Class test 3 materials Flashcards
How should a parent metal be prepared when welding cast iron materials.
*A sound weld can only be produced if the parent metal is clean and sound.
*Therefore, when a known crack or defect is repaired, it should be removed completely before welding.
* Contaminants such as scale, coatings, sand, oil, grease, moisture and any foreign materials will have to be removed prior to welding.
Why should preheat be applied when welding cast materials, list four reasons.
◊ Prevents cracking caused by thermal gradiants and stresses.
◊ Reduces the magnitude or residual stress in the casting
◊ Reduces distortion
◊ Lowers HAZ hardness
◊ Minimizes hydrocarbon contaminants
What effect does peening have when welding cast iron.
Peening each successive weld bead immediately after welding while red hot has the effect or reducing shrinkage stresses associated with welding.
Nickel based electrodes are sometimes used for welding cast materials, list four reasons.
*Nickel does not have a tendency to form carbides
*and also has a low solubility for carbon. *Therefore as the weld cools, solubility of carbon in nickel decreases and the carbon is rejected from the liquid metal as graphite.
* This results in an increase in weld metal volume thus reducing shrinkage stresses and the possibility of cracking.
List three items, which can describe corrosion as a deterioration of a substance.
◊ Destruction of metal by chemical or electromechanical reaction with its surrounding environment.
◊ Eating away of metals
◊ Destruction of metals by other than straight mechanical means, such as wear, abrasion etc.
◊ Reversion of a material to its natural state
If copper is joined to steel in the presence of an electrolyte which material will corrode at the faster rate, give a brief explanation.
the steel will corride the faster rate.
because copper is a noble metal & steel is more a reactive metal
so the steel will act as an anode & corrode quickly
this is also known as galvanic corrosion
Why is aluminum regarded as a corrosion resistant material.
Aluminum forms a thin surface film (AL₂O₃) on exposure to the atmosphere,
*which tends to insulate the aluminum metal or alloy from the environment
Under what conditions will corrosion occur, list five.?
◊ Air, particularly if its contains moisture
◊ Water-fresh, distilled or salt
◊ Rural, urban and industrial atmospheres
◊ Steam and other gases such as chlorine, ammonia, hydrogen sulphide, sulphur dioxide and fuel gases.
◊ Mineral/inorganic (strong) acids such as hydrochloric, sulphuric and nitric.
Fully explain the electrochemical theory
*The metal at the anode dissolves and becomes an ion.
*It is oxidized and loses electrons. For example, iron dissolves and loses two electrons (negatively charged particles) to become the ferrous iron FE⁺⁺.
*The iron atom detaches itself from the anode surface and goes into solution as a ferrous ion.
*The electrons (e⁻) are left in the metal and travel through it to the cathode area.
* These electrons are then accepted at the cathode area by any other positively charged ions that may be present.
Corrosion can occur on the same piece of material, list three items that can influence this ?
◊ Impurities such as oxides and other inclusions
◊ Grain boundaries
◊ Localised stressed
◊ Scratched and nicks or other rough surfaces
Explain why the grain boundaries in a metal tend to corrode faster than the grains?
As the crystals grow, any impurities present tend to stay in the liquid, as they often have the lowest melting points, so the last metal to solidify at the grain boundaries is the least pure
What effect does chromium have on stainless steels?
*Chromium provides some of the special properties by forming a thin, tight, impervious layer of chromium oxide on the surface of the metal,
*which protects the steel from the environment.
Why does stainless steel distort more than carbon steel.
Stainless steel has a lower thermal conductivity and a greater coefficient of expansion.
