I've been trying to sort out some of the heat treatment terms for steel and what they actually mean since there seems to be a lot of interchangeable usage between the two. This is what I have so far. I am asking for any corrections if there is any blatant gap missing or anything incorrect, including alternate terms for the same process.
Being harder is accompanied by being more brittle and the opposite, being more ductile is accompanied by being tougher.
The cause of brittleness or ductility of steel is due to the fact that dislocations in the crystal structure can move within the grain cannot cross the grain boundaries in the steel. So larger grains allows the dislocations to move farther before being stopped which results in a material that is more ductile and can be deformed more easily. Whereas with finer grains, the dislocations encounter the grain boundary after much less distance resulting in a harder, less ductile, more brittle material.
Critical Temperature is the temperature above which the crystal structure of the steel (or I guess metal in general) becomes fluid and can easily change. Is below the melting temperature so the steel isn't just a liquid.
AC1 Critical Temperature Insufficient knowledge of phases to try and say anything about. Seems to get very technical and unable to draw anything high-level from it as of yet.
AC3 Critical Temperature Insufficient knowledge of phases to try and say anything about. Seems to get very technical and unable to draw anything high-level from it as of yet.
Anneal (sometimes called full annealing to distinguish it from other forms of less extreme annealing) is when the metal is heated above the critical temperature and is allowed to cooled very, very slowly (i.e. in the oven). This gives the steel a lot of time for the larger crystals to grow in the material resulting in large grain sizes and high ductility. Also inherently relieves internal stresses in the process.
Normalize (sometimes called a partial annealing?) is when the metal is heated above the critical temperature and is allowed to cooled naturally. This produces smaller grains compared to annealing (or full annealing) resulting in a material that is not as ductile but still more ductile than the hardening/quenching. Also inherently relieves internal stresses in the process.
Stress Relieving is when the metal is below the critical temperature and allowed to soak (well, I guess all the heating processes requiring soaking but it is explicitly important here to mention it). It is hot enough for the crystal structure and therefore grain size remains unchanged, but it is hot enough for the the material to be a bit plastic so that things can shift internally a bit to relieve internal stress.
Hardening is when the material is heated to above the critical temperature and then rapidly cooled via quenching. This freezes the crystal structure without giving much time for large crystals to form in the material which results in a fine grain which makes the material hard and brittle to the maximum degree.
Tempering is a process applied after hardening to reduce the extreme brittleness of a material after hardening to bring a material to the final desired level of hardness vs toughness. It sounds a lot like annealing/normalizing but some sources list that the temperature is below the critical temperature which seems to conflict with the stress relieving temperature. Other sources, still, list it as above the critical temperature. There seems to be multiple critical temperatures though most layman sources only talk as if there were one. is that the difference?
Stabilization is not solely a heat treatment, but a steel where specific chemical additions are made to allow the formation of carbides which are more stable (i.e. won't migrate) than the typical carbides. Then the material is heat treated to "wipe the slate clean" and destroy the migrating carbides and allow the stable carbides to form preferentially.
Some of my sources:
- https://www.cliftonsteel.com/knowledge-center/anneal-normalize-stress-relieve
- https://inspectioneering.com/journal/2007-04-15/4115/99-diseases-of-pressure-equipm
- https://www.metalworkmasters.com/processes-treatments/annealing-normalizing-tempering-quenching-comparison/#Basics_of_Normalizing
- https://mailleisriveting.weebly.com/metallurgy-ii-annealing-normalizing-and-tempering.html
- https://knifesteelnerds.com/2019/06/10/how-annealing-of-steel-works/
