Boniszewski Self-Shielded Arc Welding

1. Auflage 1992
ISBN: 978-0-85709-324-0
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 240 Seiten

Reihe: Woodhead Publishing Series in Welding and Other Joining Technologies

A detailed original perspective from a leading expert on welding metallurgy of the self-shielded arc welding process and its applications. The author explains the basic process metallurgy of the process and its relationship with other arc welding processes. He promotes self-shielded arc welding (SSAW) as a distinct process in its own right, dispels some widely held misconceptions, and sets out to bring its existence and advantages to the attention of designers and fabricators.

Autoren/Hrsg.

Boniszewski, T.

Leseproben

2 BASICS OF PROCESS METALLURGY
Publisher Summary
This chapter focuses on the basics of process metallurgy in Self-Shielded Arc Welding (SSAW). In metallurgical processing, the handling of most molten metals in the air atmosphere requires protection from the detrimental effects of oxygen and nitrogen, which can cause various flaws, that is porosity and non-metallic inclusions, and the absorption of which can lead to property degradation. This need for protection applies equally well to fusion welding as a miniature process, as it does in bulk alloy- or steelmaking. In welding, molten metal transferred from the tip of an electrode or filler wire into the molten pool must be protected from unlimited or unrestricted reaction with oxygen and nitrogen in the air (1) to produce sound deposits, primarily free from nitrogen porosity, that is, to achieve what is known as the radiographic soundness, and (2) to achieve at least a modicum of ductility and notch toughness – the properties that still distinguish metals from oxide and nitride ceramics, which are being developed and applied for some components made from metals in the past. 2.1 Weld Metal Steelmaking in Air
In metallurgical processing, the handling of most molten metals in the air atmosphere requires protection from the detrimental effects of oxygen and nitrogen which can cause various flaws (e.g. porosity and non-metallic inclusions), and the absorption of which can lead to property degradation. This need for protection applies equally well to fusion welding as a miniature process, as it does in bulk alloy- or steelmaking. In welding, molten metal transferred from the tip of an electrode or filler wire into the molten pool must be protected from unlimited or unrestricted reaction with oxygen and nitrogen in the air, in order:- (i) to produce sound deposits, primarily free from nitrogen porosity, i.e. to achieve what is known as the radiographic soundness, and (ii) to achieve at least a modicum of ductility and notch toughness – the properties which still distinguish metals from oxide and nitride ceramics which are being developed and applied for some components made from metals in the past. In principle, there are two extreme measures [(A) or (Z)] which can be considered as means for molten metal protection in the arc welding of steel:- (A) An almost total exclusion of air from the arc environment can be arranged, by displacing it with a burden of mineral flux (submerged–arc welding) or shielding the arc with a shroud of inert gases: Ar, He or their mixtures (MIG/GMAW and TIG/GTAW). NB. In electron beam welding, the vacuum generated for maintaining the beam fulfils a similar protective role. (Z) Without any auxiliary or external shielding, a totally unprotected bare wire or rod electrode, the self-shielding one, can be used in air, but the electrode must incorporate a sufficient amount of elements (Mn, Si, Al, Ti, Zr, Ca, Mg and REM*) whose affinities for oxygen and nitrogen are greater than those of iron. As shown in Fig. 2.1, the increasing affinity of an element for oxygen and/or nitrogen is represented by the increasingly negative free energy of compound formation between a given element and O2 or N2. Thus the elements, the compound-formation lines of which lie below the FeO and Fe4N lines, have stronger affinities to oxygen and nitrogen than that of iron. Those elements react with oxygen and nitrogen in steel, to form their own oxides and nitrides, thus removing these gases from liquid and solid solutions in iron. Silicon, Al and Ti will take up enough oxygen to “kill” CO-porosity, but only A1, Ti and Zr are the elements with a strong enough affinity for nitrogen (see the dashed lines in Fig. 2.1) to prevent nitrogen-porosity formation. As shown in Fig. 2.1, the nitrides of A1, Ti and Zr are more stable than those of Si, Ca and Mg which are effective as deoxidants only, and consequently only A1, Ti and Zr are known as denitriders. By analogy with the CO-porosity in steelmaking, the denitriders are referred to here as the “killing” agents, but it should be borne in mind that the chemistry of “killing” the nitrogen porosity is different from that of “killing” the CO-porosity.
Fig. 2.1 The standard free energies of formation of oxides and nitrides as a function of temperature.
Note the high stability of the oxides (deep sub–zero positions) and the lower stability of nitrides (nearer zero levels): Mn - the weakest steelmaking deoxidant is more effective energetically in its function than the strongest denitrider - Zr. FeO is more stable than aluminium nitride -AIN. The iron nitride is completely unstable in molten steel. In practice, the total exclusion of air from the arc environment is not possible, and there are also some residual oxygen and nitrogen contents in filler metals and in the parent (base) metal, a portion of which is melted and diluted into the molten pool. Therefore, all the arc-welding consumables incorporate some combination of (i) protection/shielding and (ii) deoxidation/denitriding, but the proportions of these two measures differ considerably from one process to another, and for some types of consumables (e.g. E6010 vs. E7018 electrodes) within a process. Thus with the inert gas shielding, the TIG/GTAW filler wire need have only a minimum of 0.20% Si to suppress the oxygen activity sufficiently to kill the CO-porosity. Some shielding gases, whilst designed to exclude the nitrogen of air, are oxidizing and hence about 1.5%Mn-0.7%Si is required in the filler wire for weld metal deoxidation, as in the CO2 –shielded welding. Similar considerations apply to the MIG/GMAW utilizing shielding with Ar-CO2 and Ar-O2 gas mixtures, and to the flux-covered electrodes the coatings of which contain CaCO3 which evolves CO2 into the arc space. To deposit sound and ductile metal, a given oxidation potential must be balanced by an appropriate degree of deoxidation. Until recently, there has been no coherent perception of the different shielding/killing combinations employed in different process consumables because the formulations of flux-bearing consumables are proprietary secrets. To obtain some quantitative means for gauging the degree of protection-vs.-killing in the design of different welding consumables, it has been proposed (1) to use the “Nitrogen Scale”, i.e. the total residual nitrogen content analysed in the weld metal. 2.2 Nitrogen as the Contamination Gauging Medium
The reasons why the residual nitrogen, and not the residual oxygen in the weld metal is used to gauge the degree of shielding vs. killing is explained below. Firstly, the residual oxygen content in the weld metal can originate from non-metallic inclusion oxides in the filler and parent (base) metals, from oxides in the flux, and from the oxidizing shielding gas (i.e. CO 2) and the air. In contrast, the extra nitrogen additional to the small quantity (e.g. about 20-80 ppm) present in the materials welded can come only from the air. Thus, the contamination of the molten weld metal by air is the main and the most significant source of the residual nitrogen. Secondly, as can be seen from Fig. 2.1, stable oxides can form readily at temperatures above 1530°C in the still molten weld metal. For instance, MnO - the oxide of the weakest deoxidant (Mn) is markedly more stable than ZrN - the nitride of the strongest denitrider (Zr). Therefore, whilst substantial quantities of different nitrides may still remain in liquid solution, oxide phases separate out from the molten metal and can float out into the top slag well before solidification. This oxide separation can occur already in the droplet on the electrode tip, at arc temperatures much in excess of the melting point of iron. Consequently, a substantial quantity of deoxidation products always separates out of the weld metal well before solidification and the residual oxygen, present in the fraction of the trapped oxides, gives no indication at all as to how much the metal was protected from air during deposition. Thirdly, paradoxically as it may seem at first sight, the higher the degree of the original gaseous oxidation balanced by appropriate deoxidation, the lower the residual oxygen content in the weld metal (2, 3). This is because the more voluminous the deoxidation products, the more readily they can coagulate, and thus with the increasing buoyancy, the quicker and easier they can separate to the top slag in the short time available before solidification. This explains why the self-shielded weld metal has the lowest oxygen content among all the weld metals (Table 2.1) deposited from consumable electrodes [i.e. only about 100 ppm (4, 5)], and therefore it is the cleanest one as regards its oxide-inclusion content. Table 2.1 Examples of typical oxygen contents in the all-weld metal deposits of mild and C-Mn steel composition. Process/consumable type Oxygen ppm Flux-covered...