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:


Your first statement is misleading or wrong; Hardness=brittleness. At the same hardness there can be a wide range of "brittleness" = lack of toughness in steels. The old Bessemer steels ( such as Titanic hull) were relatively soft with satisfactory ductility, and had very poor toughness. Also, ferro-brittleness is a relatively unique property/disability, of ferritic steels, not "metals". If one wants a tough steel ,one specifies a Charpy requirement because the tensile, yield and elongation do not define toughness. There are tricks of the secret society of metallurgists to get toughness without paying for a Charpy requirement ; such as "fine grain practice" and normalize. Wikipedia has a good introduction to toughness ( in steel) under LEFM ( linear elastic fracture mechanics).It sounds like you want to join the secret society of metallurgists. Meanwhile ASM ( American Society for Metals) has very large collection of literature including their own "Metals Handbooks" in various editions. The latest edition I have is ninth, 1978, so I am obsolete and do not know the latest secrets of the society of metallurgists.

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    $\begingroup$ ASM offers shot courses and texts for "metallurgy for the non-metallurgist" and "Heat-treatment of steels" . I think these would be much better organized to learn about the subjects than finding various unrelated factoids on the internet. I can't resist saying the "Heat -treatment" was the first short course offered by ASM . And I attended the first classin 1963 taught by Dr Antone Brasunas , the leader of ASM for years. $\endgroup$ Oct 11 '21 at 18:40
  • $\begingroup$ American Society for Microbiology? Nah, you must mean the other one. Heh, This one looks okay asminternational.org/technical-books/-/journal_content/56/10192/… Pricey though. Even amongst textbook prices. $\endgroup$
    – DKNguyen
    Oct 11 '21 at 21:10
  • $\begingroup$ I forgot they added "international" for political correctness. $\endgroup$ Oct 13 '21 at 15:42
  • $\begingroup$ I mean, American Society for Materials is fine because at least it says American in the title rather than just "National" without specifying any country, which is how you know an organization is American. $\endgroup$
    – DKNguyen
    Oct 13 '21 at 18:22

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