TENNESSEE STATE UNIVERSITY
COLLEGE OF ENGINEERING
DEPARTMENT OF MECHANICAL AND
MANUFACTURING ENGINEERING
Materials Processing
MEEN 3100
Instructor: Dr. …
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Case Study 1
By: Dadyar Karim
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Overhead Conveyor Hooks in a Meat-Processing Plant
Understanding the Problem
A newly constructed meat-processing plant utilizes an overhead conveyor for sides of
beef weighing approximately 300 pounds each. After only a few weeks of service,
several hooks failed and allowed heavy sections of meat to fall from overhead. All but
one occurred shortly after the sides of beef had exited the freezer area, maintained at a
temperature of approximately 0°F (–18°C). The hooks were fabricated from a lowcarbon, plain-carbon steel.
The most likely cause of these failures is brittle fracture due to the low temperature and
material choice. The plain-carbon steels become brittle at very cold temperatures
because they can exhibit a ductile-to-brittle transition. Below this temperature, the steel
can no longer deform plastically; instead, it cracks suddenly without much warning.
1. What do you suspect is the cause of these failures?
It can be assumed that low-carbon steel hooks have lost the ability for energy
absorption and to resist impact at 0°F. Other contributing factors include:
• Notch sensitivity: The presence of sharp corners or surface scratches in the geometry
of the hook can provide sites for crack initiation.
• Impact loading: The swinging or jerking of the beef sides as they move creates small
impact forces that increase stress at the weak points within the hook.
• Thermal shock: As the hook moves from a very cold environment into a warmer area,
temperature changes can cause additional stress.
Microstructural issues include large grains or impurities within the steel, such as sulfur
inclusions, that may provide initiation sites for a crack, especially in cold steel.
Overall, the hooks probably failed due to a combination of embrittlement at low
temperature, low toughness, and stress concentration.
2. What recommendations would you make regarding the failing hooks? If replacement
is recommended, what material would you suggest as a better alternative?
A. Short-term actions
• Replace any broken or damaged hooks straight away.
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• Round off sharp corners or rough surfaces to minimize the stress concentration factor.
• Add dampers or guides to prevent the hanging sides of beef from swinging
excessively.
B. Long-term material improvement
In order to avoid the problem in future, the hooks must be made of materials that remain
tough at low temperatures. An austenitic stainless steel, like Type 304 or 316L would be
the best choice because of the following reasons:
• The material remains tough at sub-zero temperatures-it does not exhibit a ductilebrittle transition.
• It resists corrosion from moisture and cleaning chemicals used in food processing.
• It has good formability and can be polished to a smooth, sanitary finish.
If higher strength is required, the material could also be a nickel-alloyed lowtemperature steel, e.g., 3–9% Ni steel, heat-treated by quenching and tempering. These
steels are intended for operation at low temperatures in safe conditions; they also
require a fine-grain structure and a clean microstructure for resistance to cracking.
Every arithmetic operation on numbers can be generalized to matrices.
3. Do metallurgical features play a role? Consider grain size, microstructure (i.e., heat
treatment), and the type, orientation, and distribution of possible impurities.
A number of metallurgical features determine the low temperature behavior of a metal:
• Grain size: Fine grains improve toughness and lower the ductile-to-brittle transition
temperature.
• Microstructure: The presence of a tempered martensite or bainitic structure imparts
much better toughness than coarse pearlite.
• Inclusions: Impurities such as elongated manganese sulfide (MnS) inclusions can form
weak planes that assist the growth of cracks. Clean, calcium-treated steels minimize
this problem.
• Heat treatment: Proper heat treatment refines the grain structure and improves
toughness. • Orientation: Inclusions and grain flow should not be in a direction parallel
to the main stress direction in the hook.
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