What is solution treatment?

This is a process in which an alloy is heated and maintained at a suitable temperature to melt certain constituents into a solid (turned into a solid solution), and is then cooled rapidly to avoid precipitation.
This treatment is mainly used for austenitic stainless steel to improve metallurgical structure, such as removing internal stress caused by machining/welding and restoring corrosion resistance.
It is also used as a pretreatment for precipitation hardening of precipitation hardened stainless steel.

Work hardening

Work hardening, also known as strain hardening, is the hardening of metal by plastic deformation.
When austenitic stainless steel is work hardened, an austenitic phase transforms to a martensitic phase (work induced martensitic transformation), in which an originally non-magnetic material gains magnetic characteristics and also loses some corrosion resistance.
To recover from such a structural transformation, it is necessary to perform solution treatment at a proper temperature.
In such a case, we sometimes receive a request for "annealing" to lower the hardness (softening). Please note however that stainless steels will be sensitized by cooling slowly after heating up.
Also, some materials are extracted to raise the surface hardness on purpose. So, please consider that when such a material undergoes solution treatment, it will soften.

Inter-granular corrosion

In this phenomenon, metal grain boundaries are selectively corroded.
This corrosion is known to happen when chromium carbide is precipitated by sensitization of austenitic stainless steel.

Sensitization

When austenitic stainless steel is exposed to 500-850°C heat for a certain period because of heat treatment or welding, carbon bonds with chromium to precipitate chromium carbide along grain boundaries.This condition is called sensitization, in which the lack of chromium forming a passivation layer causes inter-granular corrosion, and thus compromising corrosion resistance, a characteristic of stainless steel.
The solution treatment can prevent the sensitization by heating up to and maintaining at 1000-1200°C and rapidly cooling through 900-500°C range.
If solution treatment is not an option, then a low carbon material, which is harder to be sensitized, may be an option.
Stainless steel is sometimes sensitized on purpose to determine its resistance to inter-granular corrosion.

Passivation layer

Why doesn't stainless steel rust? Because it has a thin oxide layer on its surface.
This oxide layer is very stable and passivated. So, it is called a passivation layer.
A passivation layer is generally formed when chromium content exceeds 11-12% in steel.
It is thought to form a double layer with chromium oxide on the metal side and hydroxide on the environmental side.
Its thickness varies depending on the condition but generally is around 1-3nm.
This very thin oxide layer protects stainless steel. In other words, the surface is already too oxidized (rusted) to be oxidized any further.
This passivation layer is so stable that even if it is destroyed, it is rebuilt right away in the air or water.
However, stainless steel will be corroded under certain conditions, where this rebuilding process does not work properly.

List of solution treatment temperatures for austenitic stainless steels

Heat treatment of other stainless steels

(Quenching and tempering martensitic stainless steel)
Martensitic stainless steel is chosen for its hardness, strength and resistance to rusting in the atmosphere. Yet, other corrosion resistance types are inferior to many variations of stainless steels.
SUS440s are the hardest of all. SUS440C can achieve the hardest HRC58 standard among JIS steels only with quenching and tempering.

(Annealing ferritic stainless steel)
Like solution treatment for austenitic stainless steel, annealing is performed on ferritic stainless steel to improve its metallurgical structure after machining.
This is not a type of solution treatment, but unlike normal annealing, it uses rapid cooling to avoid sensitization.
Ferritic stainless steel especially needs to avoid 475-degree brittleness by passing through that range quickly.

(Precipitation treatment (age hardening) of precipitation hardened stainless steel)
Precipitation hardened stainless steel, represented by SUS630 and SUS631, relies on fine precipitation of particles of inter-metallic compound instead of carbon to raise its hardness.
Compared to quenchable martensitic stainless steel, it is inferior in hardness but superior in corrosion resistance.
Just like martensitic stainless steel, it is suited for usage requiring hardness. It can also be molded and welded before precipitation hardening.
During precipitation hardening (age hardening), SUS630 precipitates a Cu-rich phase to harden, while SUS631 generates an inter-metallic compound between Ni (Nickel) and Al (Aluminum) to harden.