Straightening characterization?

[Fred]

We have regularly straightening problems with high strengh materials like S355MC, S550MC after a laser cut operation. The elastic limits (355 MPa, 550 MPa) are obtained by thermo-mechanical process and we control these materials by hardness measures and by their chemical analysis(%C, %Mn).... Obviously, these controls don't seem to be sufficient.

Could anyone help us to characterize the straightening ability of this kind of materials?

Thank you

 

[bklauba]

What size is the original plate (I am assuming) prior to laser cutting, and what size are the laser cut-out parts?

 

[Fred]

The plates are 370 mm wide and 6 mm thick and the cut-out parts are levers of different sizes of approximately 370x220x6.

 

[bklauba]

One possible solution is to vibratory stress relieve the entire plate before laser cutting is performed. I have personnally seen larger plates of mild steel, agreed not of such robust alloy as you descibe, behave in a far more stable manner if vibration treated prior to laser cutting. Distortion was reduced by a factor of 3X or more.

If this worked, this would take a bite out of your straightening requirements, reducing the severity of the problem.

 

[BillPSU]

Plate products made from mild steel which were produced by making into coils and de-coiled and flattened have a problem called coil memory which when cut into parts can give extreme problems. I've seen parts 50" in length with 3" of curve in the part. In the late 80's Robinson Steel started producing cold reduced and flatted steel plate from coiled plate. This process not only provided an extremely flat product but also had little coil memory. Coil memory was eliminated up to 5/16"-8mm thickness. In the 3/8" and 1/2" 55,000 HSLA we experienced intermittent problems of coil memory. Robinson now has several competitors doing similar processes, Olympic Steel and Central Steel are two of them.
I do not know the process by which your stainless was produced but being 6mm thick tells me it was probably coiled. If you are a large enough consumer of this material I would look at the possibility of getting materials first which have not been coiled. Secondly some sort of stress relieve, temperature or vibratory. Third have one of the companies toll process a coil. Caution must be taken to not to contaminate the surface of the plate with materials during cold reduction.
Flatness has always been a problem with plate and sheet fabricators and this cold reduction process aided manufacturing greatly and probably help you also.

 

[Fred]

The vibratory stress relieve seems to be a good solution but our subcontractors are not equiped with such device. The alternative could be a stress relieve but I the mechanical properties would be modified by this treatment.

I confirm that most of our materials are coiled. But we are not a large enough consumer to have a sufficient weight with our suppliers.

After a discussion with one of them, it appears that the content of Mn is very omportant concerning the amount of residual stresses (by the increase of segragtions). Indeed we have less problems with Mn<0.5% whereas Mn~1.4% are very difficult to straightened.

 

[EdStainless]

The Mn (and N) have very strong influence on the work hardening rates. High Mn and N heats will harden more when coild/uncoiled/leveled. You might need to look at alternate leveling/flattening options. Tension leveled/stretch leveled/roller leveled will all leave different amounts of residual stress and memory.

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Corrosion never sleeps, but it can be managed.

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