In the machining industry, the string of metal codes on a drawing, such as 45, 304, or Q235B, serves as the common language shared by engineers, buyers, and machine operators. A single mistake in writing or interpreting a material grade can lead to incorrect raw material purchases, unsuitable machining processes, or even the scrapping of an entire batch of parts.
However, metal designations come in many forms. Different materials and different standards follow different coding systems. For newcomers, they can be confusing, and even experienced professionals sometimes encounter unfamiliar markings.
In this guide, we have compiled the most commonly used metal grades in mechanical machining and organized them by category. Each grade is clearly explained in terms of its meaning, properties, and typical applications. Save this reference for quick comparison during drawing reviews and material sourcing, and reduce costly communication errors.
01 Carbon Steel Grades
Carbon steel is one of the most commonly used materials in machining. Its grade designation mainly reflects the carbon content. In general, the higher the carbon content, the higher the hardness and the lower the toughness.
| Grade (GB Standard) | Description | Key Characteristics | Typical Applications |
|---|---|---|---|
| Q235B | Yield strength 235 MPa, Quality Grade B (mild steel) | Medium strength, good toughness, easy to machine | Brackets, bases, connectors, non-critical structural components |
| 45 | High-quality carbon structural steel, carbon content 0.42–0.50% | Good overall mechanical properties, can be quenched and tempered | Shafts, gears, connecting rods, bolts, nuts (core mechanical parts) |
| 20 | High-quality carbon structural steel, carbon content 0.17–0.24% | Excellent toughness, good weldability, suitable for carburizing | Stamped parts, welded components, carburized gears and bushings |
| A3 (Old Standard) | Equivalent to new standard Q235A, Quality Grade A | Low cost, good machinability | Temporary supports, non-critical stamped parts (gradually replaced by Q235B) |
Quick tip: In carbon steel grade numbers, the digits typically represent the average carbon content in hundredths of a percent. For example, Grade 45 steel contains approximately 0.45 percent carbon, making it easy to remember and identify.
02 Alloy Steel Grades
Alloy steel is produced by adding alloying elements such as chromium, manganese, and nickel to carbon steel. The grade designation typically reflects carbon content plus alloy element symbols and their approximate percentages. The key advantage of alloy steel is its ability to deliver tailored mechanical properties for specific applications.
| Grade (GB Standard) | Description | Key Characteristics | Typical Applications |
|---|---|---|---|
| 40Cr | Carbon content 0.37–0.44%, Chromium 0.8–1.1% | High strength, good toughness, suitable for quenching and tempering | Critical shafts, gears, connecting rods (automotive and machine tool core components) |
| 20CrMnTi | Carbon content 0.17–0.23%, alloyed with Chromium, Manganese, Titanium | Good carburizing performance, high surface hardness after quenching | Transmission gears, spline shafts, precision gears |
| 65Mn | Carbon content 0.62–0.70%, Manganese 0.9–1.2% | High elasticity, easy heat treatment | Springs, washers, chains, blade plates |
| 38CrMoAl | Carbon content 0.35–0.42%, alloyed with Chromium, Molybdenum, Aluminum | High strength, wear resistant, suitable for nitriding | Precision spindles, piston rods, high pressure valves |
Quick tip: Alloy element abbreviations are easy to remember.
Cr stands for chromium, Mn for manganese, Ti for titanium, and Mo for molybdenum.
In most grade designations, the number at the beginning indicates carbon content, while the numbers following the alloy symbols indicate the approximate content of those elements. If no number is listed after an element, it usually means the content is 1.5 percent or less.
03 Stainless Steel Grades
Stainless steel grades are generally divided into three categories: austenitic, ferritic, and martensitic. In machining applications, austenitic stainless steels in the 300 series and martensitic stainless steels in the 400 series are the most commonly used. The grade numbers directly reflect their chemical composition.
| Grade (GB/ASTM Standard) | Description | Key Characteristics | Typical Applications |
|---|---|---|---|
| 304 (06Cr19Ni10) | Contains 19% Cr, 10% Ni, Austenitic stainless steel | Strong corrosion resistance, non-magnetic, easy to machine | Food machinery, chemical equipment, medical devices, daily hardware |
| 316 (06Cr17Ni12Mo2) | Contains 2% more Mo than 304, Austenitic stainless steel | Resistant to acids and alkalis, excellent seawater corrosion resistance | Marine equipment, chemical pipelines, parts used in highly corrosive environments |
| 410 (1Cr13) | Contains 13% Cr, Martensitic stainless steel | Heat treatable, magnetic, low cost | Knives, valves, mechanical parts (without high corrosion resistance requirements) |
| 430 (06Cr17) | Contains 17% Cr, Ferritic stainless steel | Magnetic, moderate corrosion resistance | Home appliance housings, decorative parts, non-critical structural components |
Common mistake to avoid: Do not confuse 304 and 316. Grade 316 offers stronger corrosion resistance, especially in harsh or marine environments, but typically costs 20 to 30 percent more than 304. For general applications, 304 is usually sufficient, while 316 is recommended only for highly corrosive conditions.
04 Aluminum Alloy Grades
In machining applications, the 6000 series and 7000 series aluminum alloys are the most commonly used. The first digit of the grade indicates the alloy series, while the last two digits identify the specific alloy within that series. Aluminum alloys are widely chosen for their combination of light weight and high strength.
| Code (GB Standard) | Interpretation | Core Characteristics | Typical Applications |
|---|---|---|---|
| 6061 | Al-Mg-Si alloy, versatile 6 series | Medium strength, easy to machine, anodizable | Mechanical parts, frames, connectors, automotive components |
| 7075 | Al-Zn-Mg-Cu alloy, high-strength 7 series | Strength close to steel, lightweight | Aerospace parts, high-end equipment spindles, high-load structural parts |
| 5052 | Al-Mg alloy, rust-resistant 5 series | High corrosion resistance, easy to stamp | Enclosures, panels, pipes, welded components |
| 2A12 | Al-Cu alloy, 2 series duralumin | High strength, heat-resistant | Aerospace structural components, high-pressure vessels (difficult to machine) |
Quick tip: The 6000 series is the true all rounder. It offers a good balance of strength, corrosion resistance, and machinability, and can meet around 80 percent of lightweight structural needs in mechanical manufacturing. The 7000 series provides the highest strength, but it comes at a higher cost and is more challenging to machine.
05 Copper Alloy Grades
Copper alloys are generally divided into brass, which is a copper zinc alloy, and bronze, which is a copper tin alloy. In GB designations, grades often begin with H for brass and Q for bronze. These materials are widely valued for their excellent electrical conductivity, thermal conductivity, and wear resistance.
Quick tip: In brass and bronze grade designations, the numbers usually indicate the approximate percentage of the main alloying element. For example, in many brass grades, the number reflects the copper content, making it easier to quickly estimate the material composition and performance.
06 Quick Tips to Identify Metal Grades
Identify the category by the prefix
The first letter or number in a grade often tells you the material type. Grades starting with Q typically refer to carbon steels classified by yield strength under the GB standard. H at the beginning usually indicates brass, while Q followed by specific bronze designations refers to bronze. Grades that begin with numbers are commonly carbon steel or alloy steel. Numbers starting with 3 or 4 are widely recognized stainless steel grades in industry standards.
Simple rule for carbon steel grades
Carbon steel grades are easy to remember. The number generally represents the average carbon content in hundredths of a percent. For example, Grade 45 contains about 0.45 percent carbon, while Grade 20 contains about 0.20 percent carbon.
Recognize alloy element symbols
Alloy steel grades include chemical element symbols that indicate added alloying elements. Cr stands for chromium, Mn for manganese, Ti for titanium, and Mo for molybdenum. When these letters appear in a grade designation, they confirm the presence of the corresponding alloying element.
Pay attention to standards
GB refers to Chinese national standards, ASTM to American standards, and DIN to German standards. When handling international trade orders, it is essential to accurately convert between equivalent grades under different standards to avoid material selection errors.
07 Common Mistakes: Do Not Confuse These Grades
1) 45 Steel ≠ 40Cr
Grade 45 is carbon steel, while 40Cr is alloy steel. 40Cr offers significantly higher strength and wear resistance, and its price can be roughly twice that of 45 steel. For critical load bearing shafts and gear components, selecting the wrong material can lead to serious performance issues.
2) 304 ≠ 316
Grade 316 provides much stronger corrosion resistance than 304, but it typically costs 20 to 30 percent more. For general working conditions, 304 is usually sufficient. 316 should be reserved for highly corrosive environments to avoid unnecessary material costs.
3) Q235B ≠ Q235A
Although both are carbon steels, Q235B is a killed steel with lower impurity levels and better toughness. For mechanical structural components, Q235B is generally preferred, and replacing it with Q235A is not recommended.
4) 6061 ≠ 7075
The 6000 series aluminum alloys are versatile and easy to machine, while the 7000 series focuses on ultra high strength. For lightweight parts that do not carry extremely high loads, 6061 is usually sufficient. There is no need to over specify 7075 unless the application truly requires it.
