1. HEAT TREATMENT -I

2. Heat treatment
Defined as the controlled heating and cooling of metals for the primary purpose of altering their properties (strength, ductility, hardness, toughness, machinability.

3. Purpose of Heat Treatment
To relieve internal stress
To improve machinability
To refine grain size
To soften the metal
To improve the hardness
To improve mechanical properties
To increase resistance to wear, heat and corrosion.
To improve ductility and toughness
To change the chemical composition.

4. Who uses Heat Treating ? Aircraft Industry Automobile Manufacturing
Defense Sector
Forging
Foundry
Heavy Machinery Manufacturing
Powder Metal Industries

5. Steps in Heat Treating Operation
Loading
Heating
Preheating
Soak & diffusion
Pre-cooling
Cleaning
Pre-wash with coalescence
De-phosphate system
Spray rinse
Quenching (Cooling)
Post-wash
Tempering
Surface coating
Unloading

6. Steps in Heat Treatment
Requires three basic steps:
Heating to a specific temperature
Holding (soaking) at that temperature for the appropriate time
Cooling according to a prescribed method
heating temperature range to 24000F (1316 °C)
soaking times vary from a few seconds to 3 to 4 days
cooling may be slowly in the furnace or quickly (quenched) into water, brine, oils, polymer solutions, molten salts, molten metals or gases
90% of metal parts are quenched in water, oil, polymers, or gases

7. Heat Treating Processes

8. Annealing
It refers to a heat treatment in which the material is exposed to an elevated temperature for an extended time period and then slowly cooled.
When an annealed part is allowed to cool in the furnace, it is called a "full anneal" heat treatment.

9. Types of Annealing Full Annealing Process Annealing
Stress Relief Annealing
Recrystallization Annealing
Spheroidise Annealing

10. Full annealing Soften the metal Relieve the stress
Main Objective:
Soften the metal
Relieve the stress
Refine the structure.

12. Full Annealing
Temp is ° C above the upper critical temp for hypo eutectoid steel.
° C above the lower critical temp for eutectoid steel.
Cooling is done at the furnace at the rate of 10-30°C per hour.
For hypo eutectoid steel the resulting microstructure is coarse pearlite and ferrite.

13. For hypereutectoid steel annealing temp is 30-50°C above the lower critical temp.
For hyper eutectoid steel the resulting microstructure is coarse pearlite and cementite.
This process provides high ductility and toughness.

14. Stress Relief Annealing
Stress relief or recovery annealing.
Annealing temp is at the range of °C.
Uniform cooling is mandatory.
It eliminates the stress formed during welding, cold working, casting, quenching, machining.

15. Need for SR Annealing Causes for stress:
Plastic deformation during machining
Non-uniform cooling
Phase transformations between phases with different densities.
Effect of Stress
War page
Crack
Distortion

16. Recrystallization Annealing
It is a process in which distorted grains of cool worked material are replaced by strain free new grains.
Recrystallization annealing is an annealing process at temperatures above the recrystallization temperature of the cold-worked material, without phase transformation.
The recrystallization temperature is not a constant for a material but depends on the amount of cold work, the annealing time, and other factors.
T(recrystallization) = 0.4 T (melting)

18. It reduces the Dislocation density and converts elongated grains to equ axied .
(a) after cold worked
(b) after recrystallization annealing

19. Spheroidizing Converts Lamellar Pearlite  Globular Pearlite
Plates of Cementite  Spheriods of Cementite

20. Main objectives of Spheriodising:
To soften the steel
Increase ductility and toughness
Improves machinability and formability
Reduces hardness, strength and wear resistance.
Materials mainly concentrated
Medium carbon steel
High carbon (tool steel)
Not used for Low carbon Steel

22. Three ways of Spheriodising
Prolong heating below Lower critical temperature and slow cooling.

23. Cycling between temperature and then relatively slow cooling.

24. For tool and high speed steel heating at the temperature range between 750° - 800°C then hold at this temperature and then slow cooling.

26. Normalizing
When an annealed part is removed from the furnace and allowed to cool in air, it is called a "normalizing" heat treatment.
Main Objectives:
To refine grain structure.
To remove strains
To remove internal stress
To remove dislocations
To improve mechanical properties( strength, hardness and toughness)
To improve machinability of low carbon steels.

28. Hardening Increases the hardness
Heat treatment which is used to produce Martensite predominately.
Objective:
To improve hardness
To improve wear resistance

29. Hardening
Steps:
Heating
Soaking  Complete γ
Cooling.

30. Factors affecting Hardness
Carbon Content
Increasing Carbon  Increasing Hardness
Quenching Medium
Faster the cooling  Greater the hardness.
Specimen Size
Increase the specimen size Decreases the hardness.
Other Factors
Geometry
Quenching medium
Degree of agitation
Surface conditions
Alloy elements

31. Tempering

33. Tempering
Martensite
Tempered Martensite
Heavily Tempered Martensite

34. Tempering
Martensite formed in the hardening process is converted to TEMPERED MARTENSITE
To improve ductility and toughness.
Objective
To improve ductility
To improve toughness
To reduce hardness
To remove internal stress (rapid cooling)
To impart wear resistance.

35. Tempering Heated to 250˚ - 650˚C and cooled slowly to room temp
This is also done to relieve internal stress.
Martensite(BCT) Tempered Martensite
BCT supersaturated carbon
TM stable ferrite and cementite
Hardness decreases with increasing in tempering time.

36. Types of tempering Low temp T 150-250˚C Retain hard martensite
Relieve internal stress
Med TEMP T ˚C
Increases endurance limit and elastic limit
For spring steel
High temp T ˚C
On structural steel

37. TTT of a Eutectoid Steel

38. TTT

39. TTT
Time temp transformation (or) isothermal transformation (or) S curve (or) C curve (or) Bain’s Curve.
It is created three process:
Austenitizing
Isothermal Heat treatment
Quenching
Isothermal transformation

40. TTT 723˚-550˚C Pearlite is formed.
Transformation temp decreases pearlite change from coarse to fine.
On rapid quenching Martensite is formed.
550˚C- 250˚C  Bainite is formed.
Bainite  Non-lamellar eutectoid structure.

41. TTT
Bainite
Upper Bainite 550˚- 350˚C  has large rod like cementite regions
Lower Bainite 350˚ ˚C  has much finer cementite particles.

43. CCT

44. CCT The time required for a reaction to start and to end is delayed.
The isothermal curves are shifted to longer times and lower temperatures.
No transformation below 450˚C.

47. CCT A Coarse pearlite B Fine Pearlite
C Martensite and Pearlite  Split Transformation
D Martensite
E Tangent to Nose of CCT critical cooling rate 100% Martensite.

48. Hardenability
In a ferrous alloy, the property that determines the depth and distribution of hardness induced by quenching from elevated temperatures
Hardenability Vs Hardness:-
Hardness : Function of only carbon content Hardenability : C & Alloy content,   grain size,
Component size, Quenchant etc

50. 2. Jominy Hardenability (quench) test
# Steel Sample: 25mm dia and 100mm long
# Heating the test piece to an austenitising temperature and quenching from one end with a controlled and standardised jet of water
# Take a sample from the furnace and place it on the Jominy test fixtures and observe the cooling pattern
# The cooling rate along the Jominy test specimen varies from about 225 °C s-1 (at bottom) to 2 °C s-1
# After quenching the hardness profile is measured at (0.16 mm) intervals from the quenched end after the surface has been ground back to remove any effects of decarburisation (0.38mm is removed from the surface)

51. # The hardness variation along the test surface is a result of microstructural variation which arises since the cooling rate decreases with distance from the quenched1e”nd
# Measure the distance from the quench end at where the hardness is equal to 54 HRc in the units of 0.16mm

53. Relationship between the Jominy distance and critical diameters for an ideal quench medium

56. Martempering

57. Induction hardening
Induced eddy currents heat the surface of the steel very quickly and is quickly followed by jets of water to quench the component.
A hard outer layer is created with a soft core. The slideways on a lathe are induction hardened.

58. Flame hardening
Gas flames raise the temperature of the outer surface above the upper critical temp. The core will heat by conduction.
Water jets quench the component.