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  • Pre-heating???

    Greetings all. I have a few questions about pre-heating. Is there a rule of thumb on pre-heating in regards to what the minimum metal thickness you should start preheating - I mainly work in steel. Also, what do you all use to preheat? I have a little propane torch that is pretty convenient ... will that work? Finally, how hot should you go with pre-heating, and how do you know when you are there? Awhile ago, I posted about having a few welds crack in some thicker steels right after taking the torch away, and the concensus seemed to gravitate toward lack of preheating. Ok thanks in advance for any and all advice.

    Ben

  • #2
    I'm not to sure about the pre heating part of it but I do know if you were going to they make heat crayons. You can buy different heat crayon for different temps. When it reaches it's temp it turns a glossy color. That is when your metal is at the temp you want.

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    • #3
      I was taught

      anything thick or thicker than a pencil ,preheat to at least 22 deg. f

      on big jobs we use propane weed burners. welding supply sells temp. sticks that melt, at spefic temps.

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      • #4
        I don't pre-heat anything thinner than .750, unless the weldment is colder than 50°F, or the weldment is very large, like a box-scraper or something like that. For heavy weldments, I like to get the temperature up to 250 - 300° before welding. Temperature crayons (Tempil is one brand) work really well until you get the sense of how hot 300° feels on your face! I use an O/A torch to heat, but a high-BTU output propane torch would work well also.

        I should add that my experience on thick stuff is limited. I've repaired things, like Gannon-boxes and a loader bucket bracket, but I've never eally fabricated anything from steel thicker than .375 yet.

        Hank
        Last edited by hankj; 01-24-2005, 11:36 AM.
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        • #5
          I will have to stop by the welding supply store and pick up some of those heat crayons - good tip for a newbie here. I will check back for any more suggestions/info on pre-heating.
          Thanks all.

          Ben

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          • #6
            With O/A, you blacken the metal with Acetylene, then heat it till it burns off. That's 250 to 300°. Metal should be at least 70° before welding to be safe.
            Arcin' and sparkin', Rocky D <><
            Experience is something you don't get until just after you need it.
            IF YOU CAN READ THIS, THANK A TEACHER...
            IF YOU'RE READING THIS IN ENGLISH, THANK A SOLDIER!

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            • #7
              [QUOTE=Rocky D]With O/A, you blacken the metal with Acetylene, then heat it till it burns off. That's 250 to 300°.


              see that..........you learn something every day !!

              dawg
              God Bless America

              [

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              • #8
                Freezer

                Sometimes I have to weld in the freezer which is -10f. I am welding on pallet racking that goes 8 pallets high and is made mostly of 3 to 4 inch channel.

                Are you guys telling me that I need to heat this up before I weld on it ???

                FreezerMan

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                • #9
                  Originally posted by FreezerMan
                  Sometimes I have to weld in the freezer which is -10f. I am welding on pallet racking that goes 8 pallets high and is made mostly of 3 to 4 inch channel.

                  Are you guys telling me that I need to heat this up before I weld on it ???

                  FreezerMan
                  Depends. IF the material and the weld consumables are designed for the low temperature (nil ductility temp below the working temp) you may not need to. If there is very light loading and no shock to the structure, less citical, but I woud bet that is not the case with pallet racking.

                  I woud bet that you want to preheat, that you want to confirm the compatability of the consumable with the service, and you may want to check out a standard AWS procedure for the application. Proceures are based on material, service, welding process, etc., especially of there is a liability or safety component involved.
                  I may not be good looking, but I make up for it with my dazzling lack of personality

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                  • #10
                    Ok Rocky - good tip, I have actually annealed sheet aluminum the way you described. I never actually knew the exact temperature range until you mentioned it, though. I wish the acet. was not so darn messy - I have a tiny room that tends to get really sooty really fast. Ok, thanks again all, and I will keep checking back.

                    Ben

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                    • #11
                      Originally posted by FreezerMan
                      Sometimes I have to weld in the freezer which is -10f. I am welding on pallet racking that goes 8 pallets high and is made mostly of 3 to 4 inch channel.

                      Are you guys telling me that I need to heat this up before I weld on it ???

                      FreezerMan
                      Here's an interesting read...http://www.disastercity.com/titanic/index.shtml
                      Arcin' and sparkin', Rocky D <><
                      Experience is something you don't get until just after you need it.
                      IF YOU CAN READ THIS, THANK A TEACHER...
                      IF YOU'RE READING THIS IN ENGLISH, THANK A SOLDIER!

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                      • #12
                        Hi Ben , there are many general rules of thumb regarding preheating. Before we get into that here is some preheat info :
                        [B]
                        REASON FOR PREHEAT[/B]


                        1. The most common reason for preheating is to reduce the cooling rate ( quench ) to help eliminate reduction of ductility of weld and HAZ . Obviously if the weld gets brittle ( loses ductility ) its not desirable !

                        2. Reduce shrinkage stress in a highly restraint joint ( weldment which does not allow the weld to move as it cools causes high stress in the weld ). With preheat, the shrinkage stress has more time to relieve itself in the hot weld

                        3. Reduce cooling rate to minimize underbead cracking ( 1st 2 reasons are more common )

                        NEED FOR PREHEAT ( for reason 1 )

                        Carbon Equivalent

                        The need for preheat is generally dictated by metallurgy of the base material. More specifically it is the Carbon content and to a lesser degree the amount of alloys in the material

                        Because its the carbon content which is the most significant contributor all the other alloys such as Cr , Mo , Ni etc are given a Carbon Equivalent value. For example, The %Chrome is divided by 5, but the %Ni is divided by 15 to get the Carbon equivalent. this is because Chrome has a greater effect on need for preheat than Ni does . Chrome helps greatly to increase hardenability of steels

                        Incidentally Carbon content makes a signficant difference in hardenability. The problem though is the better the hardenability of a material, the more the tendency to have reduced weld ductility ( weld gets brittle )...which kinda makes sense !

                        Fortunately steel manufactures know this and strive to control the C content


                        Won't bore you with the actual formula but just search under International Institute of Welding Carbon equivalent formula. You can also find online formulas that will crunch the numbers for you or you can buy "Preheat Calculator Wheels"

                        All these numbers are plugged into a formula which yields the carbon equivalent .


                        Thickness

                        The next determining factor is the plate thickness . If you think about it this makes sense. The thicker the plate, the more massive the heat sink ( to suck away the heat from welding ), therefore the thicker the plate the higher the quench rate. Recall REason 1 for preheating was to reduce the quench rate

                        For example for the very common T-1 type steels (quench and tempered ) typically

                        1/2" thick - about 50degF
                        3/4" thick - about 120degF
                        1" thick - about 200degF
                        1.5" thick - about 250degF

                        This is only an approximation because if the joint is highly restraint the min preheat temp increases

                        Rule of Thumbs

                        If you know the carbon content for the material a good rule of thumb is the points of carbon X1000 is the min preheat for

                        For example, if a material has .20% carbon, then min preheat is about 200degF

                        If you know the SAE classification of the material its real easy . The last 2 digits for the SAE classification is the approx carbon conent. For example, SAE4130 ( chrome moly ) is about 0.30 points of carbon . Preheat then is approx 300deg F . If you run the alloy through a preheat calculator using the carbon equivalent, it comes out a bit higher but for purposes of estimating its ok

                        Ductility and Preheat

                        As already stated, the need for preheat is mainly due to metallurgy and thickness. For example, if you were welding some quench and tempered plate such abrasion resistant ( AR400 ) wear plate. If the requirement for sake of argument is 150degF , it would be 150degF whether you undermatch with E7018 , or you want to use a alloyed electroded with great impact properties such as 1% nickel electrodes ( Nickel helps low temperature impact properties ). The electode chemistry has little to do with it

                        EXCEPT ..... ( always an exception )

                        When very low hydrogen electrodes are used the required preheat temperature decreases somewhat. For example if you were welding the above wear plate with E6010 vs a E7018-H4 you will find that the low hydrogen electrode is far more forgiving


                        When you will need prehat

                        Often people with smaller machines will need preheat not because of all the reasons above, but rather the machine does not have the current to give a good fusion ( weld looks cold ) . You see this with people welding aluminum all the time ( not really a great idea though ! )

                        You might also run into this if you are welding Quench and Tempered plate ( like T-1 etc ) or cast iron ( realatively high C content in cast irons )

                        If you work with race cars you might also run into CrMo tubing etc

                        If you are welding on relatively thin ( thinner than 1/2" ) I wouldn't get to excited about preheat on mild steels unless the plate is chilled ( outside ) . Often its more important to "put the torch to it" to help eliminate any moisture than it is to preheat it

                        If you want to preheat heavy plate don't waste your time with a small propane toch. use a Tiger torch ( inexpensive ), or Oxy Propane / Oxy Acetylene etc. Heating heads for torchs are pretty cheap

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                        • #13
                          Pre Heat

                          Originally posted by Rocky D
                          With O/A, you blacken the metal with Acetylene, then heat it till it burns off. That's 250 to 300°. Metal should be at least 70° before welding to be safe.
                          Rocky, I had a 85 year old part timer that taught me that trick for aluminum, ,,Works like a champ,Jack

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                          • #14
                            TRG-42 - GREAT lesson on pre-heating ... I am going to print this thread out to save all of the great information. I appreciate you taking the time to go over the reasoning and needs as well for pre-heat. That was all that I asked for ... and then some. Thanks again all for the help. I will start putting into practice what I have learned here and improve the quality of my welds. Thanks again.

                            Ben

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                            • #15
                              TRG-42...

                              that's really good and understandable info.

                              Thanks a lot!!!
                              tjb

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