One of the most widely used materials in the aerospace, manufacturing and automotive industries is 4140 Alloy steel. It is specifically suited for high-stress components because of its toughness and wear-resistability.
This article will introduce you to this unique type of alloy in detail, including its properties and applications.
What is 4140 Alloy Steel?
Let’s break down the term ‘4140 Alloy Steel’ for a more comprehensive understanding.
Alloy steel is a type of steel that contains elements other than iron and carbon to supplement its properties. The number 4140 tells us that its a low-alloy steel, which means that it has a lesser amount of alloying elements compared to other types of steel.
It is given the word ‘chromoly steel’, because carbon, chromium and molybdenum are its alloying elements.
Alloying enhances properties like wear resistance, abrasion and impact resistance, tensile strength, and high fatigue strength.
How 4140 Alloy Steel is Made
The production of 4140 steel is a complicated process that follows a series of steps.
1) Melting
First, you melt iron ore with scrap iron, molybdenum, chromium, and carbon and mix them in an electric or oxygen furnace at 1500°C or above. The melted result is molten steel.
2) Cooling
Molten steel is taken out of the furnace and allowed to cool either in open air or through quenching. The result is solidified steel. At this stage, 4140 steel may have internal stresses or hardness from rapid cooling.
3) Refining
The next step is refining. Hydrofluoric acid is suitable for removing rust and impurities like sulphur or phosphorus that may appear on the 4140 steel’s surface.
4) Molding
After the metal has been cleaned, it is then poured into molds to make shapes like ingots, billets or slabs.
5) Inspection
Throughout the process, the steel is tested for chemical composition, mechanical properties (strength, toughness, hardness), and dimensional accuracy to ensure it meets 4140 specifications.
Now, the fully processed 4140 steel is ready for use.
Properties of 4140 Alloy Steel
The properties of 4140 alloy steel show its great tensile strength, toughness and adaptibility, making it a great choice for many engineering applications. Properties include physical, chemical and mechanical and we will look at them one by one.
Physical Properties
4140 steel has a density of 7.85 g/cm³ (or 7850 kg/m³).
Chemical Composition
The chemical composition of 4140 alloy steel dictates its performance under heat treatment, wear resistance, and strength. Below is a detailed table of its elemental makeup:
| Element | Percentage | Notes |
| Carbon (C) | 0.38-0.43% | Provides hardness and strength |
| ABS/PC | 0.8-1.1% | Enhances hardenability and corrosion resistance |
| Molybdenum (Mo) | 0.15-0.25% | Boosts strength and wear resistance |
| Manganese (Mn) | 0.75-1.0% | Improves toughness and hardenability |
| Silicon (Si) | 0.15-0.35% | Aids in deoxidation and strength |
| Phosphorus (P) | ≤0.035% | Controlled impurity, affects brittleness |
| Sulfur (S) | ≤0.040% | Controlled impurity, impacts machinability |
| Iron (Fe) | Balance | Primary component, forms the steel matrix |
This unique blend of elements, especially chromium and molybdenum, increases 4140 steel’s durability and hardness. As a result, 4140 alloy steel is ideal for axles, crankshafts, and gears that face heavy wear.
Mechanical Properties
Mechanical properties show whether 4140 steel is suitable for applications working with heavy loads. The table below provides a detailed mindmap of all mechanical properties.
| Property | Value (Annealed) | Value (Heat-Treated Example) | Notes |
| Tensile Strength | 655 MPa | 1000-1200 MPa (e.g., tempered at 205°C) | Resistance to breaking under tension |
| Yield Strength | 415 MPa | 800-1000 MPa | The point at which permanent deformation begins |
| Hardness | 197 HB | 300-350 HB | Resistance to indentation, Brinell scale |
| Elongation | 25.7% | 12-15 | Ductility, ability to stretch before breaking |
| PSI Bulk modulus | 140 GPa | - | resistance to uniform compression |
These values tell us how versatile 4140 steel is, offering strength and ductility in its annealed state, while post-heat treatment suiting applications such as bolts, shafts, and couplings requiring toughness and fatigue resistance.
Thermal Properties
Thermal properties tell us how 4140 steel acts as a heat-treated condition.
| Property | Value | Notes |
| Thermal Conductivity | 42.6 W/m·K | Ability to conduct heat, key for processing |
| Coefficient of Thermal Expansion | 12.3 µm/m·°C | Energy required to raise temperature |
| Specific Heat Capacity | 473 J/kg·K | Boosts strength and wear resistance |
Steel’s heat conduction and expansion properties are essential for forging, welding, or heat-treating to perform well in tooling or machinery.
Advantages For Manufacturing
The advantages of 4140 steel for manufacturing arise because it can adapt to different kinds of processes.
Machinability
4140 steel is easy to machine, especially in its annealed state. As a result, you can use drills, lathes or milling machines to create precise components like drill collars, gears, shafts, bolts and piston rods and axles, all products that require the ability to withstand high stress.
Its a good choice for manufacturing intricate components because it minimizes tool wear and speeds up production process.
Forming
The ductility of 4140 steel, especially when annealed, makes it highly formable. Manufacturers can bend, stamp, or draw it into shapes such as piston rods or axles without risking cracks. So, you can easily create complex components without compromising the steel’s strength.
Welding
Welding can create strong and reliable joints suitable for large structures if you let it undergo proper heat treatment beforehand to avoid cracking.
Heat Treatment
Heat treatment enhances 4140 steel’s versatility by allowing manufacturers to fine-tune its hardness, strength, and ductility. Processes like annealing, hardening, and tempering adapt the steel for specific uses, from wear-resistant tools to tough structural parts.
This customization capability ensures optimal performance in diverse applications.
Forging
Forging is the process where you shape a heated metal via hammering, rolling or pressing at high temperatures of 900°C to 1200°C. It improves the durability of 4140 steel, making it a good fit to create crankshafts or couplings requiring good abrasion and impact resistance.
Hot Working
Hot working means shaping the 4140 steel above its recrystallization tempwerature of 400-700 °C. Hot worked steel is ductile and has a reduced strength to cater to deformation on a large scale. It removes internal tension and forms basic shapes like flat bars and plates.
Cold Working
Cold working helps to strengthen steel below its recrystallization temperature during strain hardening. It allows for a precise finishing and tight tolerance for parts like fasteners or gears.
Annealing
The annealed condition is when the steel is heat treated at 450°F – 1600°F (788°C – 871°C) and held for 1 hour (longer for thicker pieces) and cooled in a controlled way.
This cooling is slow and intentional, compared to cooling it in open air. Here, we turn off the furnace and allow the metal to cool inside. This prevents new stresses or hardness from forming, resulting in a softer material with good ductility.
Tempering
Tempering 4140 steel after you have hardened it once involves reheating it to a lower temperature to balance hardness with toughness. This reduces brittleness, relieves internal ten, and improves ductility, making the steel resilient against impact and fatigue.
Hardening
Hardening 4140 steel by heating it to high temperatures and quenching it in oil or water significantly boosts its strength and wear resistance. Quenching is the process when the heated 4140 steel is immersed in water/mineral oil to lower the elevated temperatures.
The normal HRC range (Hardness Rockwell C range) for steel after hardening is 40-60 HRC.
Ultimately, 4140 alloy steel is a versatile and reliable material that empowers manufacturers to craft high-quality components for various industries. It caters to a plethora of engineering demands, making it a highly valuable material in manufacturing.
Conclusion
AISI 4140 alloy steel is useful for many applications in engineering and manufacturing. AISI 4140 is a versatile material for its strength and toughness. Mostly, 4140 alloy steel is used in applications that need wear resistance and fatigue resistance.
This blog covers its detailed production, alloying and machining properties making it a dependable choice for your projects.
FAQs
What is 4140 steel good for?
4140 alloy steel is useful for demanding applications that require high wear resistance like gears, shafts and suspensions.
What is 4140 alloy equivalent to?
4140 alloy steel has many other names and in other material standards, it is equivalent to AISI 4140, DIN 1.7225, 42CrMo4, EN 19, and JIS SCM440.
Is 4140 considered stainless steel?
For any steel to be considered stainless, it requires a chromium content of at least 10.5%. 4140 alloy steel is not stainless steel because it has about 1% chromium.
What grade of steel is 4140?
4140 is the grade classification and 4140 is named in the molybdenum chromium alloy steel category.