Handout 8: Surface Engineering Flashcards
1
Q
Draw a simple tree diagram that categorisez the different types of surface treatments (3 levels)
A
2
Q
Describe the process of the phase change (composition unchanged) heat treatments on steels to improve surface characteristics.
A
- The surface is subjected to rapid heating and quenching. Austenite is formed to a limited depth on heating, and is then cooled sufficiently fast to form martensite (by conduction into the underlying colder material, or by a water quench).
- ‘Local’ heat treatment can be done by ‘laser hardening’ or ‘flame hardening’.
- ‘Global’ (heat whole surface then quench whole part) can be done via induction hardening.
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Advantages:
- No change in dimensions or roughness
- Easy to get hard to reach places.
3
Q
Describe the process of the phase change (composition changed) heat treatments on steels to improve surface characteristics.
A
- Carburising of steels - Carbon concentration is locally increased near the surface, followed by a direct quench (to give high carbon martensite).
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Nitriding of steels - Process differs from carburising in two main ways:
- conducted at a much lower temperature (steel is still ferritic) takes 2-4 days.
- only suitable for alloy steels containing strong nitride formers (Al, Cr, Mo, Ti, V); these alloy nitrides are very hard.
- Plasma nitriding is quicker, but more expensive.
4
Q
Explain the chemical process phosphating.
A
A traditional surfacing process used to enhance lubrication of gears, piston rings. Components are treated in a hot, dilute solution of phosphoric acid, forming a surface layer of iron phosphate. Manganese sulphate is also deposited on steels for similar applications.
5
Q
Describe the chemical process: Electroplating and Galvanizing
A
- Both processes widely used for decorative purposes, or for wear and corrosion resistance, e.g. Sn-plated cans, Zn coatings on steel.
- deposited by electrolysis (taking care not to introduce hydrogen into the steel which can lead to embrittlement).
- Electroless plating involves chemical (rather than electrochemical) reduction of the metal ions.
- Hot-dip galvanizing involves simple immersion of the object into a bath of molten zinc to deposit the zinc coating.
6
Q
Describe the different fusion processes: for thick coatings (0.1-10mm).
A
- Hardfacing - Melt deposit wear resistant alloys on steel surface, usually for wear resistance. Hardfacing materials include Ni alloys, high alloy steels, tungsten carbide-cobalt (‘cermets’).
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Thermal Spraying - Molten droplets of coating material are produced in the nozzle of a torch and sprayed on to the surface, where they flatten and rapidly solidify.
- In flame-spraying, material is fed into an oxy-acetylene torch; in plasmaspraying, powder is fed into and melted by a plasma jet (an ionised inert gas, accelerated by a high voltage electric field).
7
Q
Describe the different vapour processes for thin coatings (1 - 10um)
A
- Chemical vapour deposition(CVD) - CVD involves chemical reactions induced on the component surface at high temperature (e.g. 1000 ºC), with the reagents supplied in gaseous form: e.g. TiCl4 + CH4 = TiC + 4HCl can be used to deposit hard TiC on steel.
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Physical vapour deposition(PVD)
- Ion Plating - the vapour is ionised and accelerated by an electric field.
- Sputtering - argon ions are accelerated by the electric field on to a Ti target and Ti ions are ejected and directed onto the component surface.
- By introducing a reactive gas, hard compounds can be formed. These are low temperature and pressure processes.
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Example:
- Ti ion plating in an atmosphere of N2, which gives a coating of hard titanium nitride TiN. Operates at only 200-300 ºC, so wont over-temper metals.
8
Q
What are the advantages of laser transformation hardening?
A
- flexibility – can be manipulated to a wide range of power densities, and accurately directed, using conventional optics. One machine used for surface hardening and melting/cladding (as well as welding and cutting)
- industrial lasers can be distributed by optical fibres, and thus incorporated into robotic machine tools
- high power densities give fast processing speeds; however the heat input is low, giving low levels of distortion and thermal damage to the surrounding material
- for hardening applications, lasers can give very localised treatments, in places which are difficult to access by other means; the intense heating rate also means that the material is self-quenching
9
Q
What are disadvantages of lasers?
A
Disadvantages of lasers:
(1) Expensive machines
(2) Heavy dependence on reliability of a single machine
(3) For surface engineering, not suitable for complete surfaces
10
Q
Draw a tree diagram to show the categorization of different joining processes.
A
11
Q
Name polymers that can be welded and polymers that can’t be.
A
dielectric polymer welding:
yes - PVC, PU
no - PE, PP, PTFE
12
Q
What are simple technical design requirements that need to be thought about when considering different types of joining?
A
Does the joint need:
- to be easy to disassemble?
- to conduct heat or electricity?
- to be water-tight?
- Is joining part of an assembly line, or is fabrication on site?
- Quality: joint properties (e.g. toughness of heat-affected zone) residual stress and thermal distortion
- Economics: joining speed
13
Q
Describe adhesive bonding, how it works, why its useful, advantages, disadvantages.
A
- Adhesives are polymeric: e.g. epoxies, phenolics, acrylics, polyurethanes.
- They are mostly thermosets, made by mixing two components (resin and crosslinking initiator) before curing (some at room temperature, others at 100-200 ºC).
- Disadvantages: Adhesives are often susceptible to take-up of moisture, which can affect joint integrity and lead to degradation of properties in service.
- Joint designs are usually some form of lap joint, because adhesives are much stronger in shear than in tension (which would produce hydrostatic tension leading to void growth and failure).
- As the two materials are isolated from one another, adhesives are well-suited to joining dissimilar materials.
14
Q
Explain brazing and soldering what are their differences and their applications.
A
- A low melting point filler alloy is used to join metallic parts. The joint is designed with a small clearance so that the molten solder or brazing alloy is drawn into the gap by capillary forces (i.e. surface tension).
- So solders melt below 450 ºC, and brazing alloys melt above 450 ºC. Temperatures are low in both cases, minimising thermal damage and distortion.
- Soldering is used very widely in electronics.
- Brazing is used for mechanically loaded joints, using copper-zinc or coppersilver alloys.
- Brazing can join a wide range of metals (steels, stainless steels, aluminium alloys, copper/brass pipework), and ceramics
- Disadvantage: that the strength of the joint is limited by the strength of the filler alloy, which is often much more expensive than the metals being joined.(for both solders and brazing)
15
Q
Explain cold welding of metals(T < 0.5Tm)
A
- Uses low temperature plastic deformation to provide good metal-metal contact and break up oxide films.
- Example: Roll cladding: to clad aerospace Al alloys with pure Al for corrosion resistance.
17
Q
Explain hot plate welding in polymers.
A
23
Q
Briefly describe the common variants of arc welding.
A
Common variants of arc welding processes:
- Manual metal arc (MMA): flux-covered consumable electrode, no shielding gas. The flux forms a protective layer of slag, later removed.
- Metal-inert-gas (MIG): consumable electrode (rod or wire) to provide the filler, and shielded by an inert gas.
- Tungsten-inert-gas (TIG): non-consumable tungsten electrode, inert gas shielding, and a separate filler rod.
24
Q
Explain Power Beam welding.(Laser and Electron)
Explain their advantages.
A
- Laser and electron beam welding use a beam of high power density as the heat source. The beam vapourises a small surface spot, giving high energy coupling with the beam – a deep narrow vapour cavity forms called a ‘keyhole’.
- Thick steel plates can be welded without filler in a single pass, giving narrow melt and heat-affected zones.
- Electron beam welding is especially good for very thick plate but requires vacuum or partial vacuum
- Laser welding only requires a shrouding gas.
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Advantages: A significant advantage of laser welding over other processes is the ease with which the energy can be delivered and concentrated exactly where it is needed, and switched on and off. The total heat input is low, giving low distortion and excellent reproducibility and quality control.
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26
Q
Explain briefly what the HAZ is and list the microstructural changes that can occur in the HAZ.
A
- The heat-affected zone (HAZ) which surrounds the weld metal (but does not melt) plays an important role in determining the properties and performance of a fusion weld.
- Several microstructural changes can occur in the HAZ during the heating and cooling cycle:
- recovery, recrystallization and grain growth
- precipitate coarsening/dissolution
- phase transformations
27
Q
What factors does the HAZ microstructure depend on in welding metals?
A
The HAZ microstructure depends in complex ways on:
- the alloy composition
- initial microstructure
- the welding process variables
- joint geometry (power, speed, thickness – which determine the temperature distribution, and the heating/cooling rates)
28
Q
Explain the significance of ‘Δt8-5’.
A
- In steel welding, the cooling rate at a given point in the HAZ is commonly described by the time taken to cool between 800 oC and 500 oC: . This temperature interval is where the most important phase transformations take place: if the austenite has not transformed to ferrite, pearlite or bainite in this interval, then martensite is likely to form, leading to brittleness.
- A high cooling rate (dT/dt) is equivalent to a small Δt8-5, hence:
- small Δt8-5: fine structure, more likely to form martensite.
- large Δt8-5: coarse structure, no martensite, not brittle.
29
Q
Explain the issue of austenite grain growth in HAZ of steels and how its avoided.
A
- An important side-effect to be avoided in steel welding is grain growth in the austenite – diffusional migration of the grain boundaries during the welding cycle, driven by the grain boundary energy and reduction in boundary area per unit volume. This may increase the grain size by a factor of order 10.
- Recall that an increase in grain size increases the hardenability, making the weld more susceptible to embrittlement by martensite (cf. steels lecture). This may be prevented by ‘microalloying’ with strong carbide forming elements (Ti, V) – these carbides are stable at high temperature and ‘pin’ the austenite grain boundaries.
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32
Q
Explain the effect of the HAZ on heat-treatable aluminium alloys (age hardened).
A
- The weld initially softens, but it is possible to recover much of the strength after welding by natural ageing. Consider a weld in a peak aged alloy (T6 temper)
- The ageing curve shows two possible softening routes: (i) coarsening the precipitates and overaging the alloy (path A); (ii) dissolving the precipitates, back to supersat. solid solution (path B).
- Welds cant be artificially aged back because assembleis to large for the furnace and it will soften surrounding metal.
- But they will naturally age at room temperature, to a substantial fraction of the original unwelded strength. Suitable fillers are used such that the weld metal also naturally ages after welding.
