Introduction
4140 steel is a high-carbon chromium-molybdenum alloy steel that provides an excellent combination of strength, toughness, and wear resistance. It is commonly used in applications that require high strength and fatigue resistance such as gears, shafts, Pins, axles, fasteners, aircraft landing gear components, and structural parts.
Proper heat treating of 4140 steel is critical to achieve the desired mechanical properties for the application. The heat treating process for 4140 steel involves austenitizing, quenching, and tempering. The key processing parameters that influence the microstructure and properties include austenitizing temperature, cooling rate during quenching, and tempering temperature and time. By optimizing these parameters, the heat treatment process can be tailored to maximize the toughness and fatigue life.
This article provides a comprehensive overview of heat treating 4140 steel bar stock to achieve an optimal combination of high toughness and fatigue life for demanding applications. It covers the effects of alloying elements, austenitizing, quenching, tempering, and other secondary hardening processes on the microstructure and properties of 4140 steel. Recommended processing parameters are provided along with their impact on mechanical properties, toughness, and fatigue life.
Overview of 4140 Steel
4140 steel gets its name from the AISI-SAE designation system. The first two digits ’41’ indicate a chromium-molybdenum alloy steel. The last two digits ’40’ represent the carbon content of the steel, which is 0.40% by weight.
The chemical composition of 4140 steel is as follows:
- Carbon (C) – 0.38-0.43%
- Silicon (Si) – 0.15-0.30%
- Manganese (Mn) – 0.75-1.00%
- Chromium (Cr) – 0.80-1.10%
- Molybdenum (Mo) – 0.15-0.25%
- Phosphorus (P) – ≤0.035%
- Sulfur (S) – ≤0.040%
- Iron (Fe) – Balance
The addition of the alloying elements, especially chromium and molybdenum, provides solid solution strengthening that increases the hardness and strength. The medium carbon content improves hardenability and wear resistance.
4140 steel has the following properties in the annealed condition:
- Yield Strength: 655 MPa (95 ksi)
- Tensile Strength: 860 MPa (125 ksi)
- Elongation: 18%
- Hardness: 217 HB
With proper heat treatment, the properties of 4140 steel can be significantly enhanced. Tensile strength can reach up to 1450 MPa (210 ksi) and hardness up to 50 HRC. The fatigue strength is also very good compared to other alloy steels.
These properties make 4140 steel suitable for applications such as:
- Gears
- Shafts
- Axles
- Pistons
- Fasteners
- Aircraft and structural parts
- Dies and tooling
- Engine components
The good balance of properties, weldability, forgeability, and availability in bar stock make 4140 an excellent choice for critical components and structural parts.
Effects of Alloying Elements
The alloying elements in 4140 steel have specific effects on the microstructure and properties:
Carbon
- Increases tensile strength and hardness
- Improves hardenability by retarding the formation of pearlite during cooling
- Contributes to formation of martensite upon quenching
- Excessive carbon content decreases toughness and ductility
Silicon
- Increases strength by solid solution strengthening
- Deoxidizer which improves cleanliness
- Increases hardenability
Manganese
- Increases hardenability and tensile strength
- Improves solubility of sulfur to avoid hot shortness
- Deoxidizer which improves cleanliness
Chromium
- Increases hardenability, tensile strength, and toughness
- Provides corrosion and oxidation resistance
Molybdenum
- Increases hardenability and strength
- Enhances tempering resistance for higher strength in larger sections
Phosphorus and Sulfur
- Impurities which reduce ductility, notch toughness and fatigue strength
- Minimized by tight compositional control
By optimizing the levels of carbon, chromium, and molybdenum, 4140 steel provides an excellent combination of hardenability, strength, toughness and tempering response required for critical components. The lower impurity levels ensure good ductility and fatigue resistance.
Austenitizing
The first step in the heat treatment process is austenitizing or solutionizing. The steel is heated to a temperature above the upper critical temperature, Ac3, to transform the microstructure to austenite, a face centered cubic structure.
For 4140 steel, the recommended austenitizing temperature range is 845 – 900°C (1550 – 1650°F). Lower austenitizing temperatures below 845°C do not achieve complete transformation to austenite. Higher temperatures beyond 900°C increase grain growth which reduces toughness and fatigue strength.
The main objectives of the austenitizing step are:
- Dissolve carbides and establish chemical homogeneity
- Refine the austenite grain size
- Put carbon and alloying elements into solid solution
A fine grained austenitic structure is desired before quenching to refine the martensitic structure produced. Carbides such as iron carbide Fe3C are dissolved into the austenite. Alloy carbides of chromium and molybdenum require higher temperatures near 900°C to fully dissolve.
The time held at the austenitizing temperature, or soak time, depends on the steel section size and desired dissolution of carbides. Soak times range from 15 minutes up to 2 hours. Longer soak times are used for larger sections and when maximum dissolution of carbides is needed. Shorter times are sufficient for small sections such as bars.
Proper austenitizing and dissolution of carbides in 4140 steel sets up the microstructure for optimal response to quenching and tempering for the desired balance of strength and toughness.
Quenching
After austenitizing, 4140 steel must be quenched rapidly to form martensite. Martensite is a very hard, supersaturated metastable phase formed by the diffusionless transformation of austenite. Quenching prevents diffusion of carbon out of the crystal structure, resulting in a tetragonal lattice distortion and high hardness.
The recommended media for quenching 4140 steel is oil. Water quenching is too severe and can lead to cracking, while air cooling is too slow to produce martensite even in small sections. Medium speed oil provides rapid heat extraction to obtain martensite while minimizing the risk of distortion and quench cracking.
Agitated oil baths provide faster cooling rates compared to still oils. Polymer quenchants offer improved cooling control versus petroleum oils. The quenchant should be maintained below 45°C (120°F) for optimal heat transfer properties.
To minimize distortion during quenching, bars should be supported at multiple locations. Cooling should be interrupted periodically to equalize temperatures and relieve residual stresses.
The hardness of as-quenched 4140 steel depends on the carbon content and cooling rate, but is typically around 50 HRC. The corresponding tensile strength is 1450 – 1600 MPa (210 – 230 ksi). However, this very high hardness results in low notch toughness and reduced fatigue resistance.
Proper quenching is critical to produce the desired martensitic structure in 4140 steel needed to maximize strength. However, quenching alone produces excessive hardness and brittleness. Tempering is required to reduce these negative effects while maintaining high strength.
Tempering
Tempering involves heating quenched martensitic steel to an intermediate temperature below Ac1 to decrease hardness and increase toughness and ductility. It allows partial diffusion of carbon from martensite in a controlled manner. Tempering temperature and time are the key parameters determining the mechanical properties.
For optimal toughness and fatigue strength in 4140 steel, a double tempering treatment is recommended with the following parameters:
- 1st temper: 580 – 650°C (1080 – 1200°F) for 1 – 2 hours
- 2nd temper: 650 – 705°C (1200 – 1300°F) for 1 – 2 hours
- Cool slowly after each temper, preferably in furnace
The initial temper at 580 – 650°C relieves quenching stresses, segregates carbides, and forms transition carbides. This improves toughness. The higher second temper maximizes tempered martensite formation for the best combination of strength and ductility. Slow cooling avoids untempered martensite at the surface.
Typical mechanical properties after double tempering 4140 steel:
- Yield Strength: 1240 MPa (180 ksi)
- Tensile Strength: 1400 MPa (205 ksi)
- Elongation: 16%
- Hardness: 33 – 37 HRC
This provides over 200 ksi tensile strength with improved elongation and toughness compared to a single temper. The fatigue limit is also increased significantly.
Lower tempering temperatures below 580°C leave excessive retained austenite and brittleness. Tempering above 705°C results in over-tempering and decreased strength. Careful control of both temperature and time during tempering is essential.
Double tempering maximizes the properties of 4140 steel for applications requiring high strength combined with toughness and fatigue resistance.
Secondary Hardening Processes
In some cases, higher strength may be needed while maintaining reasonable toughness and ductility. Optional secondary hardening processes can be used after the initial tempering of 4140 steel:
Austempering
- Quench to 260 – 370°C and hold to form bainite
- Achieves tensile strength 1600 – 1700 MPa (230 – 250 ksi)
Martempering
- Quench to just above Ms and hold to form martensite
- Temper at 200 – 300°C
- Achieves tensile strength 1600 – 1950 MPa (230 – 280 ksi)
Cryogenic Treatment
- Deep freeze quenched parts to -80°C or lower
- Reduces retained austenite and increases hardness/strength
- Can reduce toughness if not tempered properly afterwards
These processes can provide substantial increases in tensile strength, but require careful control to avoid decreases in ductility and toughness. Thorough tempering is essential. Use should be evaluated for each specific application.
Effect of Section Size
The section size of 4140 steel can significantly influence its hardenability and mechanical properties. Larger section sizes make it more difficult for the interior sections to cool fast enough to form martensite during quenching. This can result in softer interior sections with lower strength.
To avoid problems in larger sections:
- Use higher austenitizing temperature near 900°C
- Increase soak time to 2 hours
- Use faster quenchant like hot polymer
- Use multiple tempers to increase tempering response
With proper processing, 4140 steel up to a 4′′ diameter can achieve full hardness and strength capability. The composition is designed for good hardenability even in medium to large cross sections. Attention must be paid to the quench severity and tempering schedule as section size increases.
Effect on Toughness and Fatigue Strength
The main goal of heat treating 4140 steel is to achieve not just high strength, but an optimal combination of strength, toughness, and fatigue resistance required for many critical applications.
Several processing factors influence these properties:
- Quenching rate – faster cooling increases hardness but reduces toughness
- Tempering temperature – lower tempers reduce ductility, higher tempers decrease strength
- Section size – large sections have lower hardenability and possible lower strength in interior
- Retained austenite – untempered martensite decreases toughness
- Prior microstructure – refined prior austenite grain size improves toughness
Proper control of austenitizing, quenching, and double tempering is necessary to obtain over 200 ksi tensile strength along with 15% minimum elongation and good notch toughness in 4140 steel. HRC should be kept between 33 and 37.
For best fatigue resistance, maintain hardness below 38 HRC while keeping tensile strength above 180 ksi. Smooth surface finish and minimized residual stresses also help improve fatigue life.
With optimized heat treating, 4140 steel provides an exceptional balance of strength, toughness and fatigue life for critical component applications.
Summary of Recommended Heat Treating Process
To summarize, the following process parameters will produce optimal toughness and fatigue life in 4140 steel:
- Normalize 25-50°C below lower critical temperature to refine grains
- Austenitize at 870 – 900°C for 1-2 hours, depending on section size
- Quench in agitated medium speed oil below 45°C
- Double temper at:
- 1st: 600 – 650°C for 1-2 hours
- 2nd: 675 – 705°C for 1-2 hours
- Slow cool after each temper
This will result in tensile strength of 1350 – 1450 MPa (>195 ksi), yield strength above 1200 MPa (175 ksi), 10-15% elongation, and 30 – 35 HRC hardness. Fatigue strength should exceed 550 MPa (80 ksi) at 107 cycles. With proper attention to detail, these properties can be achieved in sections up to 4′′ diameter.
Applications and Examples
With its exceptional balance of properties, 4140 steel has emerged as a popular alloy steel for critical components in demanding applications, including:
Gears
Used for gear shafts, ring gears and pinions. Provides high strength for torque capacity along with toughness for shock loading. Double tempered 4140 resists cracking.
Axles and Drive Shafts
Used in truck, construction equipment and off-road vehicle axle shafts and drivelines. Withstands high torque loads and shocks while providing long fatigue life.
Aircraft Landing Gear
Used for landing gear, actuators, and flight control components. Requires high strength combined with fracture toughness. 4140 achieves hardness up to 38 HRC for this application.
Fasteners
Used for high strength bolts, nuts and studs. Achieves 150 ksi proof loads when properly heat treated. Provides better fatigue resistance than higher carbon fastener steels.
Pumps and Valves
Used for plunger rods, valve stems and bodies. Resists wear and galling. Provides high fatigue resistance.
Mold and Die Components
Used for die plates, bolsters, mandrels and tooling components for molded parts and extrusions. Heat treated 4140 provides long service life in production operations.
With proper heat treating according to this guide, 4140 steel has proven to provide exceptional performance in these rugged applications.
FAQ
What is 4140 steel used for?
4140 steel is commonly used for parts requiring high strength and fatigue resistance such as gears, shafts, axles, aircraft landing gear components, fasteners, pumps and valves, molds and dies. It provides an excellent combination of toughness, wear resistance and high yield strength after heat treating.
Why heat treat 4140 steel?
In the annealed condition, 4140 steel has relatively low strength with tensile strength around 125 ksi and hardness of about 217 HB. Heat treating transforms the microstructure to impart the high strength capability of 4140. With proper hardening and tempering, tensile strength above 200 ksi and hardness greater than HRC 33 can be achieved to meet demanding applications.
What is the difference between tempering and annealing 4140 steel?
Tempering involves controlled heating of hardened quenched steel like 4140 to improve ductility and toughness. It results in tensile strength above 180 ksi. Annealing involves heating to a relatively low temperature, followed by slow cooling to remove internal stresses and soften the steel. It reduces hardness to about 217 HB to improve machinability.
What causes low toughness in heat treated 4140 steel?
Insufficient tempering, large grain size, high sulfur content, and high hardness levels above 38 HRC can all contribute to reduced toughness in 4140. Avoiding over-quenching, double tempering to 650-700°F, minimizing impurities, and restricting hardness to the 30-35 HRC range will maximize toughness.
How is fatigue strength maximized in 4140 steel?
For best fatigue resistance in 4140 steel, follow several guidelines in heat treating: avoid hardness over 38 HRC, use multiple tempers in the 600-700°F range, minimize residual stresses, provide a fine surface finish, and reduce section size as practical. With optimum processing, fatigue limits above 80 ksi at 10 million cycles can be obtained.
What size 4140 steel bar stock can be through hardened?
With proper heat treating, 4140 steel bars up to 4 inches in diameter can achieve full martensitic hardness throughout the cross section. Larger sections will require higher austenitizing temperatures, longer soak times, faster quenchant, and multiple tempers to maximize hardenability and mechanical properties.
Can 4140 steel be induction hardened?
Yes, 4140 steel can be induction hardened to impart extreme surface hardness up to 60 HRC with a hardened case depth up to 2mm for wear resistance. The core remains tough and ductile. Quench cracking may be a risk with high power density. Multiple tempering is required after induction hardening to relieve stresses.
What coating improves corrosion resistance of 4140 steel?
A black oxide finish applied after heat treating provides moderately improved corrosion resistance for 4140 steel while maintaining appearance and dimensions. For more severe conditions, nickel-chromium electroless plating or painting are recommended to protect 4140 steel from corrosion.