Flashcards
Week 7
Flashcards do conjunto (33)
- 1Carburizing & benefitsHardening process
Heats a metal in an environment with high carbon content (e.g. carbon monoxide) then quenched.
Ductility
Toughness
Hardness
Strength - 2Carburizing Pros and ConsPro:
Clean and efficient
Case depth can be controlled more accurately
Cons:
Can be expensive, furnace and gas generators are expensive.
Handling of fire hazards and toxic gases is difficult. - 3Laser hardening of steelSteel is harden by heating up the sample to near its melting temperature (where Austenite can form), breaking down the materials grain structure then quickly quenching it to lock in a tight grain structure which is harder but also brittler (Austenite to Martensitic).
- 4Types of quenchingSelf-quenching (Large object)
Convection (Thin object)
Oil or water - 5Physical vapour depositiondeposit layers of atoms or molecules from the vapor phase onto a solid substrate in a vacuum chamber.
- 6Physical vapour deposition Pros and consCan coat large complex shapes
Con
Won't fill surface imperfections - 7Sputtering processesa
- 8Chemical vapour depositiona
- 9Welded coatingsa
- 10Spray coatingsa
- 11Shot and bead peeningBy placing the surface of a material under compressive stress the component will be able to withstand a greater amount of cyclical tensile stress before suffering fatigue failure.
Glass is toughened by heating the object and rapidly cooling the outer surface. This produces a compressive stress in the ‘skin’ that increases the toughness of the glass.
Metals can be ‘shot blasted’ to produce a compressive surface with similar effects. - 12AnnealingAnnealing process occurs in 3 stages
1. Relief of Stress
gradual increase in temperature causes atoms to move into positions nearer equilibrium. There is no alteration in the distorted appearance of the structure and little effect on strength and hardness.
2. Recrystallisation
At higher temperatures recrystallisation takes place, especially where internal stress is greatest – slip planes and grain boundaries.
Crystals that are distorted due to cold working tend to form new crystal nuclei within the structure of the existing crystals as the temperature reaches the recrystallisation temperature. These will grow and absorb existing material to form small equi-axial crystals which replace the distorted crystals.
3. Grain growth
At much higher temperatures, crystals will cannibalise each other producing a coarse (larger) grain structure. This is undesirable for further deep drawing and is less ductile than fine grained material. - 13Annealing (picture)
- 14Hot working processThis is where work is carried out a temperature where recrystallisation keeps pace with deformation.
At too high temperature , grain growth occurs giving an article which is not very tough. At too low a temperature some cold work may occur. Ideally the finishing temperature should be just above the recrystallisation temperature.
Mild steel 900 – 1300 C
High Carbon steel 750 -1000 C - 15Cold working process
- 16Cold workingAn increase in stress is then necessary to initiate new dislocations and move these until they too become blocked.
A point is reached when no more mobile dislocations are available.
No more deformations are possible and any increase in stress will cause fracture – the material is then said to be work hardened.
Further cold work can only be attempted after the internal stress in the material is removed by a process such as annealing.
During the cold working process some 90% of the mechanical energy is converted to heat whilst the remaining 10% is retained as potential (strain) energy in the form of elastically balanced strains in the lattice structure.
The introduction of heat energy to the system during an annealing process permits sufficient mobility of the atoms so that dissipation of strain energy can take place.
This initial stress relief is followed by recrystallisation. This is the stage where new crystals begin to grow from the nuclei which form within the structure of the existing distorted crystals. - 17Cold Working Pros and ConsAdvantages
Work hardens most metals (mild steels, non ferrous) – increase strength, hardness and decrease in ductility.
Strength of cold material allows for drawing down (into thin wires).
Produces clean, smooth surface finish
Good dimensional accuracy possible
Harder and brittle surface produced improves machineability.
Disadvantages
Work hardening – frequent process annealing is required e.g. wire drawing, cartridge cases. - 18ElasticityAll substances when subjected to mechanical stress suffer some change which returns to the original shape when the load is removed.
When the force is applied, the particles of which the material is composed are moved through small distances from their position of equilibrium and when the displacing force is removed, they return to their original positions. - 19PlasticityIf sufficient stress is applied to a material permanent deformation takes place in individual crystals. Deformation can take place by means of slip planes, closing voids in metallic crystal, or by dislocation of the crystal structure.
- 20DislocationsInstead of slip taking place simultaneously by one block of atoms sliding wholesale over another block across the complete crystal plane, it occurs step by step by movement of faults of discontinuities within crystallographic planes. These faults, where in fact half planes of atoms are missing within a crystal, are known as dislocations.
- 21Dislocations (picture)
- 22Hot working Pros and ConsAdvantages
Recrystallisation – uniformly fine grain
Energy saving – malleability
Time saving
Product is stronger, tougher, more ductile than original cast ingot
Disadvantages
Poor surface condition (scale)
Require further cleaning (pickling) before further work
Machining to obtain accuracy
Further cold working – combination of hot rolling and final cold rolling from ingot produces metal used in car bodies. - 23StrengthThe strength depends on impeded dislocation motion within the crystal structure of the metal. This can be;
Additional (foreign) atoms
Dislocations within the matrix
Precipitates
Crystal size
Existing disloctions - 24Yield strengthYield strength measures the resistance of the material to movement of a dislocation.
We will be looking at processes that make changes to the material structure and increase the yield strength. - 25Solution strengtheningThe dissolving of ‘foreign’ atoms in a parent metal creates alloys and provides a useful mechanism for controlling material strength.
A larger atom creates compressive stress and a smaller atom creates tensile stress in a region. Both inhibit the movement of the dislocation through the material and increase the yield point.
Different atoms may have different bonding strengths to the main material and this can also inhibit the movement of the dislocation. - 26Precipitation StrengtheningPrecipitates are small particles of a different composition and structure to the parent metal. They are a more powerful obstacle to the propagation of the dislocation than atoms dissolved in an alloy.
Precipitates have the following properties;
Larger atoms than the host material.
Crystals are a different structure to the host material.
These generate high stress levels in the matrix.
Ex.
A 4.4% copper precipitate in aluminium can generate a yield stress 10 times higher than the pure aluminium. - 27Grain structureGrain boundaries are areas of severe lattice disorder and provide strong obstacles to dislocations.
Materials with a finer grain structure will give a higher yield strength due to the number of boundaries that exist.
The Hall-Petch relation describes the relationship between yield strength or hardness and grain size of the conventional polycrystalline metal materials.
We will not be studying this theory. This has led to the study of nano-structured materials with grains as small as 50nm. - 28Amorphous metalsWhen a metal is rapidly cooled from a high enough temperature the grain structure does not get time to form properly. This generally produces a very fine grain system that is not ordered. Martensite in steel is such a material.
The fine structure and high level of disorder makes the passage of a dislocation virtually impossible and yield stress is very high. This is balance by the material normally being very brittle. - 29Strain HardeningStrain hardening, work hardening and cold working are all terms that mean the same.
The material is deformed at room temperature and the grain structure is fractured by the processes. This produces an even finer structure that makes further working difficult.
This degree of deformation is generally accomplished by rolling or pressing the material into shape. - 30Surface Compressive stressGlass is toughened by heating the object and rapidly cooling the outer surface. This produces a compressive stress in the ‘skin’ that increases the toughness of the glass.
Metals can be ‘shot blasted’ to produce a compressive surface with similar effects. - 31Fine Grain StructureA reduction in grain size can result in a metal that is both high in yield strength and also ductile.
The fine grain structure has many grain boundaries that inhibit the development of cracks by changing the direction of the crack at each boundary. - 32Surface FinishA rough surface will have many possible crack generators as stress accumulates in the surface. Polishing the surface to reduce roughness will decrease the generation of small cracks and increase the toughness of the material.
- 33Surface hardeningProcesses such as carburising increase the outside surface strength of components while keeping a more ductile core. This can help improve fatigue properties.
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- University Materials Science FlashCards sobre Week 7, criado por Hugo James em 08-10-2015.
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- Hugo James
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