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.

4 Ways Nitrogen Affects Your Shop

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Nitrogen strengthens ferrite.
Nitrogen improves surface finish.
Nitrogen improves production rates.
Nitrogen can contribute to cracking during cold working.

Well 3 out of 4 ain’t bad.

Nitrogen is a chemical element that can contribute to improved surface finish, especially on side working tools. It does so by strengthening the chip, resulting in a crisp separation from the workpiece. The bulk hardness of the material increases with increased Nitrogen as well.
Nitrogen is an important factor, especially in free machining steels. Like 1215 and 12L14.

0708_nitrogen_Table-1(2) — As Nitrogen increases, so does hardness.

Nitrogen is higher in electric furnace melted steels than in steels produced in Basic Oxygen Furnaces.
The down side of higher Nitrogen is that it can result in cracking during cold work- operations such as staking, swaging or crimping.
Nitrogen is “implicitly” specified whenever purchasing chooses a steel supplier. That supplier’s melt process is a major factor on determining the Nitrogen content that you get in the shop.
For a more complete discussion of the role of Nitrogen and how it can affect your precision machining operations, see our article in Production Machining here.
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5 Ways Fine Austenitic Grain Size Affects Your Machine Shop

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While Austenitic Grain Size is a result of chemistry (composition), the changes that it evokes in our process are a result of material structure and properties, not just the chemical ‘ingredients.’
Steel that is fully deoxidized and grain refined is more sound, less susceptible to cracking and distorting, and more easily controlled in heat treat. Well worth it in final performance compared to the machinist’s increased tooling costs.
Here are 5 Ways Austenitic Fine Grained steels can affect your shop:

Poorer Machinability than Coarse Grained Steels. (The hard oxides and nitrides resulting from deoxidation and grain refinement abrade the edge of tools and coatings- this is one reason that you go through more tooling on Fine Grained Steels.)
Poorer Plastic Forming than Coarse Grained Steels.
Less Distortion in Heat Treating than Coarse Grained Steels
Higher Ductility at the same hardness than Coarse Grained Steels
Shallower Hardenability than Coarse Grained Steels.

Friday, May 16, 2008 (8).max — This is a look at Austenitic Fine Grain Steel.

Fine Austenitic Grain Size is a result of DELIBERATELY ADDDING grain refining elements to a heat of steel. Because these grain refining elements have been added, the steel has a “Fine Austenitic Grain Size.”
In order to make steels with this Austenitic Fine Grained Structure, the steel is first deoxidized , (usually with Silicon) and then Aluminum, or Vanadium or Niobium are added. Aluminum, Vanadium, and Niobium are called grain refiners.
After the Silicon has scavenged most of the Oxygen out of the molten steel, the grain refiner is added. (In this post I’ll stick with Aluminum as the example.) The added Aluminum reacts with Nitrogen in the molten steel to form Aluminum Nitride particles. These tiny particles precipitate along the boundaries of the Austenite as well as with in the Austenite grains. This restricts the growth of the grains.
Because the deoxidation and grain refinement create hard abrasive oxide and nitride particles, they machine and process differently than coarse grained steels.
Fine Austenitic Grain Size appears on the material test report as an ASTM value of 5 or greater. Values of 5, 6, 7, 8, or “5 and finer” indicate that the material is Austenitic Fine Grained. Typically 7 or 8 was reported for the Aluminum Fine Grain steels that I certified.
The methods for determining Austenitic Grain Size are detailed in ASTM Standard E112, Standard Test Methods for determining Average Grain Size.
To get the Coarse Austenitic Grain Size Story, see our post here.