HEAT TREATMENT

DEFINITION:

         Heat treatment refers to an operation or combination of operations, involving heating and cooling to alter the properties of metals, alloys plastic and ceramic materials.

          Changes in material's properties result from changes made in micro-structure of the material. During heat-treatment of a suitable rate, there occur changes in the micro-constituents of the metal. These changes in the micro-constituents of the metal may be in their nature, form, size and distribution in the metal piece. Obviously, temperature of heating and rate of cooling are the main controlling factors of changes in micro-constituents. These changes in micro-constituents then control the changes in physical and mechanical properties of heat treated metal specimen. Heat treatment can be applied to ingots, castings, semi-finished products, welded joints and various elements of machines and instruments. For various fabrication and manufacturing operations, heat treatment is a very important process.

2.1 INTRODUCTION

2.2 Object of heat treatment

2.3 Micro-Constituents of iron & steel

i. Ferrite

ii. Cementite

iii. Pearlite

iv. Bianite

v. Martensite

vi. Austenite

vii. Troosite, and 

viii. Sorbite.

2.4 IRON-CARBON EQUILIBRIUM OR PHASE DIAGRAM

2.5 Different kinds of heat treatments given to carbon steel

            2.5.1 Annealing

                    2.5.1.1 Full Annealing

                    2.5.1.2 Isothermal Annealing

                    2.5.1.3 Process or Sub critical Annealing

                    2.5.1.4 Spheroidisation Annealing

                    2.5.1.5 Diffusion Annealing

                    2.5.1.6 Recrystallisation Annealing.

            2.5.2 Normalising

            2.5.3 Distinction between Annealing and Normalising

            2.5.4 Hardening

            2.5.5 Quenching media

            2.5.6 Hardening methods

            2.5.7 Mass effect

            2.5.8 Hardenability

                     2.5.8.1 Jominy hardenability test or Jominy end quench hardenability test

            2.5.9 Tempering

                    2.5.9.1 Method of tempering

                    2.5.9.2 Temper colors

            2.5.10 Temper brittleness (Embrittlement)

            2.5.11 Quench cracks

            2.5.12 Age hardening

            2.5.13 Surface hardening

                    2.5.13.1 Case hardening

                              2.5.13.1.1 Pack hardening

                              2.5.13.1.2 Liquid Carburizing

                              2.5.13.1.3 Gas Carburizing

                    2.5.13.2 Nitriding

                    2.5.13.3 Cyaniding

                    2.5.13.4 Flame hardening

                    2.5.13.5 Induction hardening

2.6 Defects due to Heat Treatment of steels

2.7 Surface finish after heat treatment