Hardening 4140: Quench and Temper Cycles by Section Size

The standard cycle

4140 is a chromium-molybdenum steel of about 0.40% carbon, designed to be used quenched and tempered:

1. Austenitize at roughly 840–870 °C (about 845 °C is a widely used working point), long enough for the section to equalize.
2. Quench in oil for most sections; polymer or, for simple shapes in small sections, water where the extra severity is needed and cracking risk is managed. Agitation and correct oil temperature matter more than small temperature differences at the furnace.
3. Temper as soon as the part reaches near room temperature — delay invites quench cracks. The tempering temperature is chosen from the target properties, not by habit.

Tempering curve logic

As-quenched 4140 sits around the mid-50s HRC and is too brittle to use. Tempering trades hardness for toughness along a smooth curve, approximately:

• Tempering around 200 °C: still very hard and strong, limited toughness — used rarely, for wear-dominated parts.
• Around 400 °C: high strength with improving toughness; note the general tempered-martensite embrittlement region that many Cr steels show at lower tempering temperatures — critical parts are impact-tested, not assumed.
• Around 550–650 °C: the classic machinery condition — hardness roughly in the high-20s to mid-30s HRC, with the best combination of yield strength, ductility and Charpy toughness for shafts, gears and high-strength bolts.

The exact numbers shift with the actual heat's chemistry and the quench severity, which is why specifications state a hardness or tensile range and let the heat treater pick the tempering temperature to hit it.

Through-hardness by diameter

4140's hardenability is good but finite. As a widely used rule of thumb:

• Sections up to roughly 25–40 mm oil-quench to near-full martensite and temper uniformly.
• In the 50–100 mm range the core increasingly transforms partly to bainite: surface and core properties diverge, and guaranteed properties in the standards fall accordingly.
• Beyond that, treat "4140 through-hardened" with suspicion — ask where the test piece was cut (surface, half-radius or core are all used conventions), or move to a higher-hardenability grade such as 4340.

This is why EN and ASTM standards tabulate 42CrMo4/4140 properties by ruling section, and why a hardness traverse on a sample from the actual bar is worth more than a catalogue value.

Practical notes

Normalize before hardening if the incoming structure is banded or coarse from hot rolling. Machine in the annealed or pre-hardened (quenched and tempered to about 28–32 HRC) condition for the best shop economics — pre-hardened 4140 plate and bar exist precisely so small batches can skip the hardening shop entirely.

Two checks close the loop on any 4140 hardening job: verify hardness over the working surfaces after tempering (a single reading at one end of a long shaft proves little), and where the part is fatigue-critical, confirm the specification's impact requirements were actually tested from the ruling section — 4140's reputation is earned in its through-hardened range, and quietly lost outside it.