“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

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 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.

Compressive Stress is important in the forging, stamping, coining, and cold heading industries. It is compression stress that is used to change the shape of the product. This is different than in our machining industry, where we create the shape of the part by subtracting material by some means of stock removal.

Compressive stress is caused by an applied load that is acting to reduce the length of the steel in the axis of the applied load. Because the forces acting on steel are in the same axis (collinear) with the longitudinal axis of the member, these forces cause the steel to either shorten or stretch.

Compressive stress causes different failure modes in brittle and ductile steels.

Compressive strength is the limit of compressive stress that the steel can withstand before failing in a ductile failure.

  • When steel’s compressive strength is exceeded, the steel will fail in a brittle fashion, and it will shear, usually at a 30 to 45 degree angle.
  • When I see cracks at angles in the range of 30 to 45 degrees from the direction of applied load in steel, formed by cold working deformation, I know that the failure is a brittle mode.
  • This does not mean that the steel itself was too brittle, it may mean that the angles and loading in the process tooling were incorrect, causing the compressive limit of the steel to be exceeded.

When I installed a cold heading wire drawing line  in my mill, my employees preferred to call our compression or upset test the “Squeeze Test.”

We upset test (compression test ) small samples of steel from each coil of wire to see the failure mode of the material after drawing, and to see if any seams opened up as the section thickness increased.

Image from my archival copy of a chart from Steelways 1955.