HEAT TREATMENT

WHAT IS HEAT TREATMENT?

• Heat treatment is a process of combination of heating and cooling operations, timed and applied to metals and alloys in their solid state to obtain desired properties.

CLASSIFICATION HEAT TREATMENT

1. Annealing:- On the basis ofa) Heat treatment temp I) Full annealing II) Partial annealing III) Sub criticalb) Phase transformation I) First order annealing II) Second order annealing

c) Specific purposes I) Diffusion annealing II) Spheroidizing annealing III) Full annealing IV) Recrystallization annealing2. Normalising3. Tempering 4. Austempering 5. Martempring 6. Sub-zero treatment 7. Patenting

HARDENING HEAT TREATMENT

1. Through hardening2. Surface hardening (Case hardening) a) Surface hardening without changing the

surface chemistry of steel I) Introduction hardening II) Flame hardening b) Surface hardening by changing the surface

chemistry of steel

I) Carburising * pack or solid Carburising * Liquid Carburising * Gas CarburisingII) CyanidingIII) Nitriding

PROCESS DESCRIPTION

Heating the metal or alloy to pre-determine temp, holding at this temp and finally cooling at very slow rate.

Objective :-1. Relieves internal stresses developed during

solidification, machining, forging, rolling, welding etc.

2. Improve or restore ductility and toughness.3. To enhanced machinability.

Annealing

Annealing

4. Eliminate chemical non-uniformity.5. To refine grain size and grain structure.6. Enhance magnetic and electrical properties.7. Reduce gases content in steel.

Process variables annealing* Austenitizing temperature:- hypo-eutectoid dteel = Ac3 + (30-50 oC) Eutectoid steel = A1 + (30-50 oC) Hype-eutectoid steel = Acm + 50 oC*Austenitizing or soaking time:- soaking for a sufficient length of time at

austenitizing temp to convert( ferrite + pearlite ) complete austenite * Cooling:- Furnace cooling

Three stages of annealing

– Heating to the desired temperature : The material is austenitized by heating to 15 to 40C above the A3 or A1 lines until equilibrium is achieved (i.e., the alloy changes to austenite),

• Soaking or holding time: The material is held for 1h at the annealing temperature for every inch of thickness (a rule of thumb)

– Cooling to room temperature: cooling rate of 100°F/hr is typical for full annealing. Done in furnace itself.

Heating the steel to about 40-50 oC above the upper critical temp, holding for proper time and then cooling in still air or slightly agitated air to room temperature

Obejective :-To produce harder & stronger steels compared to

fully annealed steel.• Grain size refinement of steel.• Improve machinability, ductility & toughness.• Homogenize chemical composition of cast ingots.

Normalising

• Austenitizing temperature Hypo-eutectoid steel = Ac3 + 50 oC Eutectoid steel = A1 + 50 oC• Austenitizing time sufficient length of

time .• Cooling pattern cooling in still air or slighty

agitated air.

Process variable of normalising

Heat Treatment of Steel

• Most heat treating operations begin with heating the alloy into the austenitic phase field to dissolve the carbide in the iron

• Steel heat treating practice rarely involves the use of temperatures above 1040°C

• Classification– Heating and rapid cooling (quenching)– Heating and slow cooling

Figure: Heat Treatment Range for Carbon Steels

Purpose of heat treatment:

• Improvement in ductility• Relieving internal stresses• Grain size refinement• Increase of strength and hardness• Improvement in machinability and

toughness.

Factors involved

• Temperature upto which material is heated• Length of time that the material is held at

the elevated temperature• Rate of cooling• The surrounding atmosphere under the

thermal treatment.

Effects of Heat treatment

Annealing & Normalizing Hardening or Quenching

Furnace Cooling Air

Cooling Oil

Quenching Water

Quenching

Softer, less strong Harder and stronger

More ductile More brittle

Less internal stress More internal stress

Less distortion, cracking More distortion, cracking

The Iron–Iron Carbide Phase Diagram

L + Fe3C

2.14 4.30

6.70

0.0220.76

M

N

C

PE

O

G

F

H

Cementite Fe3C

Spheroidising

1. Heat to just below Lower Critical Temperature. (about 650-700 deg C)

2. Cool very slowly in the furnace.3. Structure will now be spheroidite, in which

the Iron Carbide has ‘balled up’.4. Used to improve the properties of medium

and high carbon steels prior to machining or cold working.

Tempering

• The brittleness of martensite makes hardened steels unsuitable for most applications.

• Different cooling rates between edge and core of components result in internal stresses.

• This requires the steel to be tempered by re-heating to a lower temperature to reduce the hardness and improve the toughness.

• This treatment converts some of the martensite to bainite.

Tempering Temperatures

Isothermal Heat Treatments

• Austempering - The isothermal heat treatment by which austenite transforms to bainite.

• Isothermal annealing - Heat treatment of a steel by austenitizing, cooling rapidly to a temperature between the A1 and the nose of the TTT curve, and holding until the austenite transforms to pearlite.

Induction Hardening

• Surface to be hardened is heated using inductive heating.

• Depth of hardness can be closely monitored by controlling current.

• Time required for the process is less.• Used for producing hard surfaces on

crankshafts, axles, gears etc.

The steels shown in blue can be heat treated to harden them by quenching.

Hardening Temperatures

• The temperatures for hardening depend on the carbon content.

• Plain carbon steels below 0.4% will not harden by heat treatment.

• The temperature decreases from approx 820 °C to 780 °C as carbon content increases from 0.4% up to 0.8%.

• Above 0.8% the temperature remains constant at 780 °C.

• Hardening temperature same as that for normalising

Surface Hardening

• Selectively Heating the Surface - Rapidly heat the surface of a medium-carbon steel above the A3 temperature and then quench.

• Case depth - The depth below the surface of a steel at which hardening occurs by surface hardening and carburizing processes.

• Case Hardening : Carburizing • Cyaniding , Carbonitriding• Flame Hardening

Case Hardening

• The primary purpose of case hardening is to produce a surface which is resistant to wear while maintaining the overall toughness and strength of the steel core.

• Normally used on a steel with a low carbon content and introduces carbon by diffusion (carburising) into the local surfaces requiring treatment.

• Heating steel in the presence of a solid, liquid or gas rich in carbon.

Cyaniding

• Hardening the surface of steel with carbon and nitrogen obtained from a bath of liquid cyanide solution.

• Steel is heated in molten cyanide at about 850 °C followed by quenching.

• Carbon and nitrogen are absorbed by steel.

Nitriding

• Another process called Nitriding consists of the diffusion of nitrogen.

• Nitrogen is introduced into steel by passing ammonia gas through a muffle furnace containing the steel to be nitrided.

• Temperature used is below the lower critical temperature

• Greater resistance to wear and corrosion, greater surface hardness.

Carbonitriding

• Hardening the surface of steel with carbon and nitrogen

• Steel is heated in a gaseous mixture of ammonia and hydrocarons

Flame Hardening

• Heating the surface being hardened above the upper critical temperature with an oxy acetylene flame before quenching it in a spray of water.

• This is a surface hardening process resulting in a hard surface layer of about 2mm to 6mm deep.

• The main difference between this process and other surface hardening processes is that the composition of the steel being hardened is not changed.

Strain Hardening

• Phenomenon where ductile metals become stronger and harder when they are deformed plastically is called strain hardening or work hardening.

• Increasing temperature lowers the rate of strain hardening.

• Hence materials are strain hardened at lower temperatures, thus also called cold working.

Precipitation Hardening

• Foreign particles can also obstruct movement of dislocations and increase strength of the material.

• The foreign particles can be introduced in two ways – precipitation – mixing-and-consolidation technique.

• Precipitation hardening is also called age hardening because strength increases with time.

Objectives of heat treatment (heat treatment processe

The major objectives are • to increase strength, harness and wear resistance (bulk hardening, surface

hardening)

• to increase ductility and softness (tempering, recrystallization annealing)

• to increase toughness (tempering, recrystallization annealing)

• to obtain fine grain size (recrystallization annealing, full annealing, normalising)

• to remove internal stresses induced by differential deformation by cold working, non-uniform cooling from high temperature during casting and welding (stress relief annealing)

• to improve machineability (full annealing and normalising)

• to improve cutting properties of tool steels (hardening and tempering)

• to improve surface properties (surface hardening, corrosion resistance-stabilising treatment and high temperature resistance-precipitation hardening, surface treatment)

• to improve electrical properties (recrystallization, tempering, age hardening)

• to improve magnetic properties (hardening, phase transformation)

Recovery

• Takes place at low temperatures of annealing

• The point imperfections created during plastic deformations are absorbed at grain boundaries

• Dislocations of opposite sign come together and mutually annihilate each other.

• Some of the stored internal strain energy is relieved by virtue of dislocation motion, as a result of enhanced atomic diffusion at the elevated temperature.