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Materials and Mechanics Course Notes (Part 2)
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Part 2 of the Materials and Mechanics Course Notes.
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Week 2-1 Materials and Phase Diagrams15 January 2023 15:40
Phase change enthalpy -
Enthalpy of solidification - when heat is added but temperature doesn’t rise because a state change is occurring (Latent
heat capacity)
Nucleation - where state changes start - often an impurity
Supercooling - lowering a liquid below freezing point while keeping it as a liquid. (videos of water being shook and it
instantly freezes)
Materials and Mechanics Page 1
,Week 2-2 Phase Properties
17 January 2023 10:09
Hardening
Tempering
Annealing
Diffusion Processes
Surface Treatments
Hypo Eutectic Steels:
Observe Pearlite larger grains and pearlite in grain boundaries around the white iron grains.
If Eutectic steel is all pearlite with 0.83% Carbon, estimate the carbon content here?
Est 10-15% pearlite. Therefore about 0.1% carbon
15*0.0083 = 0.1245 - around 0.1% carbon
Eutectic point - if above 0.83% carbon hyper, if below hypo
Quenching (Crash Cooling)
High rates of cooling from Austenite will not allow time for transitions: Carbon will not have time to form carbides and
pearlite lamellae and alter the phase transformation from FCC - BCC to FCC - tetragonal crystal
FCC - (face centred crystal) BCC - (body centred crystal)
Hardening and Tempering:
High rates of cooling create Martensitic transformation (martensite) which is hard and brittle. The hardness is useful
but the transformation from FCC to tetragonal can create internal stresses on its cleavage planes making it brittle.
If quenching is incomplete, often due to geometry, an intermediate phase forms called Bainite
Controlled heating can release these stresses, this is called tempering.
Quick cooling - martensite
Medium cooling - bainite
Slow cooling - pearlite
Diffusion Processes - Case Hardening:
Sometimes only local hardening is required to create wear resistance surfaces with a tough core. In a gear we want the
surface to be hard but the core to be soft and tough.
Carburizing:
By heating components in a carbon rich environment a high carbon surface can be created by diffusion. This has 2
effects.
- Creates carbides increasing hardness
- Increases surface density
Surface treatments:
Shot/ball peening
Laser peening
These processes impart compressive stresses into material surfaces which have the effect of increasing crack resisting
by increasing the tolerance to subsequently applied tensile forces.
Materials and Mechanics Page 2
, Week 2-3 Failure Mechanisms
18 January 2023 12:04
Failure can occur due to:
- Excess load
- Fatigue
- Creep
- Impact
Forensic engineers:
- Air crash investigators
- Marine accident investigation board
- Forensic road crash investigation
Sound:
When excitation frequency = resonant frequencies = bad
Wear:
Adjacent moving parts could cause wear due to friction (stiction)
Lubricants may become contaminated with metal particles from wear
Carbon from combustion (even overheating oil)
Water
Acidity/Alkalinity
Ultra Violet Degradation:
UV photon energy = bandgap energy in some carbon bonds commonly found in polymers
This can elevate some electrons into a higher shell where they may combine with atmospheric oxygen - oxidation occurs,
similar to rust in metals
Inhibitors may be used to reduce effect
Advantage may be taken of this in bio-degradation
Thermal Issues:
High Thermal conduction could cause:
- Heat loss
- Fire risk
- Risk of personal injury
- Structural weakening
(used to advantage in heat exchangers)
Thermal expansion:
Dimensional changes, if restrained could cause damage or leakage
(used to advantage in interference fitting)
Solutions:
- Thermal insulation (encasing steel beams in concrete)
- Expansion joints to control thermal expansion
Welding:
The process of joining together 2 similar materials. Heated to a shared melting point, then extra material added allowing
for bonding.
Welding can cause decarbonisation, the heat causes carbon to burn and form carbon dioxide. Can remove oxygen from
the welding environment to prevent this.
MIG welding - METAL INERT GAS
TIG welding - TUNGSTON INERT GAS
Materials and Mechanics Page 3
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