HEAT-TREATMENTBy

SUBHRAJIT GHOSH

CONTENTS• INTRODUCTION• HEAT TREATMENT PROCESSES• IRON-CARBON EQUILIBRIUM DIAGRAM• TTT / IT DIAGRAMS• CCT DIAGRAM• CRITICAL TEMPERATURES• COMMON PHASES• ANNEALING • NORMALIZING• HARDENING• TEMPERING• HARDENABILITY• PRECIPITATION AND AGE HARDENING• SECONDARY HARDENING• AUSTEMPERING• MARTEMPERING• PATENTING

INTRODUCTIONHeat treatment refers to a combination of operations involving heating and cooling of metals and alloys in the solid state for the purpose of obtaining certain desired properties. Changes in properties result from microstructural changes in the material produced by heat treatment operations.

All heat treatment processes may be considered to consist of three main parts:

1. Heating of the metal to predetermined temperature.2. Soaking of the metal at that temp until the structure

becomes uniform throughout the mass.3. Cooling of the metal at some predetermined rate to cause

formation of desirable structures within the metal for the desired purposes.

PURPOSES• Improve machinability.• Relieve internal stresses.• Improve mechanical properties (ductility,

strength, hardness, toughness).• Change the grain size.• Increase resistance to corrosion, wear etc.• Modify electrical/magnetic properties.• Change the chemical composition.• Remove entrapped gases (H,N,O).• Change the surface condition (to produce

hard surface on a ductile interior).• Soften the metals for further working.• Make the structure of metal more

homogeneous.

HEAT TREATMENT PROCESSES• Annealing• Normalizing• Hardening• Tempering• Case hardening (carburising,

nitriding, carbonitriding, cyaniding).• Surface hardening (induction, flame,

laser, plasma).• Precipitation and Age hardening.• Patenting.