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. |
