Inspection of Threads Before And After Surface Coating of Fasteners

In various engineering applications, particularly for corrosion protection or decorative purposes, fastener surface coatings are widely applied, including electroplating, non-electrolytic zinc flake coating (Dacromet), and hot-dip galvanizing. However, due to different coating processes and varying thicknesses, these surface treatments can significantly impact the thread fit and interchangeability of fasteners.

This article explores the thread inspection rules for fasteners before and after surface treatment, focusing on different coating processes and the associated industry standards.

Overview of Common Coating Processes

Electroplating:
Uses electrolysis to deposit a uniform, dense, and well-bonded metal layer on the fastener surface.

Non-Electrolytic Zinc Flake Coating (Dacromet):
This water-based coating involves dipping, brushing, or spraying a zinc-chromate solution onto steel parts, followed by baking. The resulting layer is primarily composed of zinc flakes and chromate for excellent corrosion resistance.

Hot-Dip Galvanizing (HDG):
Involves dipping fasteners into molten zinc to form a protective metal coating.

1. Electroplating - Thread Inspection Rules

Impact on Threads:
The added zinc plating layer increases both the fastener's external dimensions and thread sizes, which can interfere with the fit between internal and external threads, preventing proper assembly. Additionally, due to the shielding effect during electroplating, zinc does not deposit uniformly on the thread profile, leading to thicker deposits near the thread crest and thinner deposits at the thread root. This asymmetry can reduce the thread profile angle and affect the fit.

Coating Thickness and Thread Fit:
The uneven deposition results in thicker coatings at the bolt head and tail, with irregularities affecting the thread fit. Due to the triangular thread profile, the coating thickness impact on the pitch diameter is approximately 4 times the coating thickness. Thus, adjustments to the pitch diameter are required based on different coating thicknesses.

Standard Thread Inspection:
To assess plated threads, GO and NO-GO gauges are used according to standards such as:

ISO 4042-1999 / ISO 965-2

ASTM F1941-2015

ASME B1.13M / ASME B1.1

Post-Plating Thread Inspection:
Standards like ISO 4042-15th-2015 and ASTM F1941-2015 allow the application of a specific torque when testing threads with GO gauges to account for potential thread interference.

US Standards Compliance:

ASME B1.1 specifies that for external threads, 2A gauges should accommodate plating thickness but should not exceed 1/4 of the thread tolerance.

ASTM F1941-2015 requires post-plating external inch threads to fit 3A GO gauges and metric threads to fit 6h or 4h GO gauges.

2. Non-Electrolytic Zinc Flake Coating (Dacromet)

Coating Characteristics:
Dacromet coatings consist of a zinc flake base coat and an organic or inorganic topcoat. The base layer thickness is typically 6-12 µm, while the topcoat adds another 5-10 µm, resulting in a greater impact on pitch diameter compared to electroplating.

Standards Compliance:

ASTM F1136-2011 and ISO 10683:2014 require that post-coating threads should not exceed the basic zero line (reference size).

Inspection Adjustments:
To ensure thread interchangeability after coating, adjustments for clearance based on coating thickness (e.g., Dacromet coatings of 12-25 µm) are made. Common practice includes enlarging the internal thread clearance prior to coating.

Inspection Using Thread Gauges:
Normal inspection involves the use of GO/NO-GO thread gauges to verify post-coating thread fit. Additionally, standards like ISO 10683 permit the application of torque to thread gauges during inspection to accommodate coating thickness.

3. Hot-Dip Galvanizing (HDG) - Thread Inspection

Challenges of HDG:
Hot-dip galvanizing involves immersing fasteners in molten zinc, followed by centrifugal removal of excess zinc. However, zinc accumulates unevenly on the threads, causing serious interference with thread fit.

Coating Thickness:

ASTM F2329 specifies HDG coating thickness (average): 0.044-0.055 mm.

ISO 10684 requires a minimum coating thickness of 0.04 mm, which is 8-10 times thicker than electroplated coatings.

Thread Size Adjustments:
Two adjustment methods are outlined:

Internal Thread Reaming: Expanding the internal thread diameter after coating.

External Thread Undersizing: Reducing the external thread size before coating.

Standards for Inspection:

ASTM A307-2014 requires using oversized nuts for thread fit testing after HDG.

ASTM F2329-2015 emphasizes meeting specific thread fit requirements post-coating.

Inspection Procedure:

Prior to inspection, lubricate threads with machine oil to prevent damage to gauges.

For bolts or studs, use oversized nuts to verify the fit after hot-dip galvanizing. In case of disputes, tolerance gauges with increased clearance (marked with “X tolerance”) can be used.

Conclusion

Ensuring thread fit after surface coating is critical for maintaining fastener performance in demanding applications like rail transit, construction, and heavy machinery. Following industry standards and adjusting thread dimensions based on coating type and thickness are essential steps in ensuring thread integrity and interchangeability.