Stainless steel, a high-performance alloy, plays an indispensable role in industry and daily life due to its corrosion resistance, high strength, and aesthetic appeal. However, the world of stainless steel is much more than that. Behind its complex classifications, grades, and performance parameters lies a wealth of technical knowledge.
This article aims to provide a comprehensive guide to stainless steel, from the most basic structure and series classification to detailed grade characteristics and core technical parameters, to help you become a stainless steel expert.
Basic Knowledge and Microstructure of Stainless Steel
Stainless steel’s “rustproof” properties primarily stem from an extremely thin and durable protective chromium-rich oxide film (passivation film) that forms on its surface. Based on its metallographic structure at room temperature, stainless steel can be divided into three basic types:
Austenitic stainless steel: Contains chromium and nickel as the primary alloying elements. Its structure is stable, offering excellent corrosion resistance, ductility, and toughness, and is non-magnetic or weakly magnetic in its solid solution state.
Ferritic stainless steel: Contains chromium as the primary alloying element, with little or no nickel. It exhibits excellent thermal conductivity, strong oxidation resistance, and magnetic properties.
Martensitic stainless steel: Contains chromium as the primary alloying element, and its hardness can be significantly increased through heat treatment. It exhibits high strength, hardness, and magnetic properties.
Five major series of stainless steel: from 200 series to 600 series
Stainless steel is typically divided into several series based on its primary alloying components. Understanding these series is fundamental to understanding stainless steel grades.
200 Series (Chromium-Manganese-Nitride Austenitic):
Features: Partial nickel substitution with manganese and nitrogen results in lower cost. Excellent ductility and generally non-magnetic properties. However, corrosion resistance is relatively poor.
Representative grade: 201.
| Steel Grades | Ni | Mn | N | C | Si | P | S | Cr | Cu | Remarks |
|---|---|---|---|---|---|---|---|---|---|---|
| LH | 0.35 | 9.95 | 0.154 | 0.088 | 0.51 | 0.041 | 0.002 | 14.45 | 1.37 | Lianzhong |
| LA | 0.67 | 9.94 | 0.144 | 0.077 | 0.49 | 0.035 | 0.001 | 14.56 | 1.1 | Lianzhong |
| L1 | 1.12 | 9.3 | 0.148 | 0.073 | 0.61 | 0.046 | 0.002 | 14.41 | 1.43 | Lianzhong |
| L4 | 4.12 | 7.93 | 0.065 | 0.045 | 0.5 | 0.035 | 0.002 | 14.65 | 1.45 | Lianzhong |
300 Series (Chromium-Nickel Austenitic):
Features: The most commonly used and widely used series. Excellent corrosion resistance, heat resistance, mechanical properties, and machinability.
Representative grades: 304 (most commonly used), 304L (ultra-low carbon), 316 (chloride-resistant), 316L (chloride-resistant ultra-low carbon), 310S (high-temperature resistant), 321 (contains titanium and resists intergranular corrosion).
400 Series (Chromium-based Ferrite and Martensite):
Features: Contains no nickel and is magnetic. Relatively low-priced, it can replace the 300 series in certain applications.
Representative grades: 410S (0Cr13), 420J1 (2Cr13), 420J2 (3Cr13), 430 (ferritic, generally corrosion-resistant), 410 (martensitic, hardenable), and 443 (high chromium, corrosion resistance similar to 304).
500 Series (Heat-Resistant Chromium Alloy Steel):
Features: Primarily used in high-temperature environments, with excellent oxidation and heat resistance.
600 Series (Martensitic Precipitation-Hardening Stainless Steel):
Features: Heat treatment produces carbide precipitation, resulting in extremely high strength and hardness.
Representative grade: 630.
Common stainless steel grades, core properties and typical applications
Different stainless steel grades vary widely in performance and applications due to differences in their chemical composition. The following table lists the characteristics and typical uses of some common stainless steel grades.
| Grade | Series | Core Characteristics & Properties | Typical Applications |
|---|---|---|---|
| 304 | 300 series | Excellent overall performance, tensile strength about 520 MPa. | Household items, tableware, medical instruments, food industry. |
| 316L | 300 series | Contains molybdenum, has excellent resistance to pitting and crevice corrosion, tensile strength of about 485 MPa. | Marine environments, chemical equipment, pharmaceutical, and paper industries. |
| 430 | 400 series | Magnetic, with better thermal conductivity than austenitic stainless steels. | Washing machine drums, home appliance decoration, and automotive exhaust pipes. |
| 1Cr18Ni9Ti | 300 series | Equivalent to old Chinese grade 321, titanium-stabilized, with strong resistance to intergranular corrosion. | Corrosion-resistant containers, pipelines, and aircraft exhaust manifolds. |
| 0Cr17Ni12Mo2 | 300 series | Equivalent to 316, excellent resistance to chloride corrosion. | Seawater environments, chemical and fertilizer equipment. |
| 2Cr13 | 400 series | Martensitic stainless steel, high hardness after heat treatment. | Valves, turbine blades, surgical instruments. |
| 444 | 400 series | Highly corrosion-resistant ferritic stainless steel, with pitting and stress corrosion resistance comparable to 316. | Water tanks, solar water heaters, and food processing equipment. |
| 630 | 600 series | Precipitation-hardening stainless steel, very high strength and hardness, and good machinability. | Valves, pump shafts, high-strength fasteners. |
For general indoor use, 304 is enough. For marine or chemical use, 316 or duplex is a safer choice.
Practical Identification Method: Quickly Differentiate Stainless Steel by Magnetism
This is a very practical identification method. The magnetism of stainless steel is directly related to its microstructure.
Non-magnetic or weakly magnetic: Most austenitic stainless steels (such as 304 and 316) are non-magnetic under normal conditions.
Magnetic: Ferritic and martensitic stainless steels (such as 430 and 410) are strongly attracted to magnets due to their structural properties.
Important Note: It is important to note that austenitic stainless steels like 304 undergo structural changes after cold working (such as stamping and stretching), resulting in a certain degree of magnetism. Therefore, the quality of stainless steel cannot be judged solely by magnetism; instead, a comprehensive consideration should be given to the grade, source, and actual application scenario.
How to Choose the Right Stainless Steel
Evaluate the Environment
Indoor, outdoor, marine, or chemical exposure?
Check Corrosion Resistance Requirements
Use 316/duplex for chlorides, 304 for mild environments.
Consider Mechanical Strength
Duplex and martensitic for higher strength applications.
Assess Temperature Needs
High heat → 310 stainless steel.
Cryogenic → austenitic grades.
Fabrication & Weldability
Austenitic → best for forming/welding.
Martensitic → may require preheating.
Balance Cost vs. Performance
304 is cost-effective for general use.
316, duplex, or 317L for harsh conditions.
FAQ
What are the main differences between 304 and 316 stainless steel?
The primary differences lie in alloy composition and corrosion resistance. 316 stainless steel adds molybdenum (Mo) to 304 stainless steel. This addition significantly enhances 316’s corrosion resistance in chloride-containing environments, particularly in seawater or certain chemical media, effectively resisting pitting and crevice corrosion. Therefore, 316 is often used in applications requiring higher corrosion resistance, such as marine engineering, medical equipment, and chemical processing, while 304 is suitable for most everyday and general-purpose environments.
Will stainless steel rust?
This is a common misconception. Stainless steel is correctly called “acid-resistant stainless steel.” It’s not absolutely rust-proof, but rather extremely corrosion-resistant. Stainless steel has an invisible passive film on its surface that is self-healing and resists rust. However, under extreme conditions, such as prolonged exposure to chloride ions (such as seawater or industrial salt spray), mechanical damage, or prolonged oxygen depletion, this passive film can break down, leading to rust.
Why do some stainless steels attract magnets while others don't?
This depends on the stainless steel’s microstructure. Austenitic stainless steels (such as 304 and 316) are nonmagnetic under normal conditions due to their stable internal structure, making them unattractive to magnets. Ferritic and martensitic stainless steels (such as 430 and 410), on the other hand, are naturally magnetic. It’s worth noting that after cold working (such as stamping and stretching), the microstructure of 304 stainless steel may change, resulting in weak magnetism. However, this doesn’t necessarily indicate a quality issue.
How can I easily identify stainless steel grades?
Magnet testing can provide a preliminary distinction between stainless steel grades, but it can’t pinpoint the specific grade. Currently, the most reliable identification methods are spectral analysis or chemical reagent testing. These specialized methods can accurately analyze the chemical composition of stainless steel and determine its specific grade. If you require precise grade information, we recommend seeking assistance from a professional organization or supplier.
Conclusion: The Ultimate Guide to Stainless Steel
Stainless steel is a versatile, durable, and sustainable material that powers industries around the world. With numerous stainless steel grades available, the key to success lies in selecting the one that best suits your environment, strength requirements, and budget.
Huaxiao Metal offers a comprehensive range of stainless steel products—from 304 and 316 to duplex stainless steel and specialty alloys—ensuring a perfect match for your project needs. Our team of experts will guide you in making the right choice that balances performance and cost-effectiveness.
