conventional heat treatment of low carbon steel

CONVENTIONAL HEAT TREATMENT OF LOW CARBON STEEL

GROUP 6

140110064 : Ayush Chaurasia140110069 : Devendra Malav

140110066 : Rishabh Dosi140110070 : Aditya Kumawat

140110067 : Burhanuddin Attar Wala140110071 : Himmat Singh Rajput

140110068 : Rahul Chopra140110074: Mohd Bilal

AIM

To perform various heat treatment processes (Quenching, Normalizing and Annealing) and measure the hardness and investigate microstructure of low carbon and medium carbon steels

IRON-CARBON PHASE DIAGRAM

Source : http://www.tf.uni-kiel.de/matwis/amat/iss/kap_6/illustr/s6_1_2.html

WHY HEAT TREATMENT ?

To increase hardness, wear and abrasion resistance and cutting ability of steels To resoften the steel after it has been hardened by heat treatment or cold rolling To adjust its other mechanical, physical or chemical properties such as hardness, Tensile

Strength, Ductility, electrical or mechanical properties, microstructure or corrosion resistance To reduce or eliminate internal residual stresses To induce controlled residual stresses e.g. compressive stresses on the surface sharply

increase the fatique life of the components To stabilize the steel so that it does not show dimensions with time. To decrease or increase the grain size of steels. To produce special microstructures to increase machinability or corrosion resistance.

CONVENTIONAL HEAT TREATMENT PROCESSES Annealing

Heating material to above its recrystallization temperature, then cooling it slowly

Diffusion of atoms within the material thereby redistributing and eradicating dislocations

Relives Residual stresses Normalizing

Austenitized and cooled in open atmosphere ( Moderate cooling rate) Quenching

After heating, cooling the material in water. Here make sure you stir the water continuously to dissolve the layer of vapor forming outside the sample

HARDNESSHardness is the property of a material that enables it to resist plastic deformation, usually by indentation.

Methods to measure Hardness:

Method to achieve a hardness value is to measure the depth or area of an indentation left by an indenter of a specific shape, with a specific force applied for a specific time.

• Rockwell Hardness Test; diamond cone shaped indenter

• Brinell Hardness Test; sphere shaped indenter

• Vickers Hardness Test; right pyramid with square base shaped indenter and angle of 136 degrees between opposite faces

ROCKWELL HARDNESS TEST

Rockwell Hardness Scales :

Source : http://www.gordonengland.co.uk/hardness/rockwell.htm

EFFECT ON MECHANICAL PROPERTIES

Quenching1. Hardest of the

three2. Least Ductile3. Brittle i.e. least

toughness

Normalising1. Intermediate

Hardness2. Intermediate

Ductility3. High Toughness

Annealing1. Soft (release of

internal stresses)

2. Highly Ductile3. Slightly less

Tough

MICROSTRUCTURAL CHANGES DURING HEAT TREATMENT

Austenite (FCC) Carbon occupies the

interstitial sites in lattice

Ferrite + Austenite Ferrite being BCC

has very less solubility of carbon

Carbon diffuses to Austenite

Ferrite+Pearlite

On reaching 0.8% carbon Austenite transforms to Pearlite

In Annealing and Normalising

Source : https://m.youtube.com/watch?v=Bmd7T6SLKSE

QUENCHING

No time for Carbon to diffuse Gets trapped inside space lattice as Austenite transforms Results in distorted body centred lattice i.e. Body Centred Tetragonal

Lattice also known as ‘Martensite’ having needle shaped microstructure

OBSERVATION

Quenched > Normalized > Annealed

Heat Treatment Hardness value (HRB)

As Received 47

Annealed 45

Normalized 53

Quenched 72

MICROSTRUCTURES Microstructure of Eutectoid

steel

Single phase of Austenite transforms to a layered (alternate) structure of ferrite and cementite

Dark regions are cementite and bright regions are ferrite

Mechanical properties of pearlite is in between are in between that of ferrite (soft) and cementite (brittle)

Source : Callister

HYPO EUTECTOID STEEL

Source : Callister

CCT AND TTT DIAGRAMSTime temperature transformation diagram

Continous cooling transformation diagram

Source : Nptel Lectures of Prof. R.N. Ghosh

EFFECT OF COOLING RATE ON THE FORMATION OF DIFFERENT REACTION PRODUCTS

Very slow cooling rate (furnace cooling), typical of conventional annealing, will result in coarse pearlite with low hardness.

Air cooling is a faster cooling rate than annealing and is known as nonrmalizing. It produces fine pearlite.

In water quenching, entire substance remains austentic until the Ms line is reached, and changes to martensite between the Ms and Mf lines.

ANNEALED FROM 1223 K

Coarsened phase of pearlite

Pro Eutectoid Ferrite

NORMALIZED FROM 1223K

Fine phase of pearlite

Pro Eutectoid Ferrite

WATER QUENCHED FROM 1223K

Martensite Needles

Ferrite