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Effect of Deoxidation Sequence on Carbon Manganese Steel Weld Metal Microstructures

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Effect of Deoxidation Sequence on Carbon Manganese Steel Weld Metal Microstructures Effect of Deoxidation Sequence on Carbon Manganese Steel Weld Metal Microstructures Deoxidation sequence has a strong influence on nonmetallic inclusions formation and the subsequent weld refinement BY F-C. LIAO AND S. LIU ABSTRACT. During investigation of indi­ boundary ferrite and Widmanstatten side­ the molten metal for the purpose of vidual and combined effects of aluminum plate ferrite are often grouped together as deoxidation. Some common deoxidizers and titanium on low-carbon low-alloy primary ferrite (PF). As the weld tempera­ used in steel making are aluminum, silicon, steel weld metal microstructures, experi­ ture continues to drop, fine acicular ferrite manganese, and titanium. In arc welding, mental welds were made on 12.7-mm (AF) laths begin to nucleate intragranularly. these same elements are also used for (0.5-in.) thick A516 C70 pressure vessel Finally, the remaining austenite transforms weld pool deoxidation and alloying. They steel plates using ER70S-3 welding wire to a variety of microstructural features, enter the weld pool from the base metal, and a low-oxygen potential commercial which include bainite (B), martensite (M) electrode, or fluxes. In ladle refining of flux. In the aluminum or titanium (individ­ and pearlite (P). Together with the re­ steels, the deoxidation reactions occur at ual) addition series welds, the results indi­ tained austenite, martensite and carbides near isothermal and equilibrium condi­ cated that the final weld metal microstruc­ form the microconstituents known as tions quite different from those found in tures are related to the inclusion size dis­ MAC. The relative proportions of the dif­ most welding conditions. The nonisother- tribution and the amount of aluminum or ferent transformation products are mal, nonequilibrium nature of arc welding titanium in solid solution. In the aluminum- strongly influenced by the nonmetallic in­ makes it very difficult to identify clearly titanium or titanium-aluminum (combined) clusions in the weld metal. the effect of deoxidation sequence in the addition welds, deoxidation sequence Several models have been proposed to weld pool on weld metal microstructure. plays an important role in the formation of explain the effects of nonmetallic inclu­ In submerged arc welding, oxygen specific types of nonmetallic inclusions sions such as TiO and MnO • AI2O3 on the comes mainly from the flux, which may and in the determination of solid solution formation of acicular ferrite (Refs. 1-5). contain easily reduced oxides such as iron elements content, which are fundamental However, the influence of deoxidizers oxide, manganese oxide and silica (Ref. 6), in microstructural refinement. (individually or combined) on inclusion which often exist in a dissociated state, formation and inclusion size distribution is that is, metal cations and oxygen anions, in the weld pool. During solidification, the Introduction still not fully understood. This research fo­ cuses on the weld pool deoxidation se­ weld pool oxygen concentration estab­ In order to satisfy many critical engi­ quence to better understand the relation­ lished at high temperatures will readjust as neering applications, steel weld metals ship between weld metal microstructures a result of decreasing oxygen solubility with high strength and high toughness and nonmetallic inclusions. and the combination of oxygen with deox­ have been widely investigated in recent idizers that exist in the weld metal. Kluken years to determine the factors that control and Grong (Ref. 7) divided the weld pool Weld Pool Deoxidation Practice weld metal microstructure. During the into two reaction zones. One is the "hot" austenite-to-ferrite decomposition, the In steel making, elements with higher reaction zone, immediately beneath the first transformation product is grain affinity for oxygen than iron are added to arc, where the deoxidation products are boundary ferrite (CBF), which forms along continually separated by highly turbulent prior austenite grain boundaries. Follow­ flows that sweep those products to the ing, Widmanstatten sideplate ferrite (SP) trailing edge of the weld pool. The other nucleates and grows from grain boundary KEY WORDS is the "cold" reaction zone where most of ferrite as long needle-like laths that pro­ the precipitated products are entrapped trude into the austenite grains. Grain Deoxidation Sequence in the weld metal as finely dispersed inclu­ Weld Metal Microstructure sion particles. A516 Chemical Composition Aluminum Additions Inclusions Formation F-C. LIAO and S. LIU are with the Center for Titanium Additions Welding and joining Research, Colorado School Inclusion Composition In fusion welding, the level of residual of Mines, Golden, Colo. Weld Sequence oxygen in liquid iron is strongly dependent Transformation Paper presented at the 71st Annual AWS on the amount and kind of deoxidizers Meeting, held April 22-27, 1990, in Anaheim, Hardenability added. If only one deoxidizer is used in the Calif. welding system, the deoxidation sequence 94-s I MARCH 1992 and products are much easier to predict. When the residual oxygen content is less nominal heat input for the first pass weld­ In reality, however, multiple deoxidizers than 0.06 wt-%, CX-AI2O3 is the product, as ing was approximately 3.0 kj/mm (76 kj/ are used and complex inclusions arc described in Equation 5 (Ref. 11). in.). The welding parameters are shown in formed. The amounts of alloying elements Table 2. After the first passes were made, 2AI + 4FeO ^ and deoxidizers strongly affect the final (l) (l) V-grooves were machined from the cen­ . AI 0 -Fe0 + 3Fe i) (4) inclusion's composition, size, and shape. 2 3 {s) ( ter of the beads. Gas tungsten arc (GTA) 2AI(i, + 3FeO(l) — Al2Ol(i) + 3Fe(l) (5) welds with eight levels of aluminum addi­ Manganese as Deoxidizer tions, from 0.009 to 0.228 wt-%, were In low-carbon steel weld pools, where made with argon shielding with a nominal When a steel weld pool is deoxidized oxygen content is reasonably low, it is heat input of approximately 2.9 kj/mm by manganese alone, the final deoxidation common to observe clusters of AI2O3 (74 kj/in.). GTA welding was chosen to product will be rich in MnO with a small particles. ensure that the variations in oxygen con­ amount of FeO (Ref. 8). The deoxidation tent observed in the second passes were reaction in the weld pool is (Refs. 9,10) Titanium as Deoxidizer caused mainly by aluminum addition since all other elements were maintained con­ Mri(i) + FeO, • MnO(i) + Fe,•m (1) Using titanium as the only deoxidizer, a 10' stant. Due to the nonuniform bead shape number of oxides are formed. At increas­ with of some of the second passes, a third au­ ing oxygen content, the following se­ togenous GTA weld pass at 2.9 kj/mm AG°= -29,469+ 13.57 (2) quence of phases is expected: TiO, Ti20 , 3 was applied in the transverse direction of Ti 0 , Ti0 , FeO-Ti0 , and Fe Ti0 (Ref. In Equation 2, T is the weld pool tem­ 3 5 2 2 2 4 all welds to homogenize the weld metal. 11). The final form of the inclusions, how­ perature. The inclusion type and shape The welding sequence is shown schemat­ ever, may be rich in TiO (because of its are strongly dependent on the ratio of ically in Fig. 2. thermodynamic stability)—Fig. 1. MnO/FeO. With high manganese level However, in real arc welding processes, The second part of the experiment fol­ MnO, (high ), many inclusions will exhibit more than one deoxidizer is added to the lowed similar procedures as the first part' FeO weld pool and the deoxidation sequence with the exception of the sequence of dendritic morphology (Ref. 11). At a low- is complicated and most of the inclusions deoxidizers addition (aluminum in the first manganese level (low _ _), however, occur in a combined form, with multiple pass and titanium in the second pass). oxides. Some sulfides and oxysulfides may the inclusions are always spherical and also be present. Analyses of the Experimental single phase (Ref. 11). Welds Materials and Welding Procedure Silicon as Deoxidizer For reasons indicated previously, only the first and third passes were studied. If silicon alone is used as the deoxidizer The base metal used in this research was an ASTM A516 G70 pressure vessel They were examined using a light micro­ in the welding system, the final deoxida­ steel. A 3.2-mm (Vs-in.) diameter ER70S-3 scope to evaluate quantitatively the vol­ tion product in the weld metal is solid sil­ electrode and a high MgO-CaF low Si0 ume fractions of the various microstruc­ ica (Refs. 8,11) and the deoxidation reac­ 2/ 2 commercial flux were used in the experi­ tures. Systematic two-dimensional point tion can be written as ments. The compositions of the base counting was performed on each of the % + 2FeO(l) — Si02(s) + 2Fe(i) (3) metal and the welding consumables are weld specimens. Among all quantitative given in Table 1. methods available in phase proportions At the presence of other deoxidizers, A two-part experiment concerning the determination, the technique of system­ however, a liquid silicate product is ex­ sequence of weld pool deoxidation was atic two-dimensional point counting re­ pected to form. designed. The first part examined the tita­ sults in the smallest relative error, approx­ nium-aluminum addition sequence and imately ten percent. Aluminum as Deoxidizer the second part, the aluminum-titanium The chemical composition of the weld ln the case of deoxidation by aluminum, addition sequence. In the case of the tita­ passes were analyzed using an optical the final products can be AI2O3 or a com­ nium-aluminum addition sequence welds, emission spectrometer and reported in pound of AI2O3 and FeO. If the oxygen the first pass of each weld was made by Table 3. The carbon, sulfur, oxygen, and content is greater than 0.06 wt-%, that is, bead-in-groove submerged arc welding nitrogen content of the weld metals were low aluminum content in the molten metal, (SAW) to contain eight different levels of determined using interstitial analyzers.
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