Material and Surface Treatment Selection Guide: Bolts and Washers

1. Role of Material Selection and Surface Treatment in Joint Design

The reliability and service life of a bolted joint system depend not only on geometric dimensions and tightening parameters, but more fundamentally on the matching of material and surface treatment. Correct selection simultaneously satisfies:

• Static load and fatigue strength
• Corrosion resistance (electrochemical, environmental media)
• Wide temperature range service (low-temperature brittleness, high-temperature creep/relaxation)
• Stable friction coefficient (ensuring torque‑preload relationship)
• Anti-galling, hydrogen embrittlement prevention
• Economic efficiency

Although VDI 2230 does not have a dedicated "material selection" formula, it consistently relies on inputs such as material strength and friction coefficient. DIN 25201 wedge lock washers have explicit requirements for hardness and coating. The following provides a combination selection guide based on engineering practice.

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2. Bolt Material Classification and Characteristics

Material Category	Typical Grade/Standard	Strength Grade Range	Applicable Temperature	Corrosion Resistance	Application Scenarios
Carbon Steel	ISO 898‑1: 8.8, 10.9, 12.9	800～1200 MPa	-50°C～+200°C (standard) 10.9/12.9 requires attention to low-temperature brittleness	Requires surface protection	General machinery, automotive, steel structures
Alloy Steel	42CrMo4, 30CrNiMo8	10.9, 12.9, higher	-40°C～+300°C (after heat treatment)	Requires coating	High load, power machinery, pressure vessels
Stainless Steel (Austenitic)	A2‑70, A4‑70, A4‑80 (ISO 3506)	700～800 MPa (lower than carbon steel)	-200°C～+300°C (A2) A4 withstands up to 550°C	Excellent	Food, chemical, marine, exposed construction
Stainless Steel (Martensitic)	C1, C3	700～1100 MPa	Slightly lower than austenitic	Moderate	Applications requiring strength and corrosion resistance
Heat-Resistant Steel/Nickel Alloy	A286, Inconel 718	1000～1400 MPa	Up to 700°C	Excellent	Turbines, exhaust systems, high-temperature fasteners
Titanium Alloy	Ti‑6Al‑4V	900～1100 MPa	～300°C	Excellent	Aerospace, medical, lightweight structures

Note:
- Stainless steel bolts have lower strength: A2‑70 yield strength is only approx. 450 MPa, far below grade 10.9. Compensation is required by increasing size or quantity.
- High-strength bolts (12.9) are sensitive to hydrogen embrittlement; coating and pickling processes must be carefully selected.

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3. Common Surface Treatments and Their Effects

Surface Treatment	Friction Coefficient $\mu$ Range	Corrosion Resistance	Applicable Temperature	Thickness (μm)	Cost	Remarks
Phosphate + Oil (Phos.+Oil)	0.10–0.16	Moderate (indoor)	≤ 120°C	5–15	Low	Standard industrial treatment, stable torque coefficient
Zinc Plating (Electroplating)	0.12–0.20	Good	≤ 120°C	5–15	Low	Requires de-embrittlement to prevent hydrogen embrittlement, prone to white rust
Zinc-Nickel Alloy Plating	0.12–0.18	Excellent (> 720 h salt spray)	≤ 120°C	8–15	Medium	Common in automotive, stable friction coefficient
Dacromet/GEOMET	0.10–0.16	Excellent (> 600 h salt spray)	≤ 300°C	5–10	Medium	No hydrogen embrittlement, high-temperature resistant, coating contains aluminum flakes
Hot-Dip Galvanizing	0.18–0.30 (rough)	Excellent (> 1000 h)	≤ 200°C	40–80	Low-Medium	Thick coating, requires thread size adjustment, high and scattered friction coefficient
MoS₂ Coating (Molybdenum Disulfide)	0.08–0.12	Limited (lubrication only)	≤ 400°C	3–10	Medium-High	Very low stable friction, reduces tightening scatter, anti-galling
PTFE Coating	0.06–0.10	Good (moisture barrier)	≤ 260°C	5–20	High	Ultra-low friction, can be used to adjust torque coefficient
Nickel Plating/Electroless Nickel	0.15–0.25	Excellent	≤ 600°C	10–30	High	High-temperature and corrosion resistant, higher friction coefficient
Uncoated (Blackened/Phosphated without oil)	0.20–0.35	Poor	—	—	Lowest	Not recommended for critical joints, highly unstable friction

3.1 Effect of Surface Treatment on Preload

From the torque formula $M_A = K F_M d$, small fluctuations in friction coefficient cause significant preload variation.
- MoS₂, Phosphate+Oil provide stable low friction, resulting in low preload scatter, recommended for torque control method.
- Hot-dip galvanizing, uncoated have large friction scatter, preload can vary by ±30% or more, requiring angle control method or increased safety factor.

3.2 Hydrogen Embrittlement Risk

The electroplating process generates hydrogen atoms that penetrate the steel substrate, potentially causing delayed fracture in high-strength bolts (≥ grade 10.9). Baking at 200°C for de-embrittlement within 4 hours after plating is required.
Alternatives: Dacromet, zinc-nickel alloy, electroless coatings have no hydrogen embrittlement risk, suitable for grade 10.9/12.9 bolts.

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4. DIN 25201 Washer Material and Coating

The standard material for wedge lock washers is quenched and tempered steel, with a typical hardness ≥ 400 HV, ensuring the teeth can bite into common structural steel surfaces.
Common supply conditions:

Coating Type	Color	Hardness Guarantee	Corrosion Resistance	Suitable Bolts
Dacromet/GEOMET	Silver-gray	✓	Excellent (> 600 h)	Carbon steel, alloy steel bolts
Zinc-Nickel	Silver-white	✓	Excellent (> 720 h)	Automotive, general
Stainless Steel (Uncoated)	—	Slightly lower (~300 HV)	Excellent	Used with stainless steel bolts for highly corrosive environments

Matching Principle: - Washer hardness must be at least 30 HV higher than the clamped part surface hardness to ensure biting.
- Stainless steel washers are generally only used with stainless steel bolt connections, and hardness matching with the clamped parts must be considered (aluminum parts may be crushed).
- Coating must not over-lubricate the wedge surfaces, otherwise the locking torque $M_{lock}$ is reduced, but standard coatings already account for this.

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5. Combination Selection Matrix

Service Condition	Recommended Bolt	Surface Treatment	Recommended Washer	Key Considerations
Indoor standard machinery	8.8 / 10.9	Phosphate+Oil, Zinc plating	Carbon steel + Dacromet	Cost priority, stable μ
Automotive powertrain	10.9 / 12.9	Zinc-nickel, Dacromet	Carbon steel + Zinc-nickel	Salt spray resistance, no hydrogen embrittlement, -40°C low temperature
Outdoor steel structures	8.8 / 10.9	Hot-dip galvanizing	Carbon steel + Hot-dip galvanizing	Long-term corrosion protection, consider μ scatter
Chemical/Food equipment	A2‑70 / A4‑70	None required	Stainless steel (same material)	Corrosion resistance, note strength derating
High-temperature exhaust system	10.9 alloy steel	Dacromet, Nickel plating	Carbon steel + Dacromet	No softening at high temperature, μ not drastically changing
Aerospace lightweight	Ti‑6Al‑4V	None/Special coating	Titanium alloy washer	Prevent galvanic corrosion, strict torque control
Extreme vibration (heavy truck/mining)	10.9 / 12.9	MoS₂ + DIN 25201	Carbon steel + MoS₂	Utilize MoS₂ low μ for stable torque, wedge washer for locking

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6. Material-Related Steps in the Integrated Design Process

1. Determine load and environment → Select bolt strength grade and material category.
2. Preliminary selection (VDI 2230 R0) → Estimate diameter using $R_{p0.2}$.
3. Select surface treatment → Determine nominal friction coefficients μ_G, μ_K for torque calculation (R13).
4. Calculate preload scatter → Evaluate α_A based on possible μ range.
5. Verify strength and fatigue (R7–R9) → Use material yield strength, fatigue limit.
6. Anti-loosening and anti-slip (R11, R12, DIN 25201) → Washer material and hardness, surface friction coefficient.
7. Final validation → Confirm through salt spray tests, Junker vibration tests, etc.

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Summary:
Material and surface treatment selection is the first step in bolted joint design and is the core factor determining friction coefficient scatter, corrosion resistance, and high-temperature performance. A rational combination enables stable torque‑preload conversion, reduces design iterations, and ensures long-term reliability.