Factors influencing the toughness of submerged-arc welded line pipe welds - Part 1: Oxygen and deoxidising elements

TWI Bulletin, January/February 1989

by Peter Harrison

Peter Harrison, BSc(Eng), ARSM, PhD, MIM is a Principal Research Metallurgist in the Materials Department.

The toughness of longitudinal submerged-arc seam welds in line pipe can be sensitive to small variations in weld metal chemistry brought about by dilution effects. Part 1 of this article reviews the influences of oxygen and deoxidising elements on seam weld toughness and Part 2 will discuss the importance of weld metal hardenability, weld cooling rate and weld metal A1:0 ratio.

During the last few years, consumable manufacturers have developed consumables to a point where current weld metal specification requirements can usually be met in all weld metal samples. It is now recognised, however, that small variations in the levels of deoxidising elements, introduced through dilution from the parent material, or from the welding wire, can lead to dramatic changes in weld metal toughness. This is particularly true where high heat input submerged-arc welding (SAW) is concerned, as in line pipe seam welds.

In basic flux weld metals, the best low temperature toughness ( i.e. resistance to cleavage fracture) is achieved when microstructures consisting of a high proportion of acicular ferrite, which is a fine-grained intragranularly nucleated phase, are formed ( Fig.1). ^[1] Studies of the development of acicular ferrite in weld metals have shown that inclusion content is of considerable importance since, under favourable conditions, acicular ferrite grains may be nucleated at or around the weld metal inclusions. The following review draws together published information on the effects of oxygen and deoxidising elements, particularly Al, Ti, and Si, on the microstructure and toughness of high dilution submerged-arc welds, typical of those found in line pipe seam welds.

Oxygen

Many studies have shown that weld metal microstructural development is strongly influenced by either the nature or number of weld metal deoxidation products ( i.e. by the weld metal oxygen content). ^[2-6] In submerged-arc weld metals, it has been noted that both high and low oxygen contents may lead to poor weld toughness, whereas intermediate oxygen levels often lead to high levels of toughness because of the development of tough acicular ferrite dominated microstructures. Ito and Nakanishi ^[2] observed optimum toughness in submerged-arc weld metals with oxygen contents of about 200-250ppm, as shown in Fig.2. A similar finding was made by Terashima and Tsuboi. ^[7] At the lower oxygen levels, acicular ferrite nucleation becomes difficult and less tough 'bainitic type' microstructures are formed, with a resulting loss of cleavage resistance. At the higher oxygen levels, there is a tendency to form high temperature γ to α transformation products such as pro-eutectoid ferrite and Widmanstatten sideplates, which have a large grain or colony size and therefore have poor cleavage resistance. Dilatometric measurements made by Cochrane and Kirkwood ^[4] showed that the γ to α transformation temperature of a high oxygen content weld metal (~ 700ppm) was some 50°C higher than that of a medium oxygen content weld metal (~300ppm) of otherwise similar composition. Harrison and Farrar ^[5] noted a similar trend when they degassed a 300ppm oxygen weld metal to 60ppm and found a 30°C drop in γ to α transformation temperature. These dilatometric observations indicate that increasing weld metal oxygen content has a powerful negative effect on hardenability and thus the influence of oxygen content is opposite to that of the major alloying elements.

Aluminium

Several investigations have been conducted to study the influence of aluminium on submerged-arc weld metal toughness. ^[8-17] Investigations by Tenkula and Heikkinen ^[9] and Hannerz and Werlefors ^[10] noted that weld metal toughness was lowered by the presence of aluminium. Yoshino and Stout ^[11] and Terashima and Hart ^[12,13] noted that the influence of aluminium varied with flux type. For basic fluxes, aluminium was detrimental, but for fluxes with lower basicity or for acid fluxes, aluminium had a beneficial influence on toughness.

Most of the above investigators noted that the detrimental influence of aluminium in basic flux welds was associated with a coarsening of the microstructure. This effect was partially quantified by Terashima and Hart, who observed a decrease in the proportion of acicular ferrite in their weld metals, accompanied by an increase in the proportion of ferrite with aligned second phase, as weld metal aluminium content increased. With acid fluxes, the effect of aluminium on microstructure is less clear, and it has been suggested by Hannerz ^[18] that the beneficial influence of aluminium may be because of improved deoxidation.

A further observation on the influence of aluminium was that of Devillers et al, ^[8] who demonstrated that the optimum or 'critical' oxygen content varied with aluminium level, as illustrated in Fig.3. This was an important observation, because it demonstrated that the main influence of aluminium on toughness was to move the optimum oxygen content for acicular ferrite development. Thus, aluminium per se is not particularly detrimental to weld metal toughness, unless perhaps excessive amounts remain in solid solution.

Devillers et al recommended that flux SiO ₂ and MnO contents should be increased in order to increase weld metal oxygen contents and obtain good toughness when welding materials of high aluminium content. Saggese et al ^[19] observed a linear relationship between weld metal oxygen content and SiO ₂ + MnO +TiO ₂ oxide contents of fluxes and suggested that optimum weld metal toughness would be found when the aluminium and oxygen contents were approximately at the stoichiometric ratio for alumina formation, i.e. an aluminium/oxygen ratio of 1:12. This ratio is plotted as a dashed line in Fig.3 and is fairly close to the empirical results of Devillers. However, recent work by Thewlis ^[17] has suggested that acicular ferrite nucleation will be accelerated in the presence of fcc galaxite (MnO/Al ₂ O ₃ ) in the surface layers of inclusions. The formation of this compound was seen to be critically dependent on weld metal aluminium content, but presumably this will also depend on weld metal oxygen content during deoxidation, i.e. the Al/O ratio.

Terashima and Hart also noted that weld metal tolerance to aluminium was influenced by oxygen content, and could be increased by increasing the TiO ₂ content of the flux. More recently, Harrison ^[20] and Suzuki et al ^[15] have demonstrated that tolerance to aluminium can be increased by using semi-basic or neutral fluxes with higher oxygen potentials. Terashima and Hart also noted that weld metal tolerance to aluminium was reduced as weld metal silicon content increased ( Fig.4)

Titanium

Many investigators have studied the influence of titanium on weld metal toughness. A review of this subject has also been undertaken by Tsuboi and Terashima. ^[21] In most studies, a beneficial effect has been observed for small additions of titanium, followed by a detrimental influence at higher titanium levels. Hirabayashi et al ^[22] observed an optimum weld metal titanium content of around 0.02-0.03wt% in submerged-arc welds made in X65 linepipe steel ( Fig.5). Nakano et al, ^[23] however, noted that the optimum weld metal titanium content was highly dependent on other alloying elements such as Ni, Cr and Mo and that the optimum addition decreased as the alloy level increased.Terashima et al ^[24] observed an increase in the optimum weld metal titanium content with increasing nitrogen levels. Nakanishi and Komizo ^[25] noted an increase in the optimum titanium content with increasing oxygen content ( Fig.6).

There are thought to be two main mechanisms of toughness improvement by small additions of titanium, one involving promotion of acicular ferrite development and the other, removal of 'free' nitrogen from solution in the matrix. In the former case, it was originally proposed that TiN ^[2,26] was responsible for enhanced acicular ferrite nucleation. However, a number of more recent investigations have suggested that TiO ^[27,28] promotes acicular ferrite nucleation. An investigation by Bailey ^[29] indicated that only approximately 0.004 wt%Ti was required to change weld metal microstructure from ferrite with aligned MAC to acicular ferrite, when welding with manganese silicate flux. A similar observation was made by Bonnet and Charpentier ^[30] when welding with calcium silicate fluxes.

At high levels of titanium, weld metal toughness is degraded because of matrix hardening, and it has been suggested by Thaulow ^[31] that either the formation of microphase regions consisting of twinned martensite, or the occurrence of fine scale precipitation of TiC in ferrite may be responsible for embrittlement.

Silicon

In recent years, the influence of silicon on submerged-arc weld metal toughness has received relatively little attention compared to titanium and aluminium. However, the investigations of Ito and Nakanishi, ^[2] and Hannerz ^[32] both indicated optimum weld metal silicon contents. The results of Ito and Nakanishi indicated that the optimum silicon level lies at about 0.15-0.25 wt%Si at 1 wt%Mn. The results of Hannerz ( Fig.7) show an optimum silicon content at about 0.25-0.35 wt%Si.

Hill and Levine [33] observed an improvement in Charpy toughness of submerged-arc welds when the silicon content increased from 0.24-0.51 wt%. Similarly, Cochrane et al ^[34] reported improvement with increasing silicon from 0.19-0.28 wt%Si, and Pargeter ^[35] observed improvement in the range 0.35-0.42%. Abson, ^[36] however, observed a small decrease in toughness with increasing silicon from 0.23 to 0.57%. In all these investigations, an increase in the proportion of acicular ferrite was observed with increasingsilicon content.

Terashima and Hart, and Cochrane et al noticed that the effects of silicon and aluminium are additive with respect to their influence on weld metal microstructure. In Cochrane's work, because of the aluminium levels and flux systems studied, both elementswere seen to promote acicular ferrite development, and thus improve toughness. In the work of Terashima and Hart, it was noted that for low aluminium levels, silicon promoted acicular ferrite development and thus improved toughness,but, at high levels of aluminium, silicon reduced toughness.

Summary

Examination of the literature has shown that O, Al, Ti and Si all influence weld metal toughness, and that optimum levels of these elements exist for any given base composition and welding condition. This is thought to be closelylinked with the formation of an optimum weld metal microstructure consisting of a high proportion of acicular ferrite.

Of course acicular ferrite development is influenced by other factors, the most important of which are weld metal hardenability, weld cooling rate, and the presence or absence of suitable nucleation sites for intragranularnucleation of acicular ferrite. Part 2 will discuss these factors and present data illustrating the importance of weld metal Al:O ratio.

References