tensile strength

[ramatteson]

I am trying to calculate the tensile strength of a screw by taking the wire tensile strength and drawing it down thru a wire drawer and then cold heaing it. Example .224 302 wire 72ksi drawn down to .214 diameter and headed to .2150 dia. Is there a formula to determine how much tensile will increase per .001 that the wire is drawn?

 

[metengr]

I have not seen any published equation in metallurgical text books that specifically relates tensile strength to % cold work or %RA. I have seen tabulated data, where tensile strength and yield strength are measured values after a known reduction in area or % cold work. There are also graphs showing the effects of percent cold work on tensile and yield strength behavior for carbon, low alloy and stainless steel. These are generic curves that could result in considerable scatter of results.

 

[EdStainless]

It depends a lot on the actual chemistry of the material and the amount of reduction. The curves are all empirical.

I would not expect hte strength of drawn wire to correlate with strengths for roll formed parts anyway. Very different working methods.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[yates]

For a given alloy, a family of curves can be developed giving tensile strength versus percentage cold work for a given aging temperature. This should be available from the seller of the alloy. The reslults however are experimental and cannot be predicted by theory. Most alloys demonstrate an optimal percentage of cold work. Above this percentage tensile strength drops off again.

 

[Screwman]

We have found that the data available on strain hardening by drawing to be pretty limited; we have always had to develop our own true stress strain data.
One thing that has a large bearing on the strength after drawing is the draw die geometry. The entry angle and land width have a surpisingly large impact on the after drawing results.
The earlier comment about the effects of chemistry is very true; you cannot extrapoplate between alloys what any reliability.

 

[TVP]

ramatteson,

What you have described is a common practice in the forming and forging industry. Usually this analysis is performed in order to know what the forming loads will be at various stages of deformation, which aids in determination of appropriate tools and presses. The following equation is used to describe the stress vs. strain behavior of isotropic metals during plastic deformation:

[σ] = K[ε]ⁿ

where [σ] = the flow stress
[ε] = the true strain
n = strain hardening exponent
K = strength coefficient

For annealed austenitic stainless steels like Type 302 & Type 304, the value for K = 1300 MPa (N/mm²) and n = 0.45. Values for annealed Type 316 are K = 1100 MPa and n = 0.40. This data was obtained from the International Cold Forging Group document ICFG Document No. 11/01 Steels for Cold Forging - their behavior and selection. If you know the amount of true strain (a 20% change in diameter is a strain of 0.20), then you should be able to calculate the increase in strength during forming.

As the others have already noted, variations in alloying, wire drawing practice, etc. will affect the deformation behavior. Strain rate also has a significant effect. You should discuss this issue further with your stainless steel supplier, as they should have a technical contact who can provide more specific data. Also, you can find some graphs of increase in tensile strength vs. % cold work by using the following links:

Carpenter

http://cartech.ides.com/ImageDispla...gth+of+Popular+Austenitic+Cold+Heading+Grades

Nickel Institute

http://www.nickelinstitute.org/inde...em_id/778710/full_form/false/orig_id/3291.htm

 

[CoryPad]

Small correction to TVP's post:

A 20% change in LENGTH is an engineering strain of 0.20. For TRUE strain and DIAMETER change, the relationship is:

[ε] = ln (d₀²/d_f²)

where

[ε] is the true strain
d₀ is the original diameter
d_f is the final diameter


Regards,

Cory

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

 

[EdStainless]

The reason that draw die geometry (or any forming process tooling) has such an impact is the amount of redundant work involved. This all works out smoothly if there is little RW. In drawing operations with high RW predition is very dicey.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm