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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