Why Do Stainless Steel Bolts Have Magnetism

Subconsciously, we often believe that 304 and 316 stainless steel bolts are completely non-magnetic. Many users even judge the quality of stainless steel bolts by their magnetism, assuming non-magnetic ones are authentic and magnetic ones are inferior. However, this perception is deeply flawed and requires reinterpretation from the perspective of materials science.

I. Material Classification and Magnetic Nature of Stainless Steel Bolts

The magnetism of stainless steel is determined by its crystalline structure, not solely by its grade or carbon content:

1. Austenitic Stainless Steel (300 Series: Non-Magnetic/Weakly Magnetic)

Typical Grades: SUS304 (06Cr19Ni10), SUS316 (06Cr17Ni12Mo2)

Structural Characteristics: Predominantly austenite at room temperature (face-centered cubic structure, non-ferromagnetic), theoretically non-magnetic or weakly magnetic (permeability μ≈1.01-1.1).

Actual Magnetic Source:

Cold heading, thread rolling, and other cold working processes force partial transformation of austenite into martensite (body-centered cubic structure, ferromagnetic), generating weak magnetism (martensite content can reach 15%-20% with deformation ≥20%).

Annealing (e.g., holding at 650°C for 1 hour) can reverse martensite back to austenite, reducing magnetism.

2. Martensitic Stainless Steel (400 Series: Strongly Magnetic)

Typical Grades: SUS410 (12Cr13), SUS420J2 (30Cr13)

Structural Characteristics: Form martensite (ferromagnetic structure) after high-temperature quenching, with permeability μ≥50 and significant adsorption by strong magnets.

Design Logic:

Higher carbon content (0.1%-0.4%) enhances hardness (HRC20-50) to meet cutting requirements for self-tapping and drilling.

Magnetism is an inherent property of martensitic structure, unrelated to corrosion resistance (SUS410 resists corrosion better than carbon steel but inferior to 300 series).

II. Influence of Cold Heading on Magnetism of Austenitic Stainless Steel

1. Mechanism of Magnetism from Cold Working

During cold heading, plastic deformation causes austenite (γ-Fe) to transform into martensite (α'-Fe), with martensite content increasing with deformation:

 

At 10% deformation, martensite content is ~5%-8%, permeability μ≈1.2 (weak magnetism).

At 30% deformation, martensite content can reach 25%-30%, permeability μ≈1.5 (still weakly magnetic).

2. Relationship Between Magnetism and Performance

Mechanical Properties: Cold heading increases strength (e.g., SUS304 tensile strength rises from 520MPa to 700MPa) but reduces elongation (from 40% to 25%). Magnetism is a byproduct of work hardening.

Corrosion Resistance: Martensitic transformation does not damage the passive film (Cr₂O₃), with salt spray test (NSS) ≥48 hours, consistent with non-magnetic states.

III. Technical Details and Application Scenarios of Demagnetization Processes

1. Physical Demagnetization (Temporary)

Method: Place bolts in an alternating magnetic field device (e.g., demagnetizing coil) to eliminate residual magnetism by gradually reducing field strength.

Limitation: Magnetism may partially recover due to subsequent mechanical stress or temperature changes, suitable for temporary non-magnetic needs (e.g., temporary assembly of electronic devices).

2. Solution Treatment (Permanent Demagnetization)

Process: Heat to 1050-1100°C (austenitizing temperature), followed by rapid water cooling (cooling rate ≥50°C/s) to inhibit martensite formation.

Effect: Martensite content <5%, permeability μ≤1.03, meeting permanent non-magnetic requirements (e.g., medical precision instruments, aerospace components).

Cost: Processing cost increases by ~10%-15%, but can be amortized through mass production.

IV. Magnetic Characteristics of Free-Cutting Stainless Steel (Taking SUS303 as an Example)

1. Composition-Magnetism Correlation

Sulfur Content: 0.15%-0.30% (higher than SUS304's ≤0.03%), forming MnS inclusions to improve machinability.

Magnetic Source: Local stress during turning induces minor martensitic transformation (content ~5%-10%), permeability μ≈1.1-1.2.

2. Performance Balance

Corrosion Resistance: Comparable to SUS304 (same chromium and nickel content), salt spray test ≥48 hours.

Application Scenarios: Suitable for small-sized screws (e.g., M2-M5) processed by automatic lathes, sacrificing weak magnetism for machining efficiency.

V. Common Industry Myths and Scientific Validation

Myth 1: "Magnetic Stainless Steel = Inferior Steel"

Counterexample: SUS410 is a legitimate material under GB/T 20878-2007, widely used in high-strength applications like turbine blades and cutting tools. Magnetism is an inevitable property of martensitic structure.

Myth 2: "304/316 Must Be Completely Non-Magnetic"

Standard Reference: ASTM A276-2020 allows weak magnetism in austenitic stainless steel, only requiring compliance with intergranular corrosion tests (EPR method) and mechanical property requirements.

Myth 3: "Demagnetization Reduces Stainless Steel Performance"

Experimental Data: After solution treatment, SUS304's tensile strength, elongation, and corrosion resistance deviate ≤5% from untreated parts. Demagnetization does not affect matrix properties.

VI. Magnetic Testing and Selection Recommendations

1. Rapid Identification Methods

Testing Tool	Austenitic Stainless Steel (304/316)	Martensitic Stainless Steel (410/420)
Neodymium Magnet	Weak adsorption or no adsorption (<0.5N)	Strong adsorption (>5N)
Permeability Meter	μ≤1.5	μ≥10

2. Scenario-Based Selection Table

Application Scenario	Recommended Material	Magnetic Requirement	Demagnetization Process	Core Performance Needs
Medical Precision Instruments	SUS316L	Non-magnetic (μ≤1.02)	Solution treatment + testing	Non-magnetic + high corrosion resistance
Construction Self-Tapping Screws	SUS410	Strong magnetism allowed	No demagnetization	High hardness + self-tapping ability
Electronic Device Micro-Screws	SUS303	Weak magnetism (μ≤1.2)	Physical demagnetization (as needed)	Easy machining + small size

Conclusion

The magnetism of stainless steel bolts results from a combination of material crystalline structure (austenite/martensite) and processing techniques (cold heading/demagnetization), not an indicator of quality. The weak magnetism of 304/316 is a normal result of cold working, while the strong magnetism of 400 series is a design feature for performance. In practical applications, selection should be based on specific needs like corrosion resistance, strength, and non-magnetism, avoiding misjudgments based solely on magnetism. By dispelling the myth that "non-magnetic equals superior," we can more accurately leverage the comprehensive performance advantages of stainless steel fasteners.