4140 steel hardness

4140 steel exhibits varying hardness levels under different conditions and treatment states. Generally speaking, the primary scenarios are as follows: As-Quenched Hardness, Quenched and Tempered Hardness, Annealed or Normalized Hardness, and Surface Hardened Hardness.

What is 4140 steel?

AISI 4140 steel is a medium alloy carbon steel. It is used in applications that require good strength, toughness, and moderate hardenability. Compared with 4340 steel, 4140 has better hardenability and strength.

4140 Steel Hardness under different conditions

Let’s return to the topic of 4140 steel hardness.

As-Quenched Hardness

When it undergoes quenching treatment without tempering, its hardness is 58 HRC.

Quenched and Tempered Hardness

Tempering Temperature (℃)	205	260	315	370	425	480	540	595	650
Hardness (HRC)	57	53	50	47	45	41	36	33	29

*Normalized at 870°C (1600°F); oil quenched from 830-855°C (1525-1575°F); tempering for 2 hours.¹

Annealed or Normalized Hardness

4140 alloy steel has a maximum hardness of 197 HB after annealing at 790-845 °C (1450-1550 °F).

4140 steel bars with a diameter smaller than 25 mm (1 inch) can achieve a hardness of 302 HB after normalizing treatment at 870°C (1600°F), while those with a diameter of 100 mm (4 inches) have a hardness of 241 HB.

The overall hardness range in a normalized and tempered state is typically 260-300 HB.

Surface Hardened Hardness

Nitrided 4140 (through hardening) can have a hardness of 83.5-84.5 R15N.

Cold Drawn Condition

Cold-drawn 4140 steel has a hardness of 175-200 HB.

Summary

The hardness of 4140 steel directly relates to its mechanical properties. Hardness is roughly proportional to tensile strength. High hardness contributes to increased tool life and provides excellent wear resistance. It is suitable for statically and dynamically loaded machine components such as gears and crankshafts because of its high hardness and strength after hardening and refinement.

1. ASM International. (1991). ASM Handbook, Volume 4: Heat Treating (p. 293, Table 1). ASM International. ↩︎