Annealing is a process of heating the steel to a temperature slightly above its recrystallisation temperature and allowing it to cool at an appropriate rate - generally slowly - causing the crystals to reform without the defects caused by "working" the steel.
Annealing can restore the ductility of the steel and its corrosion resistance characteristics.
When steel contains higher levels of carbon there is a tendency for it to combine with the chromium as it cools - between 900oC and 500oC forming chromium carbide. This reduces the amount of chromium available to form the passive layer and creates intergranular boundaries that are accessible to corrosive chemicals.
This can be overcome by using Low carbon variants of the steel (designated by "L", eg 304L or 316L).
However, the lower carbon levels reduce the steel's performance at elevated temperature. If resistance to carbide precipitation and strength at high temperature are required then the addition of Titanium may be the solution. There are a number of grades available in this form - eg 316Ti.
Steels perform very differently at elevated temperatures than they do at ambient temperatures. When they are bent at ambient temperatures to below their yield point, they will spring back. At elevated temperatures, they begin to stretch, but very slowly. Some steels have better resistance to this phenomenon than others.
Steel is made up of a lattice of crystals of iron interwoven with atoms of other materials. These crystals are called grains.
The grain size is important because it affects the machining, hardness, strength, and corrosion resistance amongst other things.
Grain size can be determined both by the addition of other alloying elements and by the careful regulation of the heating and cooling processes involved in the production of the steel and by further heat treatment (annealing and quenching) following initial production, welding or "working" on the steel.
The atoms in metals are arranged into crystals (or grains) which are aligned closely with each other. In certain conditions, corrosion can attack the grain boundaries rather than the crystals themselves.
When stainless steel containing a higher percentage of carbon is heated it the chromium can react with the carbon to form chromium carbide thereby depleting the passive layer of chromium that protects the surface.
The passive layer is what makes stainless steel "stainless". It is a microscopically thin layer of chromium oxide that is impenetrable to oxygen, very hard, resists corrosion itself and is virtually transparent. This prevents oxygen and other corrosive materials reaching the iron and reacting with it.
Chromium reacts readily with oxygen with the result that should it become scratched it will repair itself providing there is free oxygen available.
This is a very localised form of corrosion that arises particularly in high chloride conditions such as the marine environment. An initial breach in the passive layer of the steel is not "repaired" by the reformation of chromium oxide. The steel beneath this breach continues to corrode often leaving no obvious signs other than a light surface staining (sometimes called "tea staining) on the surface but continuing to deepen and widen below the surface.
While localised it can result in the penetration of the entire cross section of the steel.
High contents of chromium, molybdenum and nitrogen increase the resistance to pitting corrosion. The degree of resistance can be calculated as %Chromium + 3.5 x %Molybdenum + 16 x %Nitrogen to give a Pitting Resistance Equivalent Number (PREN).
316 has a PERN of between 22.6 – 27.9. Some Duplex steels have PRENs over 40. The span of the numbers given in certain grades is the result of the specification for the quantities of the relevant chemical having max and min figures.
Also known as age hardening, is a process used to increase tensile strength. The alloy is first raised to a temperature that a produces a single phase with all the solute atoms dissolved and evenly distributed. It is then rapidly quenched before reheating to a lower temperature and holding it at that temperature for a predetermined time. At this temperature, the precipitates can clump together in a uniform and distributed manner. It is essential that the right temperature and duration at this stage of the process is correct. If the temperature is held too long it will result in oversize clumps and reduce the strength of the alloy. This is known as "over aging".
Sensitisation is the process of Carbide Precipitation - See above.
Is phase change that occurs in some stainless steels when they are heated above about 540C. This results in a dramatic loss of toughness and can lead to brittle fractures.
Stabilisation is the process of removing or protecting the steel from sensitisation - the danger of carbide precipitation which can lead to stress corrosion cracking (SCC).
There are two approaches commonly used. Low carbon variants can be used; they are inherently more stable but they perform less well at higher temperatures.
Alternatively, steel can be chemically stabilised by alloying it with titanium, niobium (sometimes still called columbium). Both of these readily form carbides thereby preserving the chromium.
However, this may not be sufficient to stabilise the steel should it be held within the carbide forming temperature band 425oC to 850oC. Should this occur during the fabrication process the problem can generally be reversed by annealing at a higher temperature.
Stress corrosion cracking occurs when chemicals attack the intergranular boundaries within an alloy. When the metal is subject to tensile stresses it can result in the sudden failure in generally ductile materials. Because the corrosion only occurs at the grain boundaries it is likely to go unnoticed since the metal will generally maintain an apparently normal surface appearance.
Work hardening is a term applied to any work done on the steel at a temperature below the metal's recrystallisation temperature.
This work includes any type of squeezing, bending, cutting/shearing or drawing.
These processes cause distortions in the crystalline structure of the metal reducing their ability to move within the metal and makes it more resistant to further deformation.
Hardening can be an advantage or a disadvantage.
- It increases tensile strength and hardness
- Reduces ductility and makes the steel more brittle.
The crystalline structure can be restored by annealing.
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