Question:Lamination and Carbon electrode

Date 04-11-2008 05:35

The lamination I have seen in steel is from a slag pocket that gets rolled out to cover a rather large area. I would tink You should be able to get the material with no lamination, but as I am not familliar with the material You mentioned, I can't say for sure.

By Richard Cook (**)

Date 04-11-2008 12:18

You would have to require such in your purcahse order, it would require that Ultrasonic testing be specified for verification. I to am not familiar with the material referenced, but what I deal with would be specified as stated above. You will be paying big bucks for this, because they will test through the batch until they find what you want $$$$$$$.

Check the material specs, my industry deals wtih the ASTM standards, for what it states about lamination and the repair of such. It is also dependent on the end use of the material whether lamination would be a factor any way, because of it's orientation. In my industry lamination up to certain limitations is accepatable, also dependent where it falls also, like at a welded joint.

good luck

By js55

Date 04-11-2008 13:25

Technically speaking there will always be laminations on rolled material. Laminations being not only the slag variety but sulphides and other non metallic inclusions that coelesce during the rolling process. They can be greatly minimized with high purity materials but they are still there. Or, as Richard stated you search through standard material to verify that any delamination is within stated tolerance.
The problem becomes delamination, to be inspected usually by UT, wherein the planar coelesced non metallic inclusions cause a detectable gross planar seperation of the material.

By G.S.Crisi (****)

Date 04-11-2008 14:45

Laminations.
Pig iron that comes out from the blast furnace has a carbon content of 3 - 4 %, due to the excess coke that has been used to make it.
This high carbon content is eliminated when making steel in the electric furnace (or open heart or Bessemer) by means of a pure oxygen injection that transforms the solid carbon in gaseous carbon dioxyde.
The ingots made out of this steel are full of carbon dioxyde bubbles and is called rimmed steel. When it is rolled, the bubbles collaps leaving a discontinuity called lamination. Of course, this is a low quality plate and ASTM standards don't accept it. I don't know in the USA, but here in Brazil steel mills roll that plate, which is called "commercial quality plate". This means that the mill doesn't give any warranty on that product and the user uses it under his responsibility.
It's often used to make parts of equipment subjected to no or little stress, as for example a hopper containing powdered material, the baseplate of a small pump or a chute between a belt conveyor and a bucket elevator.

The carbon dioxyde contained in the melted steel in the electric furnace can be eliminated giving a bubble free steel called killed steel. When rolled, this steel should not have laminations in it. This is the good quality plate, or shape, accepted by ASTM.
There are two means of eliminating the carbon dioxyde in the melted steel in the furnace: using aluminum or silicon.
In the first case, you obtain a fine grain carbon steel useful for low temperature service. An example is ASTM A-516.
In the second case you obtain a coarse grain carbon steel useful for high temperature service. An example is ASTM A-515.
Now, you've noticed that I didn't say "it doesn't have laminations". Instead, I said "SHOULD NOT have laminations". There's a difference between those two expressions.

Of course Dave Boyer is right. Laminations can also occur due to slag inclusions, and he explained very well how they show up.

Carbon electrode.
Back in my days of erector engineer, I've always used carbon electrodes to backgouge the root pass of a butt weld between two plates, as for example in a pressure vessel or a storage tank.
I personally don't like to use carbon electrodes to remove a weld defect. Theoretically is OK. In practice, however, the welder must be very good and well trained so as to remove just the weld and not damage the base material. The danger of damaging the base material is high and I prefer to avoid it.
I personally have always preferred to remove a weld defect by means of a grinding disk.

Giovanni S. Crisi
Sao Paulo - Brazil

By js55

Date 04-11-2008 15:02

Giovanni,
You need to come around a little more often. This one's gettin printed up and going into my file.

By gokhansbox Date 04-12-2008 07:05

Dear all ,i really appreciate about your answers,i really well informed.
one more thing i would like to know is ,

Giovanni you saying 'Carbon Electrode' is a danger especially during removing the weld defect by means of parent metal! i would like to know what kind of a danger(HAZ?Excessive Carbon?Decrease in Strength?) and i have 150mm thickness plates and grinding seems not a practical solution i really dont have an option but carbon electrode however i really afraid to use carbon electrode.

Regards,

Gokhan,
Kazakhstan

By 803056

Date 04-12-2008 16:58

As is the case with any welding process, training and experience is part of the learning process. No one is born the perfect welder or skilled at air carbon arc gouging.

In the right hands, air carbon arc gouging can be used to efficiently remove metal without damaging the base metal and the process is not limited to ferrous metals. The quality of the cut or gouge is dependent on the welder's skill.

I've used air carbon arc gouging to remove unacceptable welds and perform back gouging operations for many years as an ironworker. While AWS D1.1 doesn't require the back gouge to be ground to bright metal, I always did if for no other reason than to remove the light oxide that forms on the surface of the metal (similar to the oxide that results from any oxy-fuel cutting operation). I've performed air carbon arc gouging on military work, nuclear, and commercial code applications with no problems or issues passing radiographic or ultrasonic examination requirements.

In the wrong hands, the air carbon arc gouging operation can result in very rough surfaces, oxide inclusions if the gouge is not ground to bright metal, arc strikes, etc. It would be akin to handing a chain saw to a youngster. The damage to the surrounding living room furniture, walls, floors, ceilings, etc. can not be imagined until the youngster runs out of fuel and you dare to walk in to see the results for yourself. There may even be a severed limb or two on the floor.

Air carbon arc gouging doesnt lead to any unusual problems, carbon deposits, copper deposits, oxide inclusions if the proper technique is employed. It even results in less "over heating" of the base metal than oxy-fuel gouging operations. It can be used on quenched and tempered steel alloys without difficulty. However, because it removes metal fast, the proper level of care is needed to prevent removing too much metal.

I happen to like air carbon arc gouging, but as I mentioned earlier, it should only be used by people that have received proper instructions on how to perform it and they should have some experience before being "turned loose" on critical work.

Best regards - Al

By G.S.Crisi (****)

Date 04-14-2008 17:46 Edited 04-14-2008 23:18

Al,
we agree 100%. As I told on my former posting, removing defective welds by carbon air gouging is OK, provided the welder who does it is qualified and well trained.
To be on the safe side, back in my days of erector engineer I've always preferred to carry out the removing by means of a grinding wheel. If this is unfeasible, then go ahead with arc gouging, but make sure that the welder is a good one.
I've got a letter from Arcair, the inventors of carbon arc gouging, saying that, in fact, grinding to bright metal after arc gouging is not actually necessary. Of course Al, this doesn't invalidate your decision in doing it, aiming in obtaining a perfect weld.
Giovanni S. Crisi

By 803056

Date 04-14-2008 18:02 Edited 04-14-2008 18:36

Hello Giovanni;

How are you?

We're starting to see the early signs of spring here in New England. A welcome sight indeed.

As you indicate, the need to grind to bright metal is based on the level of quality required. I equate it to the mill scale found on any hot rolled steel material. In most cases, the oxide associated with air carbon arc gouging is not as severe or as thick as that found on hot rolled material, but never the less, it should be removed when the highest quality welds are necessary. The amount of carbon introduced is minimal, if any, when the process is carried out properly. Any carbon introduced into the HAZ is by diffusion, which is time @ temperature dependent. The length of time at which the temperature is high enough to promote diffusion of the carbon into the HAZ is minimal. Any hardened HAZ associated with air carbon arc gouging is most likely due to the rapid cooling from the austenizing temperatures attained while gouging. Even then, the depth of the HAZ is akin to that developed as a result of oxy-fuel cutting, no better or no worse.

In my opinion, any welding performed after gouging will temper the HAZ. Any increase in hardness or increase in the depth of the HAZ is a result of the slower travel speeds and higher heat input and rapid cooling associated with the welding operation rather than the gouging operation.

With regards to crack removal, care must be exercised to prevent the crack from traveling. I found that in ductile base metals, gouging from beyond the crack tip toward the center of the crack, stopping, and gouging from beyond the opposite end of the crack toward the center, minimized the growth of the crack. Failure to do so is a recipe for disaster.

I had a foreman ignore my recommendations on one project (this was when I still used a welding shield instead of a computer to earn my living). They attempted to gouge a crack without regard to my recommendation and chased the crack right around a vessel. I did get the overtime pay to complete the repair. Still, the foreman faulted me for not objecting more strenuously. However, the same foreman stated, "I pay you from the neck down, not the neck up!"

I learned at an early age to never call the foreman an idiot or to intentional make him look stupid. That's "job suicide." Most foreman were able do that without my help.

As for the root cause of lamination; I have to agree with my friend Giovanni on this one. The root cause of a lamination, seam, pipe, or scab originates in the ingot from which the hot rolled product is rolled. Killed steels use deoxidizers to minimize the formation of carbon dioxide (oxygen combining with free carbon) in the ingot as the liquid metal solidifies. In the event carbon dioxide does form (and it will in rimmed steels, and to a lesser extent in semi killed steels as well as killed steels), the gas can be entrapped in the ingot just as gas can be entrapped in a weld. As such, it manifests itself as a pore, only in an ingot the pore can be very large. If it isn't completely removed, the pore (gas pocket) will be flattened and elongated by the subsequent rolling operation. Since the top center of the ingot is last to solidify, the bulk (not all) of the porosity will be found in the center of the upper portions of the ingot, the lamination is most often found toward the center of the thickness of the hot rolled plate, center of the web of a beam, or center of a bar.

Likewise, dross, oxides, low melting point constituents (LMPC) such as phosphorus, lead, excess copper, sulfur, etc. tend to segregate toward the center of the top portion of the ingot because that is the last region of the ingot to solidify. That portion of the ingot is supposed to be removed, but some of the objectionable material may remain in the portion of the ingot that is rolled into blooms, billets, or slabs. Subsequently, they will occur in the final hot rolled products as segregation's (also know as banding) which can cause welding issues during fabrication. The worst cases usually occur in thick sections where the rolling operation fails to "homogenize" the LMPCs and oxide stringers. They are less problematic in the thinner materials that are subjected to more "mixing" or "homogenization" by repeated rolling operations. That isn't to say they are never a problem with thin materials, just less so.

Good to read your responses. I always look forward to your well formulated opinions.

Best regards - Al