1. HEAT TREATMENT

2. SOAKING OR HOLDING
HEATING
CONTROLLED COOLING
HEATING CYCLE

3. AUSTENITE
PHASE TRASFORMATION ON CONTROLLED COOLING
OIL QUENCH
AIR COOL
FURNACE COOL
WATER QUENCH
VERY FINE PEARLITE Rc 60
FINE PEARLITE Rc 40
COARSE PEARLITE RC 15
MARTENSITE Rc 64

4. PURPOSE OF HEAT TREATMENT
IMPROVED MACHINABILITY.
TO RELIEVE INTERNAL STRESSES.
IMPROVE MECHANICAL PROPERTIES SUCH
AS DUCTILITY, STRENGTH,HADNESS,
TOUGHNESS ETC.
CHANGE IN GRAIN SIZE.
INCREASE RESISTANCE TO HEAT AND
CORROSION.
MODIFY ELECTRICAL AND MAGNETIC
PROPERTIES.
CHANGE THE CHEMICAL COMPOSION.
REMOVE GASES.

5. ANNEALING 2. NORMALIZING 3. HARDENING 4. AUSTEMPERING 5. MARTEMPERING
HEAT TREATMENT PROCESSES
ANNEALING
2. NORMALIZING
3. HARDENING
4. AUSTEMPERING
5. MARTEMPERING
6. TEMPERING

6. 7. CASE HARDENING
A) CARBURISING
I) SOLID CARBURISING
II) LIQUID CARBURISING
III) GAS CARBURISING
B) NITRIDING
C) CYANIDING
D) CARBONITRIDING
8. SURFACE HARDENING
A) INDUCTION HARDENING
B) FLAME HARDENING
9. DIFFUSION COATING

7. NORMALISING

8. NORMALISING IS THE HEAT TREATMENT WHICH INVOLVES HEATING OF THE GIVEN STEEL TO “ AUSTENITE TEMP.RANGE” HOLDING IT & THERE AFTER COOLING TO ROOM TEMPERATURE AT SLOW RATE OF COOLING,GENERALLY “AIR COOLING”.
OBJECTIVES:
TO ELIMINATE COARSE GRAINED STRUCTURE.
TO REDUCE SEGREGATION.
TO REFINE GRAIN STRUCTURE.
TO PRODUCE HARDER AND STRONGER STEEL THAN ANNEALING.
TO OBTAIN REQUIRED MECHANICAL PROPERTIES.
TO RELIEVE INTERNAL STRESSES IN SOME CASES.

9. A TYPICAL NORMALISING PROCESS INVOLVES FOLLOWING STEPS,
HEATING OF STEEL: HERE THE STEEL,DEPENDING UPON ITS TYPE IS HEATED TO THE NORMALISING TEMPERATURE RANGE. FOR PLAIN CARBON STEEL THIS RANGE IS,
Ac FOR HYPO EUTECTOID STEELS.
Ac FOR EUTECTOID STEEL.
Acm + 50 FOR HYPER EUTECTOID STEELS.
FOR VARIOUS ALLOY STEELS THE NORMALISING TEMPERATURE RANGE IS AROUND 780 TO 850 0C,DEPENDING UPON THE TYPE OF STEELS.

10. NORMALISING TEMPERATURE RANGE

11. 2. HOLDING OF STEEL:HERE THE STEEL IS KEPT AT THIS NORMALISING TEMPERATURE FOR SOME TIME FOR EQUILIZATION OF TEMPERATURE DEPENDING UPON THE WEIGHT AND AREA OF STEEL PART.
3.COOLING OF STEEL:HERE THE STEEL IS COOLED FROM THIS NORMALISING TEMP. TO ROOM TEMPERATURE WITH A SLOW RATE OF COOLING IN THE “AIR”. HERE THE AUSTENITE IN THE STEEL IS TRANSFORMED INTO THE “ FINE PEARLITE STRUCTURE”
THE AIR COOLING USED MAY BE,
STILL AIR COOLING
FORCED AIR COOLING.

12. NORMALISING HEAT TREATMENT PROCESS

14. ADVANTAGES OF NORMALISING
USED TO ELIMINATE COARSE GRAINED STRUCTURE OBTAINED IN THE PREVIOUS WORKING OPERATION SUCH AS ROLLING,FORGING etc.
AVOIDS THE COARSE STRUCTURE THAT WOULD RESULT FROM SLOW COOLING OF REGULAR ANNEALING.
CARBURIZING STEELS AND CARBON TOOL STEELS MAY BE NORMALIZED TO BREAK UP CONTINUITY OF CEMENTITE NETWORK SURROUNDING PEARLITE.
REFINEMENT OF SIZE OF FERRITE AND PEARLITE.
IMPROVEMENT IN GRAIN STRUCTURE AND MECHANICAL PROPERTIES.
AS A PRIOR HEAT TREATMENT BEFORE SPHEROIDIZING ANNEALING
MODIFYING AND REFINING CAST DENDRITIC STRUCTURES.
NORMALISING MINIMIZES THE DISTORTION AND IMPROVES THE MACHINABILITY OF ALLOYED CARBURIZING STEELS.

15. ANNEALING

16. OBJECTIVES OF ANNEALING :
ANNEALING MAY BE DEFINED AS THE HEAT TREATMENT IN WHICH STEEL IS HEATED TO AUSTENITIC REGION AND THEN COOLING SLOWLY IN TRANSFORMATION RANGE.
OBJECTIVES OF ANNEALING :
TO IMPROVE HOMOGENEITY OF STEEL.
TO ALTER MICROSTRUCTURE TO IMPROVE PROPERTIES OF STEEL.
TO RESTORE DUCTILITY.
TO REFINE THE GRAIN SIZE.
TO RELIEVE THE INTERNAL STRESSES IN STEEL.
TO IMROVE MACHINABILITY OF STEEL.
TO REDUCE STRAIN HARDENING EFFECT OF COLD WORKING.THIS INCREASES DUCTILITY.

17. TYPES OF ANNEALING PROCESSES
FULL OR CONVENTIONAL ANNEALING.
ISOTHERMAL ANNEALING.
SPHERODIZING ANNEALING.
HOMOGENIZING ANNEALING
BRIGHT ANNEALING
BLACK ANNEALING
STRESS RELIEVING ANNEALING.

18. CONVENTIONAL ANNEALING : CONVENTIONAL OR FULL ANNEALING MAY BE DEFINED AS THE HEAT TREATMENT PROCESS IN WHICH THE GIVEN STEEL IS HEATED TO ANNEALING TEMP.RANGE,HOLD FOR SOME TIME AND THERE AFTER COOLED SLOWLY IN THE FURNACE BY SWITCHING OFF THE FURNACE LEADING TO FORMATION OF COARSE PEARLITIC STRUCTURE IN THE STEEL.
FOLLOWING ARE STEPS IN FULL ANNEALING,
1.HEATING STEEL TO ANNEALING TEMPERATURE RANGE.FOR PLAIN CARBON STEELS THIS RANGE IS,
Ac FOR HYPO EUTECTOID STEELS.
Ac FOR EUTECTOID STEEL.
Ac FOR HYPER EUTECTOID STEELS

19. ANNEALING TEMPERATURE RANGE

20. 2. HOLDING THE STEEL FOR EQUALIZATION OF TEMP. ACROSS CROSS SECTION
2.HOLDING THE STEEL FOR EQUALIZATION OF TEMP. ACROSS CROSS SECTION.THE HOLDING TIME IS A FUNCTION OF FOLLOWING FACTORS,
TYPE OF STEEL.
SIZE AND CROSS SECTIONAL AREA OF STEEL PART.
3. COOLING THE STEEL SLOWLY TO ROOM TEMPERATURE TO ROOM TEMPERATURE BY SWITCHING OFF THE FURNACE.DUE TO SLOW COOLING THE AUSTENITE IS TRANSORMED INTO “COARSE PEARLITE”

21. ANNEALING HEAT TREATMENT PROCESS

23. ISOTHERMAL ANNEALING:
MODIFICATION OF CONVENTIONAL ANNEALING PROCESS.
REDUCES THE TIME REQUIRED FOR FULL ANNEALING PROCESS.
STEEL IS QUENCHED IN A CONSTANT TEMPERATURE BATH KEPT AT REQUIRED TEMPERATURE AT WHICH A PARTICULAR FINE PEARLITIC STRUCTURE IS DESIRED.
AT CONSTANT TEMP.BATH THE AUSTENITE IN STEEL TRANFORMS INTO FINE PEARLITE.
STRUCTURE HAS IMPROVED MACHINABILITY.
AFTER THIS THE STEEL IS COOLED IN AIR OR RAPIDLY COOLED
CARRIED OUT IN SALT BATH OR VACCUM FURNACES.

24. ISOTHERMAL ANNEALING

25. SPHERODIZING ANNEALING
-SPHERODIZING ANNEALING IS DONE TO IMPROVE THE “MACHINABILITY” OF THE STEEL.
-HERE PLATE FORM OF CEMENTITE IS CONVERTED INTO GLOBULAR OR SPHERICAL FORM OF CEMENTITE.
-EFFECTIVE FOR HIGH CARBON STEELS,HIGH CARBON TOOL STEELS,ALL ALLOY STEELS,BALL BEARING STEELS.
FOLLOWING TWO METHODS ARE USED.
1. PROLONGED HOLDING BELOW Ac1 TEMP:HERE THE STEEL IS HEATED BELOW Ac1 TEMP.& HELD AT THIS TEMPERATURE FOR PROLONGED PERIOD OF TIME(6 TO7 HRS) TO CONVERT PLATE FORM OF CEMENTITE INTO GLOBULAR OR SPHERICAL FORM OF CEMENTITE.

27. 2.PENDULUM ANNEALING:
STEEL IS HEATED ALTERNATIVELY ABOVE Ac1 TEMP BY 20 DEGREE CENTIGRADE HELD THERE & COOLED BELOW Ac1 TEMP BY 20 DEGREE CENTIGRADE AGAIN HELD FOR SOME TIME AND THIS IS REPEATED FOR NO. OF CYCLES.
CARBIDE FORMING ELEMENTS LIKE CHROMIUM,TUNSTEN,MOLYBDENUM , VANADIUM INCREASES STABILITY OF CARBIDES IN STEEL
SPHERODIZING IS CARRIED ON
HIGH CARBON TOOL STEELS
BALL BEARING STEELS.
WEAR RESISTING STEELS.
TOOL STEELS.

29. HOMOGENIZING ANNEALING
PROCESS IS CARRIED OUT AT ºc.THE STEEL IS HEATED TO THIS TEMP.AND HELD FOR A LONG TIME.
DIFFUSION TAKES PLACE AND HOMOGENIZES THE STRUCTURE.
ADOPTED FOR ALLOY STEEL INGOTS IMMEDIATELY AFTER POURING.
DUE TO DIFFUSION THERE IS UNIFORM CONCENTRATION OF PRIMARY GRAINS.
STEEL IS HELD AT ºc.THEN COOLED TO ºc & THEN FURTHER COOLED IN AIR.
AFTER THIS THE STEEL MAY UNDERGO EITHER NORMALIZING OR ANNEALING TO REFINE OVER HEATED STRUCTURE.
COST OF TREATMENT IS VERY HIGH.

31. PROCESS ANNEALING
CARRIED IN BETWEEN TWO STAGES OF COLD WORKING PROCESSES LIKE WIRE ROLLING ,WIRE DRAWING ,TUBE DRAWING ETC.
AFTER ONE STAGE OF COLD ROLLING THE STEEL GETS “WORK HARDENED” i.e. ITS HARDNESS GETS INCREASED AND FURTHER COLD ROLLING BECOMES IMPOSSIBLE AS STEEL MAY GET CRACKED DURING NEXT STGE DUE TO HIGH HARDNESS.
THE STEEL AFTER FIRST STAGE OF ROLLING IS SUBJECTED TO PROCESS ANNEALING WHERE IT GETS HEATED TO “RECRYSTALLIZING TEMP”. (RECRYSTALLIZING TEMP = 0.5 TO 0.6 TIMES MELTING TEMPERATURE) IN THE FURNACE.

32. PROCESS ANNEALING
IN THIS PROCESS FOLLOWING THREE CHANGES OCCUR IN GRAINS/CRYSTALS AS UNDER,
1. RECOVERY:HERE THE STRESSES IN THE ELONGATED GRAINS ARE RELIEVED.
2.RECRYSTALIZATION: HERE THE NEW CRYSTALS GROW FROM THE OLD ELONGATED CRYSTALS.HARDNESS DECREASES,TENSILE STRENTH ALSO REDUCES.
3.GRAIN GROWTH:HERE THE NEWLY FORMED GRAINS GROW IN THE SIZE.HARDNESS COMES TO NORMAL VALUE AND STEEL GETS SOFT.

35. STRESS RELIEVING ANNEALING
INTENDED TO RELIEVE THE INTERNAL STRESSES IN THE STEEL COMPONENTS INTRODUCED DUE TO COLD WORKING PROCESSES.IT IS ALSO CALLED AS “SUBCRITICAL ANNEALING” AS THIS IS CARRIED OUT BELOW CRITICAL TEMPERATUTE AC1.CARRIED IN ELECTRIC BATCH FURNACE
STRESSES ARE INDUCED IN STEEL COMPONENTS DUE TO.
MACHINING,
FORMING OPERATIONS
HEAT TREATMENT

36. CASTING
WELDING
OTHERS INCLUDE EXPOSURE OF THE METAL TO SEVERE ATMOSPHERIC CONDITIONS SUCH AS MARINE OR CORROSIVE LIQUIDS.
HEAT TREATMENT PROCESS:
HEAT TREATMENT PROCESS CONSIST OF HEATING THE STEEL TO A TEMPERATURE BELOW AC1 TEMP.( ºc) ,HOLDING THERE FOR SOME TIME AND THERE AFTER COOLING THE STEEL TO ROOM TEMPERATURE IN FURNACE.
AT HIGH TEMP.STEEL BECOMES DUCTILE AND INTERNAL STRESSES ARE RELIEVED AS RESULT OF LOCAL PLASTIC DEFORMATIONCAUSED BY THEM.

37. STRESS RELIEVING TEMPERATURE High temperature steels
STRESS RELIEVING TEMPERATURE RANGE
STEEL
STRESS RELIEVING TEMPERATURE
High Speed steel ºc
Hot worked steel ºc
Cold worked steels
Nitriding steels ºc
High temperature steels ºc
Bearing Steels
Free cutting steel

40. BRIGHT ANNEALING
ANNEALING PROCESS CARRIED OUT IN A PROTECTIVE ATMOSPHERE.
PREVENTS DISCOLORATION OF STEEL.
ARGON,NITROGEN ,PURE HYDROGEN OR REDUCING ATMOSPHERE IS USED.
ILL EFEFCTS DECARBURISING , EMBRITTLEMENT ETC ARE AVOIDED.
CARRIED OUT ON WIRE,STRIP,SHEET,TUBE,PRESSINGS.

41. BRIGHT ANNEALING DETAILS
MATERIAL
TYPE OF WORK
FURNACE
GAS USED
Carbon Steel
Strip and Wire
Bell or pit furnace
Dry burnt amminia or lean exothermic stripped of carbon dioxide and H2O
Mild Steel
Strip and Wire in coil,sheets,strips,tubes in lengths.
Bell or Pit type furnace ,roller hearth.
Rich exothermic
Stainless steel
Strip and wire strands,small pressings etc,straight small bore tubes.
Pusher type or mesh belt
Cracked NH3 and pure hydrogen

42. BOX ANNEALING
TYPE OF FULL ANNEALING
STEEL COMPONENTS ARE PEACKED IN SEALED CONTAINER USING CHARCOAL OR CAST IRON CHIPS.
THIS REDUCES OXIDATION.
ALSO KNOWN AS BLACK ANNEALING.

43. HARDENING

44. HARDENING OF STEEL MAY BE DEFINED AS PROCESS OF IMPARTING HARDNESS,WEAR RESISTANCE,ABRASION RESISTANCE TO STEEL.
PROCESS CONSISTS OF HEATING THE STEEL TO “HARDENING TEMPERATURE RANGE” ,HOLDING THERE FOR SOME TIME AND FOLLOWED BY QUENCHING IT IN WATER OR OIL TO GET MARTENSITIC HARD STRUCTURE IN THE STEEL.
Heating of Steel: HERE THE STEEL DEPENDING UPON ITS TYPE IS HEATED TO AUSTENITIZING TEMPERATURE.THIS TEMP. RANGE IS CALLED AS “HARDENING TEMPERATURE RANGE”.
Ac FOR HYPO EUTECTOID STEELS.
Ac FOR EUTECTOID STEEL.
Ac FOR HYPER EUTECTOID STEELS

45. HARDENING TEMPERTURE RANGE

46. Holding Of Steel: HERE THE STEEL IS HOLD AT THIS TEMPERATURE FOR SOME TIME FOR EQUILIZATION OF TEMPERATURE.
Cooling Of Steel: HERE STEEL IS COOLED FROM AUSTENIZING TEMPERATURE TO ROOM TEMPERATURE IN A SUITABLE COOLING MEDIUM WITH THE COOLING RATE WHICH EXCEEDS THE “CRITICAL COOLING RATE” SO THAT AUSTENITE GETS TRANSFORMED INTO HARD AND BRITTLE “MARTENSITIC STRUCTURE”.DUE TO FORMATION OF MARTENSITIC STRUCTURE THE HARDNESS OF STEEL INCREASES.

47. HARDENING PROCESS

50. MARTESITIC STRUCTURE
DURING HARDENING ץ AUSTENITE TRANSFORMS INTO MARTENSITE.
MARTENSITE IS INTERSTITIAL SOLID SOLUTION OF CARBON INTO BCC UNIT CELL WHICH GETS CONVERTED TO “BODY CENTRAL TETRAGONAL UNIT CELL” (BCT).
THIS HAPPENS DUE TO ADDITIONAL CARBON ATOMS TRAPPED INTO BCC UNIT CELL.SINCE BCC UNIT CELL DOES NOT HAVE SUFFICIENT SPACE,IT GETS ELONGATED INTO “BODY CENTRAL TETRAGONAL UNIT CELL” (BCT).THIS LEADS TO STRESSES ON BCT UNIT CELL & MARTENSITE BECOMES HARD.HARDNESS VARIES FROM 50 TO 65 Rc DEPENDING UPON TYPE OF STEEL

51. BODY CENTER TETRAGONAL UNIT CELL OF MARTENSITE

53. Variation in hardness of martensite in steel with increase in carbon percentage in steel

54. HARDENABILITY
ABILITY OF STEEL TO GET HARDENED BY FORMING 100% MARTENSITIC STRUCTURE EVEN AT A SLOWER COOLING RATE.
HARDENABILITY DEPENDS UPON FOLLOWING FACTORS,
1.COMPOSITION OF STEEL : FOR PLAIN CARBON STEELS,AS PERCENTAGE OF CARBON INCREASES,TTT DIAGRAM SHIFTS IN THE RIGHTWARDS DIRECTION,THEREBY CRITICAL RATE OF COOLING BECOMES SLOWER AND HARDENABILITY OF STEEL INCREASES.
ON THE CONTRARY IF % OF CARBON DECREASES THEN TTT DIAGRAM SHIFTS IN THE LEFTWARDS THEREBY DECREASING HARDENABILITY OF STEEL.

57. MILD STEEL CANNOT BE HARDENED COMPLETELY DUE TO LESS CARBON

58. IN PLAIN CARBON STEELS ,AS PERCENTAGE OF CARBON GOES ON INCREASING,THE TTT DIAGRAM SHIFTS IN RIGHTWARDS DIRECTION AND HARDENABILITY ALSO GOES ON INCREASING & VICE VERSA.
IN ALLOY STEELS ,AS PERCENTAGE OF ALLOYING ELEMENTS GOES ON INCREASING, INCREASING,THE TTT DIAGRAM SHIFTS IN RIGHTWARDS DIRECTION AND HARDENABILITY ALSO GOES ON INCREASING & VICE VERSA.
IN GENERAL ALLOY STEELS HAVE MORE HARDENABILITY THAN PLAIN CARBON STEELS.

59. 2.AUSTENITIC GRAIN SIZE:
WITH INCREASE IN AUSTENITE GRAIN SIZE BEFORE QUENCHING , HARDENABILITY INCREASES.
DEPTH OF HARDENING IS MORE IN COARSE GRAINED STEEL THAN FINE GRAINED STEEL.
WITH CORASE GRAIN OF AUSTENITE THERE IS LESS TENDANCY OF NUCLEATION OF AUSTENITE TO PEARLITE & MORE POSSIBLITY OF FORMATION OF AUSTENITE TO MARTENSITE TRANSFORMATION.
WITH DECREASE IN AUSTENITE GRAIN SIZE BEFORE QUENCHING , HARDENABILITY DECREASES.
WITH FINE GRAIN OF AUSTENITE THERE IS MORE TENDANCY OF NUCLEATION OF AUSTENITE TO PEARLITE & MORE POSSIBLITY OF FORMATION OF AUSTENITE TO PEARLITE TRANSFORMATION.

60. 3.HOMOGENEITY OF AUSTENITE:
NON HOMOGENITY OF AUSTENITE TENDS TO DECREASE HARDENABILTY OF STEEL.
NON HOMOGENEITY OF AUSTENITE OCCURS DUE TO INCOMPLETE DISSOLUTION OF CARBIDES IN AUSTENITE.
4.UNDISSOLVED CARBIDES IN THE AUSTENITE:
FORMATION OF IRON CARBIDES AND ALLOY CARBIDES LEADS TO REDUCTION IN ALLOYING ELEMENT CONTENT AND CARBON CONTENT AND LEADS TO DECREASE IN HARDENABILITY OF STEEL.
SOME OF THE CARBIDE FORMING ELEMENTS TENDS TO REDUCE GRAIN SIZE AND LOWERS HARDENABILTY.
5.NON-METALLIC INCLUSIONS IN STEEL:THE NON METALLIC INCLUSIONS IN THE STEEL REDUCE HARDENABILITY AS THEY INCREASE INHOMOGENEITY OF AUSTENITE.

61. MEASUREMENT OF HARDENABILITY
JOMNY END QUENCH TEST IS USED TO MEASURE HARDENABILITY OF STEEL.
HERE STANDARD JOMNY SPECIMEN IS PREPARED AND END OF THE STEEL SPECIMEN IS QUENCHED WITH WATER JET.
AFTER THIS THE POINTS ARE MARKED FROM QUENCHED END AT AN INTERVAL OF 1.6 MM.
HARDNESS IS MEASURED FROM THE END AT THESE MARKED POINTS.
CURVE IS PLOTTED BETWEEN HARDNESS Vs DISTANCE FROM QUENCHED END CALLED AS “HARDENABILITY CURVE”.
AT A POINT THE CURVE RAPIDLY DROPS DOWN INDICATING THE SECTION IN THE STEEL SPECIMEN WITH 50% MARTENSITE AND 50% PEARLITE. THE DIATANCE OF THIS SECTION FROM THE END IS MEASURED.THIS IS “JOMNY DISTANCE”.EXPRESSED IN mm OR POINTS. LESS IS DIATANCE ,LESS IS HARDENABILTY & VICE VERSA.

63. QUENCHING MEDIUMS FOR HARDENING
WATER: GIVES FAST RATE OF COOLING.USED FOR LOW CARBON P.C.STEELS.DUE TO FAST COOLING RATE ,MORE POSSIBLITY OF QUENCHING CRACKS.
OIL:GIVES SLOWER COOLING RATES AS COMPARED TO WATER.USED FOR LOW,MEDIUM ALLOY STEELS.LESS DANGER OF QUENCHING CRACKS.
AIR:GIVE SLOWEST COOLING RATES.USED FOR HIGHLY ALLOYED STEELS AS THEY HAVE HIGH HARDENABILITY.NO QUECNCHING CRACKS AND DEFORMATION WITH AIR QUENCHING.

64. WATER AND OIL QUENCHING

65. AIR COOLING DURING HARDENING FOR HIGH ALLOY
STEELS

66. MARTEMPERING PRACTICALLY A HARDENING PROCESS.
STEEL IS HEATED TO AUSTENITIZING TEMP,THEN RAPIDLY COOLED IN SALT BATH KEPT AT A TEMP. JUST ABOVE Ms.
HOLDING TIME SHOULD BE SUCH THAT AUSTENITE SHOULD NOT TRANSFORM TO BIANITE.
IT IS THEN COOLED TO ROOM TEMP. IN AIR OR OIL TO OBTAIN MARTENSITE.
HOLDING IN CONSTANT TEMP.BATH BATH EQUILIZES TEMPERATURE FROM SURFACE TO CENTRE AND MINIMIZES WARPING AND QUECNCHING CRACKS

67. MARTEMPERING
TEMPERING

68. HARDENING DEFECTS OXIDATION AND DECARBURIZATION. QUENCHING CRACKS.
DISTORTION AND WARPAGE.
CHANGE IN DIMENSIONS.
SOFT SPOTS.
MECHANICAL PROPERITIES NOT CONFORMING TO SPECIFICATIONS.

69. TEMPERING

70. TEMPERING
HARDENED STEEL HAS HIGH HARDNESS DUE TO PRESENCE OF TETRAGONAL MARTENSITE.
HARDENED STEEL HAVE HIGH INTERNAL RESIDUAL STRESSES.
HARDENED STEEL HAS LOW VALUES OF DUCTILITY AND IMPACT STRENTH.
SUCH AS HARDENED STEEL COMPONENT CANNOT BE USED FOR PRACTICAL APPLICATIONS AS IT HAS LOW DUCTILITY,HIGH HARDNESS AND LOW DUCTILITY.
SO IT IS REQURED TO REDUCE HARDNESS,RELIEVE INTERNAL STRESSES AND IMPROVE DUCTILITY OF HARDENED STEEL PARTS.
THIS IS DONE BY SUBSEQUENT NEXT HEAT TREATMENT OF TEMPERING.
SO STEEL PARTS ARE ALWAYS “HARDENED AND TEMPERED”

71. STRUCTURE OF HARDENED STEEL

72. OBJECTIVES OF TEMPERING
TO RELIEVE INTERNAL STRESSES PRODUCED DURING HARDENING.
TO REDUCE HARDNESS.
TO IMPROVE DUCTILITY AND TOUGHNESS.
TO REDUCE RETAINED AUSTENITE.
TO OBTAIN A SPHEROIDAL STRUCTURE WHICH IMPROVES MACHINABILITY.

74. TEMPERING
TEMPERING CYCLE CONSISTS OF HEATING OF HARDENED COMPONENT TO A TEMPERATURE WELL BELOW Ac1 (100 TO 700 º C )TEMPERATURE AND HOLDING FOR A PERIOD OF 1 TO 2 HOURS THEN STEEL IS COOLED TO ROOM TEMPERATURE IN AIR OR IN SALT BATH.
TOOL STEELS ARE TEMPERED AT LOW TEMPERATURE.
HIGH ALLOY STEELS ARE TEMPERED AT HIGHER TEMPERATURE.
LOW ALLOY STEELS ARE TEMPERED ABOVE 350 º C

75. TEMPERING HEAT TREATMENT CYCLE

76. TYPES OF TEMPERING LOW TEMPERATURE TEMPERING:
CARRIED OUT BETWEEN º C.
HERE INTERNAL STRESSES ARE MAJORITILY REMOVED.
MARTENSITE DECOMPOSES AS LOW CARBON MARTENSITE AND EPSILON CARBIDE(LOW CARBON CARBIDE)
NO APPRECIABLE CHANGE IN THE PROPERTIES RELATED TO MICROSTRUCTURE OBSERVED.

77. 2.MEDIUM TEMPERATURE TEMPERING:
CARRIED OUT BETWEEN º C.
LOW CARBON MARTENSITE AND EPSILON CARBIDE(LOW CARBON CARBIDE) TRANSFORMS TO FERRITE AND CEMENTITE.
HARDNESS DRASTRICALLY DECREASES WITH IMPROVEMENT IN DUCTILITY.
3.HIGH TEMPERATURE TEMPERING:
PRODUCES SPHEROIDAL CEMENTITE IN FERRITE MATRIX.ALSO CALLED AS SPHERODIZING.
HARDNESS FURTHER REDUCES WITH IMPROVEMENT IN DUCTILITY.

78. MARTENSITE & TEMPERED MARTENSITE PHASES

79. VARIATION OF MECHANICAL PROPERTIES OF STEEL
WITH TEMPERING TEMPERATURE

80. EFFECT OF ALLOYING ELEMENTS ON TEMPERING
FOR ALLOY STEELS INTERVAL BETWEEN TEMPERING STAGES SHIFTS TOWARDS HIGHER TEMEPRATURES.
MARTENSITE BECOMES MORE RESISTANT TO TEMPERING.
CARBIDE FORMING ELEMENTS LIKE CHROMIUM, MOLYBDENUM , TUNGSTEN AND VANADIUM RETARD LOSS OF HARDNESS AND STRENTH WITHTEMPERING TEMPERATURE. ALLOYING ELEMETS RETARD PRECIPITATION OF CARBON FROM MARTENSITE AT HIGHER TEMPERING TEMPERATURE.

81. SECONDARY HARDENING
SOMETIMES IT IS OBSERVED THAT INSTEAD OF DECREASING, THE HARDNESS OF HARDENED STEEL INCREASES WITH TEMPERING TEMPERATURE.THIS IS CALLED AS SECONDARY HARDNESS.
MORE COMMON IN STEELS WITH TUNSTEN,MOLYBDENUM,CHROMIUM AS ALLOYING ELEMENTS IN THE STEEL.
THIS IS DUE TO TRANSFORMATION OF RETAINED AUSTENITE INTO MARTENSITE

82. SELF TEMPERING
TEMPERING TAKES PLACE BY THE RESIDUAL HEAT IN THE INTERIOR OF THE STEEL COMPONENT.
COMPONENTS ARE QUENCHED IN BATH UPTO Ms TEMPERATURE AND THEN COOLED IN AIR.
HEAT LEFT IN THE INTERIOR PORTION WILL BE UTILISED TO REHEAT COMPONENT AND TEMPER MARTENSITE IN QUENCHING STAGE.
USED FOR SMALL FORGINGS ,AXLES.
LESS QUENCHING CRACKS AND DISTORTION.

83. SELECTIVE TEMPERING SELECTIVE HARDENED AREAS ARE TEMPERED.
DIFFERENT HARDNESS IS OBTAINED IN DIFFERENT AREAS OF STEEL COMPONENTS.
e.g INCREASE IN TOUGHNESS IN THE SHANK PORTION OF TOOLS BY REDUCING THE HARDNESS.
INCREASE IN HARDNESS OF SPRING PORTION OF COLLET.
PROCEESS IS CARRIED BY REHEATING HARDENED PORTION FOLLOWED BY SLOW COOLING OF THE SECTION REQUIRED TO BE SOFT.

84. RETAINED AUSTENITE IN STEEL
HARDENED STEELS CONTAINING MORE THAN 0.5% CARBON & NOT COOLED UPTO Mf TEMPERATURE, AT ROOM TEMPERATURE SHOWS “MARTENSITE+ RETAINED AUSTENITE” IN ITS STRUCTURE

85. AS % OF CARBON IN THE STEEL INCREASES, THE Ms AND Mf TEMPERATURES GOES ON REDUCING.

86. AFTER 0.5% CARBON, THE Mf TEMPERATURE GOES BELOW ZERO DEGREE CENTIGRADE & FOR SUCH STEEL ,TO FORM 100% MARTENSITE,IT IS NECESSARY TO COOL THEM BELOW ZERO DEGREE CENTIGRADE.THIS IS CALLED AS “SUBZERO HEAT TREATMENT”.
IF A PART IS NOT COOLED BELOW Mf TEMP.THEN AT ROOM TEMP.IT CONTAINS MARTENSITE AND RETAINED AUSTENITE.
RETAINED AUSTENITE HAS FOLLOWING PROBLEMS.
1.REDUCTION OF HARDNESS
2. UNEVEN CUTTING PROPERTIES.
3.GRINDING CRACKS.

87. 4.DIMENSIONAL UNSTABILITY IN MEASURING INSTRUMENTS
RETAINED AUSTENITE AFTER A LONG PERIOD OF TIME CONVERTS INTO “MARTENSITE”.
THIS CONVERSION LEADS TO INCREASE IN SPECIFIC VOLUME i.e. EXPANSION OCCURS.
THIS LEADS TO DIMENSIONSL INSTABILITY IN PART.
HIGHLY UNDESIRABLE IN PRECISION MEASURING INSTRUMENTS,AS SMALL DIENSIONAL INSTABILITY WILL CAUSE ERRORS IN MEASUREMENTS.

90. METHODS OF SUBZERO TREATMENT
1.SOLID CARBONDIOXIDE METHOD:
PART COOLED UPTO -80 ºC.
SOLID Co2 REDUCES TEMP.OF ORGANIC LIQUID MIXURE AS TRICHLOROETHYLENE OR ALCOHOL.
PARTS ARE IMMERSED IN SOLUTION.
2.MECHANICAL REFRIGERATION METHOD:
PARTS ARE COOLED UPTO -100 ºc.
COMMONLY USED FLUIDS ARE METHYLCHLORIDE (-97 ºc),freon(-111 ºC), METHANOL(-97 ºc)
LOW RUNNING COSTS.
HIGH CAPITAL COSTS.

91. METHODS OF SUBZERO TREATMENT
3.LIQUID NITROGEN SYSTEM METHOD:
PARTS ARE COOLED TO -196 ºC.
PARTS ARE COOLED DIRECTLY/INDIRECTLY IN CONTACT WITH LIQUID NITROGEN.
METHOD IS EFFICIENT AND ADVANTETIOUS.
PARTS CAN BE COOLED AT A CONTROLLED RATE WITHOUTA PROBLEM.
COMMONLY USED METHOD.
GAS COOLED AND LIQUID N2 COOLED SYSTEMS ARE IN COMMON USE.

94. AUSTEMPERING OR ISOTHERMAL QUENCHING
STEEL IS HEATED TO AUSTENITIZING TEMPERATURE AND THEN RAPIDLY COOLED TO A TEMP. BETWEEN NOSE OF TTT CURVE AND Ms TEMP.
STEEL IS HELD AT THIS TEMP IN A CONSTANT TEMPERATURE BATH FOR SUFFICIENT TIME SO THAT AUSTENITE TRANSFORMS INTO BIANITE AS SHOWN IN THE FIGURE.

95. AUSTEMPERING

96. AUSTEMPERING
FULLY BIANITE (NEEDLE SHAPED PEARLITIC) STRUCTURE IS OBTAINED AFTER THIS HEAT TREATMENT.
IT HAS INTERMEDIATE PROPERTIES OF PEARLITE AND MARTENSITE.
IT HAS HIGH HARDNESS WITH NOTCH TOUGHNESS.
BETTER ELONGATION AND IMPACT STRENTH.
LESS DISTORTION AND DANGER OF QUENCHING CRACKS AS THE QUENCH IS NOT DRASTIC.
SIZE OF THE PART POSES LIMITATION.

97. COMPARISION OF AUSTEMPERING AND MARTEMPERING
COOLED IN ISOTHERMAL BATH MAINTAINED IN BETWEEN NOSE OF TTT DIAG AND Ms TEMP. i.e. 510 TO 220 0C.
COOLED IN ISOTHERMAL BATH MAINTAINED JUST ABOVE Ms TEMP. i.e. ABOVE 220 0C. THEN COOLED IN AIR.

98. 2 AUSTENITE TRANSFORMS TO BAINITE .
3 HARDNESS OF BAINITE IS Rc 60.
4 NO NEED OF FURTHER TEMPERING AS INTERNAL STRESSES ARE NOT DEVELOPED.
2 AUSTENITE TRANSFORMS TO MARTENSITE.
3 HARDNESS OF MARTENSITE IS Rc 64.
4 TEMPERING IS ESSENTIAL TO RELIEVE INTERNAL STRESSES.

99. PATENTING SIMILAR TO AUSTEMPERING.
GIVEN TO PLAIN CARBON STEELS (0.3 % & 0.6% CARBON) AND ALLOY STEELS.
STEELS ARE HEATED TO AUSTENITIZING TEMPERATURE & THEN QUENCHED IN A CONSTANT BATH KEPT JUST BELOW OR ABOVE NOSE TEMPERATURE OF TTT DIAGRAM OF STEEL.
STEEL IS HELD AT THIS TEMPERATURE UNTIL AUSTENITE TRANSFORMS INTO FINE PEARLITE OR UPPER BIANITE.

101. PATENTING IS USED IN THE WIRE DRAWING OF SPRING WIRES AS IT IMPROVES DUCTILITY AND TOUGHNESS BY FORMING A FINE PEARLITIC STRUCTURE.
DURING WIRE DRAWING THE PATENTING IS USED AS SUBSEQUENT TREATMENT.
THE DRAWN WIRE GOES SLOWLY AND DIRECTLY IN A SALT BATH MAINTAINED AT REQUIRED TEMPERATURE AND MAY BE REDRAWN AGAIN.
PATENTING PROVIDES GOOD SURFACE FINISH AND LUBRICATING SURFACE.

103. COMPARISION OF ANNEALING AND NORMALIZING
ANNEALING 1.HEATING AT A C FOR HYPOEUTECOID STEEL A C FOR HYPEREUTECOID STEEL. 2. SLOW COOLING IN FURNACE ITSELF. OR BURIED IN SAND.
NORMALIZING 1.HEATING AT A C FOR HYPOEUTECOID STEEL Acm C FOR HYPEREUTECOID STEEL 2. SLIGHTLY FASTER i.e. COOLING IN AIR.

104. ANNEALING 3. LESS HARDNESS , TS AND TOUGHNESS. 4. LARGER GRAIN SIZE
ANNEALING 3. LESS HARDNESS , TS AND TOUGHNESS. 4. LARGER GRAIN SIZE. (COARSE PEARLITE ) 5. UNIFORM DISTRIBUTION OF GRAINS. 6. INTERNAL STRESS ARE LEAST.
NORMALIZING 3. HIGHER HARDNESS , TS AND TOUGHNESS. 4. FINE GRAIN SIZE. (FINE PEARLITE ) 5. SLIGHTLY LESS DISTRIBUTION OF GRAINS. 6. INTERNAL STRESS ARE SLIGHTLY MORE.

105. THANKS !!!