The Ti-containing steel weld metals with Al contents of 0.01-0.085% were prepared. The effects of Al contents in weld metals on the inclusions evolution were in detail investigated by means of thermodynamic calculations coupled with electron probe micro-analyses. The results show that the inclusions in the 0.01% Al weld metal are mainly composed of ilmenite with more amounts of (Mn-Si-Al)-oxide and titanial-spinel. When Al content is increased up to 0.035%, a more amount of corundum and a small amount of pseudobrookite are formed. In 0.085% Al weld metal, the (Mn-Si-Al)-oxide phase disappears completely, and the inclusions contain a substantial amount of corundum, in addition to a minimal amount of pseudobrookite. Ti3O5, MnTi2O4 and MnTiO3 are the primary constituents of pseudobrookite, titanial_spinel and ilmenite solid solutions, respectively. Titanial_spinel and ilmenite have higher amounts of Mn, but lower Ti levels compared with pseudobrookite.
It is well known that some fine non-metallic inclusions in weld metals and/or steels can strongly contribute to the nucleation of acicular ferrite (AF) (Lin
However, in the case of chemical compositions of commercial steels and/or ordinary weld metals, the complex Ti-containing oxide inclusions, such as MnTi2O4 and MnTiO3, rather than simple ones mentioned above are usually formed (Kang
Therefore, the formation of Ti-containing inclusions mentioned above is essential for AF nucleation regardless of Mn depletion or crystallographic lattice match mechanisms, and accurate control of the formation of such inclusions is very important. The chemical compositions of steels and/or weld metals have strong effect on the formation, types and structures of inclusions. Kang
On the other hand, the formation of inclusions is rather complex because the inclusions found in ordinary weld metals and/or commercial steels usually consist of a mixture of several complex crystalline solid solutions and/or amorphous phases instead of simple stoichiometric compounds such as Mn2TiO4, MnTi2O4, MnTiO3 and Ti2O3 (Kang
The present study analysed how Al element affects the inclusions evolution including the types, contents and of chemical compositions of constituent phases of inclusions in Ti-containing steel weld metals using FactSage commercial thermochemical computing package combined with electron probe micro-analyzer.
Chemical compositions of welding wire (mass, %)
C | Mn | Si | S | P | Al |
---|---|---|---|---|---|
0.05 | 0.86 | 0.06 | 0.022 | 0.02 | 0.01 |
Chemical compositions of weld metals (mass, %)
C | Mn | Si | S | P | Al | O | N | Ti |
---|---|---|---|---|---|---|---|---|
0.050 | 3.01 | 0.203 | 0.014 | 0.029 | 0.010 | 0.040 | 0.0037 | 0.018 |
0.045 | 3.14 | 0.220 | 0.016 | 0.025 | 0.035 | 0.039 | 0.0041 | 0.020 |
0.056 | 2.98 | 0.271 | 0.014 | 0.026 | 0.085 | 0.037 | 0.0039 | 0.018 |
Schematic drawing showing the preparation of the weld metal.
The specimens were cut from the weld metals, and examination planes perpendicular to the welding direction were prepared by mechanical polishing. The morphology and chemical compositions of the inclusions were analysed by a JEOL JXA-8530F electron probe micro-analyzer (EPMA).
The commercial thermochemical computing package FactSageTM (version 7.2) was employed to calculate the thermodynamic stability of various inclusion phases using the FToxid, FTmisc and FSstel databases containing model parameters, the thermodynamic properties and structures of thermodynamic models of the inclusion phases as functions of temperature and composition. According to the chemical compositions of weld metals and selected databases, the major constituent phases of inclusion considered in the present thermochemical calculations are as followed:
Pseudobrookite: Ti3O5-FeTi2O5-MnTi2O5 solid solution
Titanial_Spinel: (Mn, Fe)(Ti, Al)2O4 solid solution
Ilmenite: Ti2O3-FeTiO3-MnTiO3 solid solution
Corundum: Al2O3 + (Ti2O3 in dilute amount)
Stoichiometric compounds: all relevant stoichiometric compounds
Slag phase: Al2O3-SiO2-MnO-Ti2O3-TiO2-FeO multi-component liquid oxides solution formed by oxidation reactions of several elements in weld metals
The equilibrium cooling mode was applied to predict the formation of inclusions between 1000-1600 °C. Moreover, various reactions during further cooling in the solid state, were not considered, for simplicity.
Phase Diagram module was used to generate a ternary isothermal phase diagram at 1000 °C for the system containing Al2O3-MnO-SiO2 components in order to study the effect of Al on the formation of complex inclusions.
SEM image and EPMA maps of the inclusion in WL.
SEM image and EPMA maps of the inclusion in WM.
SEM image and EPMA maps of the inclusion in WH.
Thermodynamic analyses on inclusions evolution for (a) WL, (b) WM and (c) WH.
The constituent contents and chemical compositions of solid solution in the inclusions were obtained based on the thermodynamic calculation results, and shown in
Constituent contents (a) and chemical compositions (b) of solid solutions.
Based on thermodynamic calculations shown in
As mentioned above, Al element has a strong effect on the formation and evolution of inclusions in weld metals. It is well known that during welding, a series of complex metallurgical physics chemical reactions are expected to take place in weld pool. Elements of Al, Ti, Si, Mn, etc., can combine with soluble oxygen in weld pool, which results in the formation of a variety of liquid oxides such as Al2O3, TiOx and MnO (i.e., molten slag) above liquidus temperature of slag, and the decrease in the amount of the dissolved oxygen in the weld pool. There are the competition relationships between Al, Ti, Si and Mn during oxidation process. Compared with Si and Mn elements, Al and Ti have much stronger affinities with oxygen under the same contents due to their lower oxygen potentials of forming oxides (Mitsutaka and Kimihisa,
The variations in compositions of liquid oxides solutions with Al content at 1600 °C. O_FeLQ (in ppm) represents dissolved oxygen content in liquid Fe solution at 1600 °C.
where
The liquid oxide and liquid Fe solutions can be regarded as ideal solutions. Thus, the concentrations (mole ratio) of MxOy in the liquid oxide solution and the metal element dissolved in liquid Fe solution are respectively their activities (Mitsutaka and Kimihisa,
When the Al content is increased up to a certain range (for example, 0.02-0.045%), the liquid oxide solution can contain higher amount of Al2O3, which increases the activity of Al2O3, but lowers the activities of other constituents including TiOx (i.e., Ti2O3 and TiO2) in liquid oxide solution. Therefore, according to Eq. (
During cooling after welding, different kinds and amounts of constituent phases in inclusions are expected to precipitate in light of the chemical compositions characteristics of liquid oxides. In order to further demonstrate this effect, slag compositions in equilibrium at 1600 ºC were calculated with Fe-3.1Mn-0.23Si-0.039O-
Ternary isothermal phase diagram of Al2O3-MnO-SiO2 system at 1000 °C Red solid line with symbol is calculated inclusion trajectory using Equilib with Fe-3.1Mn-0.23Si-0.039O-
In the case of 0.01% Al, the liquid oxide contains a large amount of MnO constituent and a certain amounts of TiOx, which favours the formations of titanial_spinel and ilmenite rather than pseudobrookite. Similarily, the formation of (Mn-Si-Al)-oxide is also promoted due to high amount of MnO and a certain amount of SiO2, as shown in
The kinds and amounts of the constituent phases of the inclusions are remarkably changed with the Al content in the weld metals. The inclusions in the 0.01% Al weld metal are mainly composed of ilmenite with more amounts of (Mn-Si-Al)-oxide and titanial_spinel. When Al content is increased up to 0.035%, a more amount of corundum and a small amount of pseudobrookite are formed. In 0.085% Al weld metal, the (Mn-Si-Al)-oxide phase disappears completely, and the inclusions contain a substantial amount of corundum, in addition to a minimal amount of pseudobrookite.
Ti3O5, MnTi2O4 and MnTiO3 are the primary constituents of pseudobrookite, titanial_spinel and ilmenite solid solutions, respectively. Titanial_spinel and ilmenite have higher amounts of Mn, but lower Ti levels compared with pseudobrookite.
This work was financially supported by a Project of Education Department of Liaoning Province (grant no. L2016132). Authors are grateful to Drs. H.Y. Wu, W.N. Zhang (State Key Laboratory of Rolling & Automation of Northeastern University, China), and L.Z. Kong (School of Metallurgy of Northeastern University, China) for providing helps in EPMA analyses works and thermodynamic calculations.