Solution Annealing Heat Treatment

Stainless steels, nickel-based super alloys and titanium alloys are frequently solution annealed before precipitation hardening. During this heat treatment, the alloy is heated above its critical temperature (sometimes called its transformation temperature) and then held for a length of time depending on the material type and thickness. Then the alloy is rapidly cooled to room temperature in a gas (usually nitrogen) or water, depending on the metal and application.

The solution anneal heat treatment increases the passivation properties of martensitic stainless steel X46Cr13 and improves its pitting corrosion resistance. Solution annealing heat treatment does this by transferring the carbides located outside the grain boundaries to the interior of the grains. The resulting increase in the microstructure enables easier passivation, and the carbides that remain are more evenly distributed within the microstructure.

Unlike hardening, which increases strength, solution annealing decreases metal crack sensitivity of the alloy by moving dispersed intermetallic phases into the matrix. It also improves the mechanical properties of heavily contaminated materials by enabling soluble impurities to diffuse from the grain boundaries into the grain interior and by coagulating the non-soluble components in roundish particles. During the process, large needle-shaped intermetallic phases are reduced in volume by the mutual diffusion of alloying elements and aluminum atoms into the solid solution phase. The resulting soft microstructure makes the material easier to weld.