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Math Used to Generate 4130 Information.

The Information Below; Provides the math equations that estimate the theoretical cooling rates for TIG welds in thin wall 4130 tubes. The cooling rate equations were obtained from the Welding Handbook, Volume 1, 9th Edition, page 100. Both thick and thin plate equations were developed by Professor C.A.  Adams et al.
The data is presented in a way so one can see the numbers from each part of the equation. As the thickness was increased so was the heat input.  A TIG process efficiency of 0.5 (50%) was used which is somewhat  higher than the sited Welding Handbook reference but errors on the side of predicting a slower cooling rate. The thin plate equation is for two dimensional heat flow and may be OK for a butt weld but a Tee fillet weld will have faster cooling rate. Therefore despite trying to error on the side of predicting slower cooling rates, they are all around the 1/4 in Jominy bar location. The exact cooling rate will be somewhat dependent on the diameter of the tube as well. However I would expect that perhaps 1/2 way around the joint the cooling rate would be similar to that predicted. As the complete circular weld is made the last part will have a preheat from the weld start. However if a brittle structure is formed in any area and a crack forms, that is sufficient to be a problem.
Equation Elements and Values Rc = Cooling rate at weld centerline; deg F/min k = Thermal Conductivity of metal; BTU/min in deg F p = Density of metal; lb/in3 C = Specific Heat of metal; BTU/ (lb deg F) h = thickness of metal; in Hnet = Net heat input; BTU/in [V*I / ipm*.057*process efficiency] Tc = Temperature which cooling rate is calculated; deg F To = Initial plate temperature; deg F

Welding Cooling Equations from page 100 Welding Handbook Volume 1, 9th Edition

Thick Plate Equation =Rc =-[2*3.1416*k(Tc-To)2] / Hnet
Thin Plate Equation = Rc = 2*3.1416*kpC (h/Hnet)2(Tc-To)3

Calculate estimated cooling rate @ 1100 deg F for thin wall 4130 tube welds:

Tube = .040 in wall; V (volts) = 10; I (amps) = 70; travel = 12 ipm

Using thin plate equation: -2*3.1416*0.024*0.29*0.11= -.00481 Hnet = 1.66 (h/Hnet)2 = 0.000579 (Tc-To)3 = 1.07*109 Rc = -2981 deg F / min = - 50 deg F / sec
Tube = .0625 in wall; V (volts) = 10; I (amps) = 90; travel = 11 ipm

Using thin plate equation: -2*3.1416*0.024*0.29*0.11= -.00481 Hnet = 2.33 (h/Hnet)2 = 0.000718 (Tc-To)3 = 1.07*109 Rc = -3700 deg F / min = - 62 deg F / sec
Tube = .093 in wall; V (volts) = 10; I (amps) = 110; travel = 9 ipm

Using thin plate equation: -2*3.1416*0.024*0.29*0.11= -.00481 Hnet = 3.48 (h/Hnet)2 = 0.000713 (Tc-To)3 = 1.07*109 Rc = -3671 deg F / min = - 61 deg F / sec
REFERENCES USED; US Steel Atlas of Isothermal Transformation Diagrams Republic Alloy steels R. A. Grange and Kiefer; "Transformation of Austenite on Continuous Cooling and Relation to Transformation at Constant Temperature." Walter Crafts and John Lamont; "Hardenability and Steel Selection." AWS Welding Handbook; Volume 1, 9th Addition The "Gas Blast" at MIG Weld Starts Causes Problems:

1) Shielding Gas Waste - typically over half the gas used

2) Inferior Weld Quality - due to air pulled in gas stream

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