Heat Treatment Process
1.) Annealing
Types
a) Full Annealing
(i) Heating Steel 30-50 0C above A3, holding and slow cooling.
(ii) Microstructure consists of (Ferrite + cementite)
(iii) Used for Hot worked steels at high temperature.
b) Partial / Incomplete Annealing
(i) Heating Steel a temperature between A1 & A3. holding and slow cooling.
(ii) Carried out on hot worked steels finished at normal temperature (slightly above A3).
(iii) Generally used for eutectoid & hyper-eutectoid steels. (Microstructure consists of fine Pearlite + Cementite)
c) Diffusion Annealing
(i) Applied to alloy steel ingots and heavy complex castings to eliminate chemical homogeneity/ to break dendritic structures.
(ii) Heating at 1100-1200 0C, holding (10-12 hrs) followed by slow cooling.
d) Isothermal Annealing
(i) Heating above A3, holding for sometime, cooling rapidly to a temperature below A1 (600-700 0C), holding to complete Pearlite transformation.
(ii) Cheaper than full annealing process.
(iii) Microstructure equivalent to that obtained by full annealing.
(iv) Subjected to Hypo-eutectoid steels.
2.) Spheroidizing
1) Microstructure consists of Spheroids / Globules of carbides in a matrix of ferrite.
2) Heating : (i) 20- 40 0C above A1 and held for 3 – 8 hrs and furnace cooled.
(ii) Just below A1, holding and furnace cooling.
3) Improved machinability of medium and high Carbon steels.
3.) Normalizing
Process:
Heating steel to about 40 – 50 0C. Above A3 or A cm Temperature.
Holding for Proper Time
Cooling in Air
Microstructure : Pearlitic (Finer than Annealed)
Aims :
1) To eliminate coarse grain structure.
2) To increase strength & hardness of steel.
3) To improve machinability of low Carbon steels.
4) To improve welded structures & reduce internal stresses.
Microstructures of Normalized Steels :
1) Hypo – Eutectoid Steels : Ferrite + Pearlite
2) hyper – Eutectoid Steels : Cementite + Pearlite
4.) Hardening
Process
Heating steel to high temperature, holding for sufficient time followed by quenching in oil/water/brine.
Austenitizing Temperature
i) For Hypo-eutectoid steels : 30 – 50 0C above A3.
ii) For Hyper-eutectoid steels : between A1 & Acm.
Note : hyper-eutectoid steels are austentized between A1 & Acm.
Reasons
i) Cementite is harden than martensite (The two phase structure gives better wear resistance)(Microstructure consists of Cementite + Martensite)
ii) Fine Martensite is obtained in final microstructure.
Purpose of Hardening
Improvement in hardness and wear resistance.
5.) Tempering
Process
Heating Steel (hardened) below A1, followed by cooling in air.
Purpose
1) To relieve internal stresses.
2) To improve ductility & toughness of the steel (at the sacrifice of hardness).
Stages of Tempering
Stage 1 : (100 – 400 0F)
High Carbon martensite low ‘C’ Martensite + E Carbide (Fe2-4C)
(High strength & hardness, low ductility & toughness)
Stage 2 : (450 – 750 0F)
E Carbide Fe3C, low ‘C’ Martensite BCC Ferrite RA Barite
( Carbides are too small to be resolved and entire structure etches rapidly to a black mass called formerly
“Troosite”. At 9000 Y, carbide are clearly seen. T.S. reduces to 200,000 PS; ductility increases slightly,
hardness : 40 – 60 HRC.
Stage 3 : (750 – 1200 0F)
Cementite particles coalesce to grow and more fermite matrix is seen causing structure to be etched light.
Product known as “Sorbite” resorvable at 500 X. T.S. : 125,000 – 200,000 PS;
Elongation : 10 – 20 %
Hardness : HRC 20 – 40.
Stage 4 : (1200 – 1333 0F)
Large globular cementite particles are produced. Structure is soft and tough, and similar to spheroidized cementite structure.
Note : Sorbite and Trostite are obsolete terms now and structure is only referred as “Tempered Martensite”.
