Wire Drawing

It would appear to me that in the process of drawing the larger wire through the die that you are generating more heat thus altering the molecular structure of the material.

Also, there could be a type of "forging" process going on due to compressing the larger wire by a larger percentage than the smaller wire, again, altering the molecular structure of the material.

It is probably a combination of both, but it would take a metallurgist to provide the definitive answer.

Bernard, The larger diameter rod achieves higher tensile strength because it has been subjected to greater cold work. If you care to attempt the math, do a google for strain hardening. An arithmetic description would be to compare the original diameter minus the final diameter divided by the original diameter times 100 to get the "% of reduction" or "% of cold work". Do that calculation for both starting sizes and you can see that the % reduction in drawing is different just as the tensiles differ.Strain hardening is well documented and was known in ancient times. It is what makes it more difficult to straighten a piece of coat hanger wire after you have already bent it once...

In metals we do not talk about molecular structure, we talk about crystal structure. You do not describe the details of the anneal, but properly done, an anneal should be able to achieve similar properties regardless of draft or cold work...But only if you know the starting hardness /tensile of each batch and then anneal each at the temperature needed to achieve the change. An anneal in steel will change crystal structure- lamellar perlite or spheroidize structure.

A simple stress relief can also modify (soften) structures, with out changing structure.

While there is heat produced by cold drawing through a die, that heat is a an effect, not a cause of the property changes that you have observed.

THere is NO change in density of the material after drawing! After drawing the volume of the material will be the same as starting volume, so if diameter is smaller, the resulting increase in length will make up the difference.

milo

Bernard,

The act of drawing wire causes work or strain hardening, and the larger original diameter requires more drawing. That and the fact that you are running both through the same annealing bath at the same speed and temperature explains the difference.

"Strain hardening
With strain hardening, there is an increase of strength by straining of the microstructure. It occurs, when hot-formed steel products are re-drawn, re-rolled, etc. Before a further cold forming, such products have to be (soft) annealed, unless strain hardening was the target of the cold forming."

This link offers some insight:

http://en.wikipedia.org/wiki/Strain_hardening

Regards, Greg

The more the% area-of-cross-section reduced(wiredrawn) from annealed beginning stock-the more is the yield strength you get- Irrespective of Intermediate annealing.

That is why Prestressing Steel wires are hard drawn ones to get high yield point --hence minimising Steel reqirement for Beam,Arch,Sleeper.Column-----

Hi Bernard,

I my view what you have described is normal, and understandable, since you're converting the two different size rods down to the same size with the same treatment even though their not the same size.

What you should do is to put more heat into the rod drawn from the larger (7mm) one to bring it to the same molecular structure like the other one, either by slowing the annealing speed or crank up the heat. This is something you have to work out through trial and error since i do not know your process.

Hope this'll help!

Regards,

Bernard: What kind % C steel do you speak?.- When the % C increase this problem increase too, the diferent among both reducción area; because of that you should do the patenting annealing in higth Carbon steel. Low % C degree and in subcritical annealing you need slow speed, more temperature or the larger tensile made previous relax tension.- I hope help you, and we can to improve this Knowledge- ccvallino@hotmail.com -.