1.
Heat treatment – solution anneal
Typical Parameters: 1050 OC + water quench
Purpose – soften, ensure removal of detrimental chromium carbide. No phase transformation.
2.
Pitting Corrosion:
caused by aggressive agent attacking weak spots in metal structure,
chloride is everywhere,
ordinary drinking water contains typically 50-100 parts per million
the chloride ion penetrates the passive film and sets up a differential between the inside of the pit and the outside of the pit where the passive film is still doing its job of protecting the surface.
Pitting corrosion resistance:
pitting resistance equivalent number (PREN) offers a simple method of comparing different grades PREN = Cr + 3.3(Mo + 0.5W) +16N
PREN is only a guideline and material selection is usually more complicated. The Critical Pitting Temperature is a more accurate assessment of pitting resistance.
3.
Austenitic stainless steels are non-magnetic, but as a result of hardening may show a small degree of magnetic properties (Steel begins to attract the magnet). Cold working greatly increases the mechanical properties of the material and is often done precisely for this reason.
4.
1D – annealing and pickling – to produce a saleable product, black hot band has to be annealed and descaled by mechanical and chemical means. At this stage the product is called white hot band if it is to be cold rolled or CPP (continuously produced plate) if it is to be sold. At this stage the passive film is formed.
2D - final annealing and pickling – after cold rolling the product is very hard and needs to annealed again to allow it to be sold. This time the oxide formed during annealing is relatively light and therefore can be removed by chemical means alone. The passive film is formed at this stage.
2B - this is simply 2D plus a final pinch pass operation. This improves the shape of the coil and slightly brightens the surface.
2F - is a special matt finish achieved by using roughened rolls. Other proprietary finishes can be applied by applying patterns to the roll surface.
2R - bright annealing – inert atmosphere N plus H preserves bright surface. This is the smoothest surface of all cold rolled products.
5.
dual structure of austenite and ferrite
nominally 50% of each but can range between 35% and 65%
achieved by balancing ferrite forming and austenitic forming elements
different levels of influence for each element
some properties are an average of values between austenite and ferrite but most importantly strength is higher than either austenitic or ferritic
fine grain size
difference in thermal contraction from heat treatment gives strain hardening
higher strength than austenitic or ferritic
ferrite too high - toughness reduced, loss of corrosion resistance; austenite too high - loss of strength, increase risk of stress corrosion cracking
combination of strength and corrosion resistance
used in: civil engineering (bridges, any structural use), process plant - big reduction in section size and weight due to increased strength for welded storage vessels, aggressive environments - oil and gas, desalination