Class Test 1 -material Flashcards
Why are single pass welds inferior to multi pass welds with regards to mechanical properties? [4)
They produce columnar grain structures with inferior properties, a multi pass weld will heat treat the weld below it with a subsequent increase in mechanical properties because the weld has an equiaxed grain structure
List four conditions that are necessary to promote hydrogen cracking. [4]
1.Sufficient hydrogen
2.Susceptable microstructure
3.Sufficient stress
4.Temperature in the range - 100 to 200°c
List four sources of hydrogen that may have a detrimental effect on a weldment. [4]
1.Moisture in the flux coating - (smaw)
2.Moisture in saw flux
3.Moisture in the atmosphere
4.Wet materials
What effect does a high carbon equivalent have in a welding environment?
[4]
The higher the ce,
the higher the hardenability,
the greater the probability for a full martensitic structure,
which can have a greater susceptability to hic.
Why is preheat employed to avoid hydrogen cracking and give the formula for a quick estimate for the preheat temperature. [4]
To slow down the cooling rate to avoid a martensitic structure.
Formula = 200° x ce
If a material has a Carbon Equivalent (< 0.4), will preheat be normally required. [4]
Preheat is not normally required if lh electrodes are used, maybe required when welding thick sections.
What do the letters HSLA represent? Also give a typical composition of this material. [6]
High strength low allow steel.
Typical hsla steel may contain 0.15% carbon, 1.65 manganese and low levels (under 0.35%) of phosphorus and sulphur. It may also contain small amounts of copper, nickel, niobium, nitrogen, vanadium, chromium, molybdenum, silicon or zirconium
8) Name five basic strengthening mechanisms for Fine Grained Steels ?
- Precipitation hardening
- Cold Strain (work)
- Solid solution hardening
- Grain Refainment
- Quench and temper
Give a description of precipitation Hardening?
*precipitation hardening - small microscopic particles precipitate out of solid solution with a decrease in temperature.
*These microscopic particles inhibit the movement of dislocations in a lattice, *hence dislocation pile up,
*increasing the strength of a material.
Give a description of cold strain(work) hardening?
cold strain (work) -
*work hardening occurs when certain materials are plastically deformed below the re - crystallisation temperature.
*This causes dislocation pile up within the crystal lattice at grain boundaries therefore increasing lattice strain
*and hence material strength
Give a description of solid solution hardening ?
solid solution hardening
*this strengthening mechanism works as a result of the substitution of atoms into the basic lattice of the material being alloyed.
*Different atomic volumes generate a permanent elastic strain in the lattice, *hence inhibits dislocation movement
*and increases strength.
* Used it automotive and aerospace industries
Give a description of quench and temper?
quench and temper -
*a quench and temper treatment involves strengthening by a microstructure phase change.
*This is the formation of martensite by fast cooling from the normalising temperature.
*The increased lattice strain increases material strength.
*Used in Construction of building Bridges and buildings
Give a description of grain refinement?
grain refinement -
*the principle of this strengthening mechanism is that the greater the number of grain boundaries, the greater the strength of the material.
*This is brought about by a greater disordered distribution of crystallographic
orientations,
*thus increasing the complexity of the directions and number of crystallographic planes.
* the smaller the grain boundaries results in Yield strength
*increases mechanical properties
