heat treatment section 18
TRANSCRIPT
Welding Inspector
Heat Treatment
Section 18
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Heat Treatment
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Why?
Improve mechanical properties
Change microstructure
Reduce residual stress level
Change chemical composition
How?
Flame oven
Electric oven/electric heating blankets
induction/HF heating elements
Where? LocalGlobal
Heat Treatments
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Many metals must be given heat treatment before and after welding.
The inspector’s function is to ensure that the treatment is given correctly in accordance with the specification or as per the details supplied.
Types of heat treatment available:
•Preheat
•Annealing
•Normalising
•Quench Hardening
•Temper
•Stress Relief
Heat TreatmentsPre-heat treatments
• are used to increase weldability, by reducing sudden reduction of temperature, and control expansion and contraction forces during welding
Post weld heat treatments
• are used to change the properties of the weld metal, controlling the formation of crystalline structures
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Post Weld -Heat Treatments
Post Hydrogen Release (according to BS EN1011-2)
Temperature: Approximately 250 C hold up to 3 hours
Cooling: Slow cool in air
Result: Relieves residual hydrogen
Procedure: Maintaining pre-heat / interpass temperature after completion of welding for 2 to 3 hours.
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Post Weld Heat Treatments
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(A) Normalised
(B) Fully Annealed
(C) Water-quenched
(D) Water-quenched & tempered
A B
C D
Post Weld Heat Treatments
The inspector, in general, should ensure that:
• Equipment is as specified
• Temperature control equipment is in good condition
• Procedures as specified, is being used e.g.
o Method of application
o Rate of heating and cooling
o Maximum temperature
o Soak time
o Temperature measurement (and calibration)
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Post Weld Heat Treatment Cycle
Time
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TemperatureSoakingTemperature
and time at the
attained temperature
Heating Soaking Cooling
heating rate Cooling rate
Variables for heat treatment process must be carefully controlled
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Post Weld Heat TreatmentRemoval of Residual Stress
Temperature (°C)
100 200 300 400 500 600 700
Yield
Strength
(N/mm2 )
100
200
300
400
500Cr-Mo steel - typical
C-Mn steel - typical
• At PWHT temp. the yield
strength of steel reduced
so that it it is not strong
enough to give restraint.
• Residual stress reduced
to very low level by
straining (typically < ~
0.5% strain)
Heat Treatment
Recommendations
• Provide adequate support (low YS at high temperature!)
• Control heating rate to avoid uneven thermal expansions
• Control soak time to equalise temperatures
• Control temperature gradients - NO direct flame impingement!
• Control furnace atmosphere to reduce scaling
• Control cooling rate to avoid brittle structure formation
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Post Weld Heat Treatment Methods
Gas furnace heat treatment
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Advantages:
Easy to set up
Good portability
repeatability and
temperature uniformity
Disadvantages:
Limited to size of parts
Post Weld Heat Treatment Methods
HF (Induction) local heat treatment
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Advantages:
High heating rates
Ability to heat a
narrow band
Disadvantages:
High equipment
cost
Large equipment,
less portable
Post Weld Heat Treatment Methods
Local heat treatment using electric heating blankets
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Advantages:
Ability to vary
heat
Ability to
continuously
maintain heat
Disadvantages:
Elements may
burn out or arcing
during heating