a torn edge point to a foreign object or a spall. Pits with a heat tint point to a heat event. Pits with a polished area point to wear with a roller slip. Pits at the edge point to handling damage.
The pit shape and the surface colour are read together, and the moisture record, the contamination record, and the temperature record are the reference. The inspection result and the die supplier documentation decide the cause.
Causes of Die Surface Pitting
Corrosion from Moisture or a Corrosive Material
Why it pits: The die surface is exposed to moisture during storage, washdown, or a wet material, and the moisture produces a local corrosion cell. The corrosion removes small areas of the surface, and the pit forms. A material with a corrosive component, such as a high chloride content, increases the risk, and the chloride or the other corrosive component should be measured in the material analysis rather than assumed.
What to check: The moisture at the mill inlet, the storage condition, the washdown practice, and the material analysis.
What to do: Correct the storage and the washdown, and use a material that is within the die specification. A die with corrosion pits that have changed the surface should be inspected for the remaining life, and the replacement should be considered.
Abrasion from a Hard Contaminant
Why it pits: A hard particle, such as sand or metal, is caught between the roller and the die, and it removes a small area of the die surface. The pit is local, and the edge is rough or torn. The contaminant comes from the material, and the magnet inspection, the screen inspection, and the material record are the first checks.
What to check: The die hole condition, the magnet, the screen, and the material contamination.
What to do: Correct the material cleaning, and replace a die with pits that have changed the hole geometry or the surface.
Heat Damage from a Dry Die or an Overheating Event
Why it pits: A die that runs dry or overheats expands and contracts, and the surface can spall or pit. The pit appears when the die temperature exceeds the thermal fatigue limit of the die material, and the limit should come from the machine manual and the die supplier documentation. The pit is accompanied by a discoloured or heat-tinted surface.
What to check: The die temperature record, the feeder delivery, the moisture, and the cooling.
What to do: Correct the feeder and the cooling, and replace a die with heat pits that have changed the surface.
Wear Combined with a Roller Slip
Why it pits: A roller that slips on the die surface removes the surface unevenly, and the area that slips can pit. The pit is accompanied by a polished area, and the roller surface condition, the moisture record, and the die hours are the first checks.
What to check: The roller surface, the moisture, and the die hours.
What to do: Correct the roller and the moisture, and replace a die with pits that have changed the surface.
How to Inspect a Pitted Die Safely
The die should be inspected with the machine stopped and the power isolated.
- Stop the feed, then the main motor, and lock out the power.
- Allow the die to cool according to the machine manual before the inspection.
- Clean the die surface with the method stated in the manual.
- Inspect the surface with a light and a magnifier, and record the pit location, size, and pattern.
- Inspect the die hole entrance for a chip, a blockage, or a deformation.
- Check the roller surface and the clearance at the same time.
- Record the die hours, the material, the moisture, and the temperature at the time of the pitting.
The pit location, the size, and the pattern are recorded, and the surface around the pit is described, so that the cause can be identified. The pit depth is not usually measured in the field, and the decision is made on the pit distribution, the hole condition, and the pellet quality. Where a depth measurement is needed, a depth gauge, a replica, or a 3D scan can be used by a qualified person, and the result should be compared with the die supplier limit.
When to Resurface and When to Replace
The decision depends on the pit depth, the pit distribution, and the effect on the pellet.
- Resurface only when the die manufacturer permits it, and the pit depth is within the manufacturer limit.
- Replace the die when the pits have changed the hole geometry or the working surface.
- Replace the die when the pits are concentrated at the hole entrance and the compression has changed.
- Replace the die when the pits are accompanied by a heat tint and a surface spall.
- Replace the die when the pellet quality has fallen and the pits are beyond the surface limit.
A die that is resurfaced without the manufacturer procedure can lose the compression surface and the hole geometry, and the die may produce an off-spec pellet. The resurfacing should be confirmed with the die supplier, not done on site. After a resurfacing, the compression ratio and the hole geometry should be verified against the die specification, and the verification method should be confirmed with the die supplier.
What to Confirm with the Die Supplier
The replacement die is specified against the material and the machine.
| Item to confirm | What to provide |
|---|---|
| Material | The type, the moisture, the ash content, and the corrosive component |
| Die specification | The hole diameter, the effective working length, and the compression ratio definition |
| Die material | The alloy and the heat treatment proposed for the material |
| Machine and roller | The machine model and the roller configuration |
| Pitting record | The pit location, size, pattern, and the surface around it |
| Operating record | The moisture, the temperature, and the contamination |
The die material affects the pitting sensitivity. A die with a higher corrosion resistance resists the moisture pitting, and a die with a higher hardness resists the abrasion pitting, and the alloy and the heat treatment should be selected against the material and the operating condition. The die supplier documentation is the reference for the alloy and the recommended application.
A die life figure should state the material, the moisture, the compression ratio, and the operating conditions it assumes. Where the figure does not state those conditions, it is a reference rather than a commitment, and the actual die record is the basis for the next selection.
Frequently Asked Questions
Can I keep running a die with surface pits?
A die with pits that have changed the hole geometry or the working surface should be replaced, and a die with pits within the manufacturer limit can be monitored. The pit location and the size should be recorded and compared at the next inspection.
When should I replace the die instead of resurfacing?
Replace the die when the pits have changed the hole geometry or the working surface, when the pits are concentrated at the hole entrance, when the pits are accompanied by a heat tint or a spall, or when the pellet quality has fallen. Resurface only when the manufacturer permits it and the pit depth is within the limit.
Can a pitted die be resurfaced?
A die can be resurfaced only when the die manufacturer permits it, and the pit depth is within the manufacturer limit. The resurfacing should follow the manufacturer procedure, and a die that is resurfaced without the procedure can lose the compression surface and the hole geometry.
Why did the die surface pit?
The pit comes from corrosion, abrasion, heat damage, or a roller slip. The pit shape and the surface around it separate the causes, and the moisture, the contamination, the temperature, and the roller condition are the first checks.
How is a pitted die inspected?
The die is inspected with the machine stopped and the power isolated, after the die has cooled. The surface is cleaned and inspected with a light and a magnifier, and the pit location, size, and pattern are recorded, together with the hole condition and the roller condition.
Request Die Support
Send the material type and moisture, the die specification, the die hours, the pit location and pattern, the temperature record, the roller condition, and the machine model. A die review can then be prepared against the material and the machine rather than against a general catalogue.
Pit inspection, resurfacing limits, die life, and acceptance criteria in this article must be confirmed against the die supplier documentation, the machine manual, and the production record for the same material. This article does not provide universal values for these items.
