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.

1. Hardness in Annealed Condition

Annealing 4140 is done to improve its machinability and formability before processing. The maximum hardness for 4140 in the annealed condition can be up to 197 HB (Brinell Hardness). Speeds for turning 4140 are given for materials with hardness ranges of 175-200 HB and 200-250 HB. Its machinability rating is reported as 57 in the annealed condition (compared to 100% for cold-rolled B1112 steel).

2. Hardness in Normalized Condition

Normalizing involves heating the steel to a specific temperature and then air cooling it, which refines the grain structure and can provide more uniform properties than hot rolling.

Here is the bar diameter and surface hardness data presented in a table format:¹

Diameter (mm)	Diameter (in.)	Surface Hardness (HB)
13	0.5	302
25	1	302
50	2	285
100	4	241

3. Hardness in Quenched and Tempered Condition

Quenching and tempering are the primary heat treatments used to achieve high strength and toughness in 4140 steel, producing a microstructure primarily of tempered martensite. This process offers a wide range of achievable hardness levels, with higher tempering temperatures generally leading to lower hardness but increased toughness.

The hardness of 4140 after quenching and tempering is generally 50-52 HRC.

The hardness table for 4140 after specific quenching and tempering is as follows:

Specific Tempering Temperatures (Oil Quenched from 845 °C / 1550 °F)

Tempering Temperature (°C)	Tempering Temperature (°F)	Hardness (HB)
205	400	578
260	500	534
315	600	495
370	700	461
425	800	429
480	900	388
540	1000	341
595	1100	311
650	1200	277
705	1300	235

Effect of Section Size (Oil Quenched from 845 °C / 1550 °F and Tempered at 540 °C / 1000 °F):

Diameter (mm)	Diameter (in.)	Surface Hardness (HB)
25	1	335
50	2	202
75	3	293

4. Hardness in Surface Hardened Conditions (Induction Hardened, Nitrided)

4140 steel is well-suited for surface hardening treatments, which enhance wear resistance while maintaining a tougher core.

After induction heating, the surface hardness of the 4140 steel is approximately 670 HV (Vickers hardness).² Nitrided 4140 (through hardened) showed a hardness of 83.5-84.5 R15N (Rockwell 15N scale).³

Conclusion

AISI 4140 steel offers a wide range of achievable hardnesses depending on the heat treatment. It can be relatively soft in the annealed state (around 197 HB), achieve moderate hardness when normalized (e.g., 241-302 HB depending on section size), and become very hard in the quenched and tempered condition (up to 578 HB or 57 HRC at low tempering temperatures). Surface hardening techniques such as induction hardening and nitriding can impart extremely high surface hardness (e.g., 670 HV or 83.5-84.5 R15N) while maintaining a tough core.

1. ASM International. (1991). ASM Handbook, Volume 4: Heat Treating. ASM International. ↩︎
2. Beswick, J. M. (Guest Ed.). (2009). Bearing Steel Technologies: Developments on Rolling Bearing Steels and Testing, 8th Volume (STP 1524). ASTM International. ↩︎
3. Collins, J. A., Busby, H., & Staab, G. (2010). Mechanical Design of Machine Elements and Machines: A Failure Prevention Perspective (2nd ed.). Wiley. ↩︎