Heat Treatment
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Ø Applied
after welding, forming, or bending operations.
Ø To
reduce residual stress & detrimental effects.
Ø Detrimental
effects associated with welding, bending, and forming processes are:
o
Dramatic loss of toughness,
o
Reduction of ductility,
o
Increased hardness,
o
And/or high residual stresses
Ø Consequences-can
lead to:
o
Premature, unexpected, and potentially catastrophic failures
caused by brittle fracture.
o
Fatigue Cracking, Stress Corrosion Cracking, and/or Hydrogen Embrittlement.
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Forms of Heat
Treatment
Ø Heat
treatments listed in B31.3 (Stress relieving heat treatments)
o
Stress Relief Anneal
o
Full Annealing
o
Solution Annealing
o
Normalizing
o
Tempering
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Stress Relief Anneal
Ø A
stress relief anneal is performed on carbon and alloy steels
Ø Temperature
slightly below the lower critical temperature,
Ø
Known as subcritical stress relief or subcritical anneal.
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Full Annealing
Ø A
full anneal is performed at 50 to 100_F above upper critical temperature of CS.
Ø Followed
by slow cooling, generally in a furnace & air cooled.
Ø Performed
on Carbon Steel & Alloy Steel.
Ø It
provides maximum softening, resulting in lowest hardness and strength.
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Solution Anneal
Ø Used
to dissolve one or more constituents into solid solution,@ high Temperature.
Ø Then
cooled rapidly enough to hold the constituents in solid solution.
Ø Applied
to high alloy steels and other high alloy materials.
Ø It
forms most corrosion-resistant and ductile.
Ø ASTM
A 312 pipes are supplied in the solution treated condition.
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Normalize
Ø Applied
for carbon and low alloy steels.
Ø Heating
temperature range similar to full anneal.
Ø But
the parts are allowed to cool in still air.
Ø Save
time and money compared to Annealing.
Ø Normalizing
is also effective in:
o
Refining the grain size and homogenizing the structure,
o
Resulting in better toughness,
o
Uniform mechanical properties,
o And
better ductility.
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Normalize & Tempering
Ø Tempering
is carried out below the lower critical temperature.
Ø Applied
after Normalizing & Quenching operation to transformation hardenable steels.
Ø
Used to reduce hardness, improve toughness and ductility at the
expense of reduced strength.
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