I'm not sure what AWS code you are working with but, the key words in D1.1 regarding scale, slag, rust, and etc. are, "...that would prevent proper welding or produce objectionable fumes. " In view of what you've done to prove that your processes work when welding over the laser oxides, I'd say you don't have a problem.
CHGuilford
I would also agree but others are not as sure. Do you know the chemical composition of the laser oxide present in low-med carbon steels when cut by laser. It's a blue oxide so I am assuming is isn't one of the typical oxide states.
What you've got on the steel surface inmediately after laser cutting that you say is blue color are not oxides, but just a discoloration due to the temperature the metal withstood. The blue color is not related to the chemical composition, for there are no oxides, but to the temperature. I don't remember now, nor I've gor the time to check it out, to which temperature range the blue color corrisponds. Have you ever seen the metal chips resulting from machining a steel piece in a lathe? Of course you did, at John Deere there are a lot of lathes producing tons of chips every day. Have you noticed that very often the chips are colored blue, yellow etc.? This is because of the temperature they were subjected to during machining. OK, it's the same thing with your laser cutting. If you definetely want to know to which temperature the blue color corrisponds, there are colored tables indicating that. Here in Brazil those tables are known as "Color due to normalizing and annealing". I'm quite sure at John Deere you must have a table like that.
Now, to answer your question, let me try an analog way in these times that everything is digital. The ANSI/ASME B.31 Code for Pressure Piping (there are 8 sections but all of them coincide in this point) says that the "slight discoloration" remaining on the steel surface after oxyacetilene cutting is not detrimental to the weld and doesn't need to be removed before welding, provided, of course, that the weld is done right after cutting so that oxide hasn't had time to form. I know this perfectly well because back in my days of erector engineer, during the construction of an oil refinery we (the contractor) had homeric discussions with the client regarding that matter. At a certain moment I (the Field Engineer) got fed up with such sterile discussions, grabbed the B31.3, which applies specifically to oil refineries, and showed to the client what the Code said. The discussion was inmediately over.
Now, to apply the analogy, if the discoloration due to oxyacetilene cutting is OK for the Piping Code, the one due to laser cutting, a much better method, should also be OK. Agree?
Giovanni S. Crisi
Sao Paulo - Brazil
I certainly agree with G.S.Crissi.
However, if you are dealing with people who are genuinely concerned about weld quality, and who will not accept the work you've already done that shows good weld integrity, you could have some chemical analyses done on cut and un-cut surfaces. The cost won't be that high and it might put the matter to rest.
CHGuilford
I have to respectfully disagree with Mr. Crisi's opening comments. The blue coloring on the surface is indeed an oxide, or possibly a nitride. The oxide films lead to refraction of the incoming light, and thus the coloring. The thicker the oxide, the darker the color. These films are very thin, less than microns thick. It depends on the type of oxide whether or not it presents a problem. Iron oxides will melt in the heat of an arc and are therfore not a problem. Chromium oxides are more tenacious and may present a problem. Anything that is black or brown should not be acceptable.
From a practical standpoint, I have worked with a lot of laser cut stainless steel and have not had a problem with welding on the blue colored surfaces. Also, if you are using GMAW, you would have cathodic cleaning action like the reverse cycle of an AC tig weld, so the oxide will be "ripped" off by the electrons.
Regards.
I regard this site as a high level forum where all of us have the opportunity to learn something new, and I've surely learned a lot.
I must say that I'm not fully convinced of pdweldor opinion, but, as I haven't got the time to make some further reasearch for the time being, I'll accept it as true.
Every year I give to my pupils of Mackenzie University a exercise to solve after making some research in books and publications. This year, I'm thinking that the exercise could be based on the problem we are faced to: what exactly is the blue color observed after laser cutting? Don't you genlemen think is a good idea?
Giovanni S. Crisi
By Lumpy
Date 08-14-2001 15:00
I'm going to go with pdwelder on this one, and refer you to a book published by John Wiley & Sons called Physical Metallurgy, it covers oxidation in great detail. I can appreciate that seeing is beleiving, find a decent metal process house in the area, one with a vacuum furnace. A couple of easy heat treat samples in an evacuated atmosphere will give your students something to think over.
I entirely agree with most of the postings in that this "effect" is entirely insignificant in the context of welding for most applications.
We are the students at the Engineering School of Mackenzie University in Sao Paulo who, under the supervision of Prof. Giovanni S. Crisi, carrried out a research on the chemical composition of the blue color which remains on the metal after laser cutting.
Our conclusion is that the blue film consists of Ferrous Ferrocyanide (chemical formula Fe4[Fe(CN)6]3) which forms due to chemical reaction between the ferrous base metal, the carbon it contents and the nitrogen used for purging during the laser cutting.
The ferrous ferrocyanide film is only a few thousands of a millimeter thick and does not influences adversely the welding the material will be subjected to right after.
Ana Fulvia Augusto Bonini
Liliane Vallin Rodrigues
Fabio Pereira Medrado
Ricardo Francisco Cabanas
This sounds very interesting. Have you written a report that gives more details? If so, have you published it somewhere we can get hold of it? If not, please do so. It can be published free of charge on the proknownet website:
www.proknownet.com
Let us know if you have published it so that we can also read it. Is'nt it great to see some work coming out of a question posed here? Thanks for your clarification of this!
Niekie
Indeed! Your information is much appreciated! Thank you.
CHGuilford
