Seams On Rolled Steel Products

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“Seams are longitudinal crevices that are tight or even closed at the surface, but are not welded shut. They are close to radial in orientation and can originate in steelmaking, primary rolling, or on the bar or rod mill.”– AISI Technical Committee on Rod and Bar Mills, Detection, Classification, and Elimination of Rod and Bar Surface Defects

Seam p11 — Seams are longitudinal voids opening radially from the bar section in a very straight line without the presence of deformed material adjacent.

Seams may be present in the billet due to non-metallic inclusions, cracking, tears, subsurface cracking or porosity. During continuous casting loss of mold level control can promote a host of out of control conditions which can reseal while in the mold but leave a weakened surface. Seam frequency is higher in resulfurized steels compared to non-resulfurized grades. Seams are generally less frequent in fully deoxidized steels.

Seams Lab Notebookcrop — Seams are the most common bar defects encountered. Using a file until the seam indication disappears and measuring with a micrometer is how to determine the seam depth.(Sketch from my 1986 lab notebook)

Seams can be detected visually by eye, and magnaglo methods; electronic means involving eddy current (mag testing or rotobar) can find seams both visible and not visible to the naked eye. Magnaflux methods are generally reserved for billet and bloom inspection.

Seams are straight and can vary in length- often the length of several bars- due to elongation of the product (and the initiating imperfection!) during rolling. Bending a bar can reveal the presence of surface defects like seams.

An upset test (compressing a short piece of the steel to expand its diameter) will split longitudinally where a seam is present.

Seams are most frequently confused with scratches which we will describe in a future post.

“These long, straight, tight, linear defects are the result of gasses or bubbles formed when the steel solidified. Rolling causes these to lengthen as the steel is lengthened. Seams are dark, closed, but not welded”- my 1986 Junior Metallurgist definition taken from my lab notebook. We’ve a bit more sophisticated view of the causes now.

The frequency of seams appearing can help to define the cause. Randomly within a rolling, seams are likely due to incoming billets. A definite pattern to the seams indicates that the seams were likely mill induced- as a result of wrinkling associated with the section geometry. However a pattern related to repetitious conditioning could also testify to billet and conditioning causation- failure to remove the original defect, or associated with a repetitive grinding injury or artifact during conditioning.

My rule of thumb was that if it was straight, longitudinal, and when filed showed up dark against the brighter base metal it was a seam.

Rejection criteria are subject to negotiation with your supplier, as are detection limits for various inspection methods, but remember that since seams can occur anywhere on a rolled product, stock removal allowance is applied on a per side basis.

If you absolutely must be seam free, you should order turned and polished or cold drawn, turned and polished material. The stock removal assures that the seamy outer material has been removed.

Metallurgical note: seams can be a result of propogation of cracks formed when the metal soidifies, changes phase or is hot worked. Billet caused seams generally exhibit more pronounced decarburization.

5 Ways Coarse Austenitic Grain Size Affects Your Machine Shop

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Austenitic Grain Size is a material characteristic that is usually reported on test reports and certification documents for the steel materials that we machine in our shops.
Coarse Austenitic Grain Size is a result of NOT ADDING grain refining elements to a heat of steel. Because these Grain refining elements have not been added, the steel has a “Coarse Austenitic Grain Size.”

Friday, May 16, 2008 (3).max — This is Coarse Grain Austenite. You like it for machining.

Typically this practice is applied to free machining grades such as 11XX and 12XX steels. These steels are sold primarily for their ability to be machined at high production rates.
What does Coarse Austenitic Grain Size imply for the parts that you make?

Better Machinability– Coarse Grained Steels are more machinable and provide longer tool life than Fine Grained Steels. (The elements added to make the Austenitic Grain size fine create small, finely dispersed hard abrasive particles in the steel)
Better Plastic Forming– than Fine Grained Steels
More Distortion in Heat Treat- than Fine Grained Steels
Lower Ductility at the same hardness- than Fine Grained Steels
Deeper Hardenability– than Fine Grained Steels

Coarse Austenitic Grain Size will show up on the test report as an ASTM value of 1-5. Values of 5 and higher are called Fine Grained Steels, and are the result of additions of Aluminum, Vanadium, or Niobium in North American commercial practice for most Carbon and Alloy steels.
The methods for determining Austenitic Grain Size are detailed in ASTM Standard E112, Standard Test Methods For Determining Average Grain Size.
A nice discussion can also be found HERE.
While we think that chemistry may be the controlling factor for machining performance of the steel in our machines, the contribution of austenitic grain size is also important. As long as you are ordering your free machining steels (11XX and 12XX series) to Coarse Grain Practice, Austenitic Grain Size should not be an issue in your shop.

3 Actions To Minimize Cracks In Our Shops

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Paying attention to draft, chemistry, and steel melt source processes can help you minimize the potential for cracks at your customer after cold work operations.
After a crimping, staking or swaging operation, cracks can develop. This is because the cold work needed to swage, stake, crimp, etc. was greater than the material’s available elasticity. This is the case in the part photographed here.

Cracks can develop after cold work is performed on machined parts.

In order to minimize cracking during or after crimping, or thread rolling, or other substantial cold work, take the following steps:

Specify non-renitrogenized material;
Inform your supplier of your cold work application. They can consider reducing cold draft, or changing suppliers of the hot roll to get basic oxygen process, low residual, low nitrogen steel;
Ask the customer to consider changing the grade. Resulfurized steels are capable of being somewhat cold worked, but their high volume fraction and weight percent of nonmetallic inclusions (What makes them cut so well!) is also what works against successful cold work.

To minimize the occurrence of cracks that are not a result of cold work, try this:

Assure that adequate stock removal is taken in machining;
Buying from reputable sources whose quality systems employ rototesting and eddy current testing;

When cracks are discovered in your shop, what actions do you take?