This presentation gives a brief introduction to chemical heat treatment of steels and surface hardening techniques Keywords: Carburising, Nitriding, Carbonitriding, Flame hardening, Laser hardening, Induction hardening

1. Heat treatment of steels - II
By:
Nishant S. Khatod
Assistant Professor
STC, Latur

2. Introduction
 Components like ball and tapered bearings, gears, rock drill bits, camshafts,
crankpins, axles require outer surface to be hard and wear resistant and inner
core more ductile and tougher
 Such properties ensure long service life and sufficient toughness to withstand
shock loads
 Ways to achieve such combination of properties:
 Thermochemical treatment – Change in surface composition by diffusion of C
and N2 – Carburising and Nitriding
 Phase transformation of outer surface by rapid heating and cooling –
Flame, Induction, electron, and laser beam hardening

3. Carburising
 Also known as cementation OR case carburising OR case hardening
 Carburising – Method of increasing carbon
 Applicable: Low carbon steels with % C = 0.1 to 0.25
 Process:
 Heating to 900 to 930ᵒC in presence of solid, liquid or gas rich in carbon
 Holding for definite period till desired case depth is achieved
 Cooling
 Carbon diffused into steel when heated in austenitic region
 Surface layer enriched with 0.7 to 0.9% carbon
 Fully austenitic steel is essential because solubility of carbon is more in
austenite than in ferrite
 Depending upon the medium, it can be classified as;
 Solid OR Pack OR Box carburising
 Liquid carburising or salt bath carburising
 Gas carburising
 Vacuum carburising

4. Solid carburising
 Components to be carburised packed with a compound rich in carbon in steel or
CI boxes and sealed with clay
 If not sealed properly medium comes in contact with air and burns without
carburising
 Medium: 50 to 55% hardwood charcoal, 30 to 32% coke and remaining
energiser or accelerator like BaCO3
 Process:
 Heating boxes in a furnace upto 930ᵒC
 Holding for definite period till required case depth is achieved
 Cooling
 High temperature helps in aborption of carbon on surface
 Reactions:
O2 (from box) + C (medium) CO2
BaCO3 BaO + CO2
CO2 + C (from medium) 2CO
2CO + Fe Fe (C) + CO2
Liberated CO2 + C (from medium) 2CO

5. Solid carburising
 This is indirect carburising
 Direct carburising: Carburising @ steel in direct contact with medium, Not
desirable because of local variations; Non uniform hardness.
 Maximum carbon @ the surface and case depth depend upon temperature
of carburising and holding time
 Higher the temperature, higher is the case depth but grain coarsening occurs
 Higher the holding time, higher is the case depth without change in maximum
concentration at the surface
 Carburising time of 6 to 8 hrs for case depth of 1 to 2mm @ 900ᵒ C
 Used when extreme uniformity in carbon content is not desired
 NOTE:
 Case depth: The perpendicular distance from the surface of the steel to the
point at which change in hardness, chemical composition or microstructure of
the case and core cannot be distinguished

6. Liquid carburising
 Also known as salt bath carburising
 Carburising done by immersing the steel components in a carbonaceous fused
salt bath medium containing sodium or potassium cyanide, sodium and
potassium chloride and barium chloride which acts as a activator
 Bath heated in the range of 815 - 900ᵒC
 Reactions:
BaCl2 + 2NaCN Ba (CN)2 + 2NaCl
Ba (CN)2 + Fe Fe (C) + BaCN2 (Barium cynamide)
 Some beneficial nitrogen may also diffuse through oxidation of sodium
cyanide
2NaCN + O2 2NaCNO
3NaCNO NaCN + Na2CO3 + C + 2N
 Nitriding helps in increasing hardness and wear resistance
 Carbursing time of 0.5 to1 hour for case depth of 0.1 to 0.5mm (Relatively
thinner than pack carburising) @ 900ᵒ C

7. Liquid carburising
 Advantages
 Uniform and rapid heat transfer
 Low distortion
 Negligible surface oxidation
 High uniformity in case depth and carbon content
 Disadvantages
 Highly poisonous sodium cyanide; hence care should be taken while storage,
use and disposal
 Salt sticks to the components and must be removed while washing

8. Gas carburising
 Components heated in the range of 870 to 950ᵒ C in the presence of
carbonaceous gases like methane, ethane, propane or butane diluted with a
carrier gas containing 40% N2, 40% H2, 20% CO, 0.3% CO2, 0.5% CH4,
0.8% water vapor and traces of oxygen
 Reactions:
C3H8 2CH4 + C [Cracking]
CH4 + Fe Fe (C) + 2H2
CH4 + CO2 2CO + 2H2
2CO + Fe Fe (C) + CO2
 Carburising mainly occurs due to CO to CO2 conversion
 H2 reacts with CO2 and increases CO concentration
H2 + CO2 CO + H2O
 Traces of oxygen are also present due to the following reactions;
2CO2 2CO + O2
CO2 + Fe Fe (C) + O2
 To avoid dead spots and formation of soot: Control on gas composition and
proper circulation of gas is essential for constant and uniform rate of carbon
diffusion

9. Gas carburising
 Carburising time of 1 to 2 hrs for case depth from 0.2 to 0.5mm @ 900ᵒ C
 Suited for large volume production
 Accurate control on case depth and surface carbon content
 Less labor cost but skilled labor required for accurate control

10. Vacuum carburising
 Medium: Vacuum or Reduced pressure
 Two stage process:
1. Carbon made available for absorption
 Component introduced in a furnace
 Furnace evacuated till required degree of vacuum
 Heated in the range of 925 to 1050ᵒ C
 Gaseous hydrocarbon like methane or ethane introduced in the furnace.
Amount of hydrocarbon depends upon size of component, surface area to be
carburised, case depth and concentration of carbon to be introduced
 Gaseous hydrocarbon cracks when comes in contact with surface which results
in extremely fine carbon deposition on surface
 Process continues till sufficient amount of carbon is absorbed
 Inflow of gas is stopped and excess gas removed by vacuum pumps
2. Controlled diffusion cycle commences and continues till required carbon
concentration is formed and required case depth is achieved

11. Vacuum carburising
 Oil quenching is used
 Advantages:
 Components are free from oxides, microcracks and decarburization
 Energy saving process
 Disadvantages:
 Limited to batch type production
 Limitation on size of workpiece due to limited size of vacuum furnace
 Reasons for energy saving:
 Heating is carried out by radiation , improved efficiency due to vacuum
 Heat zones occupy less volume
 Not necessary to keep the furnace ON throughout the process.
 Absence of atmosphere
 Only 1% gas required compared to conventional process

12. Post carburising treatment
 Need for post carburising:
 Overheating may occur due to high carburising temperatures which results in
grain coarsening throughout the c/s
 Objectives of post carburising:
 Improve microstructure and refine grain size of core and case
 Achieve high hardness at the surface
 Break carbide network which may be formed due high carbon content (1% C)
 Following heat treatments can be used ;
 Direct quench
 Double quench
 Other quenching cycles

13. Cyaniding
 Applicable to steels with 0.3 to 0.4% C
 Surface hardened by addition by addition of carbon and nitrogen
 Process:
 Medium: Parts immersed in liquid bath containing NaCN varying between 25%
and 90%
 Bath heated in the range of 800 to 960ᵒ C
 Measured amount of air passed through the molten bath
 Reactions:
2NaCN + O2 2NaCNO
2NaCNO + O2 Na2CO3 + CO + 2N
2CO CO2 + C
 C and N2 so formed diffuse into steel and give thin wear resistant layer of
carbonitride ϵ phase
 Quenched in oil or water
 Low temperature tempering
 Cyaniding time of 1.5 to 6hrs for case depth of 0.13 to 0.35mm @ 850ᵒ C
 Higher the temperature, higher the C diffusion (0.8 to 1.2%) on surface as
compared to N (0.2 to 0.3%)
 Case hardness: 850VHN

14. Cyaniding
 Advantages:
 Less time consuming
 Less distortion due to use of salt bath
 Disadvantage:
 Not suitable for components subjected to shock, fatigue and impact because
nitrogen has adverse effect on these properties
 Difference between cyaniding and liquid carburising:
 Absence of alkaline earth salts in cyaniding
 High % of NaCN in case of cyaniding
 High N and lower C in case of cyaniding
 Thin cases in case of cyaniding

15. Carbonitriding
 Also known as dry cyaniding, gas cyaniding, and ni-carbing
 Applicable to steels with 0.3 to 0.4% C
 Surface hardened by addition by addition of carbon and nitrogen
 Used to improve wear resistance of mild steel and low alloy steel
 Process:
 Medium: Gas mixture consisting of 15% NH3, 5% CH4 and 80% neutral carrier gas
 Heated in the range of 800 to 870ᵒ C
 C and N2 diffuse into steel
 Quenching in oil to avoid cracking
 Tempering @ 150 to 180ᵒ C
 Case depth : 0.05 to 0.75mm
 Case hardness: 850VHN
 Nitrogen is more effective in increasing hardenability as compared to carbon
 Nitrogen content depend upon ammonia and temperature
 Advantages:
 Surface hardenability, wear resistance and corossion resistance better than
carburising
 Disadvantage:
 Longer times than carburising

16. Nitriding
 Applicable to alloy steels containing nitride forming elements like Al, Cr, Mo,
V and W
 Process carried out @ 550ᵒ C; hence no phase transformation
 Proper heat treatment necessary before nitriding
 All machining and grinding operations to be completed before nitriding
 Area not to be nitrided to be covered by depositing tin by electrolysis
 Two types:
 Liquid nitriding: Same reactions as that of liquid carburising except only N
diffusion because of low temperatures
 Gas nitriding: Anhydrous ammonia gas is passed which dissociates into nascent
nitrogen and hydrogen
2NH3 2 N + 3H2
 Nitriding of alloy steels: Fe4N (White layer) + alloy nitrides (dark). Hence
YES
 Nitriding of plain carbon steels: Only white layer. Hence NO
 Treatment time depends upon case depth and size; usually 21hrs to 100hrs
 Nitriding time of 100hrs for case depth of 0.5mm @ 550ᵒ C
 Case hardness: 900-1100VHN
 Achieved properties: Good wear resistance, hot hardness, corossion resistance

17. Nitriding
 Applications: Precision gears, boring bars, forming rolls for paper and rubber,
forming dies, camshafts, crankshafts, cylinder liners
 Advantages:
 No post heat treatment; hence minimum distortions
 High fatigue life
 Better corrosion resistance than carburised and hardened components
 Excellent bearing properties (Non metallic nature of nitrides, less coefficient of
friction)
 High hardeness than carburised and hardened components
 High hot hardness
 Disadvantages:
 Applicable only to alloy steels containing nitriding elements
 Thin case depth
 White layer
 No heat treatment can be done after nitriding

18. Nitriding

19. Plasma nitriding
 Also known as ion nitriding process
 What is plasma??
 introduction to plasma.mp4
 Component acts as cathode
 Process:
 Apply high DC voltage 500-1000V
 Electrically heated in the range of 370 to 650ᵒ C
 Gas mixture of N2 and H2 supplied at 1-10 torr
 Current flows and forms ionised gas
 Nitrogen ions bombard on the surface of component
 Part of energy heats the component and allows diffusion
of nitrogen and other part cleans the surface by
displacing secondary electrons
 Bombarded ions clean the surface, heat the component
and diffuse the nitrogen
 Glow envelops the component and nitrding starts
 Component cooled in atmosphere of nitrogen
 Anode is kept cooled by surrounding water around it
 Ion (Plasma) Nitriding process at Ionitech Ltd.mp4
Plasma nitriding process
[Source: T. V. Rajan, C. P.
Sharma and Ashok Sharma,
2013]

20. Plasma nitriding
 Case depth depend upon current,
temperature and time of holding
 Advantages:
 Complex shapes, components of different
size can be nitrided
 Excellent dimensional stability
 Steels sensitive to tempering can be
nitrided at low temperatures
 Very slow white layer formation
 Accurate control
 Improved fatigue properties
 Cold worked steels can be plasma
nitrided to get high wear resistance
 Disadvantages:
 Equipment is complex, skilled labor
required for proper control
 High equipment cost
 Different size and shape part cannot be
plasma nitrided together
 Deep surfaces cannot be nitrided
 Besides these limitations, process is very
attractive
View of components during plasma
nitriding process

21. Boronizing
 Applied to carbon and tool steels
 Medium: Pack or gas
 Pack process:
 Components packed in heat resistant boxes with mixture of granules or paste of
boron carbide or other boron compounds with addition of activators and
dilutents
 Heated in the range of 900-1000ᵒ C
 Boron diffuses and layers of FeB @ outer suface and Fe2B @ interior are
formed
 FeB phase is hard and brittle; hence not desirable. High temperature, long
treatment time and high alloys favor formation of FeB
 Case hardness: 1500-2100 VHN
 Case depth: 0.012-0.15 mm
 Boronizing time of 6hours for case depth of 0.15mm @900ᵒ C
 To optimize the performance hardening and tempering can be carried out after
boronising

22. Boronizing
 Advantages:
 Increases resistance of low alloy steels to sulphuric, phosphoric, and hydrochloric acid
 Increases resistance of austenitic stainless steel to hydrochloric acid
 Selective hardening is possible
 Can be polished to high finish
 Can be applied to irregular shapes
 Increases tool and mold life by improving resistance to abrasive, sliding and adhesive
wear
 Low coefficient of friction
 Disadvantages:
 Distortions due to high temperature
 Poor fatigue and corossion resistance
 Applications:
 Due to high hot hardness and wear resistance: Hot forging dies, wire drawing dies,
extrusion dies, straightening rolls, ingot molds etc
 Nozzles, plungers, gears, shafts and rollers
 Oil and gas components like valve components, valve fittings, metal seals, coal/oil
burner nozzles
 Turbine components, pump impellors, ball valves and seats, shaft protection sleeve, and

23. Chromizing
 Applied to carbon and tool steels
 Medium: Pack or gas
 Pack process:
 Components packed with fine chromium powder and additives
 Composition of chromizing mixture: 60% Cr, < 0.1%C, 0.2% ammonium
iodide, 39% kaolin powder
 Heated in the range of 900-1020ᵒ C
 Chromium carbide formed due to diffusion of chromium
 Case hardness: 1500 VHN
 Chromising time of 12hours for case depth of 0.02-0.04mm @ 900-1020ᵒ C
 Types:
 Hard chromising: For steels with minimum % C = 0.35, hard, corossion and
wear resistant chromium layer will be formed
 Soft chromising: For steels with % C < 0.35, chromium carbide layer cannot be
formed. Chromium diffusion layer with 200micrometer and 35%Cr. Excellent
corossion and oxidation resistance while maintaining ductility

24. Toyota diffusion process
 Developed by Toyota Central Research and Development Laboratories to
develop hard and wear resistant surface for large automotive press tools
 Used for die steels, tool steels, high strength steels
 Process:
 Component kept in a medium containing salt bath of proprietary composition
based on borax (sodium tetraborate)
 Carbide forming elements like vanadium and niobium are added in the form of
ferro-alloys.
 Heated at about 1050ᵒ C
 Carbide forming elements are diffused into the steel
 Quenched and tempered
 Case hardness: 3000VHN
 Carbide layer of 5-12micrometers @ 1000ᵒ C
 Advantages:
 Extremely high hardness, impact resistance and wear resistance
 High seizure resistance and low lubricant requirement
 High peel strength
 Applications: Press tools, shafts, screws, bushes, blades, taps, pins and plugs

25. Surface Hardening with no change in
chemical composition

26. Flame hardening
 Applicable to steels with %C = 0.3 to 0.6
 Process:
 Heating above upper critical temperature (here A3) by oxyacetylene flame
 Cooling by spraying of jet of water or immersion in water
 Reheating in furnace or oil bath @ 180 to 200ᵒ C for stress releiving
 Hardness in flame hardened steel is due to lower bainite or martensite structure
 Flame Hardening of Crane Wheel.mp4
 Case depth: 3mm
 Overheating to be avoided
 High heating rate to avoid oxidation and decarburisation
 Less distortions
 Selective areas can be hardened
 Different methods of flame hardening:
 Spot or stationary: Shaft ends, large gears etc
 Progressive: Guideways, flat surfaces etc
 Spinning: Shafts, wheels, pulleys etc
 Combination of progressive and spinning: Piston rods, Rolls etc
 Applications: Crankshaft, axle, large gear, cam, bending roller etc

27. Flame hardening
Progressive flame hardening
[Source: T. V. Rajan, C. P. Sharma
and Ashok Sharma, 2013]
Progressive spin hardening
[Source: T. V. Rajan, C. P. Sharma
and Ashok Sharma, 2013]

28. Flame hardening
 Depth of hardening depends upon:
 Distance between gas flames and the component surface
 Gas pressures and ratio
 Rate of travel of flame head or component
 Type volume and application of quench

29. Induction hardening
 Applicable to steels with %C = 0.4 to 0.5 and some alloy steels
 Process:
 Heating done by electromagnetic induction
 Electromagnetic Induction.mp4
 Within a short period of 2 to 5 minutes, the temperature of surface layer comes to
above upper critical temperature
 Quenched by jet of cold water
 Low temperature tempering @ 160 to 200ᵒ C
 Induction Hardening.mp4
 INDUCTION HARDENING.mp4.mp4
 Induction hardening king pin.mp4
 Sometimes self tempering may also occur
 Skin effect: Depth of hardened layer is inversely proportional to square root of
frequency of induced current
 In addition to the direct heating of the skin by induced current, there is also some
heating of the core due to conduction of heat. Hence overall depth of hardness is
increased
 Case depth: 0.5 to 6mm
 Applications: Crankshaft, camshaft, gears, crankpins, axles, boring bars, brake
drums, etc

30. Induction hardening
 Advantages:
 Orignal toughness and ductility remain
unaffected
 Fast heating and no holding leads to
increase in production rates
 No scaling and decarburisation
 Less distortion because of heating of
only required surface
 Easy control over the depth of
hardening by control of frequency of
supply voltage and/or time of holding
 Cleanliness of working conditions
 Only a light final grinding or lapping
operation may be required after
hardening
 Disadvantages:
 Irregular shaped parts are not suitable
for induction hardening
 Not economical for small scale
production
Induction hardening process
[Source: T. V. Rajan, C. P. Sharma
and Ashok Sharma, 2013]

31. Laser beam hardening
 Lens used to reduce the intensity
 Laser beam of 1kW can produce circular spot of diameter 0.25 to 0.50mm
 Process:
 Heating the zone to be hardened in the austenitizing range
 Holding to ensure adequate diffusion of carbon
 Self quenching
 Microstructure: Laser heat treated steel consists of bainite + ferrite at the surface of
heated spot and pearlite + ferrite in the interior
 Relationship between depth of hardening and power,
Where
Y = case depth (mm)
P = Laser power (W)
Db = Incident beam diameter (mm)
V = traverse speed (mm/s)
 Laser Hardening of Tool Steel.mp4
 MATEX laser hardening technology.mp4
 Hardening of steel axles using lasers.mp4
 Case depth: 0.75mm

32. Laser beam hardening
 Independent process variables:
 Incident laser beam power
 Diameter of incident laser beam
 Absorptivity of laser beam by surface
 Transverse speed across the surface
 Dependent process variables:
 Depth of hardness
 Geometry of heat affected zone
 Microstructure and metallurgical properties of laser heat treated material
 Efficiency depends on absorption of light energy by work-piece
 Colloidal graphite, manganese phosphate, zinc phosphate and black paint are
some of the commonly used absorbent coating to avoid melting and key
formation

33. Laser beam hardening
 Advantages:
 High production rates
 Effect of heat on surrounding surface is less
 Less time than induction and flame hardening
 Localized treatment is possible
 No external quenching is needed; necessary only for small parts
 No contamination
 Process can be controlled by computer
 Difficult to harden areas can be hardened
 No final machining needed subsequent to hardening

34. Electron beam hardening
 Used to harden the components which cannot be hardened by induction
hardening
 Application: Automotive transmission clutch
 Work piece kept in vacuum at 0.06 bar pressure
 Electron beam focused on work piece to heat the surface
 In the beginning, energy input is kept high
 With time, power input is reduced as the component gets heated up, to avoid
melting
 Computer is used to control voltage, current, beam, dwell time and focus
 Case depth: 0.75mm

35. THANK YOU!!