Powder metallurgy post-processing covers every secondary operation applied after sintering: sizing and coining, impregnation and copper infiltration, heat treatment, deburring, supplemental machining, surface finishing, and final inspection. It is the last gate between a near-net-shape compact and a part that can go straight onto an assembly line.
The furnace door opens, and the parts measure 2% under print. That is expected, not a defect. It is where powder metallurgy post-processing begins. In 2024 a power-tool plant shipped us a batch of oil-impregnated bearings whose bore drifted to IT9 after sintering, and the assembly line stopped for two days. After sizing and impregnation, the same parts held IT7 and yield climbed from 82% to 99%.
If you already run powder metallurgy, you know sintering is only half the story. This guide walks through the seven families of powder metallurgy post-processing, the numbers that define acceptance, and a decision framework for choosing operations from the drawing. Yujiaxin Tech has delivered precision PM parts to automotive, appliance, and power-tool customers since 1998, with post-processing and inspection under one roof.
Key takeaways
- Sizing tightens a Ø25 mm part from IT8-IT9 to IT6-IT7, with compaction typically under 4%.
- Copper infiltration lifts density to about 7.5 g/cm³ and improves strength, hardness, and wear resistance.
- Oil-impregnated bearings can hold 12%-30% oil by volume, which is what makes them self-lubricating.
- Steam treatment at 500-560°C forms a dense Fe₃O₄ film that seals surface pores and resists rust.
- Structural parts reach roughly HRC25+ (HV0.2 450+) after quench and temper.
Post-processing sits at the end of the chain: powder preparation → compaction → sintering → post-processing. The first three steps fix shape and basic density, while post-processing decides whether the part meets the print for accuracy, strength, and surface condition.
The seven families are:
Sintered parts leave the furnace with four built-in problems: shrinkage and distortion, residual porosity, rough surfaces, and local dimensional error. Sintering shrinkage typically runs 1%-3%, and furnace temperature and atmosphere drift leaves variation behind. Conventional press-and-sinter parts carry 5%-15% porosity, which is both the function of an oil-impregnated bearing and the weak point of a structural part. To see how that step drives shrinkage and porosity, revisit the powder metallurgy sintering process.
Post-processing therefore serves three goals: accuracy, performance, and surface plus sealing. To see the full process chain first, read the powder metallurgy (PM) overview; the MPIF secondary operations reference groups these steps into repressing, impregnation, infiltration, heat treatment, and finishing.
Sizing, coining, and repressing are the three most common size-correction operations in powder metallurgy post-processing, often used interchangeably even though they do different jobs.
| Operation | Main purpose | Compaction | Typical parts |
|---|---|---|---|
| Sizing | Correct dimensions, tighten tolerance | Usually under 4% | Oil-impregnated bearings, spur gears |
| Coining | Imprint profile, raise local density | 7%-10% | Structural parts needing tooth accuracy |
| Repressing | Raise density and strength overall | Depends on print | Connecting rods, heavy-duty gears |
Sizing earns its place by narrowing the tolerance band. MPIF's secondary-operations reference and powdermetallurgy.com both report a Ø25 mm part moving from IT8-IT9 to IT6-IT7, and some cases reach ±0.01 mm, an industry-typical range that needs checking against the actual drawing. Sizing compacts under 4%, coining reaches 7%-10%, and with repressing density can approach 7.6 g/cm³.
One sequence trap is worth noting. Sizing introduces work hardening that later furnace heat releases, so dimensions can spring back, and a part needing both tight tolerance and high hardness should be heat treated first, then sized. For the forming step, review the powder metallurgy compaction process.
Porosity is powder metallurgy's double-edged sword. In an oil-impregnated bearing it stores lubricant, while in a pressurized part it becomes a leak path.
Impregnation comes in three media. Oil impregnation serves self-lubrication, and bearings absorb 12%-30% oil by volume. Resin impregnation handles air-tightness and pre-plating sealing, and plastic impregnation suits higher pressure service.
Copper infiltration melts a copper alloy into the pores, lifts density to about 7.5 g/cm³, and improves strength, hardness, and wear resistance. It also raises the tensile strength and elongation of low-alloy sintered steel. A common stack for heavy-duty gears is infiltration, quench and temper, then sizing. Welding as a secondary operation requires a minimum density of 6.8 g/cm³, so if your part will be welded, infiltration is a prerequisite rather than an option.
Powder metallurgy post-processing heat treatment splits into through hardening and case hardening, plus steam treatment, which is unique to PM.
Through hardening relies on quench and temper. Structural parts typically reach HRC25+ (HV0.2 450+) after quench and temper; the exact figure depends on material and process. Case hardening covers carburizing, carbonitriding, and nitriding for gears and cams that need a hard skin and a tough core.
Steam treatment (blackening) is PM's own trick. Parts held in steam at 500-560°C grow a dense Fe₃O₄ oxide film that seals surface pores and improves rust resistance and wear, without changing dimensions.
Deburring is the most underrated step in powder metallurgy post-processing, because it drives assembly safety and cleanliness.
Batch deburring uses vibratory finishing, burnishing, and shot blasting for small and mid-size parts. Single or precision parts go to brushing and electrolytic deburring, which reaches internal bores and cross-holes. Chamfering and edge break matter too, since sharp edges cut operators and mating parts during manual assembly.
Features that PM cannot form directly, such as cross-holes, precision threads, or tight mating faces, call for supplemental machining as part of powder metallurgy post-processing.
Supplemental machining covers drilling, tapping, turning, milling, and grinding. Two PM cautions apply: porosity makes chips intermittent and wears tools faster, and machined faces expose internal pores. If plating follows, seal with resin impregnation first. Surface finishing covers plating (nickel, zinc, chrome), phosphating, and passivation, mainly for corrosion and appearance.
Finishing is not delivery. Acceptance is the last gate.
| Metric | Method | Typical range |
|---|---|---|
| Density | Archimedes method | 6.4-7.6 g/cm³ |
| Hardness | Rockwell HRB | Material-dependent |
| Tensile strength | Tensile test | Material-dependent |
| Dimensional tolerance | Caliper / micrometer | ±0.01-±0.05 mm |
| Geometric tolerance | CMM | Per drawing datum |
Density by the Archimedes method is the most direct read on consolidation. Hardness is usually HRB, moving to HRC for hardened parts. Dimensional and geometric tolerances rely on a CMM, ideal for full checks after sizing. Acceptance criteria can be matched against MPIF Standard 35 material standards, which cover PM structural parts, MIM, self-lubricating bearings, and powder-forged steel.
More post-processing is not better. Every added operation raises cost, lead time, and distortion risk, so selection runs on two lines: the drawing and the service requirement.
For the drawing, match four metrics, tolerance, hardness, surface, and sealing, to operations one by one. For the service requirement, read the part's job in the machine: a bearing needs oil content and size, a heavy-duty gear needs density and hardness, and a gas-tight part needs a leak rate.
The cost-versus-accuracy call is simple. When tolerance is looser than ±0.05 mm and no sealing or hardness is required, deburring plus a rust finish is often enough. When tolerance enters ±0.02 mm, or the part must be gas-tight or reach HRC25+, sizing, impregnation, and heat treatment become mandatory. If you are weighing which operations a specific part needs, request a post-processing solution and our engineers will return a process recommendation from the drawing.
Powder metallurgy post-processing is the end of the process chain and the last mile of accuracy and performance. Sizing handles dimensions; impregnation and infiltration handle sealing and density; heat treatment sets hardness. Deburring protects assembly safety, machining and surface finishing add features and corrosion resistance, and inspection turns all of it into deliverable data. Which operations you skip depends on the drawing, not on habit. To see the whole chain, start with the full chain from powder to delivered part; for the shapes we can deliver, browse Yujiaxin Tech's powder metallurgy parts line.
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