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  • 1. Nitriding accompanies the introduction of nitrogen into the surface of certain types of steels (e.g., containing aluminum and chromium) by heating it and holding it at a suitable temperature in contact with partially dissociated ammonia or other suitable medium. This process produces a hard case without quenching or any further heat treatment. Process Characteristics — Case depth is about 0.381 mm. — Extreme hardness (Vickers 1100), — Growth of 0.025—0.050 mm occurs during nitriding. — Case has improved corrosion resistance. Typical Uses * Valve seats * Guides * Gears * Gauges * Bushings * Aircraft engine parts * Aero engine cylinderst * Aero crankshafts, air screw shafts * Crank pins and journals * Ball races * Moulds for plasters Table 44.1 shows that steels suitable for nitriding must contain such alloying elements as Al, Cr and V to form hard nitrides because iron nitride does not confer hardness to any extent. Table 44.1: Nltriding Steels Composition (%) Heat-treatment temp. (C) Tensile strength (N/mm2 ) Harthtess (VPN) Case Oil quench Tempering 1.C 0.39, Cr 1.6 900 650 880 269 1050 Mo 0.2, Al 1.1 2.C 0.39, Cr 3.2, 940 625 1310 380 900 Mo 1.0, V 0.12 3.C 0.5, Cr 1.3, 870 620 1450 — 700 Mo 1.0, Ni 1.8 4.C 0.2, Mn 0.45, 900 600—700 772 — 800—850 Cr 3.0, Mo 0.4 5.C 0.3, Mn 0.45, 900 600—700 1000 — 800—850 Cr 3.0, Mo 0.4 6.C 0.4, Mn 0.5, Cr 3.0,900 550—650 1390 — 850—900 Mo 1.0, V 0.25 7.C 0.35, Mn 0.5, Cr 2.0 900 600—700 741 — 750—800 Mo 0.25, V 0.15 Nitriding process — Before being nitrided, the components are heat treated to produce the required properties in the core. The normal sequence of operations are: (i) Oil quenching from between 850 and 900°C followed by tempering at between 600 and 700°C. (ii) Rough machining followed by a stabilizing anneal at 550°C for five hours to remove internal stresses.
  • 2. (ui) Finish machining, followed by nifriding. — Nitriding: The components are placed in a heat-resistant metal container which is then filled with ammonia whilst cold. When it is completely purged, it is sealed, placed in a furnace and raised to a temperature of approximately 500°C. At this temperature the ammonia dissociates. NH 3H + N, and N is absorbed in the surface layer of steel. Parts are maintained at 500°C for between 40 to 100 hours depending upon the depth of case required; after which the parts are allowed to cool in the container. Advantages of Nitriding 1. Very high surface hardness of the order of 1150 VPN may be obtained. 2. Since nitrided parts are not quenched, this minimizes distortion or cracking. 3. Good corrosion and wear resistance. 4. Good fatigue resistance. 5. Whereas in a carburized part, hardness begins to fall at about 200°C, a nitrided part retains hardness up to 500°C. 6. No machining is required after nitriding. 7. Some complex parts which are not carburized satisfactorily, can be nitrided without difficulty. 8. The process is economical when large number of parts are to be treated. Disadvantages of Nitriding 1. Long cycle times (40 to 100 hours). 2. The brittle case. 3. Only special alloy steels (containing Al, Cr and V) can be satisfactorily treated. 4. High cost of the nitriding process. 5. Technical control required. 6. If a nitrided component is accidentally overheated, the surface hardness will be lost completely and the component must be nitrided again. 44.9 CYANIDING Definition In cyaniding, carbon and nitrogen are introduced into the surface of steel by heating it to a stkitable temperature and holding it in contact with molten cyanide to form a thin skin or case which is subsequently quench hardened. Characteristics of the process — Case depth is about 0.25 mm. — Hardness is about Rc 65. — Negligible dimension change is caused by cyaniding. — Distortion may occur during heat treatment. Typical Uses * Screws * Nuts and bolts * Small gears Metals usually hardened by cyaniding — Plain carbon or alloy steels containing about 0.20% carbon. Cyaniding process — Low carbon steel is heated between 800 and 870°C in a molten sodium cyanide bath for a period of between 30 mm and 3 hour, depending upon the depth of case required. Quenching in oil or water from this bath hardens the surface of steel. — In cyaniding, the bath usually contains
  • 3. 30% NaCN, 40% Na and 30% NaCl. This mixture has a melting point of 1140°F (615.6°c) and remains quite stable under continuous operating conditions. This mixture, when used at temperatures ranging from 787 to 898°C, decomposes to free carbon and nitrogen which are then absorbed into the steel to form a hardened carbide-nitride case. AND METALLURGY also 2NaCN + 202 - Na + CO + 2N 2C0 + C 2NaCN + 02 -, 2NaCNO (Sod. cyanate) NaCN + CO - NaCNO + CO 3NaCNO - NaCN + Na + C + 2N In order to obtain hardness after cyaniding, it is necessary to quench directly into oil or water from the cyaniding bath. In cyaniding, nitrogen imparts inherent hardness, whereas the increased carbon content makes the surface of steel respond to a quench ing treatment. Probably the greatest use of cyaniding is for parts that are to be subjected to relatively light loads and that require improvement in the surface-wear resistance. 44.10 CARBONITRIDING Introduction — Cases (surfaces) that contain both carbon and nitrogen are whereas produced by liquid salt baths in cyaniding, they are produced by the use of gas atmospheres in carbonitriding. Carbonitriding implies introducing carbon and nitrogen into a solid ferrous alloy by holding above Ac in an atmosphere that contains suitable gases such as hydrocarbon, carbon monoxide and ammonia. The carbonitrided alloy is usually quench-hardened. Metals Usually Hardened by Carbonitriding. Plain carbon steels containing about 0.20% carbon. Process characteristics — Case depth is about 0.5 mm. — Hardness after heat treatment Rc 65. — Negligible dimensional changes. — Distortion is less than in carburizing or cyaniding. Typical Uses * Nuts * Gears * Bolts Carbonitriding process. Carbonitriding is a modification of gas carburizing process because anhydrous ammonia gas is added to the fur nace atmosphere to cause both Carbon and Nitrogen to be absorbed by the surface of steel at the carbonitriding temperature. — The atmospheres used in carbonitriding usually comprise a mixture of carrier gas, enriching gas (about 5%) and ammonia (about 15%). The carrier gas is usually a mixture of nitrogen, hydrogen and carbon monoxide produced in an endothermic generator, as in gas carburizing. The carrier gas is supplied to the furnace under positive pressure to prevent air infiltration and acts as a diluent for the active gases (hydro carbons and ammonia), thus making the process easier to control.
  • 4. The enriching gas is usually propane or natural gas and is the primary source for the carbon added to the surface. At the furnace temperature, the added ainnionia breaks up to pro vide the nitrogen to the surface of the steel. — Carbonitriding is carried out at lower temperatures than gas carburizing. The temperature range from 650 to 885°C with 845°C being most common. Low carbon steel is heated at this temperature for several hours in the gaseous atmosphere discussed above. Nitrogen in the surface layer of steel increases hardenability and permits hardening by oil quench (instead by water quench) and thus reduces distortion and minimizes danger of cracking. Since nitrogen increases the hardenability, carbonitriding the less expensive carbon steels for many applications will provide properties equivalent to those obtained in gas carburized alloy steels. 44.11 FLAME HARDENING Principle — Flame hardening involves (1) Rapid heating of the surface of a heat steel by means of a flame, to a temperature within or above transformation range (austenite range). (ii) Followed immediately by quenching (Fig. 44.2). The highly heated surfaces became hard but the core remains soft and tough. — Objects are heated by an oxyacetylene flame. — Steels having 0.3 to 0.6% carbon are hardened by flame harden ing. Small amounts of nickel (up to 4%) and chromium (up to 1%) can be added with advantage. — Flame hardening is essentially a shallow hardening method. Depth of the hardened zone may be controlled by an adjustment of the flame intensity, heating time or speed of travel. Overheating may result in cracks after quenching. The heating time is 7y seconds (wherey is the depth of the hardened layer in mm); the torch is traversed along with the work-piece (or vice versa) at a speed of 72/y mm per second.
  • 5. Equipment A flame hardening unit comprises the source of acetylene supply, an oxygen plant, quenching devices, hardening control desk, instruments and set of torches and tips. A torch may have single flame, slit type or multiple flame tips. Methods Four methods are in general use for flame hardening. 1. Stationary. Both workpiece and torch are stationary. This method is used for s hardening of small parts, e.g., valve stems and open end wrenches. 2. Progressive. Torch moves over stationary workpiece. Large parts such as the guideways of lathes can be hardened by this method. This technique is also suitable for hardening teeth of large gears. 3. Spinning. Torch is stationary while the workpiece rotates. The method hardens circular parts such as precision gears, pulleys, etc. 4. Progressive Spinning. Torch moves over a rotating workpiece. Long shafts and rolls are hardened by this method. Before hardening by any of the above methods, the workpiece is generally normalized, so that the final structure will consist of a martenste case about 3.75 mm thick and a tough ferrite pearlite core. After hardening (i.e., quenching) the workpiece should be stress relieved by heating in the range of 180 to 205°C and then cooling in air. Stress relieving does not appreciably reduce surface hardness. Advantages 1. Flame hardening is a useful and economical method of surface hardening. 2. The hardened zone is generally much deeper than that obtained by carburizing; it ranges from 3 to 6 mm in depth. Thinner cases (about 1.5 mm) can be obtained by increasing the speed of heating and quenching. 3. Large machine parts can be surface hardened economically. 4. Surfaces can be selectively hardened with minimum warping and with freedom from quench cracking. 5. Flame hardening is easily adaptable and involves portable equipment. 6. Electronically controlled equipment provides precise control of case properties. 7. Flame hardening can treat workpieces even after their surfaces have been finished, because there is little sealing, decarburiza tion or distortion. Disadvantages 1. To obtain optimum results, a technique of flame hardening must be established for each design. 2. Overheating can damage the components. 3. It is difficult to produce hardened zones less than 1.5 mm in depth. Applications The following components are flame hardened: * Ways of lathes * Spindles * Teeth of gears * Valve stems * Worms * Open end wrenches * Shafts * Pulleys, etc. * Mill rolls
  • 6. 44.12 INDUCTION HARDENING Introduction — The danger of either overheating or burning the surface of the metal by flame hardening may be avoided by inducing heat electrically in the surface of the metal. — Induction hardening involves: (i) heating medium carbon steel by means of an alternating magnetic field to a temperature within or above the transformation range (the hardening temperature is about 750 to 800°C). (ii) followed immediately by quenching. This process may be applied for both surface hardening and full annealing. The principle of induction hardening is similar to that employed in the induction melting of steels by the high frequency induction process in which heat is produced by currents induced in the metal charge itself. Heat generated in the metal by induction is mostly confined to the outer surface of the component to be induction hardened. The higher the frequency of current, the closer the heat is to the surface of the component. At SO cycles/second, the effective current flows through a surface layer of about 7.5 mm deep, whilst at 10,000 cycles! second, the surface layer is reduced to 0.5 mm deep. Procedure (iii) — High frequency currents are generated using (1) Motor generators with frequencies of 1,000 to 10,000 cycles/ second and capacities to 10,000 kW. (ii) Spark-gap oscillators with frequencies 01100,000 to 400,000 cycles/second and capacities to 25 kW. Vacwinj- oscillators operating at 500,000 cycles per second with output capacities of 20 to 50 kW — The component part to be induction hardened (i.e., heated) is placed in the so called Inductor or Iflductor coil or Work coil (Fig. 44.3). hardening. Inductor coil comprises one or several turns of copper tube or busbar and is water cooled. — When high frequency (alternating) current is passed through the inductor coil, it sets up a 1nag field (the intensity of which varies periodically in magnitude and direction).
  • 7. As the alternating magnetic lines thread through the surface of the component (being heated) placed in the inductor coil, they induce in component’s surface an alternating current of the same frequency but reversed in direction. Heating results from the resistance of the metal (of the component) to passage of these currents. The high frequency induced currents tend to travel at the surface of the metal. This is known as skin effect. — The component is held stationary in the inductor coil and the whole of the surface of the component is heated simultaneously. The temperature of the surface layer rises to above its upper critical temperature* (i.e., austenite range) in a few seconds. — The surface of the component (after it has been heated to the required temperature) is then quenched by pressure jets of water which pass through the holes existing in the inductor block (Fig. 44.3). As after flame hardening, the induction hardened component also needs be stress relieved. Advantages (i) Time required for hardening a component is sharply reduced. The heating time varies between 1 and 5 seconds. (ii) It can be applied to both external and internal surfaces. (iii) Components may be heated with practically no scaling and distortion. (iv) Higher hardness can be obtained in a given (%C) steel than with thermal heating. A hardness of about 60 Rc may be obtained in certain types of steels to a depth of about 3 mm. (v) Through proper design of the heating coils, the shape of the hardened portion can be controlled very closely. (vi) Depth of hardening can be controlled by selecting current of appropriate frequency Frequency, cycles/second 1000 4000 10,000 120,000 500,000 Depth of hardening(mm) 6.0 3.0 2.5 1.5 0.75t (vii) Hard case and tough core is obtained. (viii) Induction hardening process can be made nearly automatic so that it can be carried out with unskilled labour. Disadvantages (i) Cost of equipment is high. (ii) (ii) Steels having less than 0.40% carbon cannot be induction hardened. (iii) (iii) Irregular shaped components cannot be handled easily and (iv) economically. (v) (iv) It is beneficial in mass production only. (vi) (v) It associates high maintenance costs. (vii) (vi) Before induction hardening the component needs some treat- (viii) ment, e.g., normalising.
  • 8. (ix) Applications (x) Typical parts hardened by this method are (xi) * Piston Lods * Crankshafts (xii) * Pump shafts * Camshafts (xiii) * Spur gears * Automobile parts (xiv) * Cams.