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Name: Melanie
Status: Student
Grade: N/A
Location: FL
Country: USA
Date: July 2006


Question:
What are the differences between heat-treatable and non heat-treatable alloys?



Replies:
Hi Melanie,

Your question is a very broad one, and can mean a couple of things. If you are asking the most general question, the answer is that heat treatable alloys are those that can have their properties (usually their hardness) affected by some form of exposure to heat. Non heat treatable alloys are those that heat is not (or cannot be) used to increase hardness.

If you were asking what is the mechanism that makes an alloy heat treatable, compared to one that is not, then there are many reasons depending on the alloy. Perhaps some examples are most useful.

High carbon steel (that is, steel with at least 1% carbon content) is heat treatable. When you heat it to red heat, the carbon and iron form localized crystals of "Martinsite" (iron carbide). If you rapidly cool the steel such as by plunging it in water, the Martinsite crystals do not have time to break down, and remain in the matrix of iron. Martinsite is very hard, and thus the resulting steel is hard. If the red hot steel is allowed to cool slowly, the Martinsite crystals slowly revert back to iron and carbon, and the steel is said to be "annealed"; that is, very soft. Various steels are the primary alloys that are heat treatable in this way.

Essentially all pure metals, and most other alloys do not respond to heat treating. Aluminum alloys, for example cannot be hardened by this method. Aluminum alloys are often able to be hardened by another non-heat-related process called "precipitation hardening". Copper is not able to be heat treated either, but like many metals and alloys, it can be "work hardened" when it is stretched, bent, or hammered, which causes large copper crystals to break up into many smaller ones that interlock and make the metal stronger. However, it can be annealed by heating it to a read heat and cooling it. This causes the many small crystals in the metal to regrow into larger ones again.

There are many other examples, but hopefully this comes close to answering your question.

Regards,

Bob Wilson.



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