NFE 25-511 Material Compatibility Assessment: Hardness Matching Criterion

1. Engineering Significance of Hardness Matching

The functional performance of NFE 25-511 single-sided toothed conical spring washers relies on two key mechanisms: - Tooth tips biting into the mating surface, creating mechanical interlocking - Disc spring elastic body providing restoring force, compensating for relaxation

Both require the hardness of the washer and the mating surfaces (bolt head/nut, clamped parts) to satisfy strict relative relationships. If hardness mismatch occurs, the following issues may arise: - Teeth fail to bite in → loss of anti-loosening and anti-slip capability - Mating surface is crushed → preload relaxation, joint failure - Plastic deformation of the washer itself → reduction or loss of elastic compensation capacity

Therefore, reliable application of NFE 25-511 requires material compatibility assessment, with hardness matching as the core.

2. Core Hardness Matching Criteria

Based on contact mechanics and engineering practice, the hardness of NFE 25-511 washers must satisfy the following tripartite relationship:

That is: - Washer hardness $H_{washer}$ must be higher than clamped part hardness $H_{plate}$ to ensure teeth can effectively penetrate the clamped part surface - Washer hardness should be no less than bolt head/nut hardness $H_{bolt/nut}$ to prevent the upper surface teeth of the washer from being crushed by the bolt head

Recommended specific hardness differences:

3. Typical Material Combinations and Assessment

3.1 Washer Materials

NFE 25-511 washers are typically made of quenched and tempered spring steel, standard materials:

3.2 Mating Surface Materials and Hardness

3.3 Compatibility Assessment Table

4. Relationship Between Hardness and Penetration Depth

The penetration depth of washer teeth into the clamped part approximately satisfies (see "Tooth Tip Penetration Depth"):

is the clamped part hardness. Higher hardness results in smaller penetration depth. When approaches or exceeds , teeth cannot penetrate the oxide layer, and biting fails. In this case, anti-loosening relies solely on surface friction, with significantly reduced effectiveness.

Empirical minimum penetration depth $h_{min}$: - Steel parts: ≥ 0.05 mm (must break through surface roughness) - To achieve this depth, based on the inverse of the above formula, it requires $H_{plate} \le H_{washer} - 50$ HV

5. Considerations for Multi-Material Connections

5.1 Double-Sided Toothed Washers

If the washer has teeth on both surfaces (biting into the bolt head and the clamped part respectively), both mating surfaces must satisfy the hardness matching requirements.

5.2 Effect of Coatings

• Dacromet, zinc-nickel coatings: Very low hardness themselves (< 100 HV), but very thin (5–15 μm), tooth tips can easily penetrate, not affecting overall hardness matching. Assessment is based on substrate hardness.
• Thick coatings (hot-dip galvanizing, 40–80 μm): Teeth penetration is difficult, may lift the washer, leading to insufficient embedment. In this case, increase preload or use uncoated washers.

5.3 Galvanic Corrosion

When the washer material and mating surface material have a large difference in the galvanic series (e.g., stainless steel washer + aluminum alloy clamped part), galvanic corrosion may occur at the contact interface, leading to loosening over time. Materials with similar potentials should be selected where possible, or ensure complete coating coverage.

6. Hardness Matching Assessment Procedure

1. Obtain mating surface material hardness: Measured or standard hardness values of the clamped part and bolt head/nut.
2. Preliminary selection of washer material: Choose spring steel or stainless steel based on environment (temperature, corrosion).
3. Hardness comparison:
4. $H_{washer} \ge H_{plate} + 30$ HV
5. $H_{washer} \ge H_{bolt/nut}$
6. If not satisfied:
7. Clamped part hardness too high → Select higher hardness washer (e.g., above 500 HV), or switch to anti-loosening solution that does not require biting (e.g., thread locking adhesive).
8. Clamped part too soft (aluminum, magnesium) → Must use L-type wide washer to increase bearing area, and check penetration depth and surface pressure to prevent crushing.
9. Stainless steel connection → Select stainless steel washer, hardness approx. 300 HV, only usable for mating surfaces with hardness ≤ 200 HV.
10. Experimental verification: For critical connections, perform actual indentation inspection (cross-section metallography) to confirm bite depth and morphology.

Summary:
The material compatibility of NFE 25-511 washers is essentially hardness gradient matching: the washer is the hardest, the clamped part is moderate, and the hardness difference between mating surfaces is ≥ 30 HV. Correct hardness matching ensures effective tooth biting without crushing the base material, and is a prerequisite for the anti-loosening and compensation functions of single-sided toothed conical spring washers.