Secondary Operations for Powder Metallurgy Parts: When and How to Use Them

Introduction

Powder metallurgy produces near-net-shape parts with good dimensional consistency and excellent material utilization. However, not every application can be satisfied by the as-sintered condition alone.

Secondary operations extend the capabilities of PM manufacturing. They improve precision where it matters most, enhance mechanical properties for demanding applications, and add protective finishes for harsh environments.

This guide explains the most common secondary operations for PM parts, when each is appropriate, and how to select the right combination for your application.

Sizing and Re-pressing

Sizing is a cold re-pressing operation performed after sintering. The part is placed in a sizing die and compressed to improve dimensions, surface finish, and geometry.

When to Use Sizing

Sizing is the most economical secondary operation and is widely used when:

• Tighter tolerances are needed on critical dimensions
• Gear tooth profiles require correction
• Bore diameters need improved consistency
• Flatness or perpendicularness must improve

Sizing does not remove material. It compresses the surface layer slightly, improving surface density and finish while correcting minor dimensional variations from sintering.

Typical Sizing Results

Re-pressing is a related operation that applies higher pressure to increase local density.

Machining Operations

Despite PM's near-net-shape capability, some features cannot be formed during pressing and must be machined after sintering.

Common Machining Operations

Drilling and Tapping

Cross holes perpendicular to the pressing direction cannot be formed during compaction. These require drilling after sintering. Threads also require tapping or thread rolling.

Turning and Grinding

Cylindrical surfaces requiring tight roundness or very fine surface finish may need turning or grinding.

Machining Considerations for PM Parts

PM parts machine differently from solid materials. The porous structure affects cutting forces, surface finish, and chip formation.

Key considerations include:

• Use sharp cutting tools with positive rake angles
• Maintain adequate cutting speeds to prevent rubbing
• Consider resin impregnation to seal pores before machining critical surfaces

Heat Treatment

Heat treatment improves hardness, strength, and wear resistance of PM steel parts.

Quenching and Tempering

Through-hardening treats the entire part cross-section. It is suitable for materials with sufficient carbon and alloying elements.

Quenching PM parts requires special attention because porosity can absorb quenching oil or water.

Case Hardening

Case hardening creates a hard wear-resistant surface while maintaining a tougher core. Carburizing and carbonitriding are commonly used for PM gears.

Heat Treatment Response by Material

Surface Treatments

Surface treatments modify the outer layer of PM parts to improve corrosion resistance, sealing, or appearance.

Steam Treatment

Steam treatment heats parts in steam at approximately 500 C, creating a protective oxide layer.

Benefits include:

• Improved corrosion resistance
• Pore sealing that prevents fluid penetration
• Increased surface hardness
• Better appearance with a blue-black finish

Oil Impregnation

Oil impregnation fills surface-connected porosity with lubricating oil, creating self-lubricating bearings and bushings.

Typical applications include:

• Sleeve bearings for appliances and motors
• Gearbox bushings
• Sliding components requiring maintenance-free operation

Plating and Coatings

Electroplating with zinc, nickel, or chromium provides corrosion protection and decorative finishes.

When to Use Which Operation

Selecting secondary operations depends on functional requirements, cost targets, and production volume.

Typical Application Patterns

Cost-Benefit Considerations

Secondary operations add cost but enable PM to compete in applications that would otherwise require more expensive manufacturing methods.

Cost Impact Guidelines

Sizing typically adds 10-20% to the base part cost. Heat treatment adds 15-30%. Machining operations vary widely based on complexity.

The economic justification depends on comparing the PM plus secondary operations route against alternatives such as full machining or forging.

Conclusion

Secondary operations transform powder metallurgy from a basic forming process into a versatile manufacturing system capable of producing precision, high-performance components.

Sizing enables dimensional precision. Machining adds features impossible to press. Heat treatment provides mechanical properties competitive with wrought materials. Surface treatments extend service life in demanding environments.

The key to successful PM part design is selecting the right combination of these operations to meet requirements at the lowest total cost.

Need Help Evaluating Your PM Part?

If you are determining which secondary operations your PM part requires, share your drawing and application requirements with our engineering team. We can help you:

• Identify which dimensions truly need sizing or machining
• Select appropriate heat treatment for your material and application
• Recommend surface treatments for your operating environment
• Optimize the total manufacturing cost while meeting all specifications

Contact SinterWorks for a comprehensive review of your powder metallurgy component requirements.