A514 from one supplier to another = problems!

[CR100]

We have ran into an interesting problem in regards to A514 material. The pieces are 13.5" diameter discs of 3/4" thickness, the are machined down to a slim profile of about .188". The finished piece almost looks like a bowl.

Anyhow from one supplier we get very little movement after machining, from another supplier less than 20% are good due to spring back.

When comparing the mill certs, of course they both say that they meet A514, strength wise and chemically.

The piece that does not spring back has a slightly lower tensile strength at 115ksi. While the other plate with the problem has a tensile of 129ksi.

Other than that the are only minor differences in the chemical compositions.

Any ideas why this could be? and how to prevent it in the futre?

 

[swall]

Your problem is due to residual stresses in the discs, not the material it self,i.e. a processing issue. How are the discs fabricated before you get them?

 

[btrueblood]

...specifically are they annealed or stress relieved?

 

[CR100]

They will not be annealed or stress relieved as ASTM A514 only calls for Q&T. None of the mill certs from each company show any post processing other then the temper.

The disc are cut from 3/4" plate. Would flame cut or plasma cut change the stresses?

 

[metengr]

Flame cutting, in lieu of plasma cutting, can indeed induce local residual stresses after tempering.

 

[pecorp]

The problem is in the machining not the material. One vendor is taking off too much material at one time causing heat.

 

[swall]

Point of clarification needed--CR100, when you say that the finished piece looks "almost like a bowl", it is not clear to me if it is supposed to be like that or if it is supposed to be flat.

 

[Guest102023]

This is a common occurrence when machining thick section materials to fairly tight tolerances. The problem is inherent residual stresses in these sections, either hot rolled or normalized. In shops I have worked in it was common practice to anneal heavy slabs at some point before final machining. As mentioned by metengr, residual stresses can also be induced by thermal cutting.

 

[CR100]

More info from each of the suppliers.

- As it turns out the machining was changed in an effort to reduce cycle time. This definitly could be causing this problem.

- The one vendor that supplier the "good" plates plasma cut them. Waiting on the other vendor.

Swall, the part is to be flat.

Brimstoner, being the A514 would arrive in an heat treated condition, once annealed I would have to heat treat again.

Thanks a bunch everyone.