MEng 3202 Chapter Four April 11, 2023 (1)

1. ADAMA SCIENCE AND TECHNOLOGY UNIVERSITYSCHOOL OF MECHANICAL, CHEMICAL AND MATERIALS ENGINEERING DEPARTMENT OF MECHANICAL ENGINEERING • Manufacturing Engineering II (MEng3202) • Chapter 4: Property Enhancing and Surface Processing

2. The aim of this chapter is to gain an understanding of the role of heat treatment on the development of microstructure and properties of metallic materials. The course will highlight a number of commercially-significant applications where heat treatment is important.

3. An overview of important heat treatments HEAT TREATMENT BULK SURFACE NORMALIZING ANNEALING HARDENING & TEMPERING THERMO-CHEMICAL THERMAL Full Annealing Carburizing MARTEMPERING Flame Recrystallization Annealing Nitriding Induction AUSTEMPERING Carbo-nitriding LASER Stress Relief Annealing Electron Beam Spheroidization Annealing

4. Definition of heat treatment Heat treatment is an operation or combination of operations involving heating at a specific rate, soaking at a temperature for a period of time and cooling at some specified rate. The aim is to obtain a desired microstructure to achieve certain predetermined properties (physical, mechanical, magnetic or electrical).

5. Can also be used to obtain certain manufacturing objectives like: – To improve machining & formability, – To restore ductility – To recover grain size etc. – Known as Process Heat Treatment

6. The major objectives are: • to increase strength, hardness and wear resistance (bulk hardening, surface hardening) • to increase ductility and softness (tempering, recrystallization annealing) • to increase toughness (tempering, recrystallization annealing) • to obtain fine grain size (recrystallization annealing, full annealing, normalising) • to remove internal stresses induced by differential deformation by cold working, non-uniform cooling from high temperature during casting and welding (stress relief annealing)

7. to improve machineability(full annealing and normalising) • to improve cutting properties of tool steels (hardening and tempering) • to improve surface properties (surface hardening, corrosion resistance-stabilising treatment and high temperature resistance-precipitation hardening, surface treatment) • to improve electrical properties (recrystallization, tempering, age hardening) • to improve magnetic properties (hardening, phase transformation)

8. Generally Heat treatment done for one of the following objective: – Hardening. – Softening. – Property modification.

9. Common Hardening Heat Treatments: • Direct Hardening – Heating Quenching Tempering • Austempering • Martempering • Case Hardening – Case carburizing

10. Hardening Heat Treatment • Case Hardening (Contd..) – Case Nitriding – Case Carbo-nitriding or Cyaniding – Flame hardening – Induction hardening etc • Precipitation Hardening

11. Quenching • Quenched steel (Martensite) • Highly stressed condition • Too brittle for any practical purpose. Quenching is always followed by tempering to – Reduce the brittleness. – Relieve the internal stresses caused by hardening.

12. Tempering • Tempering means subsequent heating – to a specific intermediate temperature – and holding for specific time • Tempering leads to the decomposition of martensite into ferrite-cementite mixture – Strongly affects all properties of steel. • At low tempering temperature (up to 2000C or 2500C), – Hardness changes only to a small extent – True tensile strength increases – Bending strength increases

13. This may be explained by • Separation of carbon atom from the martensite lattice • Corresponding reduction in its stressed state and acicularity Martensite

14. Higher tempering temperature reduces – Hardness – True tensile strength – Yield point –While relative elongation and reduction area increases. •This is due to formation of ferrite and cementite mixture.

15. Some features of Hardening Heat Treatment • retained ferrite detrimental to uniform properties – so heating beyond Ac3 for Hypoeutectoid steel • retained Cementite is beneficial as it is more hard & wear resistant than martensite – so heating beyond Ac1, not ACM, for Hepereutechtoid steel. • addition of C shifts TTT curve to right and increases hardness of martensite • addition of Alloy elements shifts TTT curve to right and changes the shape • higher the Alloy% - Higher the stability of M • higher the degree of super cooling – Higher the amount of retained Austenite.

16. Quenching Media • Quenching media with increased degree of severity of quenching – Normal Cooling – Forced Air or draft cooling – Oil – Polymer –Water and – Brine

17. Quenching medium depends on – Material composition – Weight of job • Aim is to have a cooling rate just bye-passing the nose of TTT curve for – minimum stress – minimum warping/crack during quenching.

18. Figure: Conventional quenching and tempering process

19. Case Hardening • Objective is to harden the surface & subsurface selectively to obtain: – Hard and wear-resistant surface – Tough impact resistant core – The best of both worlds • Case hardening can be done to all types of plain carbon steels and alloy steels

20. Case Carburizing • Heating of low carbon steel in carburizing medium like charcoal • Carbon atoms diffuse in job surface • Typical depth of carburisation; 0.5 to 5mm • Typical Temperature is about 9500C • Quenching to achieve martensite on surface and sub-surface • If needed, tempering to refine grain size and reduce stresses Types of carburizing i. Pack carburizing ii. Gas carburizing iii. Liquid carburizing

21. Case Nitriding • Heating of steel containing Al in nitrogen medium like Nitride salt, Ammonia etc. • Typical temperature is about 5300C • Nitrogen atoms diffuse in job surface • Forms AlN, a very hard & wear resistant compound on surface & sub-surface • Typical use is to harden tubes with small wall thickness like rifle barrel etc.

22. Case Carbo-nitriding • Heating of low carbon steel containing Al in cynide medium like cynide salt followed by Quenching • Typical temperature is about 8500C • Nitrogen & Carbon atoms diffuse in job • Typical case depth 0.07mm to 0.5mm • Forms very hard & wear resistant complex compounds, on surface & sub-surface • If needed, tempering to refine grain size and reduce stresses

23. Induction and Flame Hardening • Employed for medium & high carbon steel or alloy steels • Local heating of the surface only either by flame or induction current • Heating to austenizing range, 30 – 500C above Ac3 (Hypoeutectoid) or Ac1 (Hypereutectoid) • Quenching in suitable quenching media • If needed, tempering to refine grain size and reduce stresses

24. Softening Heat Treatment • Softening Heat Treatment done to: –Reduce strength or hardness –Remove residual stresses – Restore ductility – Improve toughness – Refine grain size • Necessary when a large amount of cold working, such as cold-rolling or wire drawing been performed.

25. Softening Heat Treatment • Incomplete Annealing – Stress Relieving – Process Annealing – Spherodising • Full Annealing • Normalizing

26. Normalizing Vs Annealing • normalizing considerably cheaper than full annealing • no added cost of controlled cooling. • fully annealed parts are uniform in softness (and machinability) • Normalized parts, depending on the part geometry, exhibit non-uniform material properties • Annealing always produces a softer material than normalizing.

27. Hardenability • ability of a metal to respond to hardening treatment • for steel, the treatment is Quenching to form Martensite • two factors which decides hardenability – TTT Diagram specific to the composition – Heat extraction or cooling rate

28. TTT Diagram • for low carbon steel, the nose is quite close to temperature axis • hence very fast cooling rate is required to form Martensite – causes much warp, distortion and stress – Often impossible for thick sections • Carbon and Alloy addition shifts the nose to right and often changes the shape

29. Salt bath II Low-temperature for isothermal treatment Salt bath I Austenitisation heat treatment Dilatometer equipment Sample and fixtures for dilatometric measurements Figure: Equipment’s for Determination of TTT Diagrams

30. Factors affecting cooling rate • Heating Temperature • Quenching bath temperature • Specific heat of quenching medium • Job thickness • Stirring of bath to effect heat convection • Continuous or batch process

31. Hardenability • Hardenability is quantified as the depth up to which full hardness can be achieved • Amount of carbon affects both hardness of martensite and hardenability • Type and amount of alloying elements affect mostly hardenability • The significance of alloying element is in lowering cooling rate for lesser distortion and thick section

32. Property Modification Treatment These heat treatments are aimed either to • achieve a specific property • to get rid of a undesired property Example – Solution heat treatment • Refers to taking all the secondary phases into solution by heating and holding at a specific temperature • Except martensite, all other phases in steel are diffusion product • They appear or disappear in the primary matrix by diffusion controlled process • Diffusion is Time & Temperature dependent

33. Surface Processing Operations Electroplating A method of forming metallic coatings (plating films) on subject metal surfaces submerged in solutions containing ions by utilizing electrical reduction effects. Electroplating is employed in a wide variety of fields from micro components to large products in information equipment, automobiles, and home appliances for ornamental plating, anti-corrosive plating, and functional plating.

34. Electroplating Deposit metal on cathode, sacrifice from anode chrome-plated auto parts copper-plating Metal part on anode: oxide+coloring-dye deposited using electrolytic process Anodizing

35. Electro less Plating A plating method that does not use electricity. The reduction agent that replaces the electricity is contained in the plating solution. With proper re-processing, virtually any material such as paper, fabrics, plastic and metals can be plated, and the distribution of the film thickness is more uniform, but slower than electroplating. This is different from chemical plating by substitution reaction.

36. Chemical Process (Chemical Coating) The process creates thin films of sulfide and oxide films by chemical reactions such as post zinc plating chromate treatment, phosphate film coating (Parkerizing), black oxide treatments on iron and steels, and chromic acid coating on aluminum. It is used for metal coloring, corrosion protection, and priming of surfaces to be painted to improve paint adhesion.

37. Anodic Oxidation Process This is a surface treatment for light metals such as aluminum and titanium, and oxide films are formed by electrolysis of the products made into anodes in electrolytic solutions. Because the coating (anodizing film) is porous, dyeing and coloring are applied to be used as construction materials such as sashes, and vessels. There is low temperature treated hard coating also. Hot Dipping Products are dipped in dissolved tin, lead, zinc, aluminum, and solder to form surface metallic films. It is also called Dobuzuke plating and Tempura plating. Familiar example is zinc plating on steel towers.

38. Vacuum Plating Gasified or ionized metals, oxides, and nitrides in vacuum chambers are vapor deposited with this method. Methods are vacuum vapor deposition, sputtering, ion plating, ion nitriding, and ion implantation. Titanium nitride is of gold color. Painting There are spray painting, electrostatic painting, electrodeposition painting, powder painting methods, and are generally used for surface decorations, anti-rusting and anti-corrosion. Recently, functional painting such as electro-conductive painting, non-adhesive painting, and lubricating painting are in active uses.

39. Electrostatic Spray Painting Painting Spray Painting in BMW plant Silk screening

40. Thermal Spraying Metals and ceramics (oxides, carbides, nitrides) powders are jetted into flames, arcs, plasma streams to be dissolved and be sprayed onto surfaces. Typically used as paint primer bases on larger structural objects, and ceramic thermal spraying for wear prevention.

41. Thermal spraying High velocity oxy-fuel spraying Tungsten Carbide / Cobalt Chromium Coating on roll for Paper Manufacturing Industry Thermal metal powder spray Plasma spray

42. Surface Hardening This is a process of metal surface alteration, such as carburizing, nitriding, and induction hardening of steel. The processes improve anti-wear properties and fatigue strength by altering metal surface properties. Metallic Cementation This is a method of forming surface alloy layers by covering the surfaces of heated metals and metal diffusion at the same time. There is a method of heating the pre-plated products, as well as heating the products in powdered form of metal to be coated.

43. The surface properties of metals are typically changed for: • decoration and/or reflectivity • improved hardness (to maintain cutting edges and resistance to damage and wear) • prevention of corrosion.

44. Industries using surface processing/treatments The surface treatment of metals and plastics does not itself form a distinct vertical industry sector. Surface treatments do not create products; they change the surface properties of previously formed components or products for subsequent use. Printed circuit boards might be considered products but are components manufactured for use in other products, and are made by a considerable number of interdependent manufacturing stages. The surface treatment of metals and plastics is therefore largely a service to many industries and examples of key customers are given below:

45. • automotive • food and drink containers • aerospace • printing • information systems • domestic appliances • telecommunications • jewellery, spectacles and ornaments • heavy engineering • furniture • construction (building)

46. • clothing • bathroom fittings • coinage • hardware • medical. • After treatment activities • 1. Drying using hot water • After all wet processing operations have been completed, the work pieces or substrates need to be quickly and effectively dried in order to avoid staining and corrosion. The simplest method of drying is by immersing the components in hot water for a few seconds and then allowing them to dry-off in the air.

47. 2. Drying using hot air • Drying in automated jig plants is most easily accomplished on automatic lines using hot air. The jigs are placed in a tank-shaped drier at the end of the process line; the tank has the same dimensions as the vats in the line to fit into the transporter system. Hot air is evenly recirculated from the top to the bottom of the tank at temperatures of 60 – 80 °C. Hot air escaping from the top of the drier tank makes the equipment thermally inefficient. In some cases, such as the new thick film passivation’s or to reduce drying times, it is necessary to heat the substrate or work pieces to 80 °C and higher. The temperature of the air circulating in the tank-shaped driers then needs to be above 100 ºC. The air is normally heated by circulation or heat-exchangers using steam or hot oil. Direct heating systems are an alternative, using a special gas burner with an open gas flame in the circulating air. The burning gas heats the air directly with an efficiency of nearly 100 % of the energy input.

48. 3. Drying using air knives • There is a growing use of localized air drying by means of precision nozzles or ‘air knives’ that is more energy efficient than hot air tank drying.