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WELDING CONSUMABLE DEVELOPMENTS IN THE AFTERMATH OF THE NORTHRIDGE EARTHQUAKE

WELDING CONSUMABLE DEVELOPMENTS IN THE AFTERMATH OF THE NORTHRIDGE EARTHQUAKE D.J. Kotecki1, Badri Narayanan2 The Lincoln Electric Company (United States) E-mail: [email protected] 2 [email protected]

ABSTRACT The earthquake that struck Northridge, California, on 17 January 1994, caused collapse of a number of reinforced concrete and wood frame structures. Fracture occurred in welded connections in some steel frame buildings, though none collapsed. Self-shielded flux cored electrodes are popular for steel frame building erection because they are largely unaffected by welding in windy conditions. These electrodes are generally “killed” with aluminium, and the aluminium recovered in the weld deposit affects the microstructure and, therefore, the properties. The paper describes the evolution of steel weld metal microstructures from self-shielded flux cored electrodes, as a function of aluminium content. IIW-Thesaurus keywords: Filler materials; FCA welding; Arc welding; Steel construction; Earthquakes; Toughness; Mechanical properties; Microstructure.

1 INTRODUCTION Self-shielded flux cored arc welding (FCAW-S) is a quite mature process, having been developed during the 1950s [1]. Due to its exceptional tolerance for welding in windy conditions, as compared to gas-shielded welding processes, FCAW-S is widely applied in the erection of steel frame buildings and other structures that are necessarily welded outdoors [2]. The design of FCAW-S electrodes is appreciably different from that of gas-shielded flux cored electrodes because FCAW-S electrodes must produce sound weld metal while welding in a high nitrogen environment (air). To reduce the partial pressure of nitrogen in contact with the liquid metal, materials that are volatile at arc temperatures are generally added to the electrode core. These materials include various fluorides. However, the quantity of volatile materials that can be added is limited by increasing spatter with increasing volatile materials [2]. As a result, it is also necessary to include, in the core of the electrode, alloy elements which allow sound weld metal to form in the presence of still rather high nitrogen partial pressure. Designs for commercial electrodes in the USA have primarily used aluminium alloying for this purpose, although titanium and, to a still lesser extent, zirconium, have also been used. Aluminium seems to be preferred because it produces the least detrimental effects on weld metal mechanical properties [2].

Doc. ICRA-2003-11 Welding in the World, Vol. 49, n° 1/2, 2005

2 METALLURGICAL DESIGN OF FCAW-S ELECTRODES Both aluminium and titanium, as alloying elements in iron-base alloys, are powerful ferritizers. Kotecki and Moll observed that excessive aluminium and/or titanium can prevent austenite formation in an iron-base alloy at any temperature [3, 4]. The resulting weld metal is extremely coarse grained ferrite, much like fully ferritic stainless steel weld metal. Figure 1 shows the microstructure of a FCAW-S deposit in which austenite was completely prevented from forming, so that the coarse grain size can be appreciated. With such coarse grain size (grains larger than 1 mm across), weld metal toughness was found to be very low, in keeping with classi-

Figure 1 – FCAW-S steel deposit high in Al and Ti Composition: 0.03 % C, 0.4 % Mn, 0.3 % Si, 1.5 % Al, 0.6 % Ti

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Figure 2 – Fe-Rich end of binary Fe – Al diagram

cal grain size effects [5]. Further, due to the very coarse grain size, the strength was found to be exceptionally low, on the order of 170 MPa yield strength and 285 MPa tensile strength. Strength has been shown to be inversely proportional to the square root of the grain diameter [6]. The iron-rich end of the binary iron-aluminium phase diagram, generated by ThermoCalc® software and shown in Figure 2, reveals that aluminium closes the austenite phase field (“austenite loop”) when aluminium is present in excess of about 1 %. Above this level of aluminium, iron-aluminium alloys solidify as ferrite and remain ferrite at all temperatures down to ambient. So no grain refinement due to transformation of austenite to ferrite is possible, and the as-cast grain size is retained to ambient temperature, as in Figure 1. Between about 0.7 and 1 % aluminium, there is a temperature range (between about 1 300 oC and 1 100 oC) in which the alloys are part austenite and part ferrite. The austenite portion can transform to ferrite on cooling, with partial grain refinement. And when there is less than about 0.7 % aluminium in the iron, the alloy, at temperatures around 1 200 oC, is 100 % austenite, which allows for complete grain refinement during transformation to ferrite on cooling. Titanium in iron has a similar effect. Carbon steel weld metals from FCAW-S electrodes are not simple binary iron-aluminium alloys, so that Figure 2 is an over-simplification of the situation. Fortunately, the ThermoCalc® software permits calculation of pseudobinary phase diagrams that take into account carbon and manganese presence at normal levels. Normal carbon steel weld metal typically contains about 0.05 % C and about 1 % Mn. Figure 3, as compared to Figure 2, illustrates the effect these levels of carbon and manganese have on the range of aluminium contents over which complete formation of austenite is possible, and over which partial formation of austenite is possible. By comparison with Figure 2, it can be seen that these normal levels of carbon and manganese increase the maximum aluminium content which will allow complete austenite formation to about 1 %, as well as increasing the maximum aluminium content which will allow partial austenite formation to about 3 %. This is expected, since

Figure 3 – Pseudo-binary Fe – Al diagram at 0.05 % C, 1.0 % Mn both carbon and manganese are known to promote austenite formation in binary systems with iron [3]. In order to obtain grain refinement for improved strength and toughness in FCAW-S weld metals of mild steel or low alloy steel compositions, it is essential to obtain formation of austenite at elevated temperatures (e.g., 1 000 oC to 1 200 oC), so that the austenite can transform to finer structures at temperatures below 800 oC. So, addition of carbon and/or manganese should have a beneficial effect on properties, as long as hardenability is not excessive. Early commercial designs of FCAW-S electrodes were not very efficient at reducing the partial pressure of nitrogen in contact with the liquid weld metal. As a result, they required rather high aluminium, on the order of 1.5 %, to provide commercially acceptable ranges of operating conditions without porosity. That level of aluminium is predicted, by Figure 3, to produce weld metal, at normal C and Mn content, that will only partially form austenite at high temperature. The remainder of the microstructure will be the coarse ferrite that formed during solidification, and that coarse ferrite will be retained to ambient temperature. However, by increasing the weld metal carbon content, while decreasing the Mn content to reduce hardenability, the austenite loop can be expanded further, making further, but still only partial, austenite formation possible at high temperatures in the presence of 1.5 % Al. This effect is shown in Figure 4, prepared from the ThermoCalc® software for iron-rich Fe-Al alloys with 0.20 % C, 0.4 % Mn. Note the dashed vertical line at 1.5 % Al, which indicates that the composition will not be completely austenite at any temperature. Some ferrite will always be present. Figure 5 shows the as-deposited microstructure obtained at about 1.5 % aluminium in this alloy system. Ferrite grains, which co-existed with austenite at high temperatures, could grow extensively during cooling from 1 200 oC down to about 800 oC, in which temperature

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Figure 5 – FCAW-S steel deposit high in C Composition: 0.20 % C, 0.4 % Mn, 0.2 % Si, 1.5 % Al Figure 4 – Pseudo-binary Fe – Al diagram at 0.20 % C, 0.4 % Mn range both diffusion and grain growth are rapid. Nucleation of ferrite is not necessary because some ferrite exists at all temperatures up to melting. As a result, the ambient temperature microstructure of the weld deposit includes only a small amount of the finer transformation products of the austenite decomposition, which appear to be mainly pearlite. The remainder of the microstructure is very coarse ferrite, and the weld metal strength is low and toughness is poor. Kotecki and Moll observed that, if austenite forms in the presence of higher manganese (1.4 %), aluminium increases the hardenability of the steel weld metal, which may result in microstructures other than ferrite [4]. Upper bainite, with accompanying poor toughness, was found in some weld metals which had been 100 % austenite at some high temperature. As a result, the weld metal strength will be unusually high for carbon steel weld metal, on the order of 565 MPa yield strength and 715 MPa tensile strength. This observation imposes a restriction on the use of Mn to enlarge the range of Al content for complete austenite formation. So the high aluminium designs are low in Mn. Titanium was found to be more harmful than aluminium in promoting upper bainite, which helps to explain why commercial FCAW-S electrode designs have concentrated on alloying with aluminium. So it can be appreciated that there is a delicate balance required among the various alloying elements in FCAWS design in order to obtain a useful range of welding conditions for the electrode along with desirable weld metal microstructure. Lower aluminium content than that of the 1.5 % design is desirable, but the 1.5 % Al electrode design allows for a wide range of welding conditions which will produce sound weld metal. Advanced design for improved properties must also provide for a commercially acceptable wide range of welding conditions, or the design will not be commercially successful. An improvement in shielding permitted reduction in deposit aluminium to 1.2 %, which in turn permits the

microstructure, at about 1 200 oC, to become entirely austenite (see the vertical dotted line in Figure 4). It is necessary for ferrite to nucleate in such weld metal. This prevents ferrite grain coarsening during cooling at high temperature. However, the transformation of austenite to ferrite can begin at temperatures above 1 000 oC, as can be seen in Figure 4, so that some ferrite grain growth is possible. The deposit microstructure is then a mixture of slightly coarse ferrite and austenite decomposition products ferrite, bainite and pearlite. This is shown in Figure 6. The reduction of aluminium from 1.5 % in Figure 5 to 1.2 % in Figure 6 has produced a dramatic effect on the microstructure. The mechanical properties of such weld metal are comparable to those of other types of weld metal, such as that from AWS E7024 covered electrodes that have also been used in steel frame building erection and do not have a toughness requirement, e.g., 350 MPa yield strength and 500 MPA tensile strength. However, the impact properties of the weld metal of Figure 6 are still not adequate to meet the requirements of Federal Emergency Management Agency document FEMA-353, which requires minimum impact values of 27 J Charpy V-notch energy at – 18 oC, 54 J at + 21 oC

Figure 6 – FCAW-S steel deposit with reduced Al Composition: 0.22 % C, 0.4 % Mn, 0.3 % Si, 1.2 % Al

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for service in seismic areas. These requirements were imposed after the Northridge earthquake of 1994. In view of the microstructural evolution of FCAW-S weld deposits to this point (Figures 1, 5 and 6), it became abundantly clear that improvement in FCAW-S deposits would have to be made by reducing the need for aluminium in the deposits. Also, the high carbon (about 0.20 %) necessary to obtain complete austenite formation at temperatures around 1 200 oC is not considered conducive to good impact properties. The formation of pearlitic carbides or cementite at the grain boundaries decreases the toughness of weld metal by acting as initiation sites for cracks during impact toughness testing. The FCAW-S electrode whose deposit microstructure is shown in Figure 6 has a broad range of operating conditions, which is a large part of its appeal for field erection of steel frame buildings. For example, the commercial 2.4 mm diameter electrode of this design can make sound deposits over the wire feed speed range of 2.8 to 7.0 m/min (approximately 250 to 450 amperes) at contact-tip-to-work distances of 50 to over 75 mm. These conditions produce deposition rates of 3.7 to 10.0 kg/h. And most importantly, at any given set of above conditions, there is an operating range of several volts (DC electrode positive) over which sound weld metal will be produced. Any further reduced aluminium design for FCAW-S electrodes had to match these welding characteristics. Again, attention was devoted to improving the self shielding characteristics of FCAW-S electrodes to further reduce the partial pressure of nitrogen in the arc. The result was proprietary developments in fluoride ingredients in the electrode core that permitted reduction in the electrode aluminium content so that deposit compositions of about 1 % Al, or less, could be obtained. This, in turn, permitted rebalancing the carbon and manganese content of the electrode to lower C and higher Mn than the electrode of Figure 6. Figure 7 shows a pseudo-binary phase diagram for the rebalanced carbon and manganese levels of this new design. It is noteworthy that, at just below 1 % Al (indicated by the dotted line in Figure 7), this composition is com-

Figure 8 – FCAW-S deposit with less than 1 % Al Composition: 0.09 % C, 1.25 % Mn, 0.2 % Si, 0.94 % Al pletely austenite at temperatures around 1 200 oC, and ferrite cannot nucleate from austenite until the austenite is cooled to about 1 000 oC. Of course, actual nucleation, no doubt, takes place at a still lower temperature. This reduced nucleation temperature, as compared to that of the weld deposit of Figure 6, results in finer austenite decomposition products, mainly ferrite, as shown in Figure 8. Comparison of the microstructures of Figure 8 and Figure 6 reveals that the microstructure of Figure 8 contains much more acicular ferrite. The higher manganese is considered to beneficial in this respect, as found in deposits of basic low hydrogen electrodes without aluminium alloying, by Evans [7]. It is considered that, while the equilibrium temperature for ferrite to begin forming from austenite for the weld metals in Figures 6 and 8 is nearly the same (about 1 000 oC, as can be seen in Figures 4 and 7), the actual nucleation of ferrite must occur at a lower temperature in the higher Mn weld metal. That is, the Mn is beneficial in delaying the onset of austenite transformation to ferrite during cooling from high temperature, which accounts for the greater quantity of acicular ferrite in the weld metal of Figure 8. In contrast to the weld metal of Figure 6, the weld metal of Figure 8 consistently meets the toughness requirements of the FEMA-353 document. It typically produces Charpy V-notch toughness of 30 to 70 J at the required – 18 oC, and comfortably more than 54 J at + 21 oC. It is classified according to AWS A5.20 as E70T-6. And it provides a very similar range of operating conditions to those of the earlier 1.2 % Al design. In particular, it produces sound weld metal at any given set of conditions over a wide voltage range, and it can achieve over 10 kg/h deposition rate with a 2.4 mm diameter electrode. The design criteria have been met with this electrode.

3 FURTHER DEVELOPMENTS Figure 7 – Pseudo-binary Fe – Al diagram at 0.09 % C, 1.25 % Mn, 0.2 % Si

Recently, development efforts have concentrated on further reduction in the amount of aluminium necessary for production of sound weld metal. This required further

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improvement in the self-shielding materials used in the electrode core. A further benefit to toughness has been obtained by reducing the size of inclusions in the weld metal. This was achieved by adding controlled traces of zirconium and titanium, in addition the normal aluminium addition. These additions tend to lessen the agglomeration of large aluminium oxide inclusions in particular, as reported by Munz and Narayanan [8]. The result is a product with a more restricted range of operating conditions than those of the electrode above that meets FEMA-353 requirements, but still better weld metal toughness. This product utilizes a small addition of nickel as an alloying element, as well. The mechanical properties of the weld metal from this electrode have been tested numerous times, and the range of values observed is 400 to 460 MPa yield strength, 480 to 540 MPa tensile strength, 23 to 32 % elongation based on gage length to diameter ratio of 4:1, and 165 to 350 J Charpy V-notch energy at – 30 oC. The as-deposited microstructure is very fine ferrite, as can be seen in Figure 9. Due to the restricted voltage range of the product whose microstructure is shown in Figure 9, that product is not suitable today for the wide range of conditions seen in erecting steel frame buildings. However, it does see service in welding pipelines. Its outstanding mechanical properties illustrate what is possible with FCAW-S electrodes, and further developments to broaden the range of application can be expected.

Figure 9 – FCAW-S deposit with Ni and low Al Composition: 0.06 % C, 1.0 % Mn, 0.1 % Si, 0.6 % Ni, 0.86 % Al, 0.027 % Ti, 0.036 % Zr

ACKNOWLEDGEMENTS The Lincoln Electric Company provided the environment and services that made the work possible. Dan DeMiglio prepared the photomicrographs.

REFERENCES [1] Landis G.G., Patton D.M.: Method and means of bare electrode welding, 1959, US Patent 2, 909, 778. [2] Boniszewski T.: Self-shielded arc welding, 1992, Abington Publishing, Cambridge, England.

4 CONCLUSIONS FCAW-S electrodes have been developed to meet the FEMA-353 toughness requirements for steel frame building construction in seismic areas. Still further improvements in toughness of FCAW-S weld metal are ongoing. The key to developing such electrodes lies in improved self-shielding to permit reduction in aluminium content. Reduced aluminium content permits improved microstructures in the weld metal. It is essential to obtain a deposit composition that completely forms austenite at high temperatures such as 1 200 oC. It is further essential that the deposit composition permit transformation of austenite to very fine ferrite (acicular ferrite) during cooling. Pseudo-binary phase diagrams, prepared from the ThermoCalc® software are a very valuable tool in understanding the development of desirable microstructures.

[3] Kotecki D.J., Moll R.A.: A toughness study of steel weld metal from self-shielded flux-cored electrodes – Part I, 1970, Welding Journal 49 (4), 157-s to 165-s. [4] Kotecki D.J., Moll R.A.: A toughness study of steel weld metal from self-shielded flux-cored electrodes – Part II, 1970, Welding Journal 51 (3), 138-s to 155-s. [5] Brick R.M., Gordon R.B., Phillips A.: Structure and properties of alloys, 1965, New York, NY: McGraw-Hill. [6] ASM International, 1990. ASM Handbook, Tenth Edition, Volume 1. Materials Park, OH. ASM International. [7] Evans G.M.: Effect of manganese on the microstructure and properties of all-weld-metal deposits, 2000, Welding Journal 59 (3), 67-s to 75-s. [8] Munz R.P., Narayanan B.: The welding of line pipe using the Innershield process – Latest Developments, Proceedings of the International Conference on Pipeline Construction Technology, 2002, Paper 17, Wollongong, NSW, Australia.