Engineers and procurement teams in the oil and gas, marine, and manufacturing sectors frequently turn to AISI 4140 alloy steel when standard carbon steels fail to meet mechanical requirements. Known for its exceptional balance of strength, toughness, and wear resistance, this chromium-molybdenum (Cr-Mo) steel is a staple in heavy machinery and structural components. However, the raw chemistry of 4140 is only half the equation; the material’s true potential is unlocked through precise 4140 heat treat processes. 

Understanding the relationship between 4140 material properties and thermal processing is critical for specifying the right material for your application. Whether you require high 4140 tensile strength for a power transmission shaft or a specific 4140 hardness range for gears, the heat treatment condition dictates performance. 

What Is AISI 4140 Alloy Steel? 

AISI 4140 is a low-alloy steel containing chromium, molybdenum, and manganese. It falls under the general classification of medium-carbon steel but offers significantly improved hardenability and fatigue strength compared to plain carbon alternatives like AISI 1045. The "41" in its designation indicates a chromium-molybdenum alloy, while "40" represents an approximate carbon content of 0.40%. 

4140 Steel Chemical Composition 

While standards vary slightly between ASTM, SAE, and international equivalents, the typical chemical composition of AISI 4140 is: 

This specific chemistry allows AISI 4140 alloy steel to achieve deep hardening during heat treatment. The chromium adds hardness and strength, while the molybdenum enhances toughness and hardenability. This makes it far superior to plain carbon steels, which often suffer from brittleness when treated to high hardness levels. 

In its "as-rolled" or annealed condition, general AISI 4140 mechanical properties provide decent machinability but relatively low strength. To maximize its utility in high-stress environments—such as downhole drilling tools or hydraulic machinery—heat treatment is not optional; it is essential. 

For detailed product specifications and stock sizes, visit our AISI 4140 alloy steel page. 

Why Heat Treatment Matters for 4140 Steel 

Heat treatment is the controlled heating and cooling of metals to alter their physical and mechanical properties without changing the product shape. For AISI 4140, this process is the primary dial engineers turn to control the trade-off between strength and ductility. 

Microstructural Control 

At the microscopic level, heat treatment manipulates the steel's grain structure. Heating the steel above its critical temperature (austenitizing) transforms the internal structure into austenite. Rapid cooling (quenching) traps carbon atoms in the crystal lattice, forming martensite—a hard, strong structure. Subsequent heating (tempering) relaxes this structure slightly to regain toughness. 

Controlling Mechanical Outputs 

The specific heat treatment cycle directly dictates: 

• 4140 hardness: The resistance to indentation and wear. 
• 4140 tensile strength: The maximum stress the material can withstand while being stretched before breaking. 
• 4140 yield strength: The stress level at which the material begins to deform plastically (permanently). 

Without proper heat treatment, a 4140 shaft might either be too soft to transmit torque (annealed) or too brittle to withstand shock loading (as-quenched). By selecting the correct process—whether normalizing for uniformity or quenching and tempering for high strength—you ensure the component will survive its service life. 

For context on how alloy steels differ from basic grades, review our Carbon steel vs alloy steel guide. 

Common 4140 Heat Treatment Processes 

The versatility of AISI 4140 lies in its response to different thermal cycles. Below are the standard treatments available from suppliers and heat-treat facilities. 

Annealed 4140 Steel 

Annealing involves heating the steel to approximately 815°C (1500°F) and then cooling it very slowly in the furnace. The goal is to soften the steel as much as possible to improve machinability and ductility. 

• Purpose: To relieve internal stresses and prepare the material for extensive machining or cold forming. 
• 4140 Annealed Hardness: Typically ranges from 197 to 217 HB (Brinell). 
• Machinability: This is the easiest condition to machine. If you are manufacturing complex parts that will be heat-treated later, buying annealed stock is often the most cost-effective strategy. 
• Applications: Use annealed 4140 for parts requiring heavy metal removal before the final hardening process. 

It is important to note that 4140 material hardness in the annealed state is insufficient for most structural applications; it is strictly a processing state. 

Normalized 4140 

Normalizing involves heating the steel to roughly 870°C (1600°F) and allowing it to cool in still air. This cooling rate is faster than annealing but slower than quenching. 

• Difference vs. Annealed: Normalizing refines the grain size and homogenizes the microstructure. It produces a harder and stronger material than annealing. 
• Property Improvements: It provides a consistent structure that machines well and offers moderate strength (roughly 850–1000 MPa tensile). 
• Use Cases: Normalized 4140 is frequently used for large forgings or structural parts where quenching might cause distortion or cracking, but higher strength than plain carbon steel is required. 

Quenched & Tempered (Q&T) 4140 

This is the most common condition for finished engineering components. It is a two-step process: 

• Hardening (Quenching): Heating to ~845°C–870°C, holding until uniform, and quenching in oil. Oil quenching is preferred over water to reduce the risk of cracking. 
• Tempering: Reheating to a temperature between 200°C and 650°C to achieve the desired hardness. 
• 4140 Hardness Ranges (HRC): Depending on the tempering temperature, hardness can range from 28 HRC to over 50 HRC. Common pre-hardened stock (like 4140HT) is usually supplied at 28–32 HRC. 
• Tensile & Yield Strength: Lower tempering temperatures yield higher strength but lower toughness. Higher tempering temperatures increase ductility but lower the tensile strength. 

Selecting Temperatures: 

• 205°C Temper: ~50–55 HRC (High wear resistance, low toughness) 
• 315°C Temper: ~45–50 HRC 
• 540°C Temper: ~32–36 HRC (Excellent balance of strength and toughness for shafts) 

For a comparison of how this differs from tool steels, refer to our Tool steel grades explained article. 

Optional Treatments

• Induction Hardening: 4140 is an excellent candidate for surface hardening. The core remains tough (often Q&T to 30 HRC), while the surface is induction hardened to 50–55 HRC for wear resistance. 
• Nitriding: Can be applied to increase surface hardness and corrosion resistance without the distortion associated with quenching. 

 In-Stock Heat-Treated 4140 Steel Options 

For many projects, waiting for a custom heat-treat cycle is not feasible due to tight project timelines. Fortunately, Nifty Alloys stocks AISI 4140 in several pre-treated conditions to support urgent maintenance and fabrication needs. 

• Common Stocked Conditions 
• Annealed: The standard for raw stock intended for heavy machining. Available in large diameter rounds and blocks. 
• Quenched & Tempered (Pre-Hardened/HT): This is the industry "workhorse." Usually supplied at 28–32 HRC (approx. 275–320 HB). This condition is machineable (though harder than annealed) and can be put directly into service after machining without further heat treatment. 

Typical Forms 

• Round Bars: From small diameters (20mm) up to large forged bars (800mm+). 
• Shafts: Precision ground shafting (PSQ) is often available in Q&T condition. 
• Flats and Blocks: Saw-cut from large forged blocks for mold bases or structural plates. 

Benefits of Stock Material 

• Speed: Immediate dispatch eliminates the 1–2 week lead time for heat treatment. 
• Predictability: Stock Q&T bars come with defined 4140 mechanical properties verified by Mill Test Certificates (MTCs), reducing the risk of heat-treat failures during fabrication. 

Visit our 4140 alloy steel supplier page to view current inventory. 

Custom Heat Treatment Specifications 

While stock material covers 80% of applications, critical oil & gas or aerospace components often require custom mechanical properties. When issuing a Purchase Order (PO) for custom 4140, precision in specification is vital. 

How to Specify 

Avoid vague terms like "heat treat to maximum hardness." Instead, specify a range: 

• Target Hardness: e.g., "Heat Treat to 34–38 HRC." 
• Mechanical Minimums: e.g., "Min Yield Strength 100 ksi (690 MPa), Min Impact Energy 40J @ -20°C." 
• Condition: e.g., "Quenched and Tempered." 

Importance of Process Control 

When ordering custom-treated material, ensure your supplier provides: 

• Heat Treatment Charts: Verification of time and temperature cycles. 
• Mill Test Certificates (MTCs): Confirmed mechanical testing results from the specific batch (heat). 
• Non-Destructive Testing (NDT): Ultrasonic testing (UT) is crucial after aggressive quenching to ensure no internal cracks have formed. 

Custom treatment is mandatory when the required 4140 tensile strength exceeds standard stock levels (e.g., >1100 MPa) or when parts are too large to achieve uniform hardness through the cross-section in standard bar stock. 

See our Steel grades guide for more on specifying materials correctly. 

AISI 4140 Mechanical Properties & Hardness Table 

The table below provides typical values for 1-inch (25mm) round bars. Note that properties decrease as the section size (diameter) increases due to the "mass effect" during quenching. 

Engineering Interpretation: 
Engineers must note that 4140 yield strength drops significantly as tempering temperatures rise. If your design requires the material to remain elastic under heavy loads, you need a lower temper (higher hardness). However, if the part is subject to impact or shock (like a hammer head or axle), you must sacrifice some yield strength for elongation and impact toughness to prevent catastrophic brittle failure. 

For comparisons with other metals, see our Density of steel and common metals resource. 

Typical Applications by Heat Treat Condition 

Selecting the right condition extends the service life of your components. 

Annealed 4140 

• Pre-machined components: Parts with complex geometries that will be hardened later. 
• Low-stress housings: Where the alloy chemistry is needed for weldability or mild corrosion resistance, but high strength is not required. 

Normalized 4140 

• Large Shafts: Where quenching might warp the long bar. 
• Structural Tie Rods: Provides better strength than mild steel without the cost of full Q&T. 
• Oil & Gas Subs: Basic downhole tools where moderate strength is sufficient. 

Quenched & Tempered (Q&T) 4140 

• Gears and Pinions: Usually tempered to 30–35 HRC for core strength, often followed by induction hardening on the teeth. 
• Crankshafts & Connecting Rods: Requires the high fatigue strength of the Q&T condition. 
• Bolts and Fasteners: High-strength B7 studs are chemically similar to 4140. 
• Heavy Machinery Parts: Axles, spindles, and conveyor rolls in mining and cement industries. 

 AISI 4140 Equivalent Materials 

Global sourcing often requires knowledge of equivalent grades. While chemistry overlaps, always verify the AISI 4140 equivalent material standards to ensure they meet your specific mechanical requirements. 

Warning: While "equivalent" suggests identical performance, slight variations in Manganese or Chromium can affect heat treat response. Always review the MTC before substituting 42CrMo4 for AISI 4140 in critical applications. 

FAQ 

What is 4140 annealed hardness? 
Typical 4140 annealed hardness is between 197 and 217 Brinell (HB). This is the softest state, ideal for machining. 

What hardness can 4140 reach after heat treatment? 
In the as-quenched condition, 4140 can reach 55–57 HRC. However, it is rarely used in this brittle state. Practical working hardness after tempering is usually between 28 HRC and 50 HRC. 

What are typical 4140 tensile and yield strength values? 
For standard pre-hardened (Q&T) stock: 

• Tensile Strength: ~1000–1100 MPa 
• Yield Strength: ~850–950 MPa 

Is 4140 good for fatigue applications? 
Yes. The molybdenum content provides excellent fatigue resistance, making it ideal for cyclically loaded parts like crankshafts and gears. 

Does heat treatment reduce machinability? 
Yes. While annealed 4140 machines easily, Q&T 4140 (at 30 HRC) requires slower speeds and carbide tooling. Machining 4140 above 45 HRC typically requires grinding or hard turning. 

How should I specify AISI 4140 mechanical properties on a PO? 
Be specific. State the standard (e.g., ASTM A193 or A434), the condition (e.g., Quenched & Tempered), and the required hardness range or minimum yield strength.