Sintered Steel

What Is Sintered Steel?

Sintered steel is a steel part made by powder metallurgy (PM): iron or steel powder is blended with alloying elements and lubricant, compacted in a precision die at 400–800 MPa, and then sintered in a controlled-atmosphere furnace at 1,120–1,300°C. During sintering, atomic diffusion bonds the powder particles into a solid metal part — without melting the material.

The result is a near-net-shape steel component, typically 85–95% of theoretical density, with:

• Dimensional repeatability of IT7–IT8 as-sintered (IT5–IT6 after sizing)
• Material utilization above 95%, versus 40–60% for many machined-from-bar routes
• Unit costs that undercut machining, casting, and forging at medium-to-high volumes

Sintered steel is the workhorse of the PM industry: gears, sprockets, structural brackets, bearing housings, synchronizer hubs, and pump components are produced by the hundreds of millions each year.

If you are sourcing sintered steel parts, send your drawing for a DFM review and quote — engineering responds within 24–48 hours.

How Sintered Steel Is Made

1. Powder Preparation

Base iron powder (atomized or reduced) is blended with alloying additions — copper, nickel, graphite (carbon), and sometimes molybdenum — plus 0.5–1% lubricant for die compaction. Powder selection largely determines the final grade's strength and cost.

2. Compaction

The blend is pressed in a rigid die at 400–800 MPa into a "green" compact with 75–85% density. Our press lines cover 6T–400T, supporting multi-level parts with complex geometries formed in a single stroke.

3. Sintering

Green parts travel through continuous mesh-belt furnaces at 1,120–1,300°C in a protective atmosphere (typically N₂/H₂). Particles bond, alloying elements diffuse into the iron matrix, and the part reaches its final mechanical properties — typically 90%+ of theoretical density.

4. Secondary Operations (Optional)

• Sizing/coining — re-pressing for tighter tolerances (down to IT5–IT6)
• Heat treatment / sinter hardening — for wear and load-bearing surfaces
• Steam treatment — seals surface porosity, improves wear and corrosion resistance
• Copper infiltration — near-full density for pressure-tight or high-strength parts
• CNC machining — features that cannot be pressed (threads, cross-holes, undercuts)

See the complete powder metallurgy process and our sintering and heat treatment capabilities.

Common Sintered Steel Grades (MPIF Standard 35)

Values are typical as-sintered ranges per MPIF Standard 35; exact properties depend on density, sintering practice, and heat treatment. Compare grades interactively in our material comparison tool or browse the full materials hub.

Diffusion-Alloyed Grades (FD Series)

Beyond admixed grades, diffusion-alloyed powders (MPIF FD series — e.g., FD-0205 and FD-0405, known commercially under names like Distaloy) bond the alloying elements — nickel, copper, and molybdenum — to each iron powder particle in a diffusion anneal before the powder ever reaches the press. That delivers two advantages over simple premixes: alloy distribution stays uniform through transport and handling (no segregation of the blend), and hardenability improves because the alloying elements are metallurgically attached to the iron rather than admixed as separate particles.

FD grades are the standard route to high-strength, heat-treatable PM gears — particularly where consistent case-hardening response matters across high production volumes. They carry a powder cost premium over admixed FC/FN blends and are typically specified only when the added strength or heat-treatment consistency justifies it. If your drawing calls out a diffusion-alloyed grade by a trade name, send it as-is — we source and qualify the equivalent powder.

How to Read an MPIF Grade Code

Sintered steel grades follow a structured code defined in MPIF Standard 35 and mirrored in ASTM B783. Once you can parse the code, a designation like FN-0405-155HT tells you the alloy family, the nominal composition, and the guaranteed strength in a few characters.

The Prefix: Alloy Family

The four digits encode nominal composition: the first pair is the major alloying element content, the second pair the carbon content. FN-0405 therefore reads "iron + 4% nickel + 0.5% carbon," and F-0008 reads "pure iron + 0.8% carbon."

The Suffix: Guaranteed Strength

The two-digit suffix is a minimum strength guarantee in ksi (10³ psi), and what it guarantees depends on the condition:

• As-sintered grades: the suffix is the minimum yield strength. FN-0405-35 guarantees ≥35 ksi (≈240 MPa) yield as-sintered.
• HT grades: the suffix is the minimum ultimate tensile strength. FN-0405-155HT certifies ≥155 ksi (≈1,070 MPa) tensile after quench-and-temper.

This is why unlabeled strength numbers cause so much confusion in PM sourcing: two parts in "FN-0405" can differ by a factor of three in strength depending on the suffix and condition. Always specify grade and suffix on drawings. The full mapping to JIS and ISO designations is covered in our MPIF/JIS/ISO grade equivalents guide, and the standard's designation logic is explained in depth on the ASTM B783 page.

The Density-Strength Ladder

Density is the single biggest lever on sintered steel properties — tensile strength scales roughly with density squared through the practical range, and fatigue strength follows even more steeply. Buyers specify density not as an afterthought but as a cost-strength decision, because each density step adds process cost.

Three practical notes on the ladder. First, DP-DS doubles the compaction and sintering steps — a part is pressed, pre-sintered, re-pressed, and sintered again — so it is reserved for sections where the strength gain is genuinely needed. Second, warm compaction heats the powder and die to roughly 80–150°C to improve powder compressibility, buying much of the DP-DS density at lower cost. Third, infiltration fills the open porosity with molten copper during sintering, which is why FX steels exceed the 7.4 g/cm³ press-sinter ceiling legitimately — the copper occupies volume that would otherwise be void. The copper-infiltrated steel guide covers that route in detail, and the sinter-hardening FLC-4608 page shows how chemistry can substitute for a separate hardening line.

Key Properties of Sintered Steel

Density and Porosity

Standard sintered steel runs 6.6–7.3 g/cm³ — about 85–93% of wrought steel's 7.85 g/cm³. The residual porosity is a designed characteristic:

• Beneficial for oil-impregnated self-lubricating bearings (18–25% connected porosity stores lubricant)
• Acceptable for most structural parts (strength scales with density)
• A problem for pressure-tight walls — solved by copper infiltration or higher-density compaction routes

Tolerances

• As-sintered: IT7–IT8 on pressing-direction dimensions; IT8–IT9 perpendicular
• After sizing: IT5–IT6 achievable on critical features
• See powder metallurgy tolerance planning for design guidance

Surface Finish

Ra 0.8–3.2 μm as-sintered; Ra 0.2–0.8 μm after sizing or machining. Details in surface finish expectations.

Heat Treatment Options for Sintered Steel

Because porosity changes quench behavior, PM heat treatment is specified with slightly different targets than wrought. The main options, with the hardness bands we quote against:

Two cautions from production experience. Case depth must be set against porosity: open surface porosity lets carburizing gas penetrate deeper than in wrought steel, so case depth is verified on sectioned PM coupons, not assumed from wrought charts. And through-hardened FN grades are capped near HRC 42 because the nickel steels' carbon balance targets toughness, not maximum hardness — parts that need HRC 50+ surfaces get a carburized or induction-hardened case instead. The full decision logic, cycles, and tempering tables are in our heat treatment guide.

Where Sintered Steel Parts Are Used

• Automotive (largest share): transmission gears, synchronizer hubs, sprockets, connecting rods, ABS rings, shock absorber components, seat recliner mechanisms
• Power tools: gearbox planetary gears, bevel gears, clutch components — see power tool gearbox gears
• Home appliances: washing machine gears, compressor parts
• Industrial equipment: hydraulic pump gears, actuators, sprockets and chain components
• Locks and hardware: lock cylinders, latch components

Across these markets, sintered steel wins the same way every time: the geometry is moderately complex, volumes are high, and the part is loaded in ways a 90%-dense steel can carry. It loses when volumes are low (tooling never amortizes), the part is very large and simple (a machined or forged blank is cheaper), or full-density fatigue life is non-negotiable.

Sintered Steel vs Machined, Cast, and Forged Steel

Deep dives: PM vs CNC machining · PM vs casting · PM vs forging · when not to use PM

The cost structure behind that table matters as much as the table itself. PM carries tooling cost up front and very low marginal cost per part, so the break-even against machining arrives early for small, complex parts — often within the first production run — and widens every year of a program. Machining inverts this: no tooling, but every part pays again for the material removed and the spindle time. Casting and forging sit between, with cheaper per-part material but more finishing operations and looser as-made tolerances that downstream machining must absorb.

Design Rules That Make Parts Pressable

Sintered steel rewards designs that respect the compaction press. The rules are mechanical, not stylistic — each one exists because a punch must move, compact, and eject the part.

• Keep features aligned to the pressing axis. Holes, bosses, and steps that run parallel to compaction are formed directly; side holes and undercuts need machining or complex tool motions.
• Limit level count. Each distinct level in the pressing direction needs its own punch movement. Two or three levels are routine; more than five raise tooling cost and tolerance risk.
• Wall thickness 1.5 mm minimum (2 mm preferred) — thin walls crack during ejection and distort during sintering.
• Radii over sharp corners — corners concentrate ejection stress and are where sintering cracks start.
• Threads, cross-holes, and knife edges are post-sinter operations. Design them so one clamping setup finishes them.

A DFM review against these rules is free and happens before quoting — submit a drawing and we will flag the features that add cost before you commit.

Quality Control and Material Certification

Sintered steel is certified against the MPIF Standard 35 property tables using standardized test coupons sintered with production parts. A typical certification package includes:

• Chemical analysis of the sintered part (C, Cu, Ni, Mo against grade limits)
• Density by Archimedes method against the specified band
• Hardness in the specified condition, with case depth where carburized
• Dimensional report by CMM on critical features
• Metallography where microstructure-sensitive properties are contractual

For automotive programs this plugs into PPAP Level 3 documentation, and our inspection lab runs the full dimensional and metallurgical program in-house.

Sourcing Sintered Steel Parts

SinterWorks produces sintered steel components on 36 presses (6T–400T) with 4 continuous sintering lines and 2,600T annual capacity. Every program includes:

• DFM review against PM design rules before tooling
• Material certification per MPIF Standard 35
• CMM dimensional reports, full inspection lab
• PPAP Level 3 documentation for automotive programs

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