1 SURFACE HEAT TREATMENTS OF STEELS

2 SURFACE HEAT TREATMENTSIT IS THE HEAT TREATMENT OF MAKING THE OUTER SURFACE OF THE STEEL COMPONENT HARD UPTO CERTAIN DEPTH AND KEEPING THE INNER CORE OF THE STEEL COMPONENT SOFT. IN MANY COMPONENTS WHICH ARE SUBJECTED TO TWISTING LOAD ALONG WITH SUDDEN IMPACT LOADS, IT IS REQUIRED THAT OUTER SURFACE SHOULD BE HARD & WEAR RESISTANT.

3 SURFACE HEAT TREATMENTSSIMULTENIOUSLY INNER SURFACE SHOULD BE SOFT AND DUACTILE WITH HIGH IMPACT STRENGTH TO BEAR THE STRESSES. FOR EXAMPLE IN GEARS THE OUTER SURFACE UPTO CERTAIN DEPTH SHOULD BE HARD AND WEAR RESISTANT AND INNER SURFACE SHOULD BE SOFT TO SUSTAIN THE TENSILE STRESSES. HIGH IMPACT STRENGTH COUPLED WITH ELASTIC PROPERTIES ARE NEEDED AT THE CORE OF THE GEAR.

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6 SURFACE HEAT TREATMENTSEXAMPLES OF SUCH COMPONENTS ARE AXLES,CAMS,CAM SHAFTS, CRANK SHAFTS. THESE STEEL COMPONENTS LIKE GEARS ,CAMS,AXLES ARE GIVEN SURFACE HEAT TREATMENTS IN ORDER TO MAKE HARD WEAR RESISTANT OUTER CASE AND SOFT DUCTILE CORE. THESE COMPONENTS CANNOT BE THOROUGHLY HARDENED AS THROUGH HARDENING WILL PRODUCE SAME HARDNESS AND SAME IMPACT STRENGTH THROUGHOUT CROSS SECTION.

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8 IT IS NECESSARY TO FORM “HARD AND WEAR RESISTANT MARTENSITE” IN THE OUTER CASE OF THESE COMPONENTS AND “SOFT AND DUCTILE PEARLITE / PEARLITE +FERRITE / PEARLITE + CEMENTITE IN THE INNER LAYER OF THESE COMPONENTS

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10 FOR SUCH COMPONENTS “CONVENTIONAL THROUGH HARDENING” IS NOT EFFECTIVE AS THOROUGH HARDENING WILL PRODUCE THE COMPONENT WITH SAME PROPERTIES THROUGHOUT THE CROSS SECTION.

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12 SURFACE HARDENING APPLICABLE TO “HIGH CARBON STEELS AND ALLOY STEELS”(> 0.4% CARBON) STEEL IS HEATED RAPIDLY ON OUTER SURFACE TO AUSTENITIZING TEMPERATURE SO THAT AUSTENITE IS FORMED IN OUTER SURFACE. THEN STEEL IS QUENCHED SUDDENLY TO CONVERT AUSTENITE IN OUTER SURFACE INTO MARTENSITE. IN THE CORE PEARLITE +FERRITE IS FORMED WHICH IS SOFT COMPARATIVELY TO OUTER MARTENSITE. NO CHEMICAL REACTIONS OCCURS. ONLY THERMAL TREATMENT

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15 FLAME HARDENING

16 FLAME HARDENING OUTER SURFACE OF STEEL COMPONENT IS RAPIDLY HEATED BY OXY ACYTELENE FLAME AND THEN QUENCHED RAPIDLY BY A WATER NOZZLE. AUTENITE FORMED IN OUTER SURFACE IS TRANSFORMED TO MARTENSITE. FERRITE+PEARLITE REMAINS IN THE INNER SURFACE. STEELS WITH 0.4% CARBON TO 0.6%CARBON ARE HARDENED BY THIS METHOD. DEPTH OF HARDENING IS CONTROLLED BY ADJUSTMENT OF FLAME INTENSITY,HEATING TIME OR SPEED OF TRAVEL OF TORCH

17 FLAME HARDENING HEATING TIME = 7 y2 TORCH SPEED = 7 / y WHERE y = Depth of hardening in mm

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19 FLAME HARDENING FOLLOWING ARE METHODS OF FLAME HARDENING.1. STATIONARY FLAME HARDENING. 2. PROGRESSIVE FLAME HARDENING. 3. SPINNING FLAME HARDENING. 4. PROGRESSIVE SPINNING FLAME HARDENING

20 STATIONARY FLAME HARDENING.BOTH WORKPIECE AND TORCH ARE STATIONARY. USED FOR SPOT HARDENING OF SMALL PARTS. e.g. LOCALIZED PART OF A STEEL CASTING, VALVE STEMS,OPEN END WRENCHES.

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22 PROGRESSIVE FLAME HARDENINGTORCH MOVES OVER THE STATIONARY WORKPIECE. GUIDEWAYS OF LATHE , LARGE GEAR TEETHS ARE HARDENED BY THIS METHOD.

23 PROGRESSIVE FLAME HARDENING OF GUIDE WAYS

24 PROGRESSIVE FLAME HARDENING OF GEAR TEETHS FLAME HARDENING TORCHCOOLING HOLES HEATING HOLES

25 FLAME HARDENING TORCHES

26 SPINNING FLAME HARDENING.TORCH IS STATIONARY WHILE WORKPIECE ROTATES. USED FOR HARDENING OF CIRCULAR PARTS LIKE GEARS PULLEYS.

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28 PROGRESSIVE SPINNING FLAME HARDENINGTORCH MOVES OVER A ROTATING WORKPIECE. LONG SHAFTS AND ROLLS ARE HARDENED BY THIS METHOD.

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30 FLAME HARDENING BEFORE FLAME HARDENING WORKPIECE IS NORMALISED.AFTER HARDENING THE WORKPIECE IS STRESS RELIEVED BY HEATING TO A RANGE OF 180 TO 200 DEGREE CENTIGRADES. FINAL STRUCTURE IS MARTENSITE CASE WITH 3.75 MM THICK AND TOUGH FERRITE – PEARLITE CORE.

31 FLAME HARDENING GASES USED:MIXURE OF AIR OR OXGEN WITH WITH NATURAL GAS ,PROPANE OR ACYTELENE. FLAME TEMPERATURE IS AROUND DEGREE CENTIGRADE. MOSTLY OXY-ACYTELENE FLAME WITH TEMP.OF 2500 DEGREE CENTIGRADE IS USED.

32 FLAME HARDENING TREATMENT PRIOR TO FLAME HARDENINGSTRESS RELIEVING TO DEGREE CENT.FOR COMPONENTS AFTER HEAVY MACHINING. ANNEALING PRE-HEATING COMPONENT MUST BE FREE FROM SCALE,RUST,DIRT,LAPS,FINS AND FOLDS OTHER WISE SOFT SURFACE MAY RESULT.

33 FLAME HARDENING QUENCHING MEDIUMS: WATER OR DILUTE SOLUBLE OIL MEDIUM.COMPRESSED AIR CAUSTIC OR BRINE SOLUTIONS. QUENCHING SHOULD FULL HEATED ZONE.

34 FLAME HARDENING CASE DEPTH:DEPENDS ON, CHEMICAL COMPOSITION OF STEEL.HARDENING TEMP. RATE OF COOLING. PRESENCE OF ALLOYING ELEMENTS LIKE NICKEL,CHROMIUM INCREASES CASE DEPTH. ALSO DEPENDS ON “RATE OF MOVEMENT OF FLAME HEAD”. COMPONENTS HAVE MAXIMUM CASE DEPTH.

35 FLAME HARDENING

36 FLAME HARDENING SUITABLE STEELS:CARBON STEELS WITH 0.3% TO 0.6% CARBON 0.45% CARBON STEELS USED FOR CRANCK SHAFTS,ROCKER ARMS,SPINDLES,SPLINED SHAFTS. STEELS WITH 0.7% CARBON CAN ALSO BE HARDENED. ALLOY STEELS WITH 0.35 % TO 0.60% CARBON,0.12 TO 0.20% VANADIUM,1TO 1.2% CHROMIUM ARE HARDENED. STAINLESS STELLS,CAST STEELS,GREY CAST IRONS,SPHEROIDAL CAST IRON AND MALLEABLE CAST IRON CAN ALSO BE HARDENED.

37 FLAME HARDENING TEMPERING:TEMPERING IS NECESSARY AFTER FLAME HARDENING. SELF TEMPERING IS USED FOR LARGE STEEL COMPONENTS. CONVENTIONAL LOW TEMPERATURE TEMPERING IN OIL BATH OR FORCED AIR CIRCULATION IS CARRIED. AIM IS TO RELIEVE INTERNAL STRESSES. TEMPERING TEMPERATURE IS 175 – 200 oC

38 FLAME HARDENING ADVANTAGES OF FLAME HARDENINGAPPLICABLE ON WIDE RANGE OF STEELS IRRESPECTIVE OF SIZE & SHAPE OF PART. SELECTIVE AREA HARDENING POSSIBLE. LARGER DEPTH OF HARDENING. MAY BE APPLIED ON LARGE SIZED COMPONENTS LIKE LARGE GEARS,LATHE BEDS,GUIDE WAYS,ROLLERS WHERE CONVENTIONAL HARDENING IS INPRACTICABLE.

39 FLAME HARDENING MINIMUM WARPAGE & DISTORTION.ELETRONIC CONTROL OF EQUIPMENT POSSIBLE GIVING PRECISE DEPTH CONTROL. CAN HARDEN WORKPIECES EVEN AFTER THEIR SURFACES HAVE BEEN FINISHED. USEFUL AND ECONOMICAL METHOD.

40 FLAME HARDENING LIMITATIONS:EXACT TEMPERATURE ON COMPONENT IS DIFFICULT TO MEASURE DURING PROCESS. OVERHEATING MAY BE POSSIBLE. DIFFICULT TO PRODUCE A CASE DEPTH LESS THAN 1.5 mm NOT APPLICABLE FOR STEELS WITH LESS CARBON (<0.3%) EXACT CASE DEPTH NOT POSSIBLE.

41 FLAME HARDENING APPLICATIONS: LATHE BED GUDEWAYS. GEAR TEETHSSPINDLES. WORMS. VALVE STEMS. SHAFTS. MILL ROLLS. PULLEYS. OPEN END WRENCHES.

42 INDUCTION HARDENING

43 INDUCTION HARDENING HEATING THE MEDIUM CARBON STEEL WITH ALTERNATING MAGNETIC FIELD BY THE PHENOMENON OF ELECTROMAGNETIC INDUCTION TO AUSTENITIC TEMPERATURE( CENTIGRADE) FOLLOWED BY RAPID QUENCHING SO THAT AUSTENTE IN THE OUTER CASE CONVERTS TO MARTENSITE,PRODUCING A HARD OUTER LAYER AND SOFT INNER LAYER OF FERRITE +PEARLITE.

44 INDUCTION HARDENING PRINCIPLE

45 INDUCTION HARDENING MAGNETIC FIELD LINES THREAD VIA SURFACE OF WORKPIECE IN THE INDUCTOR COIL.THEY INDUCE “EDDY CURRENTS”BY ELECTROMAGNETIC INDUCTION IN THE STEEL COMPONENT OF THE SAME FREQUENCY BUT REVERED IN DIRECTION. HEATING RESULTS DUE TO RESISTANCE OF THE STEEL PART. INTENCITY OF EDDY CURRENT IS MAXIMUM IN THE OUTER SURFACE AND SLOWLY DECREASES TOWARDS CENTRE THIS EFFECT IS CALLED AS “SKIN EFFECT”.

46 INDUCTION HARDENING

47 INDUCTION HARDENING DEPTH OF HARDENING IS INVERSLY PROPORTIONAL TO FREQUENCY OF CURRENT SUPPLIED TO INDUCTOR COIL BY HIGH FREQUENCY GENERATOR. AS THE FREQUENCY INCREASES THE DEPTH OF HARDENING DECREASES AND VICE VERSA DESIRED FREQUENCY OF THE CURRENT CAN BE SET TO GET THE REQUIRED CASE DEPTH.

48 INDUCTION HARDENING EFFECT OF FREQUENCY OF SUPPLY CURRENT ONDEPTH OF HARDENING FREQUENCY OF CURRENT (Hz) 1000 4000 10,000 1,20,000 500,00 DEPTH OF HARDENING mm 6.0 3.0 2.5 1.5 0.75

49 INDUCTION HARDENING FOLLOWING GENERATORS ARE USED TO PRODUCE HIGH FREQUENCY CURRENT, MOTOR GENERATORS:PRODUCES CURRENT WITH FREQUENCIES OF 1000 TO 10,000 Hz. SPARK-GAP OSCILLATORS :PRODUCES CURRENT WITH FREQUENCIES OF TO 400,000 Hz. VACCUM TUBE OSCILLATORS:GENERATORS:PRODUCES CURRENT WITH FREQUENCIES OF Hz.

50 INDUCTION HARDENING INDUCTOR COILS:SHAPE OF INDUCTOR COIL DEPENDS UPON WORKPIECE. DEPTH OF HARDENING DEPENDS UPON SHAPE OF INDUCTOR. DISTANCE BETWEEN INDUCTOR AND ZONE TO BE HARDENED. MAGNITUDE OF POWER INPUT INTO INDUCTOR. FREQUENCY OF SUPPLY CURRENT.

51 INDUCTOR COILS

52 INDUCTOR COILS

53 INDUCTION HARDENING SETUP

54 PRIOR STRUCTURE SUITABLE REFINED STRUCTURE IS OBTAINED BY NORMALISING.FOR MEDIUM CARBON STEELS SORBITE STRUCTURE IF PREFFRED. SPHERODIZED STRUCTURE HAS POOR RESPONSE TO INDUCTION HARDENING. GENERALLY COARSE PEALITIC STRUCTURE IS AVOIDED.

55 INDUCTION HARDENING METHODS1.HARDENING WITH STATIC COIL: USED FOR SMALL PARTS. FOR HARDENING SMALL BOLT HEADS,SHAFTS,AXLES,GEARS. ROTARY MOTION IS GIVEN TO WORKPIECE INSIDE THE COIL FOR BETTER EFFECTS. SOFT SPOTS MAY BE CREATED DUE TO FORMATION OF STEAM

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57 PROGRESSIVE INDUCTION HARDENINGPARTS OF THE WORK PIECE ARE HARDENED PROGRESSIVELY. PART ROTATES AS WELL AS MOVES UP OR DOWN. INDUCTOR REMAINS STATIONARY. OR PART REMAINS STATIONARY AND INDUCTOR MOVES. SHAFTS,GEARS,AXLES ARE HARDENED BY THIS METHOD.

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59 QUENCHING PROCESS OF HARDENING IS CONTROLLED BY RATE OF COOLING.NATURAL COOLING IS NOT FAST ENOUGH TO FORM MARTENSITIC STRUCTURE. FORCED QUENCHING WITH WATER, OIL, WATER EMULSION OR FORCED AIR IS DONE DEPENDING UPON THE TYPE OF STEEL. WATER IS SUITABLE FOR PLAIN CARBON STEELS,LOW ALLOY STEELS AND CAST IRONS. OIL IS USED FOR HIGH ALLOY STEELS. DIFFERENT METHODS OF QUENCHING USED ARE , AGITATED QUENCH. PROGRESSIVE QUENCH. STATIC QUENCH.

60 TEMPERING TEMPERING OF INDUCTION HARDENED COMPONENTS IS NOT RECOMMENDED FOR THE PARTS FOR WHICH WEAR RESISTANCE OR FATIGUE RESISTANCE IS REQUIRED. FOR COMPONENTS TO BE GROUND AFTER INDUCTION HARDENING ARE TEMPERED AT DEGREE CENTIGRADES TO AVOID CRACKS DURING GRINDING.

61 ADVANTAGES OF INDUCTION HARDENINGRAPID HEATING OF COMPONENT LEADS TO LESS CYCLE TIME. CAN BE APPLIED TO EXTERNAL AS WELL AS INTERNAL SURFACES. COMPONENTS ARE HEATED WITH NO SCALING AND DISTORTION. HIGHER HARDNESS CAN BE OBTAINED IN STEEL.60 Rc WITH A CASE DEPTH OF 3mm.THIS IS DUE TO COMPRESSIVE STRESSES IN THE MARTENSITE AND FINE MARTENSITIC STRUCTURE.

62 ADVANTAGES OF INDUCTION HARDENINGTHROUGH PROPER DESIGN OF THE HEATING COIL, THE SHAPE OF THE HARDENED PORTION CAN BE CONTROLLED EASILY. DEPTH OF HARDENING CAN BE CONTROLLED BY CONTROLLING FREQUENCY OF SUPPLY. PROCESS CAN BE MADE AUTOMATIC INDUCTION HARDENED COMONENTS HAVE HIGHER FATIGUE STRENTH AND IMPACT STRENTH. LESS ENERGY UTILIZATION.

63 DISADVANTAGES OF INDUCTION HARDENINGCOST OF EQUIPMENT IS VERY HIGH. STEELS WITH LESS THAN 0.4% CARBON CANNOT BE HARDENED. USE OF METHOD IS LIMITED BY COMPLEXITY OF INDUCTORS AND PARTS. BENEFICIAL FOR MASS PRODUCTION ONLY. HIGH MAINTENANCE COST. BEFORE INDUCTION HARDENING PRIOR TREATMENT OF NOMALISING IS NECESSARY.

64 STEELS FOR INDUCTION HARDENINGGENERALLY MEDIUM CARBON STEELS ARE USED. 35C8,45C8,55C6,40C4,40Cr4Mo3,50Cr4V2,AISI1035,AISI1045,AISI1050,AISI5140,AISI4140,AISI6150,AISI8640 ARE USED. STEELS PRIOR TO INDUCTION HARDENING ARE HARDENED AND TEMPERED & NORMALISED. (Slide 65 and 66 shows designation)

65 AISI- AMERICAN IRON STEEL INSTITUTION DESIGNATION35C8 i.e %CARBON, 0.8 % Mn. AISI1035 i.e. plain carbon steel containing 0.35 % carbon. (Digit 1 indicates plain c steel.) AISI5140, AISI4140,AISI6150,AISI8640. 1st digit indicates type of steel. 2nd digit for predominant alloying element. 3rd &4th digit indicates% of carbon in hundredth.

66 e.g. 1 for plain carbon steel, 2 nickel series, 3 for nickel chromium, 31 for manganese steel. 4 for chromium steel. 41 for chromium-molybdenum steels, 51 for medium chromium, 61 Chromium-vanadium series. 92XX FOR SILICON SERIES. AISI5140 i.e. medium cr steel with 0.40%c.

67 APPLICATIONS OF INDUCTION HARDENINGGEARS AXLES SPINDLES PINIONS GUIDEWAYS CAMSHAFTS TILTING LEVERS GUDGEON PINS PISTON RODS TAILSTOCK SLEEVES CAMS CRANK SHAFTS SPUR GEARS WORM WHEELS LOCKING PINS. PINS. HELICAL GEARS PUMP SHAFTS. FLAT SURFACES

68 COMPARISION OF FLAME AND INDUCTION HARDENINGFLAME HARDENING SURFACE OF STEEL IS HEATED RAPIDLY BY OXYACETYLENE FLAME , THEN QUENCHING. SUCCESS DEPENDS ON SKILL OF OPERATOR. INDUCTION HARDENING STEEL IS HEATED BY HIGH FREQ. ELECTRIC INDUCTION CURRENT AND COOLED RAPIDLY. SUCCESS IS RELATED TO SELECTION AND DESIGN OF PROPER WORK COIL.

69 1. DISTANCE BETWEEN FLAME & WORK PIECE.OPERATING VARIABLES ARE 1. DISTANCE BETWEEN FLAME & WORK PIECE. 2. GAS PRESSURE, 3. FLAME OR WORK TRAVEL RATE, 4. TYPE, VOLUME AND APPLICATION OF QUENCH. OPERATING VARIABLES ARE INDUCED VOLTAGE FLOW OF CURRENT RESISTANCE OFFERED BY WORK SHAPE AND DESIGN OF COIL & RATE OF HEATING.

70 ANY SHAPED PARTS ARE SUITABLE FOR FLAME HARDENING. INDUCTION HARDENING IRREGULAR SHAPED PARTS ARE NOT SUITABLE FOR INDUCTION HADEENING. i.e. different coils are needed for different shaped parts.

71 CASE HARDENING

72 CASE HARDENING APPLICABLE TO LOW CARBON AND ALLOY STEELS.IN THIS METHOD THE LOW CARBON STEELS ARE HEATED IN CONTACT WITH CARBON RICH MATERIAL SO THAT CARBON ATOMS PENETRATES INTO OUTER SURFACE OF STEEL TO INCREASE CARBON CONCENTRATION IN THE OUTER SURFACE INCREASES, ON SUBSEQUENT QUENCHING MARTENSITE IS FORMED IN THE OUTER SURFACE WITH FERRITE+PEARLITE IN INNER CORE.THIS IS CALLED AS “CARBURISING” THIS INVOLVES CHEMICAL REACTIONS.

73 CASE HARDENING IN OTHER METHOD THE ALLOY STEELS ARE HEATED IN CONTACT WITH NITROGEN RICH GAS LIKE AMMONIA (NH3) to C. AT ELEVATED TEMPERATURE NITROGEN PENETRATES INTO OUTER LAYER OF STEEL COMPONENTS TO FORM HARD ALLY NITRIDES IN OUTER CASE. THIS PROCESS IS CALLED AS NITRIDING INVOLVES CHEMICAL REACTIONS.

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75 CARBURIZING PROCESS OF INTRODUCING THE CARBON IN THE OUTER CASE OF LOW CARBON STEELS IN ORDER TO PRODUCE A HARD MARTENSITIC STRUCTURE IN THE OUTER SURFACE.CARBON CONTENT IN THE OUTER CASE IS INCREASED BY PROCESS OF ABSORPTION AND DIFFUSION. LOW CARBON STEELS ARE HEATED TO 870 – 925 DEGREE CENTIGRADES IN CONTACT WITH CARBON –RICH MATERIAL FOR SEVERAL HOURS. HIGHLY ENRICHED OUTER CARBON RICH SURFACE IS HARDENED BY QUENCHING .

76 CARBURIZING CASE DEPTH IS AROUND 1.27 mm. HARDNESS ABOUT 65 Rc. Depending upon nature of carbon rich material,Carburising can be classified as, 1. SOLID OR PACK CARBURISING. 2. GAS CARBURISING. 3. LIQUID CARBURISING.

77 SOLID CARBURIZING

78 SOLID OR PACK CARBURIZINGLOW CARBON STEELS ARE HEATED TO 870 – 970 DEGREE CENTIGRADES IN CONTACT WITH CARBON –RICH SOLID MATERIAL LIKE WOOD,BONE CHARCOAL, SEMI COAK , PEAT COKE ,CHARRED LEATHER TOGETHER WITH ENERGIZER (MIXURE OF SODIUM CARBONATE AND BARIUM CARBONATE) FOR SEVERAL HOURS. FOR THIS STEEL PARTS ARE PACKED IN SOLID CAST IRON BOXES. AFTER HOLDING ,STEEL PARTS ARE QUENCHED IN WATER / OIL TO FORM MARTENSITE IN THE OUTER SURFACE OF THE STEEL COMPONENT.

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80 DEPTH OF CASE IS DEPENDENT UPON THE CARBURIZING TEMP.AND HOLDING TIME

81 SOLID OR PACK CARBURIZING1.FORMATION OF CARBON MONOXIDE. 2.DISSOCIATION OF CO WITH EVOLUTION OF ATOMIC CARBON. 2CO CO2 + C atom 3.ENRICHMENT OF STEEL SURFACE LAYER WITH CARBON. 2CO + 3Fe Fe3C +Co2 Carbon actually gets dissolved in Gamma iron to form Autenite

82 SOLID OR PACK CARBURIZINGCARBON GETS DIFFUSED IN THE OUTER SURFACE OF STEEL .RATE OF DIFFUSION DEPENDS ON TEMPERATURE. CONTAINERS USED FOR PACK CARBURISING ARE MADE FROM HEAT RESISTANT STEEL. BATCH TYPE OR CONTINIOUS FURNACES ARE USED FOR PACK CARBURISING.

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84 ADVANTAGES OF PACK CARBURISINGFOR LOW VOLUME PRODUCTION IT IS ECONOMIC. CERTAIN COMPONENTS ARE HEAT TREATED BY THIS METHOD ECONOMICALLY. CAN BE DONE IN ANY FURNACE. PROCESS IS SAFE. LESS CAPITAL INVESTMENT. NO NECESSITY OF SPECIAL SPACE FOR CARBURISED COMPONENTS. IT DOES NOT REQUIRE SPECISL CONTROLLED ATMOSPHERE SAME FURNACE CAN BE USED FOR FURANCE NORMALISING,ANNEALING AND STRESS RELIEVING..

85 DISADVANTAGES OF PACK CARBURISINGCARBURISING TIME IS LONG. NO CONTROL ON CASE OF CARBURISING. NOT SUITED FOR DIRECT QUENCHING. PROCESS IS DIRTY AND DUSTY. LOADING AND UNLODING OPERATION REQUIRES CONSIDERABLE FLOOR SPACE AND TIME

86 APPLICATIONS OF SOLID CARBURISINGSHAFTS SPINDLES GEARS AXLES PINS BRACKETS ROLLS

87 GAS CARBURIZING

88 GAS CARBURIZING THE STEEL PART IS HEATED IN CONTACT WITH GAS LIKE PROPANE, BUTANE, BENZENE UPTO DEGREE CENTIGRADE AND HOLD FOR SFFICIENT TIME DURING WHICH TIME THE CARBON ATOMS GETS DIFFUSED INTO OUTER SURFACE OF COMPONENT THEN THE PART IS DIRECTLY QUENCHED INTO BATH TO GET REQUIRED HARDNESS IN OUTER CASE AND SOFT AND DUCTILE CORE.

89 GAS CARBURIZING CARBURISING GASES: GASES USED ARE NATURAL GAS, METHANE,PROPANE,BUTANE. CARRIER GAS WITH HYDROGEN IS ALSO USED. COMMON ATMOSPHERE FOR GAS CARBURISING IS 20% CO ,40%HYDROGEN,40%NITROGEN.

90 GAS CARBURISING IS CARRIED IN A SPECIALLY DESIGNED ATMOSPHERE TIGHT FURNACE .THE FURNACE IS PROVIDED WITH A FAN.

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92 MAIN REACTION DURING THE PROCESS ARE AS UNDER,3Fe +2CO Fe3C +CO2 Fe3C + 2H Fe + CH4 3Fe +CO +H Fe3C +H2O CO2 + CH CO +2H2 CH4+ 3Fe FE3C +2H2 CASE DEPTH MAY VARY FROM 0.25 MM TO 0.37 MM DEPENDING UPON THE PARAMETERS.

93 ADVANTAGES OF GAS CARBURISINGCASE CONTROL IS ACCURATE. UNIFORM CASE DEPTH. CLEAN PROCESS. MASS PRODUCTION PROCESS. LESS TIME IS REQUIRED. DIRECT QUENCHING FROM CARBURIZING TEMPERATURE.

94 DISADVANTAGES OF GAS CARBURISINGHIGHER CAPITAL INVESTMENT FOR FURNACES. BASKETS AND FIXURES ARE EXPENSIVE. GREATER SKILL OF OPERATOR. PARTIAL CARBURISING NOT POSSIBLE.

95 LIQUID CARBURIZING

96 LIQUID CARBURIZING STEEL PART IS HEATED IN MOLTEN SALT BATH IN CONTACT WITH LIQUID CARBON RICH MATERIAL LIKE SODIUM CYNIDE OR POTASSIUM CYNIDE TO A TEMPERATURE OF 870 – 950 DEGREE CENTIGRADE STEEL PARTS IN THE WIRE BASKETS ARE HELD IN THE BATH FOR A PERIOD OF 5MINUTES TO ONE HOUR DEPENDING UPON CASE DEPTH REQUIRED. AT THIS TEMPERATURE CARBON AS WELL AS NITROGEN DIFFUSES INTO OUTER SURFACE OF STEEL PART. ON SUBSEQUENT QUENCHUNG MARTENSITE IS FORMED IN THE OUTER CASE OF THE COMPONENT.

97 LIQUID CARBURIZING BATH CONSISTS OF 20 % TO 50 % SODIUM CYNIDE TOGETHER WITH 40% SODIUM CARBONATE AND VARYING AMOUNTS OF SODIUM OR BARIUM CHLORIDES. CHEMICAL REACTIONS SODIUM CYNIDE IN PRESENCE WITH ATMOSPHERIC OXYGEN FORMS SODIUM CYANATE (NaCNO), WHICH DECOMPOSES INTO SODIUM CARBONATE (Na2CO3), CARBON MONOXIDE (CO) AND ACTIVE NITROGEN (N).

98 LIQUID CARBURIZING 2NaCN +O2 2NaCNO 4NaCNO 2NaCN +Na2CO3+CO+2N2CO C+CO2 SODIUM CYNIDE THUS REFORMED FROM CYNATE IS AVAILABLE FOR FURTHER REACTIONS,BUT THE SODIUM CARBONATE IS END PRODUCT. ATOMIC NITROGEN COMBINES WITH IRON. CARBON MONOXIDE YIELDS ATOMIC CARBON WHICH PENETRATES INTO OUTER SURFACE OF STEEL PART i.e. INTO GAMMA IRON LATTICE. TO PRODUCE A DEEPER CASE DEPTH AND TO INHIBIT FORMATION OF SODIUM CYNATE WHICH MAY BLOCK CARBON PENETRATION BY FORMATION OF NITROGEN,CATALYSTS SUCH AS BARIUM CHLORIDE MAY BE ADDED.

99 LIQUID CARBURIZING PRESENCE OF CHLORIDES HINDERS THE TAKEUP OF NITROGEN AND FAVOURS ABSORPTION OF CARBON IN STEEL. BATH SHOULD HAVE HIGH MELTING POINT SO IN ORDER TO HAVE A TEMPERATURE OF DEGREE CENTIGRADES. THE INCREASED ACTIVITY OF BATH IS ATTRIBUTED BY , 2NaCN +BaCl NaCl + C at

100 LIQUID CARBURIZING THIS LEADS TO LIBERATION OF LARGE AMOUNT OF CARBON WITH SMALL AMT.OF ACTIVE NITROGEN. IN LIQUID CARBURISING AMOUNT OF CARBON ATOMS PENETRATED IN OUTER SURFACE IS MORE AND AMOUNT OF NITROGEN PENETRATED IN THE OUTER SURFACE IS LESS. PROCESS IS MORE EFFECTIVE FOR PLAIN CARBON STEELS RATHER THAN ALLOY STEELS DEPTHS OF CASE VARIES FROM 0.10 mm to 6.25 mm.

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104 COMPOSITION OF SALT BATH IN LIQUID CARBURISINGLIGHT CASE ( C) DEEP CASE ( C) SODIUM CYNIDE 17% TO 23% 8 % TO 12 % BARIUM CYNIDE 15 % TO 40 % 45% TO 55% OTHER ALKALYNE EARTHS 0 TO 3.5 % 2 TO 10% POTASSIUM CHLORIDES -- 6% TO 20 % SODIUM CHLORIDE 20% TO 30% 0 TO 15% SODIUM CARBONATE 30% MAX 30%MAX SODIUM CYANATE 1%MAX 0.30%MAX

105 ADVANTAGES OF LIQUID CARBURISINGRAPID HEAT TRANSFER AND HEATING. LOW DISTORTION. NO OXIDATION AND DECARBURISATION. RAPID ABSORPTION OF CARBON & NITROGEN IN OUTER SURFACE. UNIFORM CASE DEPTH AND CARBON CONTENT. REDUCED CYCLE TIME. FLEXIBILITY TO HANDLE RANGE OF PARTS OF VARIED DESIGN AND VARIED CASE DEPTHS.

106 DISADVANTAGES OF LIQUID CARBURISINGCYNIDE BATHS ARE HIGHLY POISONOUS WHEN CYNIDE FUMES ARE TAKEN INTERNALLY OR WHEN IN CONTACT WITH WOUNDS. MOLTEN CYNIDE EXPLODES WHEN COMES IN CANTACT WITH WATER.SO ALL WORK HAS TO BE DRIED BEFORE IT COMES IN CONTACT WITH BATH. PARTS NEED THOROGH WASHING AFTER TREATMENT TO PRVENT RUSTING.

107 HEAT TREATMENTS AFTER CARBURISINGHEAT TREATMENTS ARE CARRIED OUT TO REFINE THE CORE AND TO REFINE AND HARDEN THE CASE. FOLLOWING CYCLES ARE USED. CARBURISING – NORMALISING – HARDENING – TEMPERING. CARBURISING – HARDENING – TEMPERING. DIRECT QUENCHING FROM CARB.TEMP.- TEMPERING.

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109 NITRIDING

110 NITRIDING PROCESS OF HEATING OF ALLOY STEELS IN CONTACT WITH NITROGEN BEARING GAS ENVIRONMENT TO A TEMPERATURE OF 500 TO 550 DEGREE CENTIGRADES AND HELD FOR A LONG PERIOD OF TIME (25 TO 100 HOURS) IN THE FURNACE. DURING HOLDING PERIOD ,THERE IS A CHEMICAL REACTION IN THE GAS AND THE FREE NITROGEN ATOMS ARE LIBERATED.

111 NITRIDING THESE ATOMS PENETRATE INTO OUTER SURFACE OF THE STEEL COMPONENT AND COMBINE WITH ALLOYING ELEMENTS TO FORM “HARD ALLOY NITRIDE PARTICLES” IN THE OUTER SURFACE OF THE STEEL.DUE TO WHICH OUTER SURFACE BECOMES EXTREMELY HARD AND WEAR RESISTANT. HARD OUTER SURFACE IS FORMED WITHOUT QUENCHING. MAXIMUM CASE DEPTH ACHIEVED IS AROUND 0.03 mm TO 0.6 mm.

112 NITRIDING EXTREMELY HARD CASES CAN BE PRODUCED BY THIS PROCESS (Vickers 1100) NITRIDED CASE HAS HIGH CORROSION AND FATIGUE RESISTANCE. ONLY EFFECTIVE FOR LOW AND MEDIUM ALLOY STEELS WITH Al, Cr, V, Mo AS ALLOYING ELEMENTS.

113 NITRIDING NITRIDING PROCESS:OPERATIONS PERFORMED ON STEEL PARTS BEFORE NITRIDING PROCESS INVOLVES. ROUGH MACHINING. HARDENING AND TEMERING. FINAL MACHINING. STRESS RELIEVING. FINISH MACHINING.

114 NITRIDING STEEL COMPONENTS ARE PLACED IN A HEAT RESISTING METAL CONTAINER (RETORT) WHICH IS THEN FILLED WITH AMMONIA WHILE IT IS COLD.THIS CONTAINER HAS INLET AND OUTLET FOR AMMONIA GAS. THIS CONTAINER IS PURGED AND ALL AIR IS REMOVED FROM RETORT / CONTAINER. CONTAINER IS SEALED AND PLACED IN FURNACE AND HEATED TO TEMPERATURE OF DEGREE CENTIGRADES.

115 NITRIDING AT THIS TEMP.FOLLOWING REACTION OCCURS,NH H + N (FREE NITROGEN ATOMS) PARTS ARE HELD AT 40 TO 100 HOURS. HERE THE FREE NITROGEN PENETRATES INTO OUTER SURFACE OF THE STEEL COMPONENTS VERY SLOWLY. THESE NITROGEN ATOMS COMBINES WITH ALLOYING ELEMENTS IN STEEL IN OUTER SURFACE AND FORMS CORROSPONDING ALLOY NITRIDES SUCH AS ALUMINIUM NITRIDES,VANADIUM NITRIDES ETC. IN THE OUTER SURFACE OF STEEL COMPONENTS.

116 NITRIDING SINCE ALLOY NITRIDES ARE EXTREMELY HARD,THE OUTER SURFACE OF STEEL COMPONENT GETS HARD. AFTER THE NITRIDING PROCESS IS COMPLETED,THE FURNACE IS SWITCHED OFF AND PARTS ARE COOLED TO ROOM TEMPERATURE.DURING COOLING CONTINIOUS SUPPLY OF AMMONIA IS MAINTAINED.NO AIR IS ALLOWED TO ENTER INTO CONTAINER THERE IS NO NECESSITY OF QUENCHING OF THE STEEL.

117 NITRIDING PROCESS

118

119 NITRIDED GEAR TEETH

120 ADVANTAGES OF NITRIDINGVERY HIGH SURFACE HARDNESS IN RANGE OF 1150 VPN IS PRODUCED. NO QUENCHING IS REQUIRED SO NITRIDED PARTS HAVE LESS DISTORTION AND WARPAGE. GOOD CORROSION OF CASE NO MACHINING IS REQUIRED AFTER NITRIDING. PARTS ARE HEATED TO LESS TEMP.RANGE SO NO OXIDATION AND DECARBURIZATION. NITRIDED PARTS CAN RETAIN THEIR HARDNESS UPTO 500 DEGREE CENTIGRADE.

121 ADVANTAGES OF NITRIDINGCOMPLEX PARTS CAN BE NITRIDED. NITRIDED CASE CONTAIN ALLOY NITRIDES WHICH ARE EXTREMLY HARD AND HAVE LESS STRESSES. NITRIDED CASE IS LESS SENSITIVE TO STRESS RAISERS CAUSED BY POOR SURFACE FINISH,SHARP CHANGES IN CROSS SECTIONS. NITRIDED PARTS CAN BE DIRECTLY USED FOR PRACTICAL APPLICATIONS.

122 ADVANTAGES OF NITRIDINGFORMATION OF HIGH NITROGEN PHASE ON THE SURFACE OF NITRIDED COMPONENT IS ASSOCIATED WITH LARGE VOLUME CHANGES WHICH LEADS TO APPEARANCE OF COMPRESSIVE STRESSES IN THE HARDENED LAYER.THESE STRESSES SUBSTANSTIALLY INCREASE THE FATIGUE RESISTANCE OF COMPONENTS.

123 IRON PILLAR AT QUTAB MINAR

124 IRON PILLAR AT QUTAB MINARFAMOUS IRON PILLAR AT QUTAB MINAR (415 AD) IS STLL IN THE UNCORRODED CONDITION. HIGHER CORROSION RESISTANCE OF THIS IRON PILLAR CAN BE EXPLAINED BY FORMATION OF NITRIDE FILM ON ITS SURFACE. IT IS BELIEVED THAT HIGH CONCENTRATION OF AMMONIA IN THE ATMOSPHERE IN COMBINATION WITH THE SUBTROPIC CLIMATE OF INDIA PRODUCED A PROTECTIVE LAYER OF IRON NITRIDES ON THE SURFACE OF PILLAR.

125 DISADVANTAGES OF NITRIDINGLONG CYCLE TIMES (25 TO 100 Hrs) BRITTLE CASE ONLY SPECIAL ALLOY STEELS CONTAINING Al,Mo,V.Cr AS ALLOYING ELEMENTS CAN ONLY BE NITRIDED. PLAIN CARBON STEELS CANNOT BE EFFECTIVELY NITRIDED. HIGH COST. TECHNICAL CONTROL REQUIRED. IF NITRIDED PART GETS ACCIDENTLY OVERHEATED.(ABOVE 500 0C ) THEN THE HARDNESS WILL BE LOST COMPLETELY.

126 APPLICATIONS OF NITRIDINGVALVE SEATS GUIDES GEARS GUAGES BUSHINGS PINS CRANK SHAFTS LEVERS AIRCRAFT ENGINE PARTS AERO ENGINE CYLINDERS AERO CRANKSHAFTS AIR SCREW SHAFTS CRANK PINS JOURNALS BALL RACES MOULDS

127 COMPARISION OF CARBURISING AND NITRIDING PROCESSBY THIS METHOD CARBON CONTENT AT THE SURFACE OF METAL IS INCREASED. CARRIED AT HIGH TEMPERATURE ( 930 0C ) AND QUENCHING IS DONE. NITRIDING BY THIS METHOD NITROGEN CONTENT AT THE SURFACE OF METAL IS INCREASED. TEMPERATURE EMPLOYED IS 600 0C ONLY AND QUENCHING IS NOT REQUIRED.

128 CARBURISING 3. HARDENING AND TEMPERING NEEDED. 4CARBURISING 3. HARDENING AND TEMPERING NEEDED. 4. BEFORE THIS REFINING IS NOT NECESSARY. 5. INFERIOR SURFACE FINISH AS COMPARED TO NITRIDING. 6. AUSTENITIC STRUCTURE IS TO BE BROUGHT. NITRIDING 3. NO NEED OF HARDENING AND TEMPERING . 4. BEFORE THIS REFINING IS NECESSARY. 5. SURFACE FINISH IS VERY GOOD. 6. FERRITE STRUCTURE IS FAVOURED.

129 CARBURISING 7. THICK CARBURISED LAYER IS OBTAINED. 8CARBURISING 7. THICK CARBURISED LAYER IS OBTAINED. 8. SIMPLE AND INEXPENSIVE. 9. CASE DEPTH AROUND 1.27 MM. 10. Applicable for low carbon steels and for alloy steels. NITRIDING 7. VERY THIN NITRDE LAYER IS OBTAINED. 8. COMPLEX AND EXPENSIVE. 9. CASE DEPTH 0.03 TO 0.6 MM Applicable for alloy steels. (best results with alloying elements such as Al, Cr, and V.

130 CYANIDING

131 CYANIDING PROCESS IS SIMILAR TO LIQUID CARBURIZINGSTEEL PART IS HEATED IN MOLTEN SALT BATH IN CONTACT WITH LIQUID CARBON RICH MATERIAL LIKE SODIUM CYNIDE OR POTASSIUM CYNIDE TO A TEMPERATURE OF 870 – 950 DEGREE CENTIGRADE STEEL PARTS IN THE WIRE BASKETS ARE HELD IN THE BATH FOR A PERIOD OF 5MINUTES TO ONE HOUR DEPENDING UPON CASE DEPTH REQUIRED. AT THIS TEMPERATURE CARBON AS WELL AS NITROGEN DIFFUSES INTO OUTER SURFACE OF STEEL PART. IN CYNIDING NITROGEN PENETRATES MORE IN THE OUTER SURFACE AND PRODUCES HARD ALLOY NITRIDES IN THE OUTER CASE .EFFECT OF PENETRATION OF CARBON IS NEGLIGIBLE.

132 THANKS !!