Age hardening of steel

[SpikeC]

Does anyone know anything about age hardening characteristics of steels? I know that some hand saws can develop brittleness at the tooth line which seems to be due to age, and I have heard of old pieces of steel becoming harder over time, but I don't know if that is true or rumor.
I have the opportunity to acquire a rather old German knife, would it's age make any difference in hardness, or maybe other factors?

 

[Larrin]

Age hardening is also known as precipitation hardening and is most often done on purpose, it is not common with knife steels, as they are not alloyed for it. Work hardening can occur but is not all that typical in knives because it happens through plastic deformation (bends that don't bend back). Another phenomena that can occur is the conversion of retained austenite to martensite through heat or impact. This is only really a concern if the knife has a high percentage of retained austenite, most common in highly alloyed steels that are autenitized at too high a temperature without a cryogenic treatment or sufficient number of tempers.

It's also possible the saw teeth would become brittle due to fatigue, or a couple other factors, but I don't have any familiarity with the failure/problem.

 

[SpikeC]

Thanks, Larrin! One thing that spurred my interest was the way that sterling silver and some gold alloys harden over time, as well as being heat treatable. I have heard rumors that some steels could act similarly. Talk about apples and oranges, tho!!