time temperature transformation curve in heat treatment process

heat treatment process

1. UNIT II HEAT TREATMENT

2. SYLLABUS 1 Definition – Full annealing, stress relief, recrystallisation and spheroidising 2 Normalizing, hardening and tempering of steel. Isothermal transformation diagrams 3 Cooling curves superimposed on I.T. diagram – C ontinuous Cooling Transformation (CCT) diagram 4 Austempering , Martempering – Hardenability, Jominy end quench test 5 Case hardening, carburizing, Nitriding, cyaniding, carbonitriding 6 Flame and Induction hardening – Vacuum and Plasma hardening 7 Thermo-mechanical treatments - elementary ideas on sintering.

3. Heat treatment : Heat treatment process may be defined as an operation or combination of operation involving heating and cooling of a metal/alloy in the solid state to obtain desirable properties . P urpose of heat treatment process: To Relive Internal Stresses To Improve Machinability. To refine Grain Size. To soften the metal. To improve the hardness of metal surface. To Improve Mechanical Properties. To increase resistance to wear, heat and corrosion. To improve magnetic and electrical properties. To improve ductility and toughness. To change the chemical composition

4. Principles of heat treatment: The theory of heat treatment is based on the fact that a change takes place in the internal structure of metal by heating and cooling which includes desired properties in it. The rate of cooling is the controlling factor in developing hard or soft structure. Rapid cooling from the critical range results in hard structure, whereas very slow cooling produces the soft structure. The variety of metal such as steel and various metallurgical processes are depending upon: i. The method and rate of heating and cooling ii. Furnaces used iii. Quenching medium used

5. STAGES OF HEAT TREATMENT PROCESS : Stage 1: Heating a metal or alloy beyond the critical temperature. Stage 2: H olding at that temperature for a sufficient period of time to allow necessary changes to occur. Stage 3: C ooling the metal or alloy at a required rate to obtain the desired properties . TYPES OF HEAT TREATMENT PROCESSESS / OPERATIONS: 1. Annealing 2. Normalising 3. Hardening 4. Tempering 5. Austempering 6. Martempering 7. Case hardening a. Full annealing b. Process annealing c. Stress relief annealing d. Recrystallisation annealing e. Spheroidise annealing Carburising Nitriding Cyaniding Carbonitriding Flame hardening Induction hardening

6. Annealing: The term annealing refers to a heat treatment in which a material is exposed to an elevated temperature for an extended time period and then slowly cooled. Stages of annealing process / Annealing cycle: 1. Heating to the desired temperature 2. Holding or soaking at that temperature 3. Cooling or quenching, usually to room temperature. Purposes of Annealing: To relieve or remove stresses. To induce softness To alter ductility, toughness, electrical, magnetic and other properties To refine grain structure To remove gases To produce define microstructure Application: It is employed in casting, forging, rolled stock and press work etc..

7. Types of Annealing: a. Full annealing b. Process annealing c. Stress relief annealing d. Recrystallisation annealing e. Spheroidise annealing a. Full annealing In fact, the actual definition of annealing describes only the full annealing. Purpose: To soften the metal To refine its crystalline structure To relive the internal stresses Material: The full annealing is specially adopted for steel castings and steel ingots.

8. Process / Method: The process consists of heating the steel to austenitic region, followed by slow cooling. Steel is heated to about 30 to 50°C above the upper critical temperature (A3 line)- between 723°C and 910°C for hypoeutectoid steels and 30 to 50°C above the lower critical temperature ( A1 line ) for eutectoid steels. The steel is held at this temperature for a predetermined period, followed by slow cooling usually in furnace. Cooling is usually done in the furnace itself by decreasing the temperature 10°C to 30°C per hour below the A1 line. Then the steel is removed from the furnace and air cooled to room temperature. Structural change: When steel is heated to the lower critical temperature , the pearlite changes to the finer austenite crystals. As the temperature is raised the coarser ferrite gets dissolved and disappears at the upper critical temperature with the production of fine austenite crystals.

9. Subsequent furnace cooling produces ferrite and pearlite for hypoeutectoid steels and pearlite for eutectoid steels. The properties obtained depends on the size of pearlite and ferrite grains and their relative distribution. Applications: This treatment improves the machinability of medium carbon steels. All steel castings, rolled stocks and forgings are subjected to this heat treatment to attain enhancement of mechanical properties

10. b. Process annealing or Subcritical annealing: Purpose: It is a heat treatment that is often used to soften and increase the ductility of a previously strain hardened metal. Material: It is extensively employed for steel wires and sheet products (especially low carbon steels) Process / Method : In process annealing, the low carbon steels (less than 0.25%C) are heated to temperature slightly below the A 1 line (i.e., between 550°C and 650 °C ). Then they are held for a period of time to achieve softening and then cooled at any desired rate. This process includes single phase morphology. Because the material is not heated to as high temperature as in full annealing, the process annealing is comparatively cheaper, more rapid and tends to produce less scaling. Application: This process has wide application in preparing steel sheets and wires for drawing

11. c. Stress relief annealing or Commercial annealing: Purpose: The stress relief annealing is a heat treatment process that is employed to eliminate internal residual stresses induced by casting, quenching, machining, cold working , welding etc.. Causes of internal residual stresses: Plastic deformation processes such as machining and grinding Non uniform cooling of a metal that was processed or fabricated at an elevated temperature, such as a welding or a casting A phase transformation that is induced upon cooling wherein parent and product phases have different densities. Effects of internal residual stresses: If the internal are not removed , then distortion or warpage of the material may result Process / Method : Steel is heated uniformly below the lower critical temperature in the range of 550°C to 650°C and held at this temperature for a sufficient period of time , followed by uniform cooling. The magnitude of stress relieved depends on the temperature and holding time. Structural Change: No microstructural changes Only relieve the internal residual stresses Little change in mechanical properties are observed.

12. d. Recrystallisation annealing; Recrystallisation: It is a process by which distorted grains of cold worked metal are replaced by new, strain free grains during heating above a specific minimum temperature. Recrystallisation temperature: The temperature at which crystallization takes place i.e., new grains are formed is called recrystallization temperature. Process / Method : In this heat treatment process, cold worked steel is heated to a temperature above recrystallization temperature held at this temperature for some time and then cooled. Structural Change: It may be noted that the recrystallization process does not produce new structures. But it produces strain free new grains. This results in increase in ductility as well as decrease in the hardness and strength. Application: This process is employed as an intermediate operation or as final treatment in industries manufacturing steel wires, sheets or strips.

13. e. Spheroidise annealing / Spheroidizing : Purpose: The purpose of this process is to improve the machinability and ductility of medium and high carbon steels and air hardening alloy steels. To soften steels To reduce hardness, strength and wear resistance. Process / Method : Various methods are available to produce spheroidised structure i. The first method consists of heating steel to a temperature just below the lower critical temperature , holding at that temperature for a prolonged period and followed by slow cooling . ii. The second method involves heating and cooling the steel alternatively just above and below the lower critical temperature. iii. The third method consists of heating the steel to a temperature above the lower critical temperature followed by slow cooling to a temperature below the lower critical temperature and holding at that temperature for a long period.

14. Structural Change: No phase change takes place. Lamellar and free cementite coalesces into tiny spheroids due to surface tension effect. Final structure consists of spheroids of carbide in a matrix of ferrite.

15. 2. Normalising Normalising is similar to full annealing, but cooling is established in still air rather than in the furnace. Purpose: To refine the grain structure. To increase the strength of the steel. To provide a more uniform structure in castings and forgings. To relieve internal residual stresses due to cold working. To achieve certain mechanical and electrical properties. Materials: The normalising process is extensively employed for low and medium as well as alloy steels. Operation: Steel is heated to about 40-50 o C above the upper critical temperature (A 3 or A cm ), held at that temperature for a sufficient period of time and then cooled in still air or slightly agitated air to room temperature.

16. Structural Change: In this process, the homogeneity of austenite increases since the temperature involved is more that that for annealing. It results in better dispersion of ferrite and cementite in the final structure. The grain size is finer in normalized structure than the annealed one . This results in a slightly higher strength and hardness but lower ductility than full annealing. Application: Rolled and forged steels possessing coarse grains are subjected to normalising treatment for grain refinement.

17. NORMALISING ANNEALING / FULL ANNEALING Normalising is more economical than full annealing since no furnace is required to control the cooling rate. Full annealing is costly Normalising is less time consuming Full annealing is more time consuming Normalising temperature is higher than full annealing Annealing temperature is lower than normalising It provides fine grain structure It provides coarse grain structure In normalising, the cooling is established in still air. So the cooling will be different at different locations. Thus properties will vary between surface and interior In full annealing, the furnace cooling ensures identical cooling conditions at all locations within the metal, which produces identical properties Comparatively higher strength and hardness Comparatively lower strength and hardness Normalising improves the machinability of low carbon steel Annealing improves the machinability of medium carbon steel

18. QUENCHING: The Process of fast or instant Cooling is Knows as Quenching. The cooling can be accomplished by contact with a quenching medium which may be a gas, liquid or solid. Most of the times, liquid quenching media is widely used to achieve rapid cooling. Types of Quenching medium: 5 – 10% Caustic soda 5 – 20% brine ( NaCl ) Cold water Warm water Mineral oil – obtained during the refining of crude petroleum Animal oil Vegetable oil Air Thus water produces the most severe quench followed by oil, which is more effective than air.

19. APPLICATIONS OF QUENCHING MEDIUM Quenching medium Applications Mineral oils Used in hardening alloy steels Water or aqueous solution of NaOH or NaCl Used for quenching carbon and low alloy steels Water and Air Used for rails, pipes and heavy forgings

20. Selection of Quenching medium: Based on the following factors, Desired rate of heat removal Required temperature interval Boiling point Viscosity Flash point Stability under repeated use Possible reactions with the material being quenched Cost Stages of Quenching: The three stages of quenching are; Stage i: Vapour – Jacket Stage: When a piece of hot metal is first inserted into a tank of liquid quenchant, that adjacent to the metal vaporises and forms a gaseous layer separating the metal and liquid.

21. In this stage cooling is slow since all heat transport must be through a gas – by conduction and radiation. This stage occurs when the metal is above the boiling point of the quenchant. Stage II: Vapour – Transport Cooling stage: This stage starts when the hot metal is cooled to a temperature at which the gaseous layer is no layer is no longer stable. In this stage, bubbles nucleate and remove the gaseous layer . Liquid contacts the metal and vapourises it and thus the process of bubbles formation continues. The second stage of quenching provides rapid cooling as a result of the large quantities of heat removed by the mechanism. Stage III: Liquid Cooling Stage: Third stage begins when the metal cools below the boiling point of the quenchant. In this stage, all heat transfer occurs through conduction across the solid-liquid interface, aided by convection or stirring within the quenchant. The third stage of quenching provides the slowest rate of cooling .

22. 3. HARDENING: Hardening refers to the heat treatment of steel which increases its hardness by quenching. Hardening normally implies heat treating operations which produce microstructures which are entirely or predominantly martensitic. Purpose: To harden the steel to resist wear. To enable it to cut other metals. To develop a high hardness level for heavy duty service. Operation: i. Heating: The steel to be heat treated is heated slowly in a furnace to a temperature 30 o C to 50 o C above the upper critical temperature- A 3 line. ii. Soaking: The heated steel is held at this temperature for considerable length of time to allow complete austenisation. iii. Cooling: The steel is cooled by quenching to room temperature . The cooling rate should be higher than the critical cooling rate in order to get the completely martensitic structure.

23. Phase transformation during hardening: Due to rapid cooling, the austenite – γ iron will be super cooled by nearly 500 o C and the driving force will be large enough to convert FCC Body Centered Tetragonal (BCT ). The resulting structure of FCC BCT is called as martensite. A new phase transformation from austenite to martensite occurs without a compositional change . Austenite ( γ iron)  Martensite FCC BCT The martensite is super-saturated solution of carbon in α -iron. Also because carbon is present in the lattice, slip does not occur easily. That’s why, martensite is very hard, strong and brittle. It has needle like structure.

24. Factors affecting the hardness: Carbon content Quenching medium Specimen size Other factors Carbon content: Hardening process is carried out on the high carbon steels – 0.35 to 0.50%C. As the carbon content in steel increases, the hardness obtainable also increases. Quenching medium: Faster the cooling greater the hardness, slower the cooling lower the hardness. Specimen size: As the specimen size increases, the hardness decreases. Other factors: Geometry - Shape of the specimen Quenching temperature Degree of agitation Surface condition of the specimen Alloying elements

25. 4. TEMPERING: It is a heat treatment process in which martensite is reheated. In this, the ductility and toughness of martensite can be enhanced by reducing the hardness of martensite. Usually hardening is always followed by tempering process . Purpose: The purpose of tempering is to relieve the residual stresses and improve ductility and toughness of the hardened steel. The gain in ductility and toughness is usually attained at the loss of hardness and wear resistance. Process: The process consists of heating the steel below the lower critical temperature- typically 150 - 630 o C followed by cooling in air or at any other desired cooling rate.

26. Structural changes: As martensite is a supersaturated solid solution, if energy is supplied by tempering, it decomposes to a two phase microstructure consisting of BCC alpha ferrite and small particles of carbide. With increasing temperature, the carbide particles coalesce and grow. Depending on the range of temperature, tempering is classified into following types. i. Low temperature tempering or first stage tempering: At this stage, the maximum temperature to which steel is heated is restricted to about 200 o C. This results in the decomposition of high carbon martensite into low carbon martensite and transition carbide (Fe 2.4 C) called ε -Carbide. This structure is fine and etches are dark. This is known as black martensite. Due to the above change in the structure there is a marginal decrease in the hardness value but toughness improves to a lesser degree. Most of the residual stresses are relieved.

27. ii. Medium temperature tempering or second stage tempering : The medium temperature tempering consists of heating steel in the temperature range of 200 o C - 400 o C . During this stage, the low carbon martensite becomes BCC ferrite and ε -carbide changes to cementite (Fe 3 C). The retained austenite gets transformed to bainite. This structure is called troostite. These changes in microstructure result in increase of ductility and toughness with a corresponding decrease in hardness and strength. ii. High temperature tempering or third stage tempering: It consists of heating the steel within a temperature range of 400 - 650 o C. During this stage of tempering, the cementite particles coalesce and grow . This cementite particles are just about resolvable by an optical microscope. The resultant structure is known as sorbite . This treated steel has better tensile, yield and impact strength and is free from internal stresses. If the third stage tempering is carried out at a temperature just below the A 1 temperature (723 o C) the structure produced consists of cementite spheroids embedded in a matrix of ferrite. This structure is very soft and tough.

28. Applications: Low temperature tempering is useful for high carbon and low alloy steels in manufacturing cutting and measuring tools . Medium temperature tempering is best suited for coil and laminated springs. High temperature tempering is suited for medium carbon steel and medium carbon low alloy steels. Connecting rods, shafts and gears are frequently subjected to this treatment.

29. Isothermal transformation diagrams Cooling curves superimposed on I.T. diagram – Continuous Cooling Transformation (CCT) diagram Refer PDF in GCR

30. Hardenability: The hardenability of a steel is defined as that property which determines the depth and distribution of hardness induced by quenching from the austenitic condition. In other words, hardenability is a measure of ease of forming martensite. Hardness Hardenability It is the property of material by virtue of which it is able to resist abrasion, indentation and scratching. It is a mechanical property related to strength and is a strong function of the carbon content of a metal. The hardness can be measured by the methods like Brinell , Rockwell and Vickers hardness test. It is susceptibility of a material to get hardened. It is affected by alloying elements in the material and grain size. The hardenability can be measured by the Jominy end-quench test method.

31. Factors affecting hardenability: The hardenability of a steel depends on the following factors, The composition of the steel. The austenitic grain structure. The structure of the steel before quenching. The quenching medium and the method of quenching.

32. DETERMINING HARDENABILITY (JOMINY END QUENCH TEST) The Jominy end –quench test method is universally adopted, because: It is relatively easy to perform It has excellent reproducibility It gives information useful to a designer as well as manufacturer End Quench Specimen: A bar of the steel to be tested is machined to a given cylinder 111.6 mm (4 inch) long and 25.4 mm (1 inch) in diameter with an upper lip. The end-quench specimen and testing arrangement are shown in figure. Testing Procedure: The standard test piece is heated above the upper critical temperature of the steel i.e., until becomes completely austenitic. It is then very quickly transferred from the furnace and immediately dropped into position in the frame of the apparatus. Here it is quenched at one end only, by a standard jet of water at 25 o C. Thus different rates of cooling are obtained along the length of the test-piece.

33. 4. When the test-piece has cooled, a flat approximately 0.4 mm deep is ground along the length of the bar. Now Rockwell C hardness readings are taken every 1.6 mm (1/16 inch) along the length from the quenched end. 5. Finally the results are plotted as shown in fig 2.23 From the fig 2.23 it is clear that the greatest hardness is at the quenched end, where martensite is formed and the lower hardness is farther away.

34. Use of hardenability curves: The main practical uses of end quench hardenability curves are: If the quench rate i.e., cooling rate for a given part is known, the Jominy hardenability curves can predict the hardness of that part. If the hardness at any point can be measured, the cooling rate at that point may be obtained from the hardenability curve for that material. Cooling rates can be determined by thermocouples embedded in the bars during the quenching operation. Fig 2.24 and 2.25 shows Jominy hardenability curves for various common engineering steels .

35. INTERRUPTED QUENCHING: The quenching i.e., rapid cooling mechanism has its own disadvantages. Some of the disadvantages of continuous rapid cooling are: Setting up severe quenching stresses Warping or distorting the object Promoting crack formation in the steel These undesirable effects may develop during uneven cooling of the heat-treated material. In order to overcome the disadvantages of continuous quenching a modified quenching procedure known as interrupted quenching is usually used. Two forms of interrupted quenching are: Martempering and Austempering

36. Martempering (Marquenching): It is a interrupted cooling procedure used for steels to minimize the stresses, distortion and cracking of steels that may develop during rapid quenching. Martempering Process: Step I : Austenitizing the steel , i.e , heating the steel above the critical range to make it all austenite. Step II : Quenching the austenitized steel in hot oil or molten salt at temperature just slightly above the martensite start temperature. Step III: Holding the steel in the quenching medium until the temperature is uniform throughout and stopping this isothermal treatment before the austenite to bainaite transformation begins. Step IV: Cooling at a moderate to room temperature (usually in air) to prevent large temperature differences between center and surface. The resulting microstructure of the martempered steel is untempered martensite.

37. Now the untempered martensite structure is transformed into tempered martensite structure by the conventional tempering heat treatment processing rapid quenching. Application: The martempering process is mostly used in alloy steels. Advantages of Martempering: Minimised quenching stresses Minimised chances of formation of quenching cracks Less distortion of warping

38. Austempering : Austempering forms bainite structure. The austempering is an isothermal heat treatment process, usually used to reduce quenching distortion and to make a tough and strong steels. Austempering process: Step I : Austenitizing the steel Step II : Quenching the austenised steel in a molten salt bath at a temperature above the martensite start temperature (Ms) of the steel. Step III : Holding the steel isothermally to allow the austenite –to- bainaite transformation to take place. Step IV: Slow cooling to room temperature in air The resulting microstructure of the austempering process is bainaite . Unlike martempering , tempering is rarely needed after austempering .

39. Application: Austempering is widely applied on small tools, springs, retainers, automobile seat belt components, link chains, lawn mover blades and various machinery parts. Advantages: Improved ductility Increased impact strength and toughness Decreased distortion of the quenched material Less danger of quenching cracks. Disadvantages: The disadvantages of austempering over quenching and tempering are: Need for a special molten salt bath. The process can be used only for a limited number of steels. Only small sections up to 9 mm thick, are suitable for austempering because big sections cannot be cooled rapidly to avoid the formation of pearlite.

40. CASE HARDENING / SURFACE HEAT HARDENING: In many applications, it is desirable that the surface of the components should have high hardness, while the inside or core should be soft. The treatment given to steels to achieve this are called surface heat treatment or surface/ case hardening. Uses: Typical parts that are case-hardened are gears, splines, cams, bearing balls, wrist pins, universal joints and valve tappets. Types of surface heat treatments: 1. Diffusion methods: Carburizing b. Nitriding c. Cyaniding d. Carbonitriding 2. Thermal Methods; a. Flame hardening b. Induction hardening Diffusion is the spontaneous movement of atoms or molecules in a substance that tends to make the composition uniform throughout.

41. 1. Diffusion methods: In this method, the hardness of the surface is improved by diffusing interstitial elements like carbon, nitrogen or both into the surface of steel components. Depending upon the different elements chosen, diffusion treatment can be classified as Carburizing b. Nitriding c. Cyaniding d. Carbonitriding a. Carburizing: Introduce carbon into surface layer of low carbon steels in order to produce a hard surface. Old and cheapest method Depth of case 0.5 to 2mm. Temp range 900-930 o C. Solubility of carbon is more in austenite state than in ferrite state, fully austenite state is required for carburizing. Achieved by heating steel above critical temp Diffusion of carbon is made by holding the heated steel in contact with carbonaceous material which may be a solid a liquid or a gas.

42. Process of Carburising: Here, when a piece of low-carbon steel is placed in a carbon saturated temperature, then the carbon will diffuse or penetrate into the steel and carburising it. Methods of Carburising: The carburising i.e., the process of adding carbon to a metal surface, can be accomplished by the following three methods, ( i) Pack carburizing (ii) Gas carburizing (iii) Liquid carburizing This classification is based on the form of the carburising medium used.

43. (i) Pack carburizing: This method of carburizing is also known as solid carburizing. In this process, steel components to be heat treated are packed with 80% granular coal and 20 % BaCO 3 as energizer in heat resistant boxes and heated at 930°C in furnace for a specific time which depends on the case depth required. Such a high temperature in furnace helps in absorption of carbon at the outer layer. The following reactions takes place: i . Energizer decomposes to give CO gas to the steel surface ii . Carbon monoxide reacts with the surface of steel : iii . Diffusion of carbon into steel iv . CO 2 formed in step (ii) reacts with “C” in the coal For a given steel at a given temperature, the depth of penetration is dependent on diffusion and can be related to the time t by the equation Generally, carburizing time varies from 6 hours to 8 hours, and case depth obtained varies from 1 mm to 2 mm.

44. PACK CARBURISING ADVANTAGES: This process does not require the use of prepared atmosphere It is efficient and economical for industrial processing of parts in small lots or for massive parts. DISADVANTAGES: This process is not suited to produce thin cases. Close control of surface carbon cannot be achieved.

45. (ii)Gas carburizing: It overcomes the drawbacks of pack carburising by replacing the solid carburising mixture with a carbon- providing gas. It can be done with any carbonaceous gas i .e., gas containing excess of CO. In general, natural gas, propane, or generated gas atmospheres are most frequently used. Procedure: It is carried out in an air tight retort furnace, capable of maintaining positive pressure. The furnace is initially purged with a carrier gas consisting of a mixture of CO, N 2 and H 2. The furnace operates at the carburising temperature of 930 o C and dew point is adjusted to about -4 o C. The components are loaded in baskets and lowered into the furnace, so that a free flow of gas could pass around them. A fan is situated at the top of the furnace which circulates and mixes the gases. When the material reaches the carburising temperature, propane or methane is added to maintain a specific carbon potential.

46. During gas carburising, the following reactions take place. The carbon monoxide in the carrier gas is the active carburising agent. 2CO CO 2 + ( C ) The carbon thus released dissolves intestinally into the surface of the steel. ii. Methane also releases carbon CH 4 2H 2 + ( C ) iii. Carbon dioxide formed in (i) reacts with methane. CO 2 + CH 4 2H 2 + 2CO The concentration of carbon monoxide is thus maintained, so that carburising continues. Gas carburising is commonly used to obtain relatively thin cases between 0.2 and 0.5 mm

47. Advantages: It is particularly adopted for large volume production This process provides accurate control of case depth and surface carbon content It is a cleaner process It allows quicker handling by direct quenching It has less cost but highly skilled personnel are required to maintain the necessary controls.

48. LIQUID CARBURISING: Also known as salt bath carburising. The carburising medium is a fused salt bath composed of sodium cyanide, sodium chloride and barium chloride. The bath is melted in a steel cast pot type furnace heated by oil or gas. The process is carried out by immersing the steel components in the bath maintained at a temperature of 815-900 o C, for a periods varying from 5 minutes up to 1 hour depending on the depth of case required . The components are then quenched. The reactions in the salt bath are as follows, 2NaCN + BaCl 2 Ba(CN) 2 + 2NaCl Ba(CN) 2 BaCN 2 + ( C ) The carbon atoms dissolve interstially in the steel. A small amount of liberated nitrogen is also absorbed. This process gives a thin end clear hardened layer up to a case depth of 0.08 mm. High temperature salt baths are used for producing a deep case.

49. Advantages: This process are rapid and uniform heat transfer, low distortion, negligible surface oxidation and rapid absorption of carbon. Highly uniform case depths are obtained with uniform carbon content. The cycle time for liquid carburising is shorter than gas or pack carburising. Disadvantages: Cyanide salts are extremely poisonous so care is required during its storage and disposal. The salt sticks to the components and must be thoroughly washed after treatment. Regular checking and adjustment of the bath composition is necessary to obtain uniform case depth.

50. 5 Case hardening, carburizing, Nitriding, cyaniding, carbonitriding 6 Flame and Induction hardening – Vacuum and Plasma hardening 7 Thermo-mechanical treatments - elementary ideas on sintering . - Refer PDF in GCR