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Up Topic Welding Industry / Technical Discussions / Factors of heat input
- - By pencilpusher Date 10-08-2007 17:42
In calculating heat input (amps x volts x 60/IPM) on a weld, I'm interested in getting opinions on how much impact preheat and interpass temps have vs. the heat input itself as calculated by the traditional formula shown.  None of the technical papers I've found incorporate anything but the four factors in the above formula, so my thought is that while preheat and interpass certainly have a role and cannot be completely discounted, their respective dominance in calculating heat is minimal (i.e., 80% calculated heat input and 20% preheat and IP temps).  Any thoughts?
Parent - By OBEWAN (***) Date 10-08-2007 19:25 Edited 10-08-2007 19:28
My suspicion is that preheat would impact heat flow.  There are equations that calculate heat flow I think, but it would be extremely technical and involve a lot of theory and higher math.  The heat input would be the same regardless of ambient temperature, but the effect of heat flow could change drastically at different temperatures.
Parent - - By PhilThomas (**) Date 10-09-2007 00:37
In welding may very-high strength steels, the better measurement is cooling rate (which takes in to account the preheat/IP temp) vs heat input which is a measure of only the energy transferred to the plate.  In other word, for heat input, it doesn't matter what the starting temp is, the energy from the arc is a function of A, V, and TS - it is only looking at the effect on the HAZ and base metal.  Cooling rate, on the other hans, looks at the deposited metal as well as the effect on the NAZ and base metal.
Parent - - By swnorris (****) Date 10-09-2007 13:50 Edited 10-09-2007 13:56
The heat from the welding process and subsequent re-cooling causes changes in the HAZ.  The extent and magnitude of property change depends primarily on the base material, the weld filler metal, and the amount and concentration of heat input by the welding process.  The thermal diffusivity of the base material plays a large role.  If the diffusivity is high, the material cooling rate is high and the HAZ is relatively small.  Alternatively, a low diffusivity leads to slower cooling and a larger HAZ.  The amount of heat input induced by the welding process plays an important role as well, as processes like oxyfuel welding use high heat input and increase the size of the HAZ.  Processes like laser beam welding and electron beam welding give a highly concentrated, limited amount of heat, resulting in a small HAZ.  Arc welding falls somewhere between these two extremes, with the individual processes varying somewhat in heat input. 

Q = (V x A x 60) x efficiency
       ----------
        S x 1000  

Q = heat input (kJ/mm), V = voltage (V), I = current (A), and S = welding speed (mm/min). The thermal efficiency is dependent on the welding process used, with SMAW having a value of 0.75, GMAW and SAW, 0.9, GTAW, 0.8.  Heat input calculated by multiplying the arc energy by the thermal efficiency factor.
Parent - - By js55 (*****) Date 10-09-2007 15:05
The information you're looking for is out there. And plenty of it. Mountains of it. The problem is, its not all in one place. There is no one stop shopping for the info you're asking for. And for good reason. 10 cackozillion transformational alloys, and varying thicknesses, and ranges of chemistries, etc.
I think on the level of technicality you are looking for you start with a CCT diagram for your particular alloy. Determine what cooling rates yield what microstructure and prescipitations, intermetallics, carbides, etc., and then determine the effects of any heat regime imposed upon that material. Your instincts I think are correct. There is 'in general' more response due to heat input than there is preheat or interpass temps, simply because, in my opinion, heat input is a measure of an actual imposing of energy upon the weld, whereas preheat and interpass temp control is a slowing down of the dissipation of the already imposed thermal energy, and the fact that the critical transformaition temps are most often well above those temps controlled by PH and IP, though there are notable exceptions.
Parent - By js55 (*****) Date 10-09-2007 15:11
Sorry, CCT = Constant Cooling Transformation diagram. They superimpose transformation C curves over cooling rates. So that you can determine that at X cooling rate you will realize Y transformation products. There are volumes of these diagrams available.
Parent - By Stephan (***) Date 10-11-2007 08:58
Jeff,

only an extremely short input from my side, if you allow.

"Heat Input" in welding is my absolute favourite for the past few years and everything I can learn on that is being absorbed as by a sponge.

I mean to have mentioned anytime to having a "very special attitude" on "Heat Input" (what ever this means) and have often tried to find the right words for expressing this attitude.

What I was not able to find words for, you were!

Quote:

"And for good reason. 10 cackozillion transformational alloys, and varying thicknesses, and ranges of chemistries, etc."

Unquote.

Thanks for that! :-)

Best regards,
Stephan
Parent - By MBSims (****) Date 10-09-2007 22:00
This equation calculates "changes" in heat input, rather than total heat input.  Given the welding process, preheat and interpass temps are the same, it is used to look at the net change.  If you want true heat input, it needs to be measured with a calorimeter.
Up Topic Welding Industry / Technical Discussions / Factors of heat input

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