Hello Gene, I take this discussion about welding and combustibles as an oopportunity to clarify a few things. 1. CO2 is used in arc welding only for old soft iron (mild steel) in order to maintain the percentage of carbon of the components also in the seam material. It's a balance reaction of consuming carbon by oxidation and returnig this amount by decomposing CO2. Without this, the properties of the material would be strongly affected, e.g. iron turning brittle jeopardizing constructions. Therefore, there are welding gases on the market containing different amounts of CO2 mixed with argon. Linde AG gave it the brand name of CORGON. For all other metals including stainless steel welding a pure inert gas is used, preferably argon because of the low price. It is obtained in the process of air liquefying as a byproduct. That means, it should cost nothing at all because air is actually liquefied to obtain liquid oxygen and nitrogen....:-) 2.) Thermite is a mixture or aluminium powder and iron oxides. When kindled, It develops temperatures above 3000 degrees C (5400 F) by burning (oxidizing) the Alu and in turn reducing the iron oxides to metallic iron with sufficient surplus energy to melt this iron at the instant and even melt the surrounding substances such as railway tracks. However, the thermite mix can't be started with a simple match, so a small strip of magnesium sheet is used as an iginiter. Alu is so reactive and, at the same time, safe, that today, it's the main component of rock blasting. 3.) CO2 is a common fire extinguishing agent especially for strange and rare combustibles. It can't give off oxygen at the temperatures considered. It is, however, dangerous for alll personell around and, at least hereabouts, there must be a alarm time before the CO2 cylinders are fired. This delay might be detrimetal to the success of fire extinguishing. Some substances like thermite can't be extinguished because they carry their oxygen inside. 4.) I don't think there is much use in applying mist to machine tools. There is way too little effect compared with flooding. Modern lathes and mills are capsuled and coolant is directed with high pressure from as many of 10 to 20 nozzles from all directions onto the tools. This makes it possible to mill stainless steel with, say, 15.000 rpm or Alu with up to 100.000. I bought a CNC mill some years ago and all the elder statesmen in the shop shook their heads when they first saw what happened. Point is: the machine adjusts its speed automatically to tool diameter and material to be processed, and all the workers, with their lifetime experience, believed that the machine must be in error and the tools won't make it longer than an hour. 5.) I can't see what all this has to do with electronic machine control...:-)))
Greeting from Germany Peter Blodow Dipl.-Phys. (and don't call me wise guy, I used to be the head of a rather special scientific work shop for more than 30 years) At 12:04 21.11.2009, you wrote: >On Saturday 21 November 2009, Erik Christiansen wrote: > >On Fri, Nov 20, 2009 at 05:10:18PM +0000, Leslie Newell wrote: > >> Carbon dioxide is 66% oxygen (CO2). As aluminum is very active it will > >> strip oxygen out of the CO2. That is also the reason why you should > >> never use a CO2 fire extinguisher on magnesium fires. > > > >However, it is extensively used in fire extinguishers precisely because > >it does not give up its oxygen even at hundreds of degrees C. I'm not > >sure of how many thousand degrees magnesium burns at, but it is more > >than 1500, because thermite (magnesium and iron oxide) combustion melts > >the iron produced by the reduction of the iron oxide. > > > >If the cutting operation is hot enough to dissociate CO2, then there'd > >better not be any oil about, especially as mist, unless Gene has his > >detonation-deadening earmuffs on tight. ;-) > > > >Wikipedia appears to be self-contradicting: > > > >"Carbon dioxide also finds use as an atmosphere for welding, although in > >the welding arc, it reacts to oxidize most metals." > > > >The closest I've come in a quick search is: > > > >http://www.hitech-inst.co.uk/pdfs/technical/heat_treatment.pdf which > >says: > > > >"The graph shows cell output against carbon monoxide/carbon dioxide > >ratio. this is plotted at 634°C and 812°C, ..." > > > >i.e. CO2 is still so completely undissociated at 812°C that ratios can > >be measured for metallurgical analysis. So the aluminium would melt > >long before the CO2 dissociated to any measurable degree. > > > >Erik > >True, but those temps are not the temps of the arc, by about a factor of 10. >My single sample that was badly decomposed, blackened and porous, I blamed on >the 75% argon/25% co2 in the shielding gas I had. I theorized that the o2 >was used up by the alu's reaction, leaving the c to blacken and contaminate >the alu puddle. I resolved to get some purer argon, but locally it is not >available. The mixed bottle, about 4" in diameter and maybe 18" tall, is >$125 at TSC. Samson's in Clarksburg are similarly priced. And that makes it >a very expensive toy for me. One that I only drag out when there is no other >choice. > >-- >Cheers, Gene >"There are four boxes to be used in defense of liberty: > soap, ballot, jury, and ammo. Please use in that order." >-Ed Howdershelt (Author) >The NRA is offering FREE Associate memberships to anyone who wants them. ><https://www.nrahq.org/nrabonus/accept-membership.asp> > >Power corrupts. Absolute power is kind of neat. > -- John Lehman, Secretary of the Navy, 1981-1987 > > >------------------------------------------------------------------------------ >Let Crystal Reports handle the reporting - Free Crystal Reports 2008 30-Day >trial. Simplify your report design, integration and deployment - and focus on >what you do best, core application coding. Discover what's new with >Crystal Reports now. http://p.sf.net/sfu/bobj-july >_______________________________________________ >Emc-users mailing list >Emc-users@lists.sourceforge.net >https://lists.sourceforge.net/lists/listinfo/emc-users ------------------------------------------------------------------------------ Let Crystal Reports handle the reporting - Free Crystal Reports 2008 30-Day trial. Simplify your report design, integration and deployment - and focus on what you do best, core application coding. Discover what's new with Crystal Reports now. http://p.sf.net/sfu/bobj-july _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
