Views: 5896 Author: Site Editor Publish Time: 2025-04-19 Origin: Site
1. Introduction: A Subtle Difference with Big Consequences
In industries such as compressors, valves, and fasteners, "stainless steel" and "magnetic stainless steel" (often called "stainless iron") are frequently confused. In fact, "stainless iron" is not a formal grade but a common term for magnetic martensitic or ferritic stainless steels. The differences in composition, microstructure, performance, and application are significant. Wrong material selection can lead to rust, fracture, or dramatically shortened service life. This article clarifies these differences from three perspectives and provides quick identification methods.
2. Comparison of Three Microstructure Types
Type | Typical Grades | Carbon Content | Chromium Content | Magnetic | Key Features |
|---|---|---|---|---|---|
Martensitic | 1Cr13, 2Cr13, 3Cr13, 4Cr13 | 0.1–0.45% | 12–14% | Yes | Hardenable by heat treatment; high hardness; corrosion resistance decreases as carbon increases |
Ferritic | 0Cr13, 1Cr17, 1Cr17Ti | <0.15% | 12–30% | Yes | Better corrosion resistance than martensitic; good weldability but poor toughness; 475°C embrittlement |
Austenitic | 201, 202, 304, 321, 316L | Low carbon | 18% + Ni 8–11% | No | Highest corrosion resistance; excellent formability; most widely used |
3. Detailed Analysis of Each Type
3.1 Martensitic Stainless Steel (One Type of "Stainless Iron")
Composition & Microstructure: 12–14% Cr, 0.1–0.45% C. Typical grades: 1Cr13 to 4Cr13.
Performance: Can be strengthened by quenching + tempering.
1Cr13, 2Cr13 (high-temperature tempering at 600–700°C) → tempered sorbite → used for turbine blades, structural parts.
3Cr13, 4Cr13 (low-temperature tempering at 200–300°C) → tempered martensite, hardness up to HRC 50 → used for medical instruments, cutters, oil pump shafts.
Rule: Higher carbon → higher strength and hardness → lower corrosion resistance.
3.2 Ferritic Stainless Steel (Second Type of "Stainless Iron")
Composition & Microstructure: 12–30% Cr, <0.15% C. Typical grades: 1Cr17, 1Cr28.
Performance: Single-phase ferrite from room temperature to high temperatures. Cannot be strengthened by heat treatment. Better corrosion resistance, plasticity, and weldability than martensitic grades.
Key Defects:
475°C embrittlement: Occurs after staying at 450–550°C. Can be eliminated by heating to 600°C and rapid cooling.
Sigma phase embrittlement: Occurs after long-term heating at 600–800°C.
Intergranular corrosion and grain coarsening: Occurs when rapidly cooled from above 925°C.
Applications: Corrosion-resistant parts, widely used in nitric acid and nitrogen fertilizer industries.
3.3 Austenitic Stainless Steel (True "Stainless Steel")
Composition & Microstructure: 18% Cr + 8–11% Ni. Typical grade: 1Cr18Ni9 (304).
Performance: Nickel expands the austenite region, giving a single-phase austenite structure at room temperature. Highest chemical stability and corrosion resistance.
Solution Treatment: Heating to 1100°C followed by water cooling dissolves carbides, producing single-phase austenite and preventing intergranular corrosion.
Applications: The most widely used stainless steel, covering food, chemical, medical, construction, and many other fields.
4. Three Quick Identification Methods
Method | Procedure | Result Interpretation |
|---|---|---|
1. Magnetic Test | Use a common magnet | Magnetic → Martensitic/Ferritic ("stainless iron") |
2. Spark Test | Grind on a grinding wheel, observe spark pattern | "Stainless iron": long, numerous, heavily branched sparks |
3. Copper Sulfate Test | Apply copper sulfate solution to clean surface (remove oxide layer) | No color change → Stainless steel |
4. Material Tester | Use handheld spectrometer or alloy analyzer | Directly reads Cr and Ni content for precise grade identification |
5. Conclusion
"Stainless iron" is not a deception but a common name for martensitic and ferritic stainless steels. The core differences from austenitic stainless steel are: magnetism, nickel content, and corrosion resistance level. For high corrosion resistance requirements (humid environments, food contact, chemical media), choose austenitic grades like 304 or 316. For high strength and hardness with mild corrosive conditions, "stainless iron" may be suitable. The simplest quick test is the magnet test – one touch tells you a lot.
