Cold working and residual stresses 6

[aetayar08]

Hello there

Quick question for you experienced folk.

Any idea as to why cold working increases strength and induces residual stresses (what microstructural effect specifically)? Is it simply just an increase in dislocations? Seems to easy...

TIA!

 

[arunmrao]

Please read Mechanical Metallurgy by Dieter.

I'm just one step away from being rich, all I need now is money.
( read somewhere on the internet)

 

[CoryPad]

Yes, you are on the right track regarding increased dislocation density.

 

[FennLane]

Strain Hardening/Work Hardening (cold working) occurs when most metals are plastically deformed and makes metals stronger.

Work hardening typically obeys an exponential law Ơ=kƐ^n - This is a little simplistic but a reasonable starting point.

The plastic deformation causes dislocations to move and additional dislocations to be generated.

The more dislocations there are within a material the more they will interact and become pinned or tangled.

This will result in a decrease in the mobility of the dislocations and a strengthening of the material as the forces needed to cause dislocations to move will increase.

A typical annealed steel will have around 10^8 dislocations per cm^2 and this will increase in a fully cold worked form to around 10^28 dislocations per cm^2.

Plastic deformation must occurs at a temperature low enough that atoms cannot rearrange themselves. When a metal is worked at higher temperatures (hot-working) the dislocations can rearrange and little strengthening is achieved.

 

[aetayar08]

Thank y'all

 

[EdStainless]

But the two factors, cold work and residual stress, are not always tightly linked.
I can create very high residual stress (= to yield strength, which is as high as is possible) with fairly small amounts of cold work (just a few %).
Likewise I can heavily cold work a material significantly raising its strength and leave it with fairly modest amounts of residual stress.
I depends on how you do the work.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[tbuelna]

Cold working a metal can also relieve stress. Heat treatable aluminum alloy bar stock is made more dimensionally stable by a mechanical stress relief (stretching) after a thermal heat treatment without any appreciable loss in strength.