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why exactly is cold worked metal stronger longitudinal
4

why exactly is cold worked metal stronger longitudinal

why exactly is cold worked metal stronger longitudinal

(OP)
I would like to know why exactly a cold worked piece of metal is stronger in it's longitudinal direction.

I'm not sure!

 

RE: why exactly is cold worked metal stronger longitudinal

Trick question.

Cold worked material is stronger in the longitudinal direction because cold working increases the dislocation density, resulting in increased hardness and yield strength.  This increase occurs in all directions, so it necessarily occrus in the longitudinal direction.

rp

RE: why exactly is cold worked metal stronger longitudinal

(OP)
and why not in transvers direction?


 

RE: why exactly is cold worked metal stronger longitudinal

The reason for this observation is called anisotropic behavior of metals. This behavior is related to crystallographic texture and direction of hardening from working. Alignment of grains (and grain boundaries) along a preferred working direction alters mechanical properties, similar to fibers in a piece of wood. The strength of wood is a function of fiber orientation.

RE: why exactly is cold worked metal stronger longitudinal

(OP)
but why exactly?

why do the newly developed dislocations only effect longitudinal strength

RE: why exactly is cold worked metal stronger longitudinal

Read met's post again.  Metal crystals (grains) have been strain-hardened in direction of rolling, which also means they are elongated in that direction, and create an intertwined/interlocking structure similar to wood.  

Another way to think of it is to assume a bubble-shaped void in a cast bar, which is subsequently rolled - the bubble has elongated into an ellipsoid with its long axis aligned to the rolling direction.  The defect has essentially narrowed in the rolling direction, and now creates much lower stress concentration for tensile loading in the that direction.  But, in the transverse direction, it has started to look like a crack.

RE: why exactly is cold worked metal stronger longitudinal

Mechanical properties vary with grain size. That's a given. As you deform in one direction the grain size becomes smaller in proportion to the reduction in area in that direction. Transversely, the grains change shape, but keep the same area; hence no difference in the grain size contribution. Thus the transverse properties are not increased via the grain size contribution while the longitudinal are.

This is separate from grain re-orientation by deformation, which causes grains to rotate into the direction of easier flow, which has a off-setting effect on yield strength, but decreases transverse elongation and toughness. This effect varies significantly with crystal structure. FCC has a lower grain orientation effect than BCC.

Good question.
 

Michael McGuire
http://stainlesssteelforengineers.blogspot.com/

RE: why exactly is cold worked metal stronger longitudinal

Like I said above, it's a trick question.

Cold working increases strength in both the longitudinal and transverse directions.  The increase in more pronouced in the longitudinal direction for the reasons stated by others, but increases are seen in all directions.  The OP wanted to know EXACTLY why this increase in strength occurs, which is due to the increase in dislocation densities.  The changes in fibers, grain orientation, etc..., occur with both hot working and cold working but the original question deals only with cold working.

rp

RE: why exactly is cold worked metal stronger longitudinal

(OP)
thank you all, top!

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