Warm it up!

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Cutting steel with a torch, also known as flame machining, is more challenging than welding.  Really precise cutting takes a lot of attention to detail.  Among the many different factors that will help you make clean, low slag cuts is to warm the steel up with the preheat flame before you start to cut.  Although it sounds crazy, steel is not a homogeneous solid.  Steel is a mixture of microscopic particles.  The metalurgists call it a "solid solution".  I compare it more to a sponge.  Steel, especially mild steel, will soak up all kinds of crap.  Water in particular.  Did you ever pass an oxy-acetylene flame over a piece of steel and see a small patch of condensation appear on the steel?  That is water that has soaked into the surface of the steel.  Steel will also soak up oil, oxygen, nitrogen, and other contaminants.  The best way to eliminate those contaminants is to use the preheat flame to bring the steel up to near cutting temp before you initiate the cut.  If the steel you are working with has been outside in the weather, or its very humid, or the steel is wet, or old, or painted this little practice will help a lot.  Torch flame "popping" is familiar to anyone that regularly uses a cutting torch.  Frequently, that "pop" is made because a minute inclusion of some contaminant, usually water, has rapidly gasified and momentarily extinguished your torch flame.  During that split second, unburned gas mixture builds up around the torch tip, then ignites from the molten metal present.  That's the "pop".  Cooking out contaminants ahead of time reduces that a lot.  Even if you don't get a "pop", cold steel just doesn't cut right.  Once the steel has been preheated, you can greatly reduce the length of your preheat flames, torch tip size, and oxygen pressure.  Warm it up before you weld too.  The higher the alloy, the harder the metal, the more some preheat helps.  Most welding supply stores sell tempil sticks.  These sticks are made from a hard wax that melts at very specific temperatures.  I don't use the tempil sticks myself, but many high end critical weld shops must use them every time.  Power plant welders, pipe welders, ship welders, oil refining plant welders, boilermakers, etc. must use them to pass very stringent weld inspections.  It will help the quality and appearance of your welds too.  Easy on the preheat for stainless steel.  SS steel is a very poor heat conductor. The heat balls up right at the weld and doesn't commute to the surrounding area. This is why stainless is so prone to warping.One exception!  Never preheat autobody metal, or other really thin sheet metal.  It'll warp instantly.  Autobody welding I defer the the Body shop metal techs.
Reply:Topdeadcenter, I'm no metallurgist, but I have doubts about your observation:"Steel, especially mild steel, will soak up all kinds of crap. Water in particular. Did you ever pass an oxy-acetylene flame over a piece of steel and see a small patch of condensation appear on the steel? That is water that has soaked into the surface of the steel."I believe that the water you see on the surface of steel after passing a flame over it is the water vapor in the combustion products in the flame condensed onto the cool steel surface, not water drawn out of the steel.  Try the same thing on a cold material that you believe does not absorb water and you will see the same condensation (assuming similar thermal mass and conductivity).I have no doubt at all about the benefits of preheat (under most, but not all circumstances), but I think you may have an exaggerated impression of how much crap steel absorbs.I also question your observation of cutting torch popping being a result of water pockets in the steel vaporizing.  I believe popping is usually a result of poor torch condition (especially seals), improper gas pressures, or poor operator technique.  I reached the point with my OA cutting torch that it was popping almost continuously.  I don't think my work piece materials had gone bad or were being manufactured at lower quality.  I think my torch seals were going bad.  Unfortunately, I never proved that since I got a plasma cutter about the time and just retired the cutting torch.I'm not saying that steel does not get contaminated at all.  However, I don't think it is as big a problem as you describe for our routine, non-critical projects.  I think we should look at our equipment and technique, rather than blaming the material, for most such problems (assuming, of course, that we have properly cleaned the work piece).Of course, cast iron IS a sponge and is easily or usually contaminated.  Preheat or baking out is especially beneficial in that case.awrightLast edited by awright; 01-20-2007 at 03:07 PM.
Reply:Yup,  maintaining your equipment definitely eliminates many welding and cutting problems.  No doubt.  My intent was to suggest easy best practices that would eliminate frequently encountered problems.  We all develop our own style and tricks of the trade.  If my suggestions help someone just starting out, then I've made my contribution.
Reply:Wow.... just, wow...  Originally Posted by TopdeadcenterCutting steel with a torch, also known as flame machining, is more challenging than welding.
Reply:First I have to pick myself off the floor after rolling around in laughter.  Steel does not "absorb" water.  You are seeing the water vapour which is a product of combustion condensing on the surface.  The reason that you pass your torch quickly over the surface is to pop off the oxide scale and debris so that when you are cutting the preheat cones effectively transfer heat into the plate in order to raise it to kindling temperature.  Oxide expands at a different rate than the steel underneath resulting in it popping off.  Years ago flame cleaning of steel was a valid way of preparing surfaces prior to painting.  If your preheat flame is inadequate such as when you are beveling a edge often you have to preheat the edge raising the temp to several hundred F.  An angled torch has trouble heating the plate surface.  For everyone's information you can very effectively cut sheet metal bodywork to within a sixteenth using a triple zero tip, an extreme angle, and rapid travel.  Too much preheat will result in the top edge of the plate melting and rounding.   The melted material ends up as slag hanging underneath.  "Flame Machining" indeed is an accurate description.  I can order a one inch plate from a custom CNC cutting supplier and expect the dimensions to be plus or minus 1/64.  That is plus or minus 15 thousandths of an inch!  If I brow beat the operator he can get even closer.  I have seen this done by operators who strive every day to get better.  Some of them are amazing once they know their machine.
Reply:Just to touch a little on the subject, if you live in an area that gets very cold and has plenty of moisture in the air reguardless of weather steel soaks water like a sponge. Take a good look while preheating and watch the moisture content in the steel sweat out. As far as my personal experiences pre heating cannot hurt and if anything in conditions as stated above will help your cuts to have less slag and have a uniform desire apearance. And I love it when peeps talk about interpass heat because that is a mistake which causes adverse problems for most welders. They tend to rush the job and getting the innerpass heat above what manufactures suggest is exceptable. Also break time rolls around the innerpass temp is up and techs are not taking precautions to contain the innerpass heat and sometimes it quickly cools which we all know what happens then! The industry I work in has all sorts of problems and has taken very little effort to overcome the problems we find when working in extreme or undesired conditions such as cold weather,wind and built in stresses that cannot be normalized. There are so many conditions set by the industry reguarding the life of a piece of equipment, and the almighty dollar dictates otherwise, currently we are working past the operating hours on hitachi shovels and management scatches they're heads wondering why the equipment is running itself to destruction. They are not using the info provided, but like I said earlier are letting the dollar dictate repairs and modifications. The reason I'm pointing this out is topdead is right to a point but has elaborated on some areas, but non the less is right in informing tradesmen on ways that can enhance they're work. Just my 2 cents.Last edited by Dipper Welder64; 01-20-2007 at 10:57 PM.
Reply:I've had enough... ...zap!I am not completely insane..Some parts are missing Professional Driver on a closed course....Do not attempt.Just because I'm a  dumbass don't mean that you can be too.So DON'T try any of this **** l do at home.
Reply:Dipper Welder64 said: "...steel soaks water like a sponge. Take a good look while preheating and watch the moisture content in the steel sweat out."Dipper Welder64 and Topdeadcenter, consider this:If the moisture you see on a steel surface preheated by flame was due to water sweating out of the steel, wouldn't you see water along the outer edge of the HAZ along an electrical weld of any technology on steel?  You have very high temperatures at the weld bead and a continuous decrease of temperature out to the body of the steel at ambient temperature.  Somewhere along that temperature profile you should find the range of temperatures at which you postulate that water "sweats out" of the steel due to heat.  I have never observed such moisture alongside an electrical weld bead.  Have you?  (Distinguish between water sweated out of steel and water vaporized out of flux on improperly stored rods, since many fluxes are quite hygroscopic.)Another thing to consider:  If moisture was soaked into steel, why wouldn't the paint blister or pop off of properly painted steel whenever it was heated to less than the degradation temperature of the paint?  I don't think I've seen that, either.Additionally, if water was soaked into steel, along with contaminant gases, wouldn't the steel rust from within, despite the best of paint coatings or plating?awrightLast edited by awright; 01-21-2007 at 02:07 AM.
Reply:Originally Posted by TensaitekiWow.... just, wow... Flame machining? That's a new one on me. I've heard it called flame cutting, oxy/fuel cutting, and plate burning but never flame machining.
Reply:Well, I guess it was me that learned something today.  I must be a lot funnier guy than I thought.  Here, I've wasted 31 years of my life with this welding stuff, that I'm obviously all delusional about, when I could have been a stand up commedian!  I could go on "Last comic standing" and say doddering, hilarious things like, "Warm up your work.", or "Verify a good ground." and all you young fellows and your instructors at school will fall out of your seats and roll round on th floor.  Yeah, I think that's what I'll do.
Reply:OK My turn..Besides if a roller has shrunk fit heads in it that was just done and the roller is already hot..I dont heat anything up before welding..Why? Because i dont..Thats why.. ...zap!I am not completely insane..Some parts are missing Professional Driver on a closed course....Do not attempt.Just because I'm a  dumbass don't mean that you can be too.So DON'T try any of this **** l do at home.
Reply:And by the way..Steel does not absorb water like a sponge.. If it did your car would not last 3 weeks going from cold nights to warm days..all that moisture would rot it out that fast..and my car is mint and 45 years old so throw the absorbtion theory out the window.. Or submit it to Myth Busters.....zap!I am not completely insane..Some parts are missing Professional Driver on a closed course....Do not attempt.Just because I'm a  dumbass don't mean that you can be too.So DON'T try any of this **** l do at home.
Reply:Here is a link you should read.http://www.lincolnelectric.com/knowl...nt/preheat.asp
Reply:Done..Well thats all well and good but not once does the article mention anything about steel absorbing water..And thats that.....zap!I am not completely insane..Some parts are missing Professional Driver on a closed course....Do not attempt.Just because I'm a  dumbass don't mean that you can be too.So DON'T try any of this **** l do at home.
Reply:OK, lets get past the fact that steel doesn't absorb water. It has been a myth/legend for years and yet some people still subscribe to it even though it's been proven not to be true.TopDeadCenter still brings up a good point about preheating as SOME metals require preheat. Not all weldments will need preheat but certain materials require it.I handle the production welding for a company that manufactures a series of "The Jaws-of-Life" and other rescue equipment. I weld all of the tools steel cutting blades for these tools. It's an S7 Air hardening tool steel that requires preheat. If I didn't preheat to a specific temp before welding, the part would crack faster than a saltine cracker.A lot of us know that thick Aluminum can benefit from a preheat before trying to weld but some still prefer to let the root pass bring the part up to temp. Some materials REQUIRE it while other can BENEFIT from it. A lot of it comes down to personal preference while some if it comes down to metallurgy.Let's all keep this discussion on a professional level and keep the "laughing" to a minimum. We're all here to help and learn. - Paulhttp://all-a-cart.comWelding Cart Kits and accessories
Reply:what paul saidChuckASME Pressure Vessel welder
Reply:Well said Paul..Getting the hang of this moderator gig was easy for you  Keep it up!...zap!I am not completely insane..Some parts are missing Professional Driver on a closed course....Do not attempt.Just because I'm a  dumbass don't mean that you can be too.So DON'T try any of this **** l do at home.
Reply:Sorry for laughing but after thirty plus years of hearing this myth I am starting to object rather than walking away when I was younger.  The picture I am including is of a stack of plate cut into strips with a bevel at one end.  The material is over two inches thick.  As you can see a saw couldn't cut much cleaner.  A CNC burning table is a delight and can take days off fitting plate.  This particular operator was extremely fussy and over the months I saw his work keep improving.  I would keep saying that it was good enough but he would have none of it.  In the end we could rely on this man cutting us strips that were dead on.  We could roll the strips and end up with cans that were exactly on size and never outside a range of plus or minus 1/32 on the diameter. Attached Images
Reply:Ever see stuff that was cut out with a Waterjet machine.. Unreal..Clean..No burrs..+ - .005!!!Most can do up to 5" of ANYTHINGSteel..Alum..Plastics..Composites..Granite..Your arm if your not careful..These things are amazing.....zap!I am not completely insane..Some parts are missing Professional Driver on a closed course....Do not attempt.Just because I'm a  dumbass don't mean that you can be too.So DON'T try any of this **** l do at home.
Reply:There is a waterjet with a 40 foot table installed locally.  I saw a presentation in it last week.  They did some art work on one inch plate.  There was 80 hours of cutting on the one plate which was about 20 feet long.  Very impressive.  The presenter told us that they didn't want to damage the pattern after the cut and lifted it very carefully.  The plate had increased in value by 75,000 dollars.  Ouch!
Reply:Originally Posted by TensaitekiI'm assuming that you are talking about preheating the entire length of the material to be cut. A properly set torch requires no preheat of the cut. The preheat flames are only there to get the spot where the cut begins up to a high enough temperature for the oxygen/iron reaction to be self supporting.At school we use torches to bevel the plate that we practice on. When we have everything set properly we can start the cut (without preheating the line of cut) and turn off the preheat gas and continue cutting on oxygen alone.
Reply:Originally Posted by zapsterEver see stuff that was cut out with a Waterjet machine.. ...zap!
Reply:To address one of the original issues of the appearance of moisture on the metal surface during heating with a torch which might lead to the perception of significant moisture coming from the metal, it helps to glance at the combustion process of fuels, O/A being one of them. One of the byproducts of burning hydrocarbon fuels is H2O itself. Burning a hydrocarbon fuel in an oxygen rich environment, coupling that with acetylene which is hydrogen rich, exagerates this for demonstration purposes. Add to that low surface temps that are conducive to condensation and any pre-existing humidity and it is easy to understand where this chemical reaction could lead one to believe that the moisture you see had to come from the metal itself and the heat applied is burning it out.I'm sure there are more influences involved but combustion is one of the major contributors. Is there some moisture in the metal or trapped in the pores of plain mild steel---I'm sure there is, nothing on this planet is moisture free. Is that amount measurable by ordinary means or significant----I wouldn't know. On a cold damp night is there moisture on the metal---I'm sure there is. There's moisture on my windshield from time to time.
Reply:I have a buddy with a WaterJet. He cuts everything from steel to custom bricks. He does some amazing work but even at a "friends" price he was more than twice the cost of getting my parts laser cut
Reply:Originally Posted by zapsterSomebodys gotta pay for the machine..And its not gonna be "your friend"..  ...zap!Zap show me some proof that supports your opinion I did and if you read it you'll see they mention hydrogen(When Should Preheat be Used?In determining whether or not to preheat, the following array of factors should be considered: code requirements, section thickness, base metal chemistry, restraint, ambient temperature, filler metal hydrogen content and previous cracking problems.If a welding code must be followed, then the code generally will specify the minimum preheat temperature for a given base metal, welding process and section thickness. This minimum value must be attained regardless of the restraint or variation in base metal chemistry; however, the minimum value may be increased if necessary. An example is illustrated in the next section. When there are no codes governing the welding, one must determine whether preheat is required, and if so, what preheat temperature will be appropriate. In general, preheat usually is not required on low carbon steels less than 1 in, (25 mm) thick. However, as the chemistry, diffusible hydrogen level of the weld metal, restraint or section thickness increases, the demand for preheat also increases. There are several methods to determine the required preheat temperature for a given base metal and section thickness that will be discussed in the next section.) , I understand your from mass and it might be difficult to interpet what lincoln electric writes but to some degree wether in the surface or condensation from gases used in preheat I still believe in preheating to get the results that will make the weldment hold up. Tomorrow I'll bust out the torch and the welder and take pictures. I Have brought plates 2 inches thick into the shop ambient temp out side will be in the 20's and the shop 65-70 degrees and the minute you try to cut you have nothing but moisture.So be ready for my pictures I'm curious to see what you have to say then.Last edited by Dipper Welder64; 01-21-2007 at 08:58 PM.
Reply:Try heating the plate from the underside and see if any moisture appears on the top surface.   The Lincoln preheat calculator is a handy little device to calculate preheat as long as you know the alloy and thickness.  It is basically a circular slide rule.
Reply:Yes I 'll try that I have a piece of 3/4 x6 FB Thats about 6" long It should be  a good specimen.
Reply:Originally Posted by Dipper Welder64Zap..., ...I understand your from mass and it might be difficult to interpet what lincoln electric writes ...
Reply:Rather than heating from the underside with a flame, which will still bathe the cold object in water vapor, heat from the underside with a heat source that does not produce water vapor (as almost any gas flame does) like an electric hot plate, and see if water is driven out of the metal.awright
Reply:Sheesh, I'm sorry I said anything.  I didn't expect everyone to start debating how many angels can dance on the head of a pin.  All I meant was a little preheat helps, even on mild steel.  It was meant for the beginners and non-trade guys.  You don't wanna preheat?  OK.
Reply:I've decided after reading awright's post there is no sense in trying to prove any point. i've asked for proof to prove your side in 2 posts prior I have read numerous articles about hydrogen and yet have not seen anything from others. As for topdeadcenter I understand were your coming from. So this will be the end of any posts pertaining to preheat you'll see from me.Those of us who work outdoors know what we have to do to make good welds and maybe some of you will experience it and then realize it's not as easy as said.
Reply:True..this started with "preheat"Now its deteriorated to this.. Personal attacks on ANYONE will not be tolerated on this site. Consider this your first warning Dipper Welder64.
Reply:True..this started with "preheat"Now its deteriorated to this.. Personal attacks on ANYONE will not be tolerated on this site. Consider this your first warning Dipper Welder64.
Reply:Topdeadcenter, just because we're all responding energetically to your post does not mean it should not have been posted.  In fact, I'd say that a post that stimulates the level of response and multiple opinions shown here (not to mention the rapid-fire speed of the dialogue) is a service to the welding community.  It gives the subject a good working over, gives us all an opportunity to learn something new, and engenders a stimulating discussion.Dipper Welder64, I didn't realize that I was expected to express a new understanding of hydrogen problems in welds.  As far as I know, no one challenges the fact that that is an issue of concern.  My comments are limited to the questions of whether or not water is absorbed in steel stored under less than ideal conditions (I say not), and whether the water seen on the surface of flame heated steel is "sweated out" of the steel or is condensate from the water vapor in the flame (I say condensate).  I think I can agree that hydrogen can be a problem in welds without conceding that the hydrogen comes from water absorbed in the steel due to storage.  The link that you provided to the Lincoln tech note was quite interesting and informative (confirming my point about learning), but says nothing about absorption of water into stored steel.awright
Reply:awright this is from a AWS article and may clarify my opinion.Hydrogen.The source of hydrogen is the moisture retained within the electrode coating. Baking of electrodes to ensure the moisture is removed prior to welding is essential. Other potential sources of hydrogen involve paint and grease and failure to remove moisture from the weld area prior to welding. Material hydrogen levels can also contribute to HAZ cracking.
Reply:I don't know if I should say anything here or not but I have something of interest, maybe.  I have done some welding on steel that was submerged in hydraulic oil. The only way I can get a good clean weld is to preheat the steel to get all the oil out of it. I have cleaned the part with parts cleaner and even steam cleaned it and when the heat hits it the oil starts to bleed to the surface. I mean you can actually wipe it off with a rag. I have even seen this on oil pan repairs on auto's. I never mentioned this before as I thought some steels could absorb liquids to some degree.I love opinions as we all have them and debating them can be a learning experience and even fun. They may change yours and it may not. In the end stay cool.Miller DVI2Lincoln Precision Tig 225Thermodynamics Cutmaster 38Everything else needed.
Reply:Dipper Welder64, I agree completely with the AWS article you posted and see no conflict with my position about the absence of water absorption in steel or the source of water on the surface of flame heated steel.awright
Reply:What a hoot.  Steel absorbing water.  If that occurred, steel pipes and tanks would leak over their entire surface, especially if containing materials that would move through easier than water, such as many solvents and gases.Preheat can be useful in many situations but not because the steel has absorbed water.
Reply:Hello, Come on ladies, If you want to play with this concern a bit, take a small specimen of this offending "special steel" that posses the same properties as a desiccant, and weigh it on a gram sensitive digital scale then evaporate all the water out of it by preheating and then re weigh it. You will have your answer. Unless it is determined that water is weightless!!! While the preheating does in many cases improve/ speed up the cutting process that is all it is doing in my opinion. Perhaps during the time the preheat is being applied and the hydrogen is being driven from the steel and it mixes with the oxygen in the atmosphere it becomes....WATER!!! The moisture is simply a byproduct of the heating and combustion process.Heat treaters and Platers both deal with this phenomenon daily it leads to Hydrogen Embriddlement. The fastner business is very aware of this situation as well since Hydrogen Embrittlement is a major cause of bolt/fastner failure after heat treatment and or plating. Aluminum and Cast Iron however are well documented "sponges for any moisture and contaminants but not steel. I suspect that since "Flame Machining" is Tabu here due to its name then we probably don't want to discuss Thermal Deburing do we? Every one responding to this issue had something pertinent to share so lighten up a bit and enjoy the upcoming weekend.ric