A logistics problem in engineering disguise
The customer assembles cordless drills and impact drivers. The planetary reduction gear in question was already a powder metallurgy part — pressed and sintered in FN-0205, then shipped to an external heat treater for quench and tempering to reach the required 28–34 HRC, then shipped back for final inspection and packing.
That round trip added nine days to every batch and introduced two extra handling stages where parts could be mixed, damaged or delayed. When the heat treater had a furnace down for maintenance, the customer's assembly line felt it two weeks later.
There was also a quality dimension. Hardness across batches ran anywhere from 26 to 36 HRC depending on load position in the external furnace. The drawing tolerance absorbed it, but the customer's gear noise testing showed a correlation between the soft end of the range and early wear complaints in the field.
Why sinter-hardening was the answer
Sinter-hardening uses an alloy designed to transform to martensite during the accelerated cooling stage of the sintering furnace itself. There is no separate austenitising and quench — the part exits the sinter furnace already hard, needing only a temper.
The material change from FN-0205 to FLN2-4405 was the enabler. FLN2-4405 contains nickel, molybdenum and manganese in proportions that give sufficient hardenability at the cooling rates a production sinter furnace can achieve. We had run the grade for three years on other gear programs, so the process window was already characterised.
What had to be verified
A material change on a load-bearing gear is not a paperwork exercise. The validation covered:
- Tooth root strength. Sinter-hardened FLN2-4405 at 7.05 g/cm³ delivers higher tensile strength than quench-and-tempered FN-0205, but we verified it rather than assuming, with single-tooth bending tests on production parts.
- Dimensional change. Sinter-hardening changes the shrink behaviour slightly. We adjusted the die dimensions and confirmed tooth geometry on a gear measuring instrument across three trial lots.
- Hardness uniformity. Nine positions per part, five parts per lot, ten consecutive lots. Spread came in at ±2 HRC around a 31 HRC mean.
- Field-equivalent durability. The customer ran 500-hour accelerated life testing on assembled gearboxes before release.
Results
- Nine days removed from lead time. Parts now go from press to sinter to temper to inspection without leaving the campus.
- Hardness spread tightened from ±5 HRC to ±2 HRC, which eliminated the soft-end population the customer associated with early wear.
- Unit cost down 11%. Modest, because FLN2-4405 powder costs more than FN-0205 — but external heat treatment charges and double freight more than offset it.
- Two handling stages eliminated, with the corresponding drop in transit damage and mix-up risk.
- Traceability improved. One furnace charge record now covers sintering and hardening, rather than a record here and a certificate from a third party.
Where sinter-hardening does not apply
It is worth being clear about the limits, because sinter-hardening gets oversold. It works when the required hardness sits in the 25–40 HRC band and the section thickness is moderate. If you need a carburized case over a tough core — say 58 HRC surface with a ductile centre — sinter-hardening will not get you there and a separate carburizing cycle is still the right answer.
It also assumes the part tolerates the slight dimensional shift that comes with the transformation. On a gear where tooth geometry is compensated in the die, that is manageable. On a part with a tight bore that cannot be machined afterwards, it may not be.
For this customer, the fit was good and the payback was three months. For the next one, we will run the same assessment before recommending it.
Talk to the engineer who ran this program
If your part sits in the same territory — similar size, similar volume, similar frustration with the current process — the fastest route is a drawing and your current unit cost. We will come back with a process recommendation, a tooling estimate and a break-even volume, usually inside 48 hours.