cold working and age hardeing

[LeeEccles]

Is the strength of a cold worked or age hardened material likely to change with time?

I believe if a component strengthened by either of the two above mechanisms is exposed to increased temperatures during operation then the strength would change, but does it also change over time if the component remains at approximately ambient temperatures?

 

[CoryPad]

Generally there is no change in strength at ambient temperatures when a material is strengthened by these two mechanisms.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[NickE]

I think that that is very dependant on the mechanism. For example:

Cold work iduces large numbers of dislocations into the structure. These dislocations are resposible for strengthening the material. (Unless there is a phase x-formation that is a result of the deformation.) They have a specific activation energy requred for them to move through the structure and annihilate each other. Until this energy is reached there shouldnt be any reduction in strength.

Age hardening is (often) the result of precipitation of hard coherent (and usually very small) particles that strain the matrix. They limit dislocation movement providing strength. Heating these metals above the original aging temperature will casue the these particles to increase in size until they no longer provide the same or as strong of an effect on strength. In this case temperatures below the origninal aging temperature will generally not have a large effect at short times. At lower temps again we run into an activation energy that has to be present for the precipitation reaction to occur.

So from the above I think that the answer is no. Cold worked and aged metals should not suffer a reduction in strength at resonable times in ambient environments.

(If I am wrong some one like maui will come by and let you know.)

Nick
I love materials science!

 

[metengr]

YES. Certain low alloy carbon steels can become strengthened (increase in yield strength, loss of ductility) over time between ambient and say 500 deg F, after some level of initial cold work has been applied during forming. This is known as strain aging, and is a function of carbon, nitrogen and oxygen levels in the steel.

Certain low carbon steels that are quenched can also be age hardened at ambient temperature over time based on the precipitation of certain carbides. Again, this is related to Carbon, Nitrogen and Oxygen levels in the steel. As the temperature is increased, aging becomes more rapid.

Age hardening itself deals with the precipitation of carbides that effect mechanical properties.