Guide
How Much Do Density, Sizing and Roll Densification Matter for Powder Metal Gear Strength?
Powder metal gear strength is the tooth-bending and flank-contact capacity of a sintered steel gear, and it depends mainly on how dense the steel is through the tooth and how deeply the surface has been densified. A drawing that says only PM steel leaves both open; the figures below show how much they move the result.
By Saola Manufacturing engineering team · Published · Updated
On this page
What does the strength of a sintered gear depend on?
Core density, the depth of any surface densification compared with the case depth, the alloy and its hardenability, and the heat-treatment route. Porosity is the underlying reason: pores raise local stress and shorten fatigue life.
The Höganäs review concludes that the key to a high performance sintered steel gear is a surface densification depth equal to the demanded case-hardened depth, combined with a properly high core density for the application (Dizdar, Höganäs, 2010). It also reports that once the core density exceeds about 7.4 to 7.5 g/cm³, case hardening of the sintered steel becomes as simple as case hardening wrought steel.
Below roughly 7.0 to 7.1 g/cm³ the same paper says the pore system is fully connected, so carburizing gas penetrates deeply and soaking times are a fraction of those for wrought parts, and carburizing of unwanted surfaces has to be prevented, which can add masking cost. Density therefore affects not only strength but how the heat treatment behaves.
On definitions, PickPM, Engineering Properties describes density as mass per unit volume in grams per cubic centimeter, says structural PM parts generally fall at 80 percent to above 95 percent relative density, and says the density method is set out in MPIF Standard 42. For the water-displacement method itself, see ASTM B962-23, which covers PM products using Archimedes’ principle.
How much does core density change the numbers?
A great deal. In the Dizdar, Höganäs, 2010 tests on case-hardened gears, going from 7.3 to 7.6 g/cm³ roughly doubled tooth-root strength and pushed pitting resistance from about 400 MPa toward 1000 MPa.
The figures come from one laboratory and a few alloys, so read them as an order of magnitude rather than a design allowable. They are the clearest density-to-strength data in the sources opened for this guide (Dizdar, Höganäs, 2010). The roller figures are for rolling contact fatigue rollers, which rank materials and processes but, per the paper, cannot be transferred directly to gears.
The paper also lists the densities that process routes typically deliver: cold pressing for machinery gears commonly gives 6.9 to 7.1 g/cm³, warm and warm-die pressing give 7.2 to 7.3, and high density pressing or double press and double sinter reach up to 7.7. Elsewhere it says a single press and single sinter with warm compaction can exceed 7.2, double press and double sinter exceeds 7.4, and fully densified flank or root surface of 7.8 can be reached by gear rolling, shot peening or a proprietary densification process.
| Sintered density | Gear pitting resistance (MPa) | Gear tooth-root strength (MPa) | Roller rolling-contact fatigue (MPa) |
|---|---|---|---|
| About 7.0 g/cm³ | not reported | not reported | Close to 1000 |
| 7.3 g/cm³ | 400 | 400 | not reported |
| 7.6 g/cm³ | Approaching 1000 | 800 | 1500 (rollers at 7.6 to 7.7) |
| Wrought reference | Not tabulated here | 1000 (ISO 6336, material quality MQ); 1300 for small module | 2100 (16MnCr5, case depth 1.0 mm) |
What does roll densification add on top of core density?
It moves the pitting and root figures toward or past the wrought reference by densifying the flank and root surface to a stated depth. In the sources, the densified zone needs to match the case depth, and the surface finish after densifying matters.
In the Höganäs gear tests (Dizdar, Höganäs, 2010), shot peening a 7.5 g/cm³ Astaloy CrL gear made a densified layer 0.15 mm deep, but the orange-peel surface caused adhesive wear failure; adding burnishing or gear rolling to smooth the surface lifted pitting resistance to about 1200 MPa, and radial rolling to 0.3 mm gave the same level, with re-sintering adding about 100 MPa (same paper).
For tooth-root strength the paper (Dizdar, Höganäs, 2010) reports that rolling 7.1 to 7.4 g/cm³ gears to a densification depth of 0.3 mm gave 1000 to 1200 MPa in one alloy and 1100 to 1300 MPa in another; a 7.5 g/cm³ gear rolled and then shot-peened after hardening reached up to 1400 MPa; and for roller contact fatigue, rollers densified to deeper than 1.0 mm by radial rolling met the 2100 MPa wrought reference at both 7.1 and 7.5 g/cm³ core density.
An older figure comes from Sanderow, Gear Technology, 1996: it reports, citing a 1982 SAE paper, an improvement in bending fatigue strength of 32 percent and a 3.5 times increase in contact fatigue stress for case-hardened 4600-type PM steel gears after roll densification. The same article lists warm compaction, which was reported to raise tooth break load by 30 percent, as another route. It is 1996 evidence; the 2010 figures above are the better guide to what a plant can reach today.
Rolling also changes accuracy and finish. The Höganäs paper says gear rolling lifts quality to DIN 6 to 7 (8) and gives flank average roughness smoother than 0.2 µm, and that a die can compensate for the teeth distortion that case hardening adds (Dizdar, Höganäs, 2010).
| Route | Result reported | Source |
|---|---|---|
| Shot peening, DD 0.15 mm, 7.5 g/cm³ | Orange-peel surface; adhesive wear failure in pitting test | Höganäs, 2010 |
| Shot peening plus burnishing or gear rolling | Pitting resistance about 1200 MPa | Höganäs, 2010 |
| Radial rolling, DD 0.3 mm | About 1200 MPa pitting; plus about 100 MPa with re-sintering | Höganäs, 2010 |
| Gear rolling, DD 0.3 mm, core 7.1 to 7.4 g/cm³ | Tooth-root 1000 to 1300 MPa depending on alloy | Höganäs, 2010 |
| Roll densification, 4600-type, case hardened | Bending fatigue strength +32 percent; contact fatigue stress 3.5 times | Gear Technology, 1996 |
| Radial rolling deeper than 1.0 mm, rollers | 2100 MPa, meeting the wrought roller reference | Höganäs, 2010 |
What does sizing do, and does it help fatigue life?
Sizing is an accuracy step, and the sources opened here do not credit it with fatigue life. Treat fatigue life as the job of density and surface densification, and ask a supplier for data before assuming sizing contributes.
Sanderow, Gear Technology, 1996 lists sizing, coining, repressing and re-sintering among the optional operations after sintering, and says an AGMA Q8 or Q9 gear needs a sizing or boring operation, which makes sizing a way to hit dimensions and tooth accuracy. That is a different job from densifying the tooth surface to a stated depth, which the sources attribute to rolling, shot peening and similar processes.If a quote proposes sizing in place of rolling, ask what it does to the surface of the tooth and what test data support the fatigue claim. If the answer is a dimensional benefit only, it is an accuracy step and should be priced as one.
How do you write density and surface densification on the drawing?
Write a minimum core density with its test method, a surface-densified zone on the flank and root with a minimum depth and a definition of how depth is measured, and the case depth and hardness. Do not write only PM steel.
The Höganäs paper defines the densified depth as the depth at which the density falls to 98 percent relative density, which is the kind of definition a drawing note needs (Dizdar, Höganäs, 2010). A drawing callout can then read, for example: core density minimum stated in g/cm³, method water displacement; flank and root densified to a stated depth in millimeters, with the depth threshold named; case depth and hardness stated; tooth accuracy class stated with its standard.
PickPM, Engineering Properties places case-depth evaluation of heat-treated PM parts in MPIF Standards 51 and 52 and apparent hardness in MPIF Standard 43; use them as the references for the heat-treat callouts. Our guide on effective case depth covers how a case depth is defined and measured on wrought steel.Add the questions the callout cannot answer: which alloy, which process route achieves the density, how the densified depth is verified, and what lot sampling is used. The route and the verification belong in the RFQ and the quality plan, not in the title block.
How do you prove the gear survives the gearbox life test?
Test sintered samples in the real gearbox or in a back-to-back rig, and sample each lot with tooth-breaking tests. Tables from papers rank routes; they do not replace a life test.
The Höganäs paper measured pitting in a force-square back-to-back test rig and tooth-root strength in a high-frequency resonance pulsator following ISO 6336 and DIN 3990 requirements, and it warns that conversion from a wrought to a sintered gear raises questions because many gears in service never had a load-capacity calculation or experimental verification (Dizdar, Höganäs, 2010).
For lot control, Sanderow, Gear Technology, 1996 says mechanical testing of a statistically representative sample from each production lot is used to demonstrate quality, with tooth torque or tooth breaking strength tests, and that inspection uses the same equipment as machined gears. The same article offers early rules of thumb, such as bending fatigue strength of about 30 percent of tensile strength and a mean fatigue life closer to 32 percent for heat-treated PM steels; treat these as screening numbers from 1996 only.
Saola places powder metal work through a partner plant it manages and does not name, and the quote states the production site and country of origin. Finish machining of bores, hubs and cross features is ordinary CNC turning and CNC milling. For the route decision itself, see our guide on powder metal or machined gears.
Frequently asked
Is 7.2 g/cm3 dense enough for a powder metal gear?
It depends on the load. The Höganäs paper says warm compaction in a single press and sinter can exceed 7.2, but its case-hardened test gears at 7.3 reached about 400 MPa in pitting and tooth-root tests. For gears with real load, the paper points to higher core density or surface densification.
What density makes case hardening behave like wrought steel?
The Höganäs paper puts the point at about 7.4 to 7.5 g/cm3 core density. Below roughly 7.0 to 7.1 the pores are connected, and the carburizing atmosphere penetrates deeply.
Do I need roll densification on every powder metal gear?
No. It is for gears whose load needs it. In the sources, the largest gains came from densifying the flank and root to the case depth, so ask for it where the life test says it is needed.
Can sizing replace roll densification?
The sources opened for this guide treat sizing as an accuracy operation. Ask the supplier for fatigue data before assuming it contributes to fatigue life.
How should I specify powder metal gear density on a drawing?
State a minimum core density with the test method, the surface-densified depth for flank and root with how it is measured, the case depth and hardness, and the tooth accuracy class. Avoid a bare PM steel note.
Sources
- Dizdar, S., High performance sintered steel gears for use in transmissions and machinery: a critical review, Höganäs AB, presented at the International Conference on Gears, Munich, October 2010 (a powder supplier’s paper)
- Sanderow, H. I., Gear Design and Inspection (powder metal gears), Gear Technology, September/October 1996 (old; read for mechanisms, not current limits)
- PickPM, Engineering Properties, MPIF powder metallurgy design center, accessed October 2026 (2026)
- ASTM B962-23, Standard Test Methods for Density of Compacted or Sintered Powder Metallurgy (PM) Products Using Archimedes’ Principle, ASTM International store page (updated 2023)