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Co related to chapter. Select different heat treatment processes as per application. Visit for more Learning Resources. HEAT TREATMENT OF STEEL. HEAT TREATMENT. HEAT TREATMENT IS DEFINED AS SEQUENCE OF HEATING AND COOLING TO GET DESIRED COMBINATION OF PROPERTIES IN THE STEEL.

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  1. Co related to chapter • Select different heat treatment processes as per application. Visit for more Learning Resources

  2. HEAT TREATMENT OF STEEL HEAT TREATMENT HEAT TREATMENT IS DEFINED AS SEQUENCE OF HEATING AND COOLING TO GET DESIRED COMBINATION OF PROPERTIES IN THE STEEL. • CHANGE IN THE PROPERTIES IS THE FUNCTION OF • PHASE TRANSFORMATION • CHANGE IN STRUCTURE. 2

  3. HEAT TREATMENT OF STEEL TRNSFORMATION FROM PEARLITE TO AUSTENITE INITIALLY STEEL CONTAINING PEARLITE,CARBIDE,FERRITE,DIFFERENT ALLOYING ELEMENT DEPEND UPON CARBON PERCENTAGE. WHEN WE START HEATING SUCH STEEL, AND TEMP REACHES AT LOWER CRITICAL TEMP (727 ˚C),CHANGE OF PHASE OCCURS, AND PEARLITE IS CONVERTED IN TO AUSTENITE. THIS IS BECAUSE FORMATION OF NUCLEATION. RATE OF NUCLEATION INCREASES WITH INCREASE IN TEMP BECAUSE OF FERRITE AND CEMENTITE ALSO START TO DISSOLVE . 3

  4. HEAT TREATMENT OF STEEL TRNSFORMATION FROM PEARLITE TO AUSTENITE WHEN TEMP REACHES TO UPPER CRITICAL TEMP THE STRUCTURE BECOMES COMPLETELY AUSTENITE. HOWEVER SOME UNDISOLVED CARBIDE IS PRESENT, THESE ARE CONVERTED BY HOLDING STEEL TO SAME TEMP. FORMATION OF AUSTENITE IS DEPEND UPON TEMP AND TIME REQ. FOR HEATING. 4

  5. HEAT TREATMENT OF STEEL TRNSFORMATION FROM PEARLITE TO AUSTENITE FOUR STAGES TO FORMATION OF AUSTENITE. 1)FORMATION OF NUCLEATION 2)TRANSFORMATION OF CEMENTITE AND FERRITE 3)DISSOLVATION OF CARBIDE 4)FORMATION OF AUSTENITE. 5

  6. HEAT TREATMENT OF STEEL TTT DIAGRAM FOR EUTECTOID STEEL Austenite is stable above 727, below this temp it gives various stages as further……. 6

  7. HEAT TREATMENT OF STEEL TTT DIAGRAM RAPID COOLING • FIG.SHOWS ISOTHERMAL TRANSFORMATION DIAGRAM FOR A EUTECTOID STEEL TEST PIECE WHICH HAS BEEN RAPIDLY COOLED IN A BATH AT SET TEMP. HELD FOR A TIME AND WATER QUENCHED. • IT CAN BE SEEN THAT IF THE TRANSFORMATION IS ALLOWED TO TAKE PLACE AT HIGHER TEMP THEN ,AS ABOVE COARSE PEARLITE IS FORMED. • IF THE TEST PIECE IS COOLED TO A LOWER TEMP IN A BATH A FINER PEARLITE STRUCTURE RESULTS. • IF THE TEST PIECE IS RAPIDALY COOLED TO A TEMP BELOW VALUE Ms PHASE IS PRODUCED CALLED MARTENSITE. • IF THE TEST PIECE IS COOLED RAPIDLY AT A TEMP. BETWEEN 220 AND 550 A PPHASE STRUCTURE BETWEEN PEARLITE AND MARTENSITE IS FORMED. THIS CALLED BAINITE. 7

  8. HEAT TREATMENT OF STEEL BAINITE • AUSTENITE COOLED BELOW 530 BAINITE (FERRITE+ CEMENTITE) • ABOVE M BAINITE HARDER STRONGER TOUGHER PEARLITE • BAINITE FORMED AT HIGHER TEMP UPPER BAINITE (40-50RC) 8 • BAINITE FORMED AT LOWER TEMP LOWER BAINITE (50-60RC)

  9. HEAT TREATMENT OF STEEL MARTENSITE • AUSTENITE COOLED AT VERY LOW TEMP MARTENSITE. • AUSTENITE PHASE TRANSFORMATION MARTENSITE • DIFFUSIONLESS (TIME INDEPENDENT) HARDER STRONGER BRITTLE 9

  10. HEAT TREATMENT OF STEEL HEAT TREATMENT HEAT TREATMENT IS DEFINED AS SEQUENCE OF HEATING AND COOLING TO GET DESIRED COMBINATION OF PROPERTIES IN THE STEEL. HEAT TREATMENT PROCESSES • ANNEALING • NORMALISING • HARDENING • TEMPERING • SURFACE HARDENING. 10

  11. HEAT TREATMENT OF STEEL ANNEALING WHEN STEEL IS HEATED ABOVE (30-50˚C)A3 LINE IN CASE OF HYPOEUTECTOID STEEL AND Acm LINE IN CASE OF HYPEREUTECTOID STEEL, KEPT AT THIS TEMP FOR SOME TIME, AND ALLOW SLOW COOLING IN FURNACE UP TO ATM TEMP ,BY AIR CONVECTION IS CALLED AS ANNEALING.(FULL ANNEALING) 11

  12. HEAT TREATMENT OF STEEL FULL ANNEALING STEEL (30-50˚) A3, Acm   SLOW FURNACE (20˚C/HR) UP TO ATM. TEMP COLLING MEDIUM IS AIR. 12

  13. HEAT TREATMENT OF STEEL BRIGHT ANNEALING OXIDATION : WHEN A METAL COMES IN CONTACT WITH OXYGEN A REACTION TAKESPLACE WHICH REMOVES A ELECTRON FROM METAL ,VALANCE IN ELEMENT IS INCREASED. IN CASE OF IRON IT IS SLOW BURNING PROCESS RESULT IN TO BRITTLE BROWN SUBSTANCE WE CALLED AS RUST. EFFECT OF RUST: 1)ORANGE COLOUR APPEARS 2)SURFACE BECOMES ROUGH 3)CORROSION MAY TAKESPLACE 4)HOLE MAY BE DEVELOPED 5)IN CASE OF SLIDING OBJECT,THEY MAY JAM. 6)WEAKER

  14. HEAT TREATMENT OF STEEL BRIGHT ANNEALING WHEN ANNEALING IS CARRIED IN PRESENCE OF SOME PROTECTIVE MEDIUM IN ORDER TO AVOID OXIDATION AND DISCOLOURATION ,IS CALLED AS BRIGHT ANNEALING. IT IS CALLED AS BRIGHT ANNEALING ,AS IT KEEPS SURFACE BRIGHT. THE PROTECTIVE MEDIUM USED IS INNERT GASES SUCH AS ARGON OR NITROGEN OR REDUCING GAS (15%H2,10%C0,5%CO2,1.5%CH4 AND REMAINDER N2).

  15. HEAT TREATMENT OF STEEL BOX ANNEALING IN THIS TYPE ANNEALING IS CARRIED OUT IN SEALED CONTAINER IN ORDER TO MINIMIZE OXIDATION. OBJECT IS COVERED WITH SMALL IRON CHIPS,CHARCOAL,CLEAN SAND AND ANNEALED SAME AS THAT OF FULL ANNEALING. BLACK ANNEALING=POT ANNEALING=CLOSE ANNEALING

  16. HEAT TREATMENT OF STEEL ISOTHERMAL ANNEALING IN THIS TYPE ANNEALING STEEL IS HEATED ABOVE UPPER CRITICAL TEMP.(A3),KEPT THEIR FOR SOME TIME ,THEN TEMP IS REDUCED BELOW LOWER CRITICAL TEMP(A1),KEPT THEIR FOR SOME TIME, AND THEN ALLOW TO COOL UP TO ATM TEMP. IN PRESENCE OF AIR.

  17. HEAT TREATMENT OF STEEL ISOTHERMAL ANNEALING STEEL UPPER CRITICAL TEMP (A3)   STEEL LOWER CRITICAL TEMP (A1)   ROOM TEMP (AIR MEDIUM)

  18. HEAT TREATMENT OF STEEL ISOTHERMAL ANNEALING IT REDUCES ANNEALING TIME IT IMPROVES MACHINABILITY IT IMPROVES SURFACE FINISH AFTER MACHINING

  19. HEAT TREATMENT OF STEEL SPHEROIDISE ANNEALING ANY HEAT TREATMENT THAT PRODUCES A STRUCTURE OF GLOBULES (ROUNDED PARTICLES) IN CEMENTITE IN FERRITE MATRIX IS CALLED SPHEROIDISE ANNEALING. THIS TYPE OF ANNEALING IS DONE IN HIGH CARBON STEEL(0.6-2%C) TO SOFTEN THE MATERIAL AND INCREASE MACHINABILITY.

  20. HEAT TREATMENT OF STEEL SPHEROIDISE ANNEALING • METHODS TO PRODUCE SPHEROIDISED STRUCTURE • BY HOLDING FOR LONG TIME AT JUST BELOW THE LOWER CRITICAL TEMP,CEMENTITE FROM PEARLITE GLOBURISES. • THERMAL CYCLING AROUND A1 : DURING HEATING ABOVE A1 , CEMENTITE TRY TO DISSOLVE AND DURING COOLING THEY TRY TO FORM, THIS REPEATED ACTION SPHEROIDISES THE CARBIDE OR CEMENTITE PARTICLE

  21. Spheroidite: Nonequilibrium Transformation • Fe3C particles within an a-ferrite matrix • diffusion dependent • heat bainite or pearlite at temperature just below eutectoid for long times 10

  22. HEAT TREATMENT OF STEEL SUBCRITICAL ANNEALING PROCESSES AS THESE ANNEALING PROCESSES CARRIED OUT BELOW LOWER CRITICAL TEMP. THESE ARE CALLED AS SUBCRITICAL ANNEALING . 1)STRESS RELIEF ANNEALING 2)RECRYSTALLIZATION ANNEALING 3)PROCESS ANNEALING • STRESS RELIEF ANNEALING • IN THIS PROCESS,COLD WORKED STEEL IS HEATED TO TEMP. BETWEEN 500-550˚C I.E. BELOW ITS RECRYSTALIZATION TEMP. , KEPT THERE FOR 1-2 HOURS AND COOLED TO ROOM TEMP IN AIR.

  23. HEAT TREATMENT OF STEEL STRESS RELIEF ANNEALING STEEL BELOW CRITICAL TEMP (A1)   ROOM TEMP (AIR MEDIUM)

  24. HEAT TREATMENT OF STEEL STRESS RELIEF ANNEALING PROCESS IS APPLICABLE ONLY FOR HYPOEUTECTOIDSTEEL BECAUSE ONLY THESE STEEL CAN BE COLD ROLLED. DUE TO THIS PROCESS INTERNAL STRESSES ARE RELIEVED WITHOUT AFFECTING HARDNESS AND STRENGTH. IT INCREASES CORROSION RESISTANCE.

  25. HEAT TREATMENT OF STEEL RECRYSTALLIZATION ANNEALING PROCESSES THIS PROCESS IS DONE BELOW A1 TEMP THAT IS LOWER CRITICAL TEMP. HEATING RANGE IS 625-675˚ C DUE TO THIS PROCESS FERRITE RECRYSTALISES AND CEMENTITE TRIES TO SPEROIDISE. NOT ONLY STRESSES ARE REMOVED BUT ALSO MATERIAL GET SOFT AND DUCTILE.

  26. HEAT TREATMENT OF STEEL PROCESS ANNEALING THIS PROCESS IS DONE ABOVE ITS RECRYSTALIZATION TEMP. THIS PROCESS IS SIMILAR TO THAT OF RECRYSTALIZATION PROCESS AND BOTH THE PROCESS INVOLVES CRYSTALLIZATION AND FORMATION OF NEW STRESS FREE GRAINS. APPLICABLE FOR LOW CARBON STEEL. DUE TO THIS PROCESS FERRITE RECRYSTALISES AND CEMENTITE TRIES TO SPEROIDISE.

  27. HEAT TREATMENT OF STEEL PURPOSE OF ANNEALING • TO RELIVE INTERNAL STRESSES YHAT ARE DEVELOPED FROM WELDING AND COLD ROLLING. • TO REDUSE HARDNESS • TO REFINE THE GRAIN SIZE • TO MAKE MATERIAL HOMOGENEOUS IN RESPECT OF CHEMICAL COMPOSITION • TO INCREASE MACHINABILITY • TO MAKE STEEL SUITABLE TO SUBSEQUENT HEAT TREATMENT LIKE HARDENING APPLICATION OF ANNEALING • IN STEEL USED IN SHEET AND WIRE DRAWING • HIGH CARBON STEEL • BALL BEARING STEEL • CASTING OF CARBON AND ALLOY STEEL.

  28. HEAT TREATMENT OF STEEL NORMALISING PROCESS INVOLVES HEATING TO ABOVE UPPER CRITICAL TEMP. (A3) FOR HYPOEUTECTOID STEEL AND ABOVE ACM FOR HYPEREUTECTOID STEEL ABOVE 30 TO 50˚C , HOLDING LONG TIME AT THIS TEMP. FOR HOMOGENEOUS AUSTENITIZATION AND COOLOING TO ROOM TEMP. IN STILL AIR.

  29. HEAT TREATMENT OF STEEL NORMALISING STEEL (30-50˚) A3, Acm   FAST IN AIR MEDIUM ROOM TEMP. THE PROCESS OF ANNEALING IS SAME AS THAT OF NORMALISSING BUT DIFFER IN FOLLOWING MANNER.

  30. HEAT TREATMENT OF STEEL NORMALISING

  31. HEAT TREATMENT OF STEEL PURPOSE OF NORMALISING • THE PURPOSE OF PROCESS IS SAME AS THAT OF ANNEALING . TO ELIMINATE CEMENTITE NETWORK IN HYPEREUTECTOID STEEL ,FORMED DUE TO SLOW COOLING IN THE TEMP RANGE FROM ACM TO A1. • PRODUCE UNIFORM STRUCTURE • PRODUCE HARDER AND STRONGER STEEL • INCREASE STRENGTH AND DUCTILITY • TO REFINE GRAIN SIZE. APPLICATION OF NORMALISING • USED TO IMPROOVE MICROSTRUCTURE IN STEEL • IT IS APPLIED ON WELDED STRUCTURE TO IMPROOVE UNIFORMITY. • IT IS MOSTLY APPLIED ON LOW AND MEDIUM CARBON STEELS • TO REFINE GRAIN STUCTURE IN ROLLED AND CAST STEEL.

  32. HEAT TREATMENT OF STEEL HARDENING THE CONVENTIONAL HARDENING PROCESS INVOLVES HEATING THE STEEL ABOVE A3 TEMP FOR HYPOEUTECTOID STEEL AND ABOVE A1 TEMP FOR HYPEREUTECTOID STEEL BY 50˚ C ,AUSTENISING FOR SOME TIME AND COOLING WITH A RATE JUST EXCEEDING THE CRITICAL COOLING RATE OF THAT TO ROOM TEMP. OR BELOW ROOM TEMP.

  33. HEAT TREATMENT OF STEEL HARDENING –QUENCHING MEDIUM A PROPER QUENCHING MEDIUM IS USED SUCH THAT COMPONENT GETS COOLED AT A RATE JUST EXCEEDING THE CRITICAL COOLING RATE OF STEEL. ALLOY STEELS AIR LESS CRITICAL COOLING RATE HIGH CARBON STEEL OIL SLIGHTLY MORE COOLING RATE MEDIUM CARBON STEEL WATER HIGH CRITICAL BRINE COOLING RATE FOR LOW CARBON STEEL WHICH IS LESS THAN 0.1% CARBON CAN NOT BE HARDENED BECAUSE OF THEIR HIGH CRITICAL COOLING RATE.

  34. HEAT TREATMENT OF STEEL HARDENING • NEED OF HARDENING • TO HARDEN THE STEEL TO THE MAXIMUM LEVEL BY AUSTENITE TO MATENSITE TRANSFORMATION. • TO INCREASE THE WEAR RESISANCE AND CUTTING ABILITY OF TOOLS. • HARDENING DEFECTS • OXIDATION AND CARBURIZATION • CRACKS AFTER QUENCHING • DISTORTION AND WARPAGE • CHANGE IN DIMENSION • APPEARANCE OF SOFT SPOTS • REDUCTION IN MECHANICAL PROPERTIES.

  35. HEAT TREATMENT OF STEEL TEMPERING THE PROCESS CONSIST OF HETING THE HARDENED COMPONENT S TO A TEMP. BETWEEN 100 TO 700˚C (BELOW A1) ,HOLDING AT THIS TEMP. FOR SPECIFIC PERIOD OF TIME (1-2 HOURS) AND COOLING TO ROOM TEMP. USUALLY IN AIR. HARDENED COMPONENT (100-700) BELOW A1 (1-2 HOURS) AIR MEDIUM ROOM TEMP. .

  36. HEAT TREATMENT OF STEEL TYPES OF TEMPERING 1)LOW TEMP. TEMPERING.(100-200 C) AFTER HARDENING HEAT TREATMENT PROCESS STEEL CONTAINS MARTENSITE AND RETAINED AUSTENITE DURING THIS PROCESS MARTENSITE DECOMPOSES AND GIVES LOW CARBON MARTENSITE AND TRANSITION CARBIDE CALLED &-CARBIDE. 200 C MARTENSITE LOW CARBON MARTENSITE + & CARBIDE HARDNESS BRITTLNESS INTERNAL STRESSES

  37. HEAT TREATMENT OF STEEL TYPES OF TEMPERING 1)MEDIUM TEMP. TEMPERING.(200 -500 C) DURING TO HEATING IN THIS TEMP RANGE CEMENTITE IS FORMED AND VERY FINE DISTRIBUTION OF CEMENTITE PARTICLES IS OBSERVED IN THE MATRIX OF FERRITE. HARDENED STEELMARTENSITE + RETAINED AUSTENITE RETAINED AUSTENITEBAINITE OR (MARTENSITE+CARBIDE) MARTENSITE LOW CARBON MARTENSITE AND +& CARBIDE LOW CARBON MARTENSITE AND +& CARBIDE FERRITE + CEMENTITE HARDNESS TOUGHNESS DUCTILITY

  38. HEAT TREATMENT OF STEEL TYPES OF TEMPERING 1)HIGH TEMP. TEMPERING.(500-700 C) DURING THIS STAGE OF TEMPERING ,CEMENTITE PARICLES BECOMES COARSE. EXCEPT THIS THERE IS NO CHANGE IN THE STRUCTURE. STRUCTURE UNDER MICROSCOPE IS OBSERVED AS SPEROIDITE. HARDNESS TOUGHNESS MACHINABILITY

  39. HEAT TREATMENT OF STEEL VARIATION IN PROPERTIES WITH VARYING TEMPERING TEMP

  40. HEAT TREATMENT OF STEEL NEED OF TEMPERING • TO REDUCE INTERNAL STRESES DEVELOPED DUE TO RAPID COOLING IN HARDENING PROCESS • TO REDUCE HARDNESS AND INCREASE DUCTILITY AND TOUGHNESS • TO ELIMINATE RETAINED AUSTENITE.

  41. HEAT TREATMENT OF STEEL MARTEMPERING IN THIS PROCESS AUSTENISED STEEL IS COOLED RAPIDALY AVOIDING THE NOSE OF THE I.T. DIAGRAM TO TEMP BETWEEN NOSE AND Ms SOAKED AT THIS TEMP. FOR SOME TIME BUT NOT ENOUGH TO PERMIT THE FORMATION OF BAINITE AND THEN COOLED TO ROOM TEMP.IN AIR OR OIL.

  42. HEAT TREATMENT OF STEEL MARTEMPERING PROCESS IS APPLICABLE FOR HIGH CARBON STEEL AND LOW ALLOY STEEL. LESS WARPING LESS POSSIBILITY OF QUENCHING CRACKS

  43. HEAT TREATMENT OF STEEL AUSTEMPERING IN THIS PROCESS AUSTENISED STEEL IS COOLED WITH RATE EXCEEDING THE CRITICAL COOLING RATE IN A MOLTEN BATH HELD AT SOME CONSTANT TEMP. BETWEEN THE NOSE OF TTT DIAGRAM AND Ms TEMP I.E. IN BAINITE REGION , HOLDING AT THIS TEMP. FOR A SUFFICIENT PERIOD FOR THE COMPLETION OF BAINITIC TRANSFORMATION AND COOLING TO ROOM TEMP AT ANY DESIRED RATE.

  44. HEAT TREATMENT OF STEEL AUSTEMPERING THE ADVANTAGE OF THIS AUSTEMPERING IS THAT IT PRODUCES STRUCTURE AND PROPERTIES MUCH SIMILAR TO TEMPERED MARTENSITE WITHOUT INVOLVING IN MARTENSITIC TRANSFORMATION. DEPENDING UPON TEMP.OF TRANSFORMATION , THE PRODUCT MAY BE UPPER BAINITE OR LOWER BAINITE . PROPERTIES OF BAINITE ARE INTERMEDIATE TO THOSE OF MARTENSITE AND PEARLTE ARE VERY MUCH SIMILAR TO THAT OF TEMPERED MARTENSITE.

  45. HEAT TREATMENT OF STEEL PATENTING THE PROCESS IS VERY MUCH SIMILAR TO AUSTEMPERING EXCEPT IN THE RANGE OF TEMPERATURE USED FOR THE ISOTHERMAL TRANSFORMATION. IT CONSIST OF QUENCHING AN AUSTENITISED STEEL IN MOLTEN BATH MAINTAINED AT SOME CONSTANT TEMP. SLIGHTLY ABOVE OR BELOW THE NOSE OF TTT DIA., HOLDING AT THIS TEMP. UNTIL THE TRANSFORMATION IS COMPLETE, AND COOLING TO ROOM TEMP AT ANY DESIRED RATE.

  46. HEAT TREATMENT OF STEEL PATENTING THE MICROSTRUCTURE PRODUCED IS VARY FROM THE PEARLITE TO UPPER BAINITE AS TEMP. IS LOWERED FROM SLIGHTLY ABOVE THE NOSE TO BELOW THE NOSE OF TTT DIAGRAM. THIS PROCESS IS CHIEFLY USED IN WIRE DRAWING INDUSTRY BECAUSE THE MICROSTUCTURE PRODUCED HAVE GOOD TOUGHNESS. IT IS CHIEFLY USED FOR PLAIN CARBON STEEL WITH CARBON % 0.3-0.6 , AND IS EQUALLY APPLICABLE FOR ALLOY STEEL.

  47. HEAT TREATMENT OF STEEL SURFACE HARDNING NEED OF SURFACE TREATMENT: IN MANY APPLICATION SUCH AS CRANKSHAFT ,CAMSHAFTS, GEARS ETC., HARD AND WEAR RESITANCE IS REQUIRED WITH TOUGH CORE(AT CENTRE) TO WITHSTAND IMPACT LOADS . SUCH A REQUIREMENT IS DIFFICULT TO ACHIEVE BY USING A STEEL OF UNIFORM COMPOSITION . LOW CARBON STEEL ARE TOUGH BUT CANT HARDENED, HIGH CARBON STEELS CAN BE HARDENED BUT ARE NOT TOUGH. ALSO THEY ARE VERY MUCH LIKELY TO CRACK DURING HARDENING .

  48. HEAT TREATMENT OF STEEL SURFACE HARDNING • MEDIUM CARBON STEEL ARE INTERMEDIATE IN PROPERTIES • TO THOSE OF LOW AND HIGH CARBON STEEL AND DO NOT • SATISFY REQUIREMENT TO OPTIMUM LEVEL. • SUCH PROBLEM ARE SOLVED BY : • INCREASING THE CARBON ON THE SURFACE OF A LOW CARBON OR LOW CARBON LOW ALLOY STEEL . • INTRODUCING NITROGEN IN SURFACE OF A TOUGH STEEL. • INTRODUCING CARBON AND NITROGEN IN THE SURFACE OF TOUGH STEEL. • HARDENING THE SURFACE WITHOUT CHANGE OF COMPOSITION OF SURFACE.

  49. HEAT TREATMENT OF STEEL CARBURIZING / CASECARBURISING / CASE HARDENING THE METHOD OF INCREASING THE CARBON ON THE SURFACE OF THE STEEL IS CALLED AS CARBURISING. WORKING: IT CONSIST OF HEATING THE STEEL IN AUSTENITE REGION IN CONTACT WITH CARBURISING MEDIUM, HOLDING AT THIS TEMP. FOR SUFFICIENT TEMP.FOR A SUFFICIENT PERIOD AND COOLING TO ROOM TEMP. IN AUSTENITIC REGION ,THE SOLUBILITY OF CARBON IS MORE AND HENCE THE CARBON FROM MEDIUM DIFFUSES IN THE STEEL. HIGH CARBON CONTENT ON SURFACE DOES NOT MEAN HIGH HARDNESS ON THE SURFACE, UNLESS CARBON IS PRESENT IN MARTENSITIC FORM.

  50. HEAT TREATMENT OF STEEL • CARBURIZING / CASECARBURISING / CASE HARDENING • HENCE AFTER CARBURISING ,HARDENING TREATMENT IS • NECESSARY TO BRING THE CARBON IN THE MARTENSITIC • FORM. • TYPES OF CARBURIZING: • SOLID OR PACK OR BOX CARBURISING • GAS CARBURIZING • LIQUID CARBURISING

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