HEAT TREATMENT PROCESSES (Part 1) · Heat treatment •Defined as the controlled heating and...
Transcript of HEAT TREATMENT PROCESSES (Part 1) · Heat treatment •Defined as the controlled heating and...
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HEAT TREATMENT PROCESSES
(Part 1)
by
Dr. SANJIB BANERJEE
DEPARTMENT OF MECHANICAL ENGINEERING
TEZPUR UNIVERSITY
2020 Dr.
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Heat treatment
• Defined as the controlled heating and cooling of metals for the primary purpose of altering their properties (strength, ductility, hardness, toughness, machinability.
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Purpose of Heat Treatment
• To relieve internal stress
• To improve machinability
• To refine grain size
• To soften the metal
• To improve the hardness
• To improve mechanical properties
• To increase resistance to wear, heat and corrosion.
• To improve ductility and toughness
• To change the chemical composition.
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Who uses Heat Treating ?
• Aircraft Industry
• Automobile Manufacturing
• Defense Sector
• Forging
• Foundry
• Heavy Machinery Manufacturing
• Powder Metal Industries
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• Heating • Preheating• Heating• Soak & diffusion • Pre-cooling
• Quenching (Cooling)• Post-wash
•Tempering• Surface coating
Steps in Heat Treating Operation
Loading
• Unloading
o Cleaning o Pre-wash with coalescenceo De-phosphate systemo Spray rinse
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Steps in Heat Treatment
Requires three basic steps:
❖Heating to a specific temperature
❖Holding (soaking) at that temperature for the appropriate time
❖Cooling according to a prescribed method
o heating temperature range to 24000F (1316 °C)
o soaking times vary from a few seconds to 3 to 4 days
o cooling may be slowly in the furnace or quickly (quenched)
into water, brine, oils, polymer solutions, molten salts, molten
metals or gases
o 90% of metal parts are quenched in water, oil, polymers, or
gases
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Heat Treating Processes
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Annealing
• It refers to a heat treatment in which the material is exposed
to an elevated temperature for an extended time period and
then slowly cooled.
• When an annealed part is allowed to cool in the furnace, it is
called a "full anneal" heat treatment.
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Types of Annealing
• Full Annealing
• Process Annealing
• Stress Relief Annealing
• Recrystallization Annealing
• Spheroidise Annealing
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Full annealing
• Main Objective:
• Soften the metal
•Relieve the stress
•Refine the structure.
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Full Annealing
• Temp is 30 - 50° C above the upper critical temp for hypo eutectoid steel.
• 30 - 50° C above the lower critical temp for eutectoid steel.
• Cooling is done at the furnace at the rate of 10-30°C per hour.
• For hypo eutectoid steel the resulting microstructure is coarse pearlite and ferrite.
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• For hypereutectoid steel annealing temp is 30-50°C above the lower critical temp.
• For hyper eutectoid steel the resulting microstructure is coarse pearlite and cementite.
• This process provides high ductility and toughness.
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Stress Relief Annealing
• Stress relief or recovery annealing.
• Annealing temp is at the range of 550-700°C.
• Uniform cooling is mandatory.
• It eliminates the stress formed during welding, cold working, casting, quenching, machining.
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Need for SR Annealing
❖Causes for stress:
• Plastic deformation during machining
• Non-uniform cooling
• Phase transformations between phases with different densities.
❖Effect of Stress
✓War page
✓Crack
✓Distortion
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Recrystallization Annealing
• It is a process in which distorted grains of cool worked material are replaced by strain free new grains.
• Recrystallization annealing is an annealing process at temperatures above the recrystallization temperature of the cold-worked material, without phase transformation.
• The recrystallization temperature is not a constant for a material but depends on the amount of cold work, the annealing time, and other factors.
• T(recrystallization) = 0.4 T (melting)
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• It reduces the Dislocation density and converts elongated grains to equ axied .
(a) after cold worked
(b) after recrystallization annealing
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Spheroidizing
❖Converts
➢Lamellar Pearlite → Globular Pearlite
➢Plates of Cementite → Spheriods of Cementite
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❖Main objectives of Spheriodising:▪ To soften the steel
▪ Increase ductility and toughness
▪ Improves machinability and formability
▪ Reduces hardness, strength and wear resistance.
❖Materials mainly concentrated• Medium carbon steel
• High carbon (tool steel)
• Not used for Low carbon Steel Dr.
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Three ways of Spheriodising
➢Prolong heating below Lower critical temperature and slow cooling.
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➢Cycling between temperature and then relatively slow cooling.
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• For tool and high speed steel heating at the temperature range between 750° - 800°C then hold at this temperature and then slow cooling.
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Normalizing
• When an annealed part is removed from the furnace and allowed to cool in air, it is called a "normalizing" heat treatment.
Main Objectives:
✓To refine grain structure.
✓To remove strains
✓To remove internal stress
✓To remove dislocations
✓To improve mechanical properties( strength, hardness and toughness)
✓To improve machinability of low carbon steels.
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