Scabs are irregularly shaped, flattened protrusions caused by splash, boiling or other problems from teeming, casting, or conditioning.-AISI Technical Committee on Rod and Bar Mills, Detection, Classification, and Elimination of Rod and Bar Surface Defects
(Teeming refers to the process of filling an ingot mold with molten steel from the ladle. We’ll point out some continuous casting analogs later in this post.)
Scabs have scale and irregular surfaces beneath them; they tend to be round or oval shaped and concentrated to only certain blooms or billets. Scabs are always the same chemistry as the steel bloom or billet.
(If the gross irregular surface protrusion characteristic is appearing on all product, it is not likely to be a scab. If the protrusion is a different analysis, it is likely to be mill shearing.)
To differentiate between scabs and rolled in scale, scabs are ductile when bent while scale is brittle and crumbles.
If the protrusion is brittle, it may be rolled in scale.
Scabs are primarily an ingot process issue related to teeming, but we have seen them on continuous cast products as a result of mold and tundish anomalies.
Scabs present with scale beneath; Cracks may (but are not always) be present associated with the scab due to stress concentration causing the material underneath to crack. (Not the crack causing the scab…)
Ingots or blooms showing scabs should be conditioned to remove the scabs. Thermal conditioning of billets (hot scarfing or torch conditioning) can sometimes leave artifacts which present as scabs upon rolling.
While scabs can be confused with slivers, shearing, rolled in scale, or tearing, their ductility eliminates them as rolled in scale. Scabs are distinct from shearing as scabs are isolated by occurrence and have an irregular surface beneath them, while shearing usually presents as multiple instances in a single orientation along the bar. Tearing is characterized by chevron shaped breaks rather than oval shaped protrusions.
We remember that not all returned from “that cause for which they gave the last full measure of devotion.”
Many of the people in our industry have served in the armed forces.
Some of our companies have employees on active duty.
Some of us have family serving.
You may have sung the Star Spangled Banner a thousand times. You may have heard it 10,000 times.
But you hear it with your whole body when it is your daughter or son that is deployed.
This Memorial Day, we will remember.
We are not at war.
Our politicians are not at war.
Some of our employees, some of our sons and daughters are at war.
This Memorial Day, we will remember.
We will show we are grateful for their service.
This memorial day we hope that you remember too:
“…that these dead shall not have died in vain, that this nation under God shall have a new birth of freedom, and that government of the people, by the people, for the people shall not perish from the earth.”
Excerpts from Abraham Lincoln’s Gettysburg Address
“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.
Polyamide is used in a wide variety of automotive technologies- yarn for airbags and tires; engineering plastics under the hood, transmission elements, cooling circuits, hand brakes, hub caps and shells for exterior mirrors.
We just saw a press report that a unit of Solvay group declared force majeur on its Polyamide production due to a fire at its plant in Paulinia, Brazil:
“On May 16th, 2012, at around 8.30pm, a fire occurred at our site of Paulinia (SP – Brazil), which severely damaged the electric cabin of our adipic acid production unit. As a consequence, Rhodia Poliamida e Especialidades is obliged to declare Force Majeure. We are currently investigating all possible alternatives to mitigate the impact on our customers. Allocation measures are being elaborated as stipulated by the Force Majeure regulation. Force Majeure will be lifted as soon as normal production activities recover.”- Rhodia press release
Force majeur is a clause that is included in contracts to clarify performance obligations when a catastrophe not under the control of the parties prevents the fulfilling of the contract. In the old days these were often called “Acts of God” clauses.
When we were kids playing hide and seek we had a term for Force Majeure too: Ollie ollie in free. It meant that it was safe to come out from hiding you couldn’t be tagged “it.”
It originated from ‘All ye, All ye, outs in free’ taken from the the British usage.
This Rhodia declaration of force majeure comes less than a month after the explosion and fire at Evonik Industries in Marl Germany that we discussed here.
Why are we posting about this on a precision machining blog?
If your automotive releases seem to slow down – it may not be due as much to the lack of consumer demand…
…as it is due to the wisdom of single sourcing critical materials and the decision to use lean supply chains.
Effective August 20, 2012, this standard provides guidance for organizations regarding:
Operational polices including:
Business and personal use of vehicles;
Driver-owned and leased vehicles;
“This standard gives employers a credible tool that is reflective of best practices. Employers who operate noncommercial fleets can learn from commercial fleet operators on matters of common interest,” ASSE professional member and ANSI/ASSE Z15 committee member Tim Healey, director of safety, at The Hartford Steam Boiler Inspection and Insurance Co. (HSB), said. “After all, common management goals, regardless of who the fleet operator is, should include crash avoidance – and all of the fatalities, injuries, property damage and expenses that result – as well as economical/efficient operation. No employer can afford to have an employee operate under the influence, text, or fall asleep while driving on company business, whether it is in an 80,000 pound tractor trailer or a 3,600 pound sedan.”
If you do not now have a “written motor vehicle safety program that defines organizational requirements, responsibilities and accountabilities for drivers and motor vehicle safety; a written safety policy that includes communicating management’s concern for the health and well-being of drivers throughout the organization; a system of responsibility and accountability in order to ensure effective implementation of a vehicle safety program,” this new ASSE standard can help you implement these best practices.
Compliance comments: “The use of American National Standards is completely voluntary; their existence does not in any respect preclude anyone, whether he/she has approved the standards or not, from manufacturing, marketing, purchasing, or using products, processes, or procedures not conforming to the standards.”
SPEAKINGOFPRECISION SAYS: It is likely that this standard will be come the basis for regulatory enforcement as its best practices are adopted.
Below are the 2012 TRI reference documents for PMPA member companies. These materials will help you understand the requirements, explain the issues, and will help you to make the calculations needed for your Form R Reporting.