Incoming Steel Inspection...

[sehunnic]

I'm looking for a good cost-effective test method for incoming steel, typically ETP material. Currently, if forming issues are encountered on the press, there's no method for testing the steel to rule it out.

I've read some about eddy current testing for hardness and possibly shear strength, but don't believe it works on coated materials well. I've also heard that basic hardness tests can be an indicator of formability problems.

Any info would be appreciated. Thanks.

[erickam]

I usually defer such questions to my Friends at EQSgroup.com
They have specialty in steel inspection. Maybe they can find your answer.

[Danny]

Unfortunately, hardness testing is not really a good test for formability. It's a test that is good for determining resistance to indentation, which is why it is used in testing dies and heat treated parts. The best reasons to use it in sheet metal formability applications are that it is cheap and fast. It just doesn't give you useful information. A hardness test can discriminate between a high strength steel and a mild steel, but it is not appropriate for characterizing formability of the same grade.

A much better test is the tensile test. This test gives you the yield strength, tensile strength, and total elongation of the sheet metal in any orientation of interest (in the rolling/grain direction, perpendicular to the rolling direction, or some angle off the rolling direction).

Even the total elongation (TE) is not the best parameter. Total Elongation is a measure of how much you can stretch the metal until is breaks. That's fine if total fracture is your failure criterion. But for most applications, the part is considered bad if there is a neck (aka, "smile" or visible strain). In some metals, this happens right before total fracture, while in other metals (like most steels), there is considerable deformation that occurs after the neck appears.

To measure up to the beginning of a neck, the parameter of interest is called uniform elongation (UE). This describes the amount of deformation the sample can uniformly distribute over the test area. (Once the strain localizes, that's where a neck forms).

Another parameter that is closely related to uniform elongation is "N-value", also known as the strain hardening exponent. It is this n-value that along with thickness is used in determining forming limit curves of low carbon steels.

Your test lab should be able to provide UE and n from the tensile test as long as the load-displacement data is collected in a file. This digital stress/strain curve is one of the best inputs into your forming simulation software, since it will describe the full deformation characteristics of the sheet metal, rather than just inputting the discrete points of yield strength, tensile strength, and elongation to fracture.