“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 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 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.
Art Gonzales says:
Are there rolling-related defects that are transverse to rolling direction; i.e. perpendicular to seams?
speakingofprecision says:
Great Question! During hot rolling, there can be “repeats” impressed into the bar surface if an imperfection or buildup on a roll persists. The strict periodicity of the repetition is a clue to look for a damaged roll. Remember, the bar is moving longitudinally at miles per hour speeds toward the end of the rolling, so any contact with the soft metal by rolling mill guides etc, will cause a longitudinal oriented imperfection because of that speed. I also remember one case where a single “divot” or “notch” occurred what seemed to be sporadically throught an order. Eventually we determined that it was caused by the bar strand striking an inline shear blade just above the line- when the shear had been repaired, the millwrights had reassembled it off by one tooth on the gearing. This resulted in it sitting a few inches lower and when the bar slack caused it to rise, it could just nick the shear blade hanging there…
If you are seeing a repetitive longitudinal pattern of short lines or small chevrons near the end of a cold drawn bar, those can be pusher marks that were not cropped off or Gripper marks that were not cropped off from the drawbench by the cold finisher. I do not have a photograph, but do have some sketches and will create a post based on your question. THANKS!
Rajeshkumar Trivedi says:
Sir,
What is allowable limits of Seams ( length wise & Depth wise width wise) for different Bar sizes ?
speakingofprecision says:
On non resulfurized bar products, it is 0.001″ per sixteenth inch of bar diameter PER SIDE. That would be a stock removal of 0.016″ per side (maximum allowable seam depth) or 0.032″ off the diameter.
On steels where sulfur is deliberately added to improve machinability (Resulfurized Steels) the allowance is 0.0015 per sixteenth of bar diameter per side so on a one inch bar, that would be 0.024″ maximum seam depth and Stock removal , or 0.048″ off diameter.
This standard was originally AISI.
The ASTM A 108 Standard Table A1.1.8 for level 1 products is 1.6% maximum surface discontinuity depth for carbon and alloy nonresulfurized; 2.0% for Resulfurized to (0.08-0.19^ sulfur; and 2.4% for Resulfurized (0.20-0.35% Sulfur)
The ASTM spec has a wiggle room statement that “The information in the chart is the expected maximum surface discoontinuity depth within the limits of good manufacturing practice. Occassional bars in qa shipment may have surface discontinuities that exceed these limits.
Level 1 means no stock removal has been taken by the producer.
Sidra Naseem says:
is that seem apper in resulphurized seel also?
Kareem says:
I’m working as a quality control engineer in EZDK company (Egypt) which produce deformed steel rebars. We use QTB technology for producing rebars. Recently, discontinuous cracks appeared on the Deformed 12 mm rebars surface. Micro structure test showed that the crack depth is about 0.8 mm. I need to know the possible reasons could lead to existence of such cracks.
speakingofprecision says:
YES.
Ken says:
I do believe ASTM A108 is for cold finished bars and not hot finished.
speakingofprecision says:
Hi Ken. Yes that is true. Obviously the Hot rolled bars will need to meet those same standards to make cold finish. Thanks. Miles
Rasheed Khot says:
Recently I have done a lot of trials to eradicate seams on sizes 25mm and above.
It included
1. increasing the Si content in the chemistry
2.Addition of aluminium blocks
3.Increasing the CaSi cored wire
All the above actions were taken to kill the steel properly.
There is a significant improvement on the surface of rebar.
But the elimination of seams on rebar is not yet 100%.
what would be the other probables that are still causing the seams?
Gary Caudill says:
What NDT method is used to detect seam cracks? what ASTM spec is used for this kind of sort?
speakingofprecision says:
for bar product http://www.astm.org/Standards/E543.htm
for products try this company:
http://www.modalshop.com/ndt.asp
Miles
speakingofprecision says:
Reduce sulfur and copper content.
Rasheed Khot says:
is there any preferable range of “S” & “Cu” to have seams free surface?
speakingofprecision says:
Rasheed, I can’t get you to seam free, but for best quality I would hold Copper at 0.020 max. Sulfur limits low as possible, but it is really more about the ratio of Manganese to sulfur. I would want to be high range of Managanese and low range of sulfur to minimize surface irregularities such as seams.
Rasheed Khot says:
Thanks for valuable information
Appreciated the quick response
robin kumar Bagchi says:
Is there any co relationship between banding and seams?
AJ says:
thank you for posting as this helps me in my everyday inspection on rolled products
speakingofprecision says:
Only that they both seem to be attributes of rolled steel products. Seams tend to be associated with Sulfur. Banding is independent of the sulfur in steel, which is tied up in the manganese sulfides.