Seams On Rolled Steel Products

May 24, 2012

“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.

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Leaded Steels-What You Should Know

November 9, 2010

Lead is NOT banned by the European Union’s End of Life Vehicles Regulations for machining purposes in steel, aluminum and brass.

PDF HERE

Not banned in every application...

Lead is NOT banned by the European Unions Restriction of Hazardous Substances (RoHS) Directive.

The exemption for “Lead as an alloying element in Steel containing up to 0.35% lead by weight, aluminum containing up to 0.4% lead by weight, and as a copper alloy containing up to 4% by weight.” This exemption is located in article 4.2 and Annex, line 6.

UK link to RoHS exemptions

If even the European Union recognizes that additions of Lead in materials for machining is worthy of exemption, Lead must provide some significant benefits…

  • “Boosts machinability 25% at lower cost”- Pat Wannell, La Salle Steel April 1994, quoted in Modern Metals Magazine
  • “Cutting Speeds can normally be increased from 15-25% above those employed for the standard grade”- Monarch  Turning Manual
  • “Lead, found mainly enveloping manganese sulfide inclusions, promotes machinability in two ways, possibly three. By forming a layer of liquid lubricant at the tool chip interface, it reduces the stress required to overcome friction. By acting as an initiator of microcracks and, possibly, by causing some liquid metal embrittlement, it reduces the deformation stress.” American Machinist Special Report 790.
  • In our experience we have found leaded steels to  lower cutting temperatures and reduce wear rates on tools, resulting in greater up time. Surface finish on leaded materials  is superior to those on non leaded equivalents.

Increasing speeds and production, reducing power needed (and thus greenhouse gas emissions), and improving surface finish are some powerful advantages that are provided by the addition of lead to materials for precision machining.

What’s the down side?

In this photo lead is visible as tails (pointed out by arrows.)

1) Lead is not soluble in iron.  It is therefore a separate phase in the steel, usually visible enveloping the manganese sulfides as tails, though sometimes appearing as small particles.

2) Lead has a greater density than iron. This means that it will tend to segregate given enough time while the metal is liquid.

3) Lead has a relatively low melting point (liquidus) compared to steel. This can mean that at processing temperatures for heat treatment, leaded steel parts can ‘exude’ lead

These three factors mean that if you ABSOLUTELY MUST HAVE parts that are free from possible segregation, parts that will not have potential hollows or porosity after being exposed to high temperatures, and absolutely no visible indications of a separate phase in the steel (ie. what the shop guys call  “lead stringers,” you probably ought to forego the leaded grade.

And forego  the 25-30 % savings that it gives you on the piece part machining cost…

You want highest machinability or highest product integrity?

Take your pick.

Periodic

Photo of Lead on Manganese sulfides from L.E. Samuels Optical Microscopy of Steels.

Coin Flip

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Killed Steel

July 20, 2010

Some things you want to have bubbles, some you don’t.

Usually, Bubbles are good.

In beermaking, yeast consumes the sugars in the wort and convert them into CO2 gas bubbles- carbonation.

In steel making the main reaction is the combination of Carbon in the melt with Oxygen to form a gas. At the high temperatures involved, this gas is very soluble in the molten bath.

If the Oxygen that is available for this chemical reaction isn’t completely removed before the steel is cast the gases will continue to be forced out of the melt during solidification, resulting in porosity in the steel.

Bubbles and where the gas goes can be important in your steel part.

In order to control the evolution of gas, chemicals called deoxidizers are added to the steel. These chemicals, Silicon or Aluminum, Vanadium, Columbium, Niobium scavenge the available oxygen in the molten steel, react chemically to form solid oxide particles dispersed throughout the steel, rather than bubbles of Carbon Dioxide.

The amount and type of deoxidizer added determines the type of steel. If sufficent deoxidizers are added, no gas is evolved from the solidifying steel, and the steel is said to be “killed.” The ingot drawing labelled number 1 shows a fully killed (deoxidized) steel showing only a shrinkage cavity, and no bubbles or porosity. ( This shrinkage cavity would be cropped off in normal rolling practice.)

Because gas is still evolving, this beer is NOT KILLED.

Killed steel has more uniform chemical composition and properties than rimmed, semi-killed, or non-killed steels, and generally less segregation. The uniformity of killed steel and and its freedom from porosity makes these steels more suitable for critical components and for applications involving heat treatment.

Killed steels generally contain 0.15-.35 weight percent Silicon as a deoxidizer, and may contain  some of the other elements as mentioned above. These other elements may be used as deoxidizers or as grain refiners.

Steel grades with a Carbon maximum of 0.30 weight % and above, and all alloy steels are typically provided as “killed steels.”

Free machining steels such as 12L14, 1215, and some 11XX series steels are not “killed” with Silicon, Aluminum, etc., due to their deleterious effects on tool life and machinability. The high amounts of Manganese  in these steels form Manganese Sulfides to promote machinability, and also the Manganese scavenges excess Oxygen, preventing  evolution of CO2.

Killed steel is specified so your critical parts won't have bubbles in them.

Killed steel- for critical parts. Non-killed beer for critical  after work down time.

Cheers!

Beer Bubbles Photo Credit

Ingot scan from a handout in my files originally after Making Shaping and Treating of Steel.

 Beer Head Photo Credit

Bread with Holes

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