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