A gear tooth has to satisfy two requirements that pull in opposite directions, and the entire logic of gear steel selection follows from that conflict.
The conflict
The tooth flank is in rolling and sliding contact with a mating tooth for millions of cycles. It needs to be hard, or it will pit and wear. The tooth root is in bending fatigue and takes shock loading. It needs to be tough, or it will crack.
Hardness and toughness trade against each other in a uniformly heat treated steel. Through-harden a gear to resist wear on the flank and the root becomes brittle enough to fracture. Leave it soft enough to be tough and the flank pits out.
Case hardening resolves it
The answer is not to make the whole tooth the same. A low carbon alloy steel is machined soft, then carburised: carbon is diffused into the surface at 880–980 °C, and the part is quenched. The carbon-rich case transforms to hard martensite at 58–62 HRC; the low carbon core does not, and stays tough.
One part, two structures, each doing the job that region needs. This is also why base carbon must be low — you want carbon at the surface to come from the furnace atmosphere, under your control, not from the steel.
Why alloy, not plain carbon
Chromium and manganese raise hardenability — the depth to which martensite forms during quenching. On anything beyond a small gear, plain carbon steel simply does not harden deep enough: the case forms but the core beneath never develops proper strength. The Mn-Cr family exists to make case hardening work at real section sizes.
Choosing between the three
- 16MnCr5 — the general purpose carburising alloy. Correct for moderately loaded gears and gear blanks, and for job-shop work where the heat treatment cycle is set per batch.
- 16MnCr5H — the same steel with a certified Jominy hardenability band. Correct for volume production running a fixed carburising cycle, and effectively mandatory where an IATF 16949 customer specifies it.
- 20MnCr5 — higher carbon and chromium, core tensile 1000–1300 MPa against 780–1080 MPa. Correct for heavily loaded teeth and larger sections where 16MnCr5 would leave a soft centre.
What decides the outcome besides grade
Grade selection is necessary but not sufficient. Three process points matter as much.
- Supply condition. The bar must arrive soft annealed — ≤ 207 HB for 16MnCr5 — because all machining happens before carburising.
- Distortion control. Carburising and quenching move the part. Gear blanks are routinely machined with grinding stock and finish ground after heat treatment.
- Batch consistency. A furnace cycle proved on one lot must remain valid on the next, which is precisely the guarantee an H grade provides and a standard grade does not.
And underlying all of it, documentation: for automotive work, being able to show which heat a batch of gears came from is part of the specification, not an administrative extra.
Written by the technical team at Kalika Enterprises, Waluj MIDC. Guidance here is general; the values that govern your part are those on the test certificate supplied with your consignment.