What is Alloy Steel?
Alloy steel is carbon steel with additional elements like chromium, nickel, molybdenum, vanadium or manganese. These additions change how the steel behaves. Depending on what’s added and how much, the steel can become harder, tougher, more resistant to corrosion, or better at handling heat.
The basic idea is that plain carbon steel has limits. It works well for general use, but won’t hold up well under high stress, extreme temperatures, or corrosive environments. That’s where alloy steel steps in.
Alloy steels are broadly split into two categories, low alloy and high alloy. Low alloy grades have less than 8% total alloying elements. High alloy grades go beyond that. Stainless steel, for example, falls under the high alloy category because of its high chromium content.
Popular Alloy Steel Grades
There are dozens of grades in any alloy steel grades list, but a few come up repeatedly across industries.
| Grades | Specifications |
| 4130 | Contains chromium and molybdenum. Commonly used in aerospace components, automotive parts and structural tubing. Good strength-to-weight ratio and welds well. |
| 4140 | Similar to 4130 but with higher carbon content, which makes it stronger and harder. Used in gears, shafts, bolts and machine parts where wear resistance matters. |
| 4340 | Nickel-chromium-molybdenum steel. High strength, good toughness. Preferred for heavy-duty applications like aircraft landing gear and large shafts. |
| 8620 | A case-hardening grade. The surface hardens while the core stays tough. Common in gears and bearings. |
| 52100 | High carbon, high chromium. Almost exclusively used in bearing components due to its hardness and contact fatigue resistance. |
| D2 | A high-chromium tool steel is hard and wear-resistant. Used in cutting tools, dies, and punches. |
This isn’t a full list of alloy steel grades, but these are the ones most engineers and buyers often use.
AISI/SAE Classification System
The AISI/SAE system is the most widely used method for identifying alloy steel grades. It assigns a four-digit number to each grade. The first two digits identify the alloy type. The last two (or sometimes three) digits indicate the carbon content in hundredths of a percent.
A quick breakdown:
| Series | Alloying Elements |
| 13xx | Manganese |
| 41xx | Chromium-Molybdenum |
| 43xx | Nickel-Chromium-Molybdenum |
| 51xx | Chromium |
| 86xx | Nickel-Chromium-Molybdenum (low) |
| 92xx | Silicon-Manganese |
So when you see 4140 on an alloy steel grades chart, the “41” tells you it’s a chromium-molybdenum steel, and “40” means approximately 0.40% carbon.
This system makes it straightforward to compare grades without needing to look up every detail separately.
What Are Some Specialised Grades?
Beyond the standard list, some alloy steels are built for very specific conditions.
H-grades: These are standard grades with tighter hardenability limits. The “H” suffix (like 4140H) means the steel is tested to ensure it hardens consistently within a defined band. Useful when heat treatment results need to be predictable.
Maraging steels: Very low carbon, high nickel content. They get their strength from ageing treatment rather than carbon hardening. Used in aerospace tooling and high-performance applications.
Chromoly (Cr-Mo) variants: Grades like P91 and P22 are used specifically in high-temperature piping systems in power plants. Their alloy specification is tightly controlled for creep resistance.
Spring steels: Grades like 5160 (chromium steel) are designed to handle repeated flexing without permanent deformation. Leaf springs and coil springs for automotive use.
These grades don’t always appear on a standard alloy steel grades chart because they serve narrow industrial needs.
Alloy Steel Specifications & Grades
An alloy steel specification goes beyond just the grade number. It defines the chemical composition, mechanical properties, heat treatment requirements, and testing standards, and different standards bodies publish these specs. ASTM is used heavily for structural and pressure vessel applications.
ASTM A29 covers general alloy steel bars, and ASTM A193 covers bolting materials, with each spec controlling what the material must contain and how it must perform. AISI/SAE is primarily composition-based and does not specify mechanical properties on its own, only the chemistry. EN/DIN are the European equivalents; for example, EN 1.7225 is roughly equivalent to SAE 4140. BS, or British Standard, is still referenced in some industries, especially in older infrastructure and oil and gas work.
JIS covers Japanese standards used across Asia, and SCM440 under JIS is comparable to 4140. When reviewing any alloy steel specification, you will typically see limits on carbon, manganese, phosphorus, sulfur, and the primary alloying elements. Mechanical property requirements like tensile strength, yield strength, and hardness are usually tied to heat treatment conditions, whether annealed, normalised, or quenched and tempered.
How To Choose The Right Alloy Steel Grade?
Start with the mechanical requirements. What tensile and yield strengths are required? If very high strength is required, grades like 4340 are helpful. For moderate strength with good machinability, 4140 is often the default choice. Then comes the environment. Corrosive conditions usually need a higher alloy content or stainless steel territory. High heat points towards Cr-Mo grades with proven creep resistance.
Fabrication-specific grades like 4130 are easier to weld than 4340. If welding is a major part of the process, options are limited. Cost plays a major role, too. Higher alloy content means higher material cost. Nickel and molybdenum aren’t cheap. Grades like 1040 or 1045 (lower alloy) will cost less and still work fine for many applications.
Finally, check if there’s an existing alloy steel specification required by the applicable standard. In pressure equipment or aerospace, the grade is often already specified. The function of the structure/material and the surrounding conditions affect the choice.
Conclusion
Alloy steels cover a wide range of compositions and properties. The AISI/SAE numbering system makes it easier to navigate the alloy steel grades list, but understanding what each grade actually offers takes more than reading a chart.
Pick the grade based on what the application demands. Considering the following factors helps better: mechanically, environmentally, and in terms of fabrication. Match it to the right alloy steel specification for your industry. When in doubt, consult the relevant ASTM or EN standard or work with a materials supplier who can confirm the right fit.
FAQs
What is the most commonly used alloy steel grade?
4140 is probably the most common. It offers a good balance of strength, hardness, and machinability, which makes it useful across a wide range of industrial applications, including gears, shafts, and tooling.
What alloys are used in steel alloys?
The most common additions are chromium, nickel, molybdenum, manganese, vanadium, and silicon. Some grades also include cobalt, tungsten, or boron, depending on the intended use.
How are alloy steel grades classified?
Mostly through the AISI/SAE four-digit system in North America, and through EN or DIN numbers in Europe. The classification is based on chemical composition, particularly the type and amount of alloying elements and carbon content.
How do you read alloy steel grade specifications?
A spec will list the chemical composition range (min/max percentages for each element), the required mechanical properties (tensile strength, yield strength, elongation and hardness), and the applicable heat treatment condition. The grade number tells you the composition family; the full alloy steel specification tells you the performance requirements.
What standards define alloy steel grades?
ASTM, AISI/SAE, EN (European Norm), DIN (German), BS (British Standard) and JIS (Japanese) are the main ones. Which standard applies depends on the industry and geography. In the US, ASTM and SAE are the most common, and in Europe, EN standards are used.
