4140 steel technical overview

4140 steel is a versatile medium-carbon low-alloy steel. It contains chromium and molybdenum as its principal alloying elements. This steel is known for its good hardenability, allowing it to achieve high strength and hardness through heat treatment processes like quenching and tempering. It is commonly used in applications requiring a good balance of strength and toughness, such as shafts, gears, and other machinery components. It can be subjected to various heat treatments, including annealing for softening, normalizing for improved microstructure and moderate strength, and hardening followed by tempering to achieve the desired mechanical properties.

1. Chemical composition

Carbon (C)	Manganese (Mn)	Phosphorus (P)	Sulfur (S)	Silicon (Si)	Chromium (Cr)	Molybdenum (Mo)
0.38% – 0.43%	0.75% – 1.00%	≤ 0.035% (max)	≤ 0.040% (max)	0.15% – 0.35%	0.80% – 1.10%	0.15% – 0.25%

2. Applications

Based on its properties, 4140 steel finds applications in various demanding engineering components due to its good hardenability, high strength and hardness potential, and a decent level of toughness when properly heat treated. Here’s a breakdown of applications based on these properties:

2.1 Components Requiring High Strength and Hardness (Achieved through Quenching and Tempering):

• Statically and dynamically loaded parts of car engines and machines, such as crankshafts. The high hardness achieved after hardening makes it suitable for these demanding applications.
• Machine parts with large cross-sections where high strength can be obtained after refinement. Its good hardenability allows for through-hardening in significant thicknesses.
• Components for moderately severe service conditions where a balance of strength and toughness is needed.
• Gears that require through hardening to achieve the necessary strength and wear resistance.
• Pins and shafts in various engineering applications, where high strength and toughness are crucial. For higher duty applications, it can be hardened and tempered to achieve core strengths in the range of 100-140 kgf/mm².
• Components require resistance to wear, making it suitable for surface hardening techniques like flame and induction hardening.
• High-demand automobile parts such as axles, benefiting from its strength and hardenability.
• Alloy steel tubes for general and structural applications, where its strength after heat treatment is advantageous.
• Bars and forgings for various engineering purposes are often heat-treated to specific strength levels.

2.2 Components Requiring Surface Hardness and Wear Resistance (Achieved through Nitriding or Induction Hardening):

• Gears requiring a deep case and surface hardness below 60 HRC can be made from 4140 steel and subjected to nitriding.
• Gears and other components benefit from induction hardening for a hard, wear-resistant surface layer.

2.3 Tooling Applications:

While not primarily considered a tool steel, 4140 is popular in the toolroom for various applications requiring moderate hardenability and good strength and toughness.

3. Physical properties

3.1 Tensile Strength of 4140 Steel

Condition	Tensile Strength (MPa)	Tensile Strength (ksi)
Annealed	434-620	63-90
Normalized	Can range from 483 to 690 depending on carbon content	Can range from 70.0 to 100.0 depending on carbon content
Quenched & Tempered @ 205 °C (400 °F)	1965-1980	285-287
Quenched & Tempered @ 425 °C (800 °F)	1450-1500	210-217
Quenched & Tempered @ 540 °C (1000 °F)	1140-1240	165-180
Quenched & Tempered @ 650 °C (1200 °F)	900-1020	130-148
Cold Drawn and Annealed	Approximately 620-703	Approximately 90-102
Drawn at 1000°F (540 °C)	Approximately 903-1054	Approximately 131-153

3.2 Yield Strength (0.2% Offset) of 4140 Steel

Condition	Yield Strength (MPa)	Yield Strength (ksi)
Annealed	Approximately 201-434 depending on carbon content	Approximately 29.1-63 depending on carbon content
Normalized	Can range from 247 to 355 depending on carbon content	Can range from 35.8 to 51.5 depending on carbon content
Quenched & Tempered @ 205 °C (400 °F)	1740-1860	252-270
Quenched & Tempered @ 425 °C (800 °F)	1340-1365	195-198
Quenched & Tempered @ 540 °C (1000 °F)	985-1160	143-168
Quenched & Tempered @ 650 °C (1200 °F)	790-860	114-125
Cold Drawn and Annealed	Approximately 620-703	Approximately 90-102
Drawn at 1000°F (540 °C)	Approximately 903-1054	Approximately 131-153

3.3 Ductility Properties (Elongation & Reduction in Area) of 4140 Steel

Property	Condition	Value
Elongation (in 50 mm/2 in.)	Annealed	Typically around 18-27%
Elongation (in 50 mm/2 in.)	Quenched & Tempered	Decreases with increasing tempering temperature. For example, it can range from 11% (at 205 °C/400 °F) to 23% (at 705 °C/1300 °F)
Reduction in Area	Quenched & Tempered	Generally ranges from 39% to 65% depending on the tempering temperature

3.4 Hardness of 4140 Steel

Condition	Hardness Value
Annealed	Approximately 185 HB
As-Quenched	Can reach around 601 HB
Quenched & Tempered	Varies widely with tempering temperature, from around 578 HB (~53 HRC) at 205 °C (400 °F) down to 235 HB (~24 HRC) at 705 °C (1300 °F)

3.5 Impact Strength of 4140 Steel

Property	Condition	Observation / Value
Impact Strength	Quenched & Tempered	Increases with increasing tempering temperature.
Izod Impact Energy	Quenched & Tempered	For instance, Izod impact energy can range from 15 J (11 ft·lb) at 205 °C (400 °F) to 136 J (100 ft·lb) at 705 °C (1300 °F).
Charpy V-notch	Quenched & Tempered	Values also show a similar trend [to Izod].

3.6 Fatigue Strength of 4140 Steel

Condition	Observation / Value
General	Sensitive to notch and transition on machine parts subjected to fatigue loading.
Quenched & Tempered (Hardness 46-50 HRC)	Fatigue strength under rotating bending can be around 270 MPa (39 ksi).
Shot Peened (after Q&T)	Shot peening can significantly increase fatigue strength.

3.7 Other Mechanical Properties of 4140 Steel

Property	Condition	Value
Elastic Modulus	Tension	Around 115 GPa (17 × 10⁶ psi)
Shear Strength	H04 temper rod	Can range from 200-205 MPa (29-32 ksi) depending on the diameter.

4. Heat treatment

4.1 Annealing: This process is used to soften the steel, improve machinability, and relieve internal stresses. The general steps for annealing 4140 steel are:

• Heating: Heat the 4140 steel to 830 to 870 °C (1525 to 1600 °F), ensuring uniform heating throughout the section. Slow heating is generally recommended for alloy steels to minimize thermal stresses3.
• Holding (Soaking): Maintain this temperature for a sufficient period. The holding time typically depends on the part’s section thickness or the furnace load.
• Cooling: Slowly cool the steel in the furnace at a rate of approximately 15 °C/h (30 °F/h) down to around 480 °C (900 °F), followed by air cooling to room temperature. This slow cooling allows for the formation of a softer microstructure. The maximum hardness achievable after annealing is around 197 HB4. Isothermal annealing methods, involving controlled cooling to a specific temperature range and holding before final cooling, can also be employed to obtain a predominantly pearlitic structure.

4.2 Normalizing: This treatment refines the grain structure, improves uniformity, and increases strength and hardness compared to the annealed state. The steps are:

• Heating: Heat the 4140 steel to a temperature range of 845 to 925 °C (1550 to 1700 °F), which is approximately 55 to 85 °C (100 to 150 °F) above its upper transformation temperature. This ensures complete transformation to austenite. For a specific normalizing temperature, you mentioned 1600°F (approximately 870°C), which falls within this range.
• Holding: Hold at this temperature for a minimum of 1 hour or 15 to 20 minutes per 25 mm (1 inch) of maximum section thickness. This allows for the formation of homogeneous austenite.
• Cooling: Cool the steel in still air to room temperature. The faster cooling rate, compared to annealing, results in a finer pearlitic structure and higher hardness.

4140 steel normalized at 1600 °F (870 °C) will exhibit a refined grain structure, improving strength and hardness compared to its as-forged or hot-rolled state. It serves as a common preparatory heat treatment before subsequent hardening and tempering to achieve the final desired mechanical properties for your factory’s specific applications. The exact hardness after normalizing will depend on the dimensions of the part and the cooling rate achieved. Remember that this normalized state is often an intermediate step, and further heat treatment, such as tempering, is typically required to optimize the balance of strength, ductility, and toughness for the intended service conditions.

4.3 Hardening (Quenching): This process aims to achieve a hard martensitic structure. It involves:

• Preheating (Optional but Recommended): For 4140 steel, a preheat at around 650 °C (1200 °F) for 10 to 15 minutes can be beneficial, especially for complex shapes, to reduce thermal shock and minimize distortion.
• Austenitizing: Heat the steel to the austenitizing temperature, typically in the range of 845 to 925 °C (1550 to 1700 °F). Some sources specify a temperature of 855 °C (1575 °F). Hold at this temperature for a sufficient soak time to ensure complete transformation to austenite, which depends on the section thickness (e.g., add 5 minutes per each inch of smallest cross-section after reaching the austenitizing temperature). Oversoaking, especially at higher temperatures, can lead to undesirable austenite grain growth.
• Quenching: Rapidly cool the steel from the austenitizing temperature in a suitable quenchant. For 4140 steel, oil quenching is the most common and recommended method to achieve hardening while minimizing the risk of cracking associated with faster quenchants like water. However, water quenching might be used for larger sections depending on hardenability requirements. The effectiveness of quenching depends on the quenchant temperature. If you take 4140 steel annealed at 1600 °F and then oil quench it from the appropriate austenitizing temperature, you will achieve a high hardness and strength characteristic of martensite, but the material will be brittle with low ductility and toughness. A subsequent tempering process would be necessary for most engineering applications.

4.4 Tempering: Hardened martensite is generally brittle and contains internal stresses. Tempering is performed to reduce brittleness, relieve these stresses, and improve toughness while retaining sufficient hardness and strength. The steps are:

• Heating: Reheat the quenched steel to a specific tempering temperature, which is always below the austenitizing temperature. The tempering temperature for 4140 steel typically ranges from 205 to 705 °C (400 to 1300 °F), depending on the desired mechanical properties. It is critical that tempering takes place as soon as the parts reach 52 to 65 °C (125 to 150 °F) after quenching to prevent cracking. Tempering between 230 and 370 °C (450 and 700 °F) is generally avoided for 4140 steel to prevent blue brittleness.
• Holding: Hold at the tempering temperature for a specific time, typically 1 to 2 hours18 or 2 hours per inch (25 mm) of cross-section. This allows for the diffusion of carbon and alloying elements and the formation of tempered martensite with the desired properties.
• Cooling: Cool to room temperature in air or by quenching in water or oil. The cooling rate after tempering is usually not critical. Often, a second tempering cycle at a slightly lower temperature might be used.

4.5 Spheroidizing: This is a specialized annealing process that produces a microstructure of globular carbides in a ferritic matrix, resulting in maximum softness and improved formability. For 4140 steel, this can be achieved by Heating to 760 to 775 °C (1400 to 1425 °F) and holding for 4 to 12 hours, followed by slow cooling.

4.6 Surface Hardening: To enhance wear resistance while maintaining a tougher core, 4140 steel can undergo surface hardening processes such as:

• Induction Hardening: This involves rapidly heating the surface layer to the austenitizing temperature using an induction coil, followed by quenching. This creates a hard surface case. After induction hardening, tempering is usually performed.
• Nitriding: This is a thermochemical process that introduces nitrogen into the surface of the steel at relatively low temperatures, forming hard nitride compounds and improving wear and fatigue resistance. Tempering prior to nitriding is often performed.

5. 4140 Vs D2 steel

• 4140 steel is your workhorse for structural and machinery components requiring a good balance of strength and toughness with moderate wear resistance that can be enhanced. It offers versatility through various heat treatments, including surface hardening.
• D2 steel is your go-to for tooling applications demanding high abrasion resistance and dimensional stability during hardening. Its higher carbon and chromium content provide the necessary hard carbides for wear resistance, but this comes at the cost of lower toughness compared to 4140.
• In summary, 4140 is likely the better choice for machine parts that need high strength and toughness. If you are dealing with tooling that requires exceptional resistance to wear and abrasion, D2 steel would be more appropriate.

6. 4140 Vs 4130 steel

• 4140 steel possesses a higher carbon content, resulting in greater hardenability, strength, and hardness after heat treatment compared to 4130. It is typically oil-quenched.
• 4130 steel has a lower carbon content, leading to lower to intermediate hardenability and generally lower strength and hardness than 4140 after similar heat treatments. It is often water-quenched.
• In summary, if higher strength is paramount, 4140 is generally the preferred choice. If moderate strength with potentially better weldability or machinability is sufficient and section size is a significant constraint for hardening, 4130 might be considered.

7. Effect of Tempering Temperature on 4140 Steel Properties

8. 7075 billet aluminum vs 4140 steel

• 4140 steel billet offers higher strength and hardness potential than 7075 aluminum, along with good toughness. It is also denser and more readily weldable (with precautions). It requires heat treatment to achieve its optimal properties and is susceptible to corrosion.
• 7075 aluminum billet provides a significantly lower density with high strength, making it advantageous in weight-sensitive applications. It has good corrosion resistance but is generally more challenging to weld. Its strength is achieved through specific heat treatment tempers.

9. 4140 steel for knives

4140 steel can be heat-treated to achieve the necessary hardness for a knife blade, but its lack of stainless properties makes it a less conventional choice than steels specifically designed for cutlery. If you require a corrosion-resistant knife, you would be better served by exploring stainless steel alloys. If strength and toughness are the primary requirements and corrosion can be managed, 4140 could be considered, but careful attention to heat treatment will be crucial.