Lecture 3 Chemical Reaction in The Welding Zone

1. Lecture 3 Chemical Reaction in The Welding Zone 助理教授：王惠森 huei@mail.isu.edu.tw 您可以在以下的網址 Down Load 本檔案 http://140.127.180.27/www/index.htm

2. Overviews-1

3. Overviews-2 • Gas-Metal Reaction • Slag Metal Reaction • Defects of Weld

4. Gas-Metal Reaction • Nitrogen –Metal • Oxygen-Metal • Hydrogen-Metal

5. Nitrogen –Metal The solution of nitrogen in liquid iron can be expressed as follows: 1/2 N2 (gas) = N (dissolved in iron) The free energy of solution of nitrogen in liquid iron is G = U − TS + pV = H − TS G。= 860 + 5.71 T cal/mole N

6. N Solubility Vs. Pressure If the dissolved nitrogen is sufficiently dilute that it obeys Henry's law. If follows the Sieverts’ Law the equilibrium constant for the solution of nitrogen can be written as follows: K= 即金屬液中的氣體飽和溶解度與外界總壓力的平方根成正比。

7. N Solubility Vs. Temperature The solubility of nitrogen in iron as a function of temperature is shown in Figure. As can be seen, the solubility of nitrogen decreases dramatically as liquid iron solidifies.

8. Nitrogen in the weld metal-1 Nitride • of the weld metal nitrogen in the weld metal can significantly affect its mechanical properties • the acicular structure of iron nitride (Fe4N) significantly reduce both the ductility and the impact toughness of the weld metal • The sharp ends of such a brittle nitride act as ideal sites for crack initiation • such a nitride or other similar nitrides can significantly reduce both the ductility and the impact toughness Nitride

9. Nitrogen in the weld metal-2 • nitrogen is an austenite stabilizer for austenitic stainless steels • In the self-shielded arc welding process, strong nitride formers (such as Ti, Al, Si, and Zr) are often added to the filler wire. The nitrides formed enter the slag, and nitrogen in the weld metal is thus removed. • other arc welding processes,such as GTAW, GMAW, or SAW, should be used if weld nitrogen contamination is to be minimized

10. Oxygen-Metal • The solution of Oxygen in liquid iron can be expressed as follows: 1/2 O2 (gas) = N (dissolved in iron) The free energy of solution of Oxygen in liquid iron is G = U − TS + pV = H − TS G。 = -28000 -0.69Tcal/mole O

11. The effect of gas composition in oxyacetylene welding • When too much oxygen is used, the weld metal has a high oxygen level but low carbon level. • On the other hand, when too much acetylene is used, the weld metal has a low oxygen level but high carbon level (the flame becomes carburizing). • When the oxygen/acetylene ratio is close to 1, both the impact toughness and the strength (proportional to hardness) are reasonably good.

12. Arc voltage Vs. contamination of oxygen • The higher the arc voltage, the more the weld metal contamination of oxygen and nitrogen. • The greater the arc length, and, therefore, the less effective the protection of the weld metal from air • Also, alloying elements in the filler wire cannot be effectively transferred to the weld metal owing to oxidation

13. Effects of Oxygen Contamination in Mech. Properties • Oxygen contamination can deteriorate the mechanical properties of theweld metal. Figure shows that the strength, toughness, and ductility ofmild steel welds all decrease with increasing oxygen contamination. • Foraluminum and magnesium alloys, the formation of insoluble oxide films onthe weld pool surface during welding can even cause the incomplete fusionof the weld.

14. Hydrogen-Metal Reactions • Hydrogen in the welding zone can come from several different sources: • the combustion products in oxyfuel welding; • decomposition products of cellulose-type electrode coverings in shielded metal arc welding; • hydrated oxides, • moisture, or grease on the surface of the work-piece or electrode; • moisture in the flux or electrode coverings; • hydrogen in the workpiece or the shielding gas.

15. Hydrogen level of Shielded metal arc welding (SMAW) • Class 6 electrodes are the basic electrodes containing 20-50% CaCO3 in the covering--the so-called low-hydrogen electrodes. • Rutile-type electrodes (classes 2 and 3) and Cellulosic-type electrodes (class 1) contain organic matter and cannot be baked at more than 200oC it has been reported that shielded metal arc welding (SMAW) • using cellulose-type electrodes can pro-duce welds with hydrogen levels as high as 42 ml/100 g of deposit

16. Hydrogen level of Flux-Cored Welding • Flux-cored electrodes, though considered to be low-hydrogen, contain in their flux some hygroscopic materials, which tend to absorb moisture when exposed to a high-humidity atmosphere. Consequently, flux-cored CO2 welding can still produce welds with high hydrogen levels.

17. Solution of Hydrogen • The solution of hydrogen in liquid iron can be expressed as follows: 1/2 H2 (gas) = H (dissolved in iron) The free energy of solution of Oxygen in liquid iron is G = U − TS + pV = H − TS G。 = 7490 -10.81Tcal/mole H log K=-1637/T+2.36 K=

18. Solubility of hydrogen Vs. temperature • Solubility of hydrogen in iron as a function of temperature • Excessive hydrogen porosity has been shown to severely reduce both the strength and ductility of aluminum welds

19. Reducing hydrogen content • First, hydrogen-containing shielding gases should be avoided • Second, the electrode covering, flux, filler wire and work piece should be cleaned and dried to remove moisture and grease • Third, the weld hydrogen content can be reduced by controlling the composition of the flux, electrode covering, or shielding gas. • Fourth, the weld hydrogen content can be reduced by postweld heating, • Electromagnetic stirring has been reported to reduce hydrogen porosity by helping bubbles escape from the weld pool

20. Quick Quiz-1 • (a)Will decreasing welding speed help reduce weld porosity in TIG welds of aluminum if the source of hydrogen is on the workpiece surface? (b) What about if the source of hydrogen is in the shielding gas? Explain why or why not?

21. Quick Quiz-2 • Al-Li Alloys are known to have severe hydrogen porosity problems due to the hydration of oxide. To avoid porosity , propose your suggestion??

22. Slag Metal Reaction Welding fluxes can be categorized into the following three groups according to the types of main constituents : 1. Halide type fluxes, such as CaF2-NaF, CaF2-BaCI2-NaF, KCl-NaCl-Na3AIF6, and BaF2-MgF2-CaF2-LiF. used for welding titanium and aluminum alloys 2. Halide-oxide type fluxes, such as CaF2-CaO-A1203, CaF2-CaO-SiO2,CaF2-CaO-A1203-SiO2, and CaF2-CaO-MgO-Al203. used for welding high-alloy steels. 3. Oxide type fluxes, such as MnO-SiO2, FeO-MnO-SiO2, and CaO-TiO2-SiO2 low-carbon or low-alloy steels.

23. The basicity index • The basicity index, BI, an oxide-type: • BI= • The simplest Definition: BI= • More Lengthy Definition: BI=

24. The BI correlate with the contents of elements • The BI correlate with the contents of elements such as silicon, sulfur, and oxygen in the weld metal. • These elements tend to decrease with increasing basicity of the flux. • A weld made with a higher basicity flux tends to have a lower inclusion content and hence better mechanical properties.

25. Effective Equilibrium Temperature of the Weld Pool • Effective equilibrium temperature at which the equilibrium composition of the reaction system becomes identical to the actual weld composition after Welding • Effective equilibrium temperature can in turn be employed to calculate the weld metal composition using the available thermodynamic information, such as the activity coefficients of components in the reaction system and the equilibrium constant. • Effective temperature is only an arbitrary temperature chosen for the convenience of calculations.

26. Actual kinetic conditions VS. the equilibrium condition • Curves AB and A'B' represent the composition-temperature relationships according to actual kinetic conditions and the equilibrium condition • Co and Cd are respectivety the initial and the final concentrations of a component of interest in the weld • Composition Cd corresponds to temperatures Ts and Teq according to the actual kinetic and equilibrium conditions, espectively.

27. Thermodynamic Model for Slag-Metal Reactions • The basicity index (BI) was defined according to Eq. at page 23. • Using the data shown in Figure at page29, the weld metal SIO2 content was calculated as a function of the BI for SiO2-CaO-FeO slags over an Fe-Si melt.

28. Solution of Hydrogen • The solution of SiO2 in liquid iron can be expressed as follows: SiO2 (liq) = [%Si]+2[%O] log K=-28360/T+10.61 K= Fe (liq)+[%O] = FeO(liq) log K=6372/T-2.73 K= Belton el a1 have derived the following thermodynamic data for the Si02 reaction in the temperature range 17 13 to 2000 'C.

29. relationship between the equilibrium percent SiO, andthe flux basicity • The relationship between the equilibrium percent SiO, and • the flux basicity index for a given weld metal Si content in the • SO2-MnO-FeO system at 2000 'C.

30. Example and Practice • A base metal of steel containing 0.35% Mn is welded with a filler wire containing 0.60% Mn and a flux of composition 31% CaO-4% MnO-65% A1203. The ratio of the amount of the base metal dissolved into the weld to the total amount of the weld metal, the so-called dilution ratio, is 40%. From the right hand side figure determine the range of Mn content in the weld metal.