![Effect of Alloying elements upon carbide
ii) Effect of Alloying elements upon carbide:-[Intermediate Alloy
phase (or) compound]
Elements like Ni, Si, Al, Zr, V(Vanadium), Ti, W (Tungsten),
increases carbide formation by combining with carbon.
Increases hardness and brittleness.
Forms hard structure.
Increases wear resistance.
Influences the hardening time and soaking time.](https://image.slidesharecdn.com/msmunit-iii-250507124837-879bf635/85/MSM_-Unit-III-pptxMSM-_-UNIT-V-pptx-material-science-and-metullargy-4-320.jpg)
MSM_ Unit-III.pptxMSM _ UNIT - V.pptx material science and metullargy
- 1. UNIT – III Heat treatment of Alloys: Effect of alloying elements on Fe-Fe3C system, Annealing, normalizing, hardening, TTT diagrams, tempering, hardenability, surface - hardening methods, Age hardening treatment, Cryogenic treatment of alloys.
- 2. Alloying Alloying is done either to increase or to decrease the mechanical properties. Effect of alloying elements on Fe-Fe3C system: i) Solid Solution hardening (or) strengthening:- Most of the alloy elements are soluble in ferrite. These form solid solutions when added to steel. These will Increases hardness and strength. Mostly used alloying elements are Mn, Cr, W, Mo, V, Ti, Ni, Si, Al, Zr, P, ……. Among them P, Si, Mn and Ni are most effective. Alloying elements forms solid solutions like interstitial (or) substitutional solid solutions.
- 4. Effect of Alloying elements upon carbide ii) Effect of Alloying elements upon carbide:-[Intermediate Alloy phase (or) compound] Elements like Ni, Si, Al, Zr, V(Vanadium), Ti, W (Tungsten), increases carbide formation by combining with carbon. Increases hardness and brittleness. Forms hard structure. Increases wear resistance. Influences the hardening time and soaking time.
- 5. iii) Effect of Alloying elements upon ferrite:- Ferrite a ceramic-like material with magnetic properties that are useful in many types of electronic devices. Ferrites are hard, brittle, iron- containing, and generally gray or black and are polycrystalline—i.e., made up of a large number of small crystals. Elements like Ni, Al, Si, Cu, Co are largely dissolved in ferrite. Increases hardness and strength. Increases tensile strength. Changes the mechanical properties with Cr, W, Mo, V, Si, Al, B, Zr, Nb (Niobium), P, Ti…
- 6. iv) Shifting of critical temperature and eutectoid carbon:- Critical temperature (of a substance) can be defined as the highest possible temperature value at which the substance can exist as a liquid. Alloying elements will raises or lowers the transition (or) critical temperature of steels. Austenite stabilizers like Ni, Mn lowers the eutectoid temperature. Ferrite stability like Ti, Mo raises the eutectoid temperature. Most of the alloying elements shift the eutectoid carbon to lower values.
- 7. v) Lowering the critical cooling rate:- Most of the alloying elements decrease the critical cooling rate. This increases the hardenability of steel. Mostly used elements are Mn, Mo (Molybdenum), Cr and Ni.
- 8. vi) Formation of inclusions:- They may combine with oxygen and form oxides when added to steel. Examples are Si, Al, Mn, Cr, V, Ti ………… vii) Other effects:- The transformation may become sluggish (slow). The corrosion and oxidation resistance may increases (Ex : Chromium) Carbides increase the creep resistance. (Creep is a type of metal deformation that occurs at stresses below the yield strength of a metal, generally at elevated temperatures.) May increase fatigue strength. (fatigue is the initiation and propagation of cracks in a material due to cyclic loading. )
- 9. Any Heat treatment consists of three steps H e a t i n g : … … … … … Soaking; ………… 3 C o o l i n g : … … … … . . 1 Heating:…………… 2 Soaking; ………… 3 Cooling : …………..
- 11. Conventional Annealing (Full annealing) Annealing :- Annealing is a heat treatment process that changes the physical and sometimes also the chemical properties of a material to increase ductility and reduce the hardness to make it more workable. The annealing process requires the material above its recrystallization temperature for a set amount of time before cooling. Purpose: To relieve internal stresses. To reduce hardness and increase ductility. To refine grain size To make subsequent heat treatment. To increase the uniformity of phase distribution (Isotropic) To make homogeneous in respect of chemical composition. To increase machinability. Full annealing is the process by which the distorted cold-worked lattice structure is changed back to one which is strain-free through the application of heat (simply getting original lattice by heating process). This process is carried out entirely in the solid state and followed by slow cooling in furnace.
- 12. Annealing is divided into three stages : Recovery, Recrystallization Grain growth.
- 13. Recovery:- Relief from internal stresses, (residual stresses) Also called stress relief annealing. Sometimes electrical conductivity increases. Prevents stress corrosion cracking.
- 14. Re crystallization:- Takes place by a combination of nucleation and growth of nucleation. This time is called Incubation period or recrystallization time. A typical recrystallization curve at constant temp is shown in figure. Incubation period is starting of the processes. Recrystallization temperature at which a highly cold worked material will completely recrystallizes in 1 hour. A pure metal low Recrystallization temperature Alloying metal have high Recrystallization temperature
- 15. Grain growth:- Annealing involves nucleation and grain growth. Rapid nucleation and slow growth will result in fine grained material.
- 16. Effects on properties:- 1. Restores the material to a strain free lattice structure. 2. Softening processes. 3. Properties changed during plastic deformation restored. 4. Returns very near to its original properties.
- 18. Normalizing :- (Draw the previous diagram) Normalizing is a process in which a metal is heated to a temperature below its melting point and allowed to cool in air in order to make it more ductile. Normalizing is a process in which a metal is cooled in air after being heated in order to relieve stress. Purpose: For grain refinement To improve mechanical properties Strength & hardness are higher than annealing. Normalizing temperature is higher than annealing and higher than hardening. For hypo eutectoid steels normalizing temperature is A3 For hyper eutectoid steels normalizing temperature in Acm. For steels soaking time is less. Cooling at room temperature in the open air.
- 19. Air Quenching :- Air quenching is the process of cooling metal using air or inert gas. This rapid cooling process occurs after metal has been heated, to attain the desired properties. The air or gas is forced over the metal to cool down the material. Purpose: Micro structure consists of ferrite and pearlite. It is done on forged or cast products and also forging products. Sometimes hot rolling components also. To remove internal stresses. To refining the coarse grain structure.
- 21. Hardening :- The hardening process consists of heating the components above the critical (normalizing) temperature, holding at this temperature for one hour per inch of thickness cooling at a rate fast enough to allow the material to transform to a much harder, stronger structure, and then tempering. Purpose: A heat treatment process to get martensite micro structure. Steels heated 30° - 50°C above A3 temperature. Holding some time (or soaking) at that temperature. Sudden cooling in water, oil, salt bath (quenching) Considerable part of cementite is retained. During quenching – residual stresses are formed Martensite structure is very brittle and hard. Hardening is carried out on – cutting tools, machine parts. alloy steels, high carbon steels and high speed steels.
- 22. Factors Considered during hardening :- 1. Carbon percentage – minimum 0.5%C. 2. Heating furnace rate and Heating size of component time. 3. Quenching medium and quenching rate. 4. Size of machine part i) Long, ii) Heavy iii) Recess Surface conditions – cleaning surfaces by hot water – wire brust – sand blasting
- 23. Hardening Methods:- 1. Quenching in single medium – water or salt bath 2. Quenching in double medium i) water or salt bat ii) air or oil 3. Hardening with self – tempering i) hammers, puches ii) chisels 4. Martempering (or steeped quenching) 5. Austempering
- 24. Hardenability : Definitions : Ability of steel component can be hardened by martensite transformation easily. 1. The ease with which the steel becomes hard structure on quenching is called hardenability. 2. Ability to harden through its cross-section without having to resort to drastic quenching. 3. Rate at which hardness drops into interior of a part. The materials having lower cooling rate higher hardenability. The materials having higher cooling rate lower hardenability. (Write Jominy End Quench Test)
- 25. Tempering :- Hardening is followed by tempering. In hardening steels become Brittle Quench cracks Residual stresses In tempering above effects are removed. Toughness is increased. Hardness is reduced. If the temperature is above 300°C then, all residual stresses are completely removed.
- 26. Three Tempering Temperature :- Low tempering temperature :upto 200°C retains hard martensite. Medium tempering temperature : 175°C to 275° - for springs troostite micro structure. High tempering temperature : 275° to 375° - Sorbitic structure is obtained – Internal stresses fully removed.
- 27. Surface hardening methods :- 1. Flame hardening 2. Induction hardening 3. Laser hardening 4. Electron beam
- 28. Flame hardening: This process is performed on low alloy (or) plain carbon steels. A combustile gas flame is source for heating. Flame makes austenizing the steel. Austenizing steel is moved to rapid water quenching. Rapid water quenching gives martensitic structure. Ex : Gears, wheels, sheaves and cast iron bushing are flame hardened (write you own theory about : Martensite, Austenite, Quenching)
- 29. Induction Hardening :- In induction hardening process. Electric current is used to heat the work piece. Increases surface hardness by heating & quenching. Hardened surface depth depends on current frequency. The range is 1000 Hz to 100000 Hz of supply voltage. The surface hardened depth is 0.5 to 6 mm.
- 30. Laser hardening Laser hardening—also referred to as laser case hardening— is a heat treating process used to improve the strength and durability of component surfaces. It employs the use of high- powered diode lasers that apply energy to heat localized areas of the component surface.
- 31. Electron beam An electron beam heats up the outer surface layer of workpieces to austenitization temperature so that a martensitic structure of great hardness form towards the inside of the part to be hardened due to the steep temperature gradient causing intrinsic quenching.
- 32. AGE HARDENING OR PRECIPITATION HARDENING Age hardening is a heat-treatment process used to strengthen metal alloys. A major advantage of precipitation hardening is that it can be used to increase the yield strength of many metallic materials The strength-to-density ratio of an alloy can be improved substantially using age hardening. Nickel-based super alloy. (alloys based on Ni, Cr, Al, Ti' Mo' and C) are precipitation hardened by precipitation of a Ni3Al-like γ' phase that is rich in Al and Ti. Step 1: Solution Treatment. In the solution treatment, the alloy is first heated above the solvus temperature and held until a homogeneous solid solution is produced. Step 2: Quench. After solution treatment, the alloy, which contains only a in its structure, is rapidly cooled, or quenched. The atoms do not have time to diffuse to potential nucleation sites, so the θ does not form. Step 3: Age. Finally, the supersaturated e is heated at a temperature below the solvus temperature. At this aging temperature, atoms diffuse only short distances.
- 34. CRYOGENIC TREATMENT OF ALLOYS Cryogenics is the study of how to get to low temperatures and of how materials behave when they get there. Cryogenic processing is a supplementary process to conventional heat treatment process in steels. Cryogenic temperatures are defined by the Cryogenic Society of America as being temperatures below 120K (-153C). Cryo tempering is a permanent, non-destructive, non- damaging process, which reduces abrasive wear (edge dulling), relieves internal stress, minimizes the susceptibility to micro cracking due to shock forces, lengthens part life, and increases performance
- 35. CRYOGENIC CYCLE RAMP DOWN: Lowering the temperature of the object SOAK: Holding the temperature low RAMP UP: Bringing the temperature back up to room temperature TEMPER RAMP UP: Elevating the temperature to above ambient TEMPER HOLD: Holding the elevated temperature for a specific time
- 36. ADVANTAGE OF CRYOGENIC PROCESS Homogenizes the Crystal Structure. Grain Structure refinement. Improved structural compactness. Reduces Deformation significantly. Retained austenite is converted to a fine martensite matrix. Mechanical Properties like micro-hardness, Tensile Strength etc. are the same across any cross-section Significant. Improvement in dimensional stability. Relieves residual Stresses. Several fold improvement in hot hardness. Significant improvement in material toughness. Produces stronger, denser parts for better performance and longer service life. The abrasion resistance of the metal and the fatigue resistance will be increase.
- 37. CRYOGENIC MEDIAS Liquid oxygen used in rocket propulsion. Liquid nitrogen is used as a coolant. Helium, which is much rarer than oxygen or nitrogen, is also used as a coolant.
- 38. APPLICATIONS Gun barrels: increases the wear life of the barrel and makes cleaning easier and faster. Grinding: allows a better cut, less wheel dressing, a better finish, and less tensile residual induced into the work piece. Engine parts: Engines turn more freely. There is up to a four percent increase in the torque across the rpm range. Aluminum piston alloy structure :more wear resistant surface, higher yield and ultimate strength. Significant abrasive wear improvement. Compact Discs: The effect is a permanent increase in the quality of sound coming from the disk.
- 39. INDUSTRIALAPPLICATIONS Machining: lathes, drill bits, cutting and milling tools. Pulp and Paper: saws, chippers, millers and cutters. Oil and Gas: drilling, compression, pumps, pump jack gears valves and fittings Mining: drill bits, drilling steel, smasher teeth and face cutters. Food Processing: grinders, knives and extruding dies. Textiles: scissors, needles, shears and cutting tools. Wood Fabricating: saws, drill bits, routing bits and planes. Dental and Surgical Instruments.
- 40. TTT diagrams Depending on the temperature transformation. Austenite may transform into Pearlite, Bainite, Martenite The above transformation are indicated by TTT diagrams. These diagrams indicates the phases existing at various times and temperatures. These are useful for heat treatment. These are useful for proper cooling cycles. These are useful for desired transformation products (or micro structures)