20 9 月, 2026 · Blog

Sintered Iron: Grades, Density and Properties

Powder Metallurgy · Material Reference

Sintered Iron: Grades, Density and Properties

A grade-level reference for engineers specifying iron-base PM material: what the MPIF 35 designations actually guarantee, where the density ceiling sits, and when another process wins.

By Zhou Bo, PM Process Engineer · Reviewed by Yao Qiqiang · Updated 20 Sep 2026

Direct answer

Sintered iron is an iron-base powder metallurgy material family, not a single grade. Its density, strength and fatigue life are set by powder composition, compaction pressure, sintering cycle and any secondary treatment. Common MPIF 35 grades run 6.6 to 7.2 g/cm³ as-sintered, with tensile strength from roughly 200 MPa for plain iron to over 1,000 MPa for heat-treated low-alloy PM steel.

What matters most

The trailing number in a grade code is a guarantee, not a description. In FC-0208-50, the minus-50 means minimum 50 ksi (345 MPa) tensile: a floor the supplier certifies to, not the typical value.
Density is the lever, but it is not free. Going from 6.8 to 7.2 g/cm³ [IR] usually needs double-press/double-sinter, warm compaction or copper infiltration, and each of those changes the cost curve.
Porosity hurts fatigue far more than it hurts static strength. Iron-base PM fatigue limits land near 30 to 40% of tensile strength [IR], against roughly 40 to 50% for wrought steel. Pores are stress concentrators; static tests do not see them the same way.
Not every part wants maximum density. Interconnected porosity is the entire operating principle behind oil-impregnated bushings.
Rust is a real specification item on iron-base PM and is missing from most material datasheets. Decide the surface route before the drawing is frozen.
Specify the grade plus density plus required properties. A drawing that reads only “sintered iron” cannot be quoted, inspected or rejected properly.

1. The material, in one paragraph

Iron or iron-alloy powder is blended, compacted in a rigid die to a green shape, then heated below its melting point so the particles bond by solid-state diffusion. What comes out is a consolidated part that still contains a controlled amount of porosity. That last clause is the whole story of this page: the pores are why PM can make a near-net-shape gear in one press cycle, and also why a PM gear does not behave like a machined one.

Iron powder → blending → compaction → green compact → sintering → optional sizing, heat treat, impregnation

For the process itself in depth, see

2. Composition systems and how to read a grade code

Almost every specification argument about “sintered iron” is really an argument about which alloy system was selected. There are five you will meet in practice:

Table 1: Iron-base PM alloy systems [IR]
System	Typical additions	What it buys	Main trade-off
Plain iron	none	Ductility, magnetic softness, lowest cost, easy compaction	Low strength and wear resistance; not hardenable
Carbon steel (Fe-C)	0.3 to 0.9% C	Strength and hardness via pearlite/martensite	Dimensional growth during sintering; lower ductility
Copper steel (Fe-Cu-C)	1 to 4% Cu, 0.3 to 0.9% C	The workhorse structural system; strength with modest cost	Cu swells the part; growth must be absorbed in tool design
Nickel steel (Fe-Ni-C, often diffusion-bonded)	1 to 8% Ni, 0.3 to 0.9% C	Higher toughness and hardenability than Fe-Cu-C	Higher powder cost; segregation risk if not diffusion-alloyed
Low-alloy PM steel (pre-alloyed / diffusion-bonded)	Ni, Mo, Cr, Cu	Through-hardening grades above 900 MPa after quench and temper	Needs controlled-atmosphere hardening; dimensional control is harder

Decoding the MPIF 35 designation

The Metal Powder Industries Federation code is more informative than it looks. Once you can parse it, you can read a competitor’s drawing without asking questions.

Table 2: MPIF 35 designation decoder [IR]
Element	Meaning
`F` prefix	Iron base
`C`	Copper addition; the following two digits are weight % × 100
`N`	Nickel addition; same convention
`D`	Diffusion-bonded: Ni and Cu metallurgically bonded to the iron particles rather than mechanically mixed
`L` / `M`	Pre-alloyed low-alloy steel, e.g. Mo-bearing
Digits 1-2 vs 3-4	`FC-0208` = 2.0% Cu, 0.8% C. First pair is the first alloying element, second pair is carbon × 100
Trailing `-NN`	Minimum tensile strength in ksi, guaranteed. `-50` = 345 MPa floor
`HT` suffix	Heat-treated condition; the minimum is stated for the treated state

Why this matters commercially. When you quote FC-0208-50, you are buying a certified 345 MPa minimum, not “about 400 MPa”. Two suppliers offering the same grade at the same density are offering the same floor. If one is visibly cheaper, the difference is in density, secondary operations or inspection, not in the grade.

ISO 5755 covers the same ground internationally with a different numbering scheme. If you are sourcing across regions, map the grade through the equivalent

3. MPIF 35 property data

This is the table most “sintered iron” pages omit, and it is the only part of the page an engineer actually needs. Values below are representative typical properties for single-press/single-sinter iron-base structural materials, as-sintered unless the grade carries an HT suffix. They are industry reference values from MPIF Standard 35 and its ISO 5755 counterpart, reproduced with the usual caveat: they describe the material at the stated density with the stated treatment. Change either and the numbers move.

Table 3: Typical sintered properties, iron-base PM structural materials [IR] · MPIF Standard 35 / ISO 5755
MPIF grade	Nominal composition	Density g/cm³	Tensile MPa	Yield (0.2%) MPa	Elong. %	Apparent hardness	Fatigue limit MPa, un-notched
`F-0000-20`	Plain iron	6.6	207	124	10	45 HRB	~105
`F-0000-25`	Plain iron	7.0	276	172	14	55 HRB	~140
`FC-0205-40`	2% Cu, 0.5% C	6.8	345	241	3	60 HRB	~145
`FC-0208-50`	2% Cu, 0.8% C	6.8	414	276	2	70 HRB	~170
`FC-0208-60`	2% Cu, 0.8% C	7.1	483	331	2	75 HRB	~195
`FN-0205-45`	2% Ni, 0.5% C	7.0	414	276	3	65 HRB	~180
`FN-0208-55`	2% Ni, 0.8% C	7.0	483	345	2	75 HRB	~200
`FD-0405-65`	4% Ni diffusion-bonded, 1.5% Cu, 0.5% C	7.1	586	448	2	85 HRB	~230
`FLN2-4405-95`	Low-alloy, 4% Ni, 0.5% C, pre-alloyed	7.2	690	552	2	25 HRC	~250
`FLN2-4405-130HT`	Same, quenched and tempered	7.2	1,000	896	<1	40 HRC	~310

Fatigue figures are typical un-notched rotating-bending endurance limits and are the least reproducible column in any PM datasheet: notch the part and they fall sharply. Verify against the licensed edition of the standard and the supplier’s own certification for the actual density and treatment you are buying.

The density ladder

Density is not a free variable; each band corresponds to a different manufacturing route. Theoretical density of pure iron is 7.87 g/cm³. Everything iron-base PM makes sits somewhere below it.

Table 4: Achievable density by route [IR]
Route	Sintered density	% of theoretical	Where it is used
Single press / single sinter	6.2 to 7.0 g/cm³	79 to 89%	General structural parts, the default
Warm compaction	7.2 to 7.4 g/cm³	91 to 94%	Gears and higher-load parts at single-sinter cost
Sinter-hardening	6.9 to 7.2 g/cm³	88 to 91%	Hardened parts without a separate quench
Double press / double sinter	7.2 to 7.5 g/cm³	91 to 95%	High-strength structural, high fatigue duty
Copper infiltration	7.3 to 7.6 g/cm³	93 to 97%	Sealed, denser parts with improved thermal conductivity
Sinter + surface densification (rolling)	7.4 to 7.6 g/cm³ at the flank	94 to 97% local	Gear flanks, where contact fatigue lives

Density inside a single part is not uniform either. Pressing creates a density gradient that follows the compaction geometry, and that gradient is why the fatigue limit of a real component is normally below the coupon value. The relationship is covered in detail on

In production. At JH PM, green and sintered density are checked on production samples with the Archimedes method per ASTM B962, and parts are pressed on 19 automatic presses from 6 to 1,000 t across a 3,000+ t/yr sintering shop with ten furnaces including two continuous mesh-belt lines. [SC]

4. Strength, and why fatigue is the stricter master

Static strength scales with density because pores reduce the load-bearing cross-section. Fatigue scales with density and with pore shape, and it punishes sharp ones. Two consequences follow that catch people out:

The fatigue ratio is lower than for wrought steel. Iron-base PM fatigue limits cluster around 30 to 40% of tensile strength, against 40 to 50% for wrought [IR]. A PM part selected at the same static safety factor as a machined one may still fail in fatigue.
Notching is worse here than there. A pore is already a stress concentrator. A keyway or a sharp fillet on top of it compounds the effect, which is why fillet radii and surface finish matter more on PM parts than the tensile number suggests.

Practical remedy, in order of cost efficiency: raise density if the geometry allows, roll or shot-peen the critically loaded surface, apply a compressive surface treatment, then consider heat treatment. The

5. Corrosion resistance, and what to specify instead

Iron-base PM rusts. It is worth stating plainly because it is the requirement most often discovered late, usually after a shipping container has sat on a dock. Porosity makes it worse than wrought steel in two ways: the open pore network exposes internal surface area, and trapped process fluids or cleaning solutions can weep out afterwards.

Table 5: Surface options for iron-base PM [DG]
Treatment	Effect	Watch for
Oil impregnation	Short-term indoor protection, and the operating principle of self-lubricating bushings	Not a corrosion finish on its own; oil migrates under load
Steam treatment	Magnetite layer plus pore sealing; commonly quoted as several-fold improvement in salt-spray life [IR]	Dimensional growth of a few micrometres; surface hardness up, ductility down
Resin / polymer impregnation	Seals interconnected porosity prior to plating or painting	Temperature limit of the resin
Zinc or zinc-nickel plating	Sacrificial protection	Seal first. Plating bath wicked into open pores bleeds out afterwards as staining and blistering
Black oxide / phosphate	Cosmetic and mild barrier, good paint base	Limited salt-spray performance alone

The rule that prevents most complaints: if the part will be plated, close the porosity by steam or resin before it enters the bath. This is a design decision, not a plating-house decision.

6. Two properties people ask about, and where they belong

Magnetic behaviour

Plain iron and low-alloy iron PM grades are soft-magnetic, which is why they appear in solenoid cores, actuator parts and motor components. But magnetic performance is a material system in its own right: soft magnetic composites and high-purity iron powders are specified separately, with their own loss and permeability data. If magnetics is a functional requirement on your part, this page is the wrong datasheet; specify the magnetic grade explicitly rather than assuming a structural grade will do.

Machinability

Near-net-shape does not mean no machining. Critical bores, threads, sealing faces and tight-tolerance locating surfaces usually still get cut after sintering. The standard industry lever is a manganese-sulphide addition, typically 0.3 to 0.5% [IR], which breaks chips and extends tool life. It is not free: the inclusions reduce ductility and somewhat lower fatigue performance. Add MnS when the machining volume justifies it, not by default.

For what still has to be machined and to what tolerance, and how much stock to leave, the

7. Sintered iron against the alternatives

Three comparisons matter, and they are not interchangeable. The first decides whether the material can take the load; the third decides whether the business case works.

Against cast iron

Table 6: Sintered iron vs cast iron
Factor	Sintered iron	Cast iron
Feedstock	Metal powder	Molten iron
Porosity	Uniform, controlled, predictable	Process-dependent defects: shrink, gas, sand inclusion
Geometry freedom	Constrained by press direction and tooling	High: cores, undercuts, internal cavities
As-formed tolerance	Typically tighter without machining	Generally needs machining on functional surfaces
Strength per unit volume	Lower at matched density; density itself is limited	Higher; fully dense
Economic window	High volume, small-to-medium part, suitable geometry	Broad, including large and low-volume parts

They are not substitutes. Cast iron wins on size and geometry freedom; sintered iron wins on repeatability and material utilisation once volume is high enough to amortise tooling.

Against wrought steel

Table 7: Sintered iron vs wrought steel
Property	Sintered iron	Wrought steel
Density	6.2 to 7.6 g/cm³, residual porosity	Fully dense
Ductility / elongation	Often 1 to 3% at structural strength	Typically 10 to 25% depending on grade
Impact toughness	Limited by porosity; poor at high strength	Substantially higher
Fatigue ratio (limit / UTS)	~30 to 40%	~40 to 50%
Machining content	Low if geometry suits	High

Where the load case is dominated by impact, high-cycle fatigue or a ductility requirement, wrought steel is the honest answer and no amount of density will close the gap economically.

Against machining from bar

This is usually the comparison that decides a project, because it is a pure manufacturing-cost question rather than a materials question:

Bar → turning → milling → drilling → grinding → finished part
Powder → compaction → sintering → limited secondary machining → finished part

The PM route cuts material waste and, for a suitable shape, most of the cycle time. It adds a tooling investment that must be amortised. That single trade is the whole break-even argument, and it is modelled on the

8. What actually drives the price

There is no meaningful price per kilogram for sintered iron, because the part is bought finished. What you pay for decomposes like this:

part cost = material + tooling amortisation + pressing + sintering + secondary operations (sizing, heat treat, impregnation, machining) + inspection

Two of those terms behave non-linearly and dominate the decision:

Tooling amortisation falls roughly with annual volume. At low volume it is the largest line item; at high volume it becomes a rounding error.
Secondary operations rise steeply if the drawing demands what pressing cannot hold. A tolerance that forces 100% post-machining can erase the entire PM advantage even on a shape that presses beautifully.

So the useful break-even question is not “how many parts” but “how much of the final geometry can be formed in the die, and how much has to be cut afterwards”. Volume only matters through that lens. Expecting a full cost comparison without a drawing, an annual volume and a tolerance statement is not realistic, and any supplier who quotes one is guessing.

9. Design rules that decide the material grade

The grade question cannot be answered before the geometry question, because geometry sets the density gradient, and density sets the useable strength. Four checks settle most cases:

Press direction. The part must compact and eject along a practical axis. Cross-holes, undercuts and re-entrant internal features need tooling movements or a secondary operation. See “>minimum wall thickness page.
Density gradient versus load path. The high-density region is not always where the load is. If the critical stress sits in a low-density corner, no grade change fixes it.
Flatness and dimensional behaviour after sintering. Sintering moves the part; carbon content makes it grow, copper makes it grow more. “>tolerance capability need to be settled before the drawing is released, not after first articles.

Working with us. Feasibility feedback on a drawing is returned within 24 to 48 hours. Tooling takes about 20 working days, and first-article parts (T0) follow in roughly 25 working days. Tooling is amortised from 2,000 pieces per part number. [SC]

10. Where sintered iron is used

The pattern is consistent: a small-to-medium part, a shape that presses cleanly, and volume. Beyond that, the grade choice follows the load case.

Table 8: Application map [DG]
Component	Typical grade family	The controlling requirement
Spur and helical gears	FC-0208, FD-0405, FLN2-4405 with surface densification	Tooth-root bending and flank contact fatigue
Sprockets, hubs, pulleys	FC-0205, FC-0208	Strength plus dimensional repeatability at volume
Oil-impregnated bushings	Plain iron, controlled porosity	Pore structure and oil retention, not strength
Cams and rocker components	FC-0208, FN-0208, heat treated	Contact stress and surface hardness
Structural brackets and plates	F-0000, FC-0205	Stiffness and near-net-shape economics

Downstream detail sits on the “>automotive conversion page.

11. When not to use it

This section exists because a supplier who cannot name the losing cases should not be trusted on the winning ones. Take PM off the table when:

Full density is functionally required: pressure-tight sealing, high vacuum, or optical-grade surfaces.
Significant impact toughness or ductility is a design driver.
The geometry cannot be pressed and ejected without undercuts or cross-features that force another process anyway.
Annual volume is too low to carry tooling, and the part will not be revisited.
The tolerance statement forces near-100% post-machining, which removes the cost case regardless of volume.

Alternatives to evaluate in those cases: CNC machining, forging, casting, metal injection moulding for small complex parts, and metal additive manufacturing for low-volume or topology-driven geometry. Compare the finished part, not the feedstock price.

12. What to require at inspection

The useful supplier question is not “do you have quality control” (everyone says yes). It is: which characteristic is controlled at which stage, and what record proves it? For iron-base PM, that normally means dimension, density, hardness and microstructure, with tensile or fatigue testing on coupons only where the design actually depends on those numbers.

Our own control plan runs an IATF 16949:2016 and ISO 9001:2015 system with three Zeiss CMMs for dimensional verification. [SC] The purchasing-side checklist is on the

Frequently asked questions

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References

MPIF Standard 35, Materials Standards for PM Structural Parts, Metal Powder Industries Federation, current edition.
ISO 5755, Sintered metal materials: Specifications.
ASTM B962: Standard Test Methods for Density of Compacted or Sintered Powder Metallurgy (PM) Products Using Archimedes’ Principle.
ASTM B328: Standard Test Method for Density, Oil Content, and Interconnected Porosity of Sintered Metal Structural Parts and Oil-Impregnated Bearings.
ASTM E8/E8M: Standard Test Methods for Tension Testing of Metallic Materials.
ISO 2738: Permeable sintered metal materials: Determination of density, oil content and open porosity.
ISO 4507: Sintered ferrous materials, nitrocarburised: Determination of nitrocarburising depth.
CQI-9, Special Process: Heat Treat System Assessment, AIAG.
MPIF Standard 35 (full data tables) and supplier material certifications: for the values that must be guaranteed on an order.

Send us the drawing

Grade selection and press feasibility are best answered against a real part. Send the drawing and annual volume, and we will come back with a grade recommendation, a density target and an honest read on whether PM is the right route, including when it is not.

Minimum order 2,000 pieces per part number · IATF 16949:2016 · ISO 9001:2015 · non-implant medical parts only

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