- 2070 Chaitra (old course) · 3+5 marks
Mechanical vs heat treatment, tempering
Short answer
Mechanical treatment applies force to change shape and size (e.g., rolling, forging), while heat treatment uses controlled heating and cooling to alter internal structure without changing shape. Isothermal annealing: heat above upper critical, cool rapidly to just below lower critical (600-700°C), hold for transformation, then air cool. Tempering: reheat hardened steel to 150-650°C, hold, cool in air to reduce brittleness and increase toughness.
Differentiate between mechanical treatment and heat treatment of steel. Explain isothermal annealing and tempering process of heat treatment with neat sketch.
Mechanical treatment versus heat treatment
| Point | Mechanical treatment | Heat treatment |
|---|---|---|
| Method | Applying force: rolling, forging, drawing, extruding | Heating, soaking and cooling at a controlled rate |
| Effect | Changes shape and size; refines grain | Changes internal structure, shape unchanged |
| Purpose | To shape the steel; remove casting defects | To adjust hardness, strength, ductility, toughness |
| Hot/cold | Hot working or cold working | Always uses heating |
| Examples | Hot rolling, cold drawing, forging | Annealing, normalizing, quenching, tempering |
Isothermal annealing
Isothermal annealing is a variant of full annealing in which steel is heated above its upper critical temperature (austenitised), cooled rapidly to a temperature just below the lower critical point (about 600-700 C), held at this constant temperature until the austenite fully transforms to pearlite/ferrite, and then cooled in air.
Temp
^
| ____ austenitise (A3 + 30-50 C)
| / \
|-/------\------------- A1 (723 C)
| / \_________
|/ rapid cool | hold (isothermal)
| |_______ air cool
+--------------------------> Time
It saves time compared with full annealing, gives uniform structure and good machinability, and is used for alloy steel forgings and castings.
Tempering
Tempering is the reheating of hardened (quenched) steel to a temperature below the lower critical point (150-650 C), holding it there, and then cooling in air or oil. It reduces the brittleness and internal stresses of martensite and increases toughness and ductility, with some loss of hardness.
Temp
^
| ______ temper temp (150-650 C)
| / \
| / \ cool in air
| / \
+--------------------------> Time
Lower tempering temperature (150-300 C) retains hardness (cutting tools); higher (400-650 C) gives more toughness (springs, shafts).
✓ Answer checked by Claude Opus 5.5 (high effort)
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