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Case Study #3 — Fastener Failure Workshop

Paper Mill
 

Industry:

Paper Mill – Very Large
Log Debarking (First Stage of Papermaking Process)

Problem:

Large 1-1/4" diameter, Grade 5 hex cap screws were used to attach cogs to the inside wall of the debarking barrel. The barrel, 40' (12.2m) diameter by 200' (61m) long, would be half filled with water and logs before rotated. The wedge shaped cogs would beat the bark from the logs. However, every shift at least ten bolt heads would break off. This sent cogs and broken fasteners into the paper mill — a very significant problem.

Injuries:

None.

Damages:

Various damaged pieces of expensive equipment, and an enormous amount of man-hours were regularly spent replacing hundreds of broken fasteners.

Observations:

The purchased fasteners had an electroplated zinc finish, but, when in use only a short time, they corroded. The failed products were highly corroded. Fracture surfaces of the failures resembled rock candy.

Investigation:

An independent laboratory performed a failure analysis on some of the broken fasteners. Using a scanning electron microscope, a failure mechanism of stress corrosion was determined. The fasteners were under stress because they were tightened; the corrosion made them crack. The fasteners had dramatic sudden failures. Interestingly though, an analysis of the actual corrosion matched the blends of common fertilizers — and quite potent.

Conclusion:

Spray-on tree fertilizers, mixed with water, became the enemy of the bolts. The seemingly logical switch to a 300 series stainless steel fastener, would not provide the corrosion protection for this application.

Recommendations:

A select variation of Inconel®, a high nickel alloy, was specified as the solution for the situation. Further, a particular heat treatment of the material was specified to ensure the strength required to last.

 

As appeared in Fastener Technology International magazine.

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