What is stainless steel?
Stainless steel is actually an abbreviation for "stainless acid-resistant steel," referring to a class of alloy steels with iron as the base material and a chromium content of no less than 10.5% (by mass). This metal can be modified by adding alloying elements such as nickel, molybdenum, manganese, and nitrogen to adjust its internal composition and properties. For instance, incorporating nickel into stainless steel enhances its corrosion resistance and high-temperature tolerance. Consequently, this metal material finds extensive applications across various industries after processing.
Classification of Stainless Steel
Based on the microstructure after heat treatment, stainless steel is primarily categorized into the following five major types.
Austenitic Stainless Steel
Typical grades include 304 and 316. This type contains high levels of austenite-forming elements such as nickel, manganese, and nitrogen. It is characterized by non-magnetic properties, excellent ductility and toughness, superior weldability, and generally good corrosion resistance. However, it cannot be strengthened through heat treatment.
Ferritic Stainless Steel
Ferritic stainless steel primarily uses chromium as its alloying element and exhibits a body-centered cubic (BCC) structure. A typical grade is 430. It is characterized by magnetic properties and high strength, but exhibits poor low-temperature toughness, prone to grain coarsening in the heat-affected zone during welding, and lower corrosion resistance compared to austenitic steel.
Martensitic Stainless Steel
Typical grades include 410 and 420. They exhibit high strength and hardness, with mechanical properties adjustable through tempering, but offer relatively poor corrosion resistance.
Precipitation-Hardening Stainless Steel
By adding elements such as Cu, Nb, and Al, followed by solution treatment and precipitation hardening to form strengthening phases, these steels combine high strength with good corrosion resistance. A typical grade is 17-4PH.
Duplex Stainless Steel
Composed of both austenitic and ferritic phases, such as 2205. Combines the toughness of austenitic steel with the strength of ferritic steel, offering excellent resistance to stress corrosion cracking.
Properties of Stainless Steel metal
| Property Category | Key Parameter | Description and Typical Values |
|---|---|---|
| Mechanical Properties | Tensile Strength | Typically in the range of 500-1000 MPa, depending on the grade and condition. |
| Yield Strength | Typically from 200 MPa to over 550 MPa (lower for austenitic steels, higher for martensitic and precipitation-hardening steels). | |
| Elongation | Austenitic steels usually >40%; ferritic and martensitic steels are relatively lower. | |
| Hardness | Measured by Brinell (HB), Rockwell (HRB/HRC), etc., covering a wide range. | |
| Physical Properties | Density | Approximately 7.7 - 8.0 g/cm³. |
| Melting Point | Approximately 1370 - 1450 °C, depending on composition. | |
| Thermal Conductivity | Relatively low, about 15-30 W/(m·K), which is 1/3 to 1/2 that of carbon steel. | |
| Coefficient of Thermal Expansion | Higher for austenitic steels (e.g., ~17×10⁻⁶/K for 304), requiring attention to thermal stress. | |
| Magnetism | Austenitic steels are typically non-magnetic (may become slightly magnetic after working); ferritic/martensitic steels are magnetic. | |
| Chemical Properties | Corrosion Resistance | Includes resistance to uniform corrosion, pitting, crevice corrosion, intergranular corrosion, etc. |
| Pitting Resistance Equivalent Number (PREN) | PREN = %Cr + 3.3×%Mo + 16×%N. A higher value generally indicates better pitting resistance. | |
| Manufacturing Properties | Machinability | Varies significantly, influenced by hardness, toughness, work-hardening tendency, and additives (e.g., sulfur). |
| Weldability | Generally good for austenitic steels; reduced toughness in the HAZ of ferritic steels; martensitic steels require preheating and PWHT. | |
| Formability | Austenitic steels exhibit excellent cold working properties (stamping, deep drawing); other types are relatively poorer. |
What are the surface finish grades for stainless steel?
After undergoing custom CNC machining, stainless steel forms Custom Stainless Steel Parts. Its surface can further achieve diverse appearances and functionalities through rolling, polishing, and chemical treatments.
Rolled surfaces: such as No.1 (hot-rolled + annealed), 2B (cold-rolled + bright annealed), BA (bright annealed);
Mechanical Polishing: Includes Honed (HL), Short-grain (SB), and Mirror (8K).
Stainless Steel vs. Titanium Alloys
| Property | Stainless Steel | Titanium Alloy |
|---|---|---|
| Density (g/cm³) | 7.9 | Approximately 4.5 |
| Specific Strength | Medium | Extremely High |
| Corrosion Resistance | Excellent (in oxidizing media) | Excellent (especially in chloride ion environments) |
| Cost | Low to Medium | Very High |
| Machining Difficulty | Medium (Processing difficulty varies depending on the grade.) | High (severe tool wear) |
| Typical Applications | Structural parts, daily-use products, chemical containers | Aerospace, medical implants, seawater equipment |
FAQ
Q: Under what conditions can stainless steel still rust?
A: Stainless steel will corrode in environments where its passivation film is continuously damaged or cannot self-repair, such as: media with high chloride ion concentrations (seawater), reducing acid environments, crevice structures, or galvanic corrosion caused by contact with non-metals.
Q: How to choose between 304 and 316 stainless steel?
A: 304 is suitable for most atmospheric, freshwater, and general chemical environments. 316, containing molybdenum, is suitable for environments with chloride ions (e.g., seawater, chemical plants) or where higher pitting corrosion resistance is required.
Q: What is "passivation treatment"? What is its purpose?
A: Passivation treatment is a chemical post-processing technique typically using nitric acid or citric acid solutions to remove free iron particles from the stainless steel surface (primarily resulting from contact with steel tools during fabrication) and promote the formation of a complete, uniform chromium oxide film. Its purpose is to maximize the inherent corrosion resistance of stainless steel and prevent surface rusting (stain caused by iron contamination).
Q: How does stainless steel perform at different temperatures?
A: Austenitic stainless steels (e.g., 304, 316) exhibit excellent low-temperature toughness, making them suitable for cryogenic environments like liquid nitrogen or oxygen. 304/316 grades can be used intermittently below 925°C (1632°F) and continuously below 870°C (1572°F), demonstrating good oxidation resistance. However, prolonged exposure between 450-850°C increases the risk of carbide precipitation, necessitating the use of L-grade or stabilized grades (e.g., 321).