Industry:
Wood Processing
Flake Board Manufacturing
Problem:
The fasteners holding knives on a log flaker were breaking. When
the fasteners
were broken, the sharp knives were then thrown through the air.
Injuries:
None.
Damages:
One of the knives released with such force that it was thrown through
a wall made of concrete block. Fortunately no one was
in its path. Naturally,
when a blade comes loose, this stops production and subsequent income,
but more importantly this created an extremely dangerous situation.
Observations:
The fasteners breaking were 5/8" - 11 x 6-1/2"
(165mm) socket
cap screws. All failures were at the head-to-body juncture. Three screws
were used on each knife to secure them to the carrier arm disk. There
are a total of 24 knife pockets with each holding 2 knives 18-1/2"
(470mm) long. They are mounted end-to-end to make a cutting width of 37"
(940mm). These machines cut tree lengths or shorter logs into flakes for
the production of flakeboard.
Investigation:
We learned that the carrier disk revolves at about 200RPM and that
each
knife weighed 177lbs (80kg). This clearly explained the inertia and force
that allowed the knife to penetrate the concrete wall. We also learned
that the knife's edges are finished to very sharp conditions, as the normal
cutting thickness of the wood flakes is only 0.025" (0.6mm). The screws
had a plain finish, and corrosion products "red rust" were
very
obvious, particularly under the heads. There was also a rather strong
odor coming from the screws. Laboratory tests of the fastener's metal
showed a proper core hardness and a well-tempered martensitic structure
with an ideal amount of slight decarburization. The fasteners also had
been carefully installed in a proper manner with calibrated torque wrenches.
A lab evaluation with a scanning microscope (SEM) showed
that the fracture
surface had a rock candy intergranular appearance. An EDAX microprobe
scan of that surface showed a high concentration of chlorine with a fair
amount of potassium. The chlorine was obviously the source of the odd
smell on the parts. The fracture was 360° around the fillet radius
area and the cracking initiation was directly attributed to the corrosion
products.
Conclusion:
This was a clear case of stress corrosion cracking (SCC). The plain
finished
products were susceptible to cracking just by virtue of normal rusting.
This generates atomic hydrogen and will lead to the cracking. However,
the element of chlorine magnifies this opportunity as chlorine develops
hydrogen at a very fast rate. The source of the chlorine and potassium
is believed to have come from the fertilizers used to spark the tree's
growth.
Recommendations:
With a high concentration of chlorine, plating or coatings would
prove
ineffective. Therefore, the best solution would be to make the screws
from Inconel 718.
As appeared in Fastener Technology
International magazine. |
