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Plating Initiated Cracks of Nickel Plated Screwdriver Blades

ENVIRONMENT:

Standard Use

EQUIPMENT:

Screwdriver Blades

MATERIAL:

Alloy Steel

FAILURE MODE:

Plating Initiated Cracks

 

Alloy steel screwdriver blades are coated with carbide particles brazed to the steel substrate at the tip. A nickel plating process follows the brazing. Several of these blade tips had failed due to fractures, Figure 1.

The blades that had been stressed, either by testing or from use, had cracks visible on the surface of the nickel-plated carbide coated tips. These cracks extended through the nickel plate and down into the brazing alloy. For the most part, they appeared to stop at the interface of the alloy steel substrate. The new, unused blade did not show these cracks.

Residual stresses in nickel plate can be as high as 50% of the tensile strength of the plate. Cracks are initiated in the nickel plating when the combined residual plating stresses and the service stresses exceed the strength of the nickel plate, Figure 2. The cracks appear to propagate selectively through tungsten-rich phases in the brazing system. If the cracks are not arrested in the brazing system or at the alloy steel interface, they enter the alloy steel where they may result in blade failure.

 

Figure 1. Fractured surface of screwdriver blade (20X Original Magnification) Figure 2. Crack traveling through coating cross-section (200X Original Magnification)

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