Metal powder additions in submerged-arc welding

TWI Bulletin, May/June 1988

Part 2 Techniques and properties

Part 1

by Kevin Middleton

Kevin Middleton, BSc (Hons), was a Research Engineer in the Flux Processes and Surfacing Section of the Arc Welding Department.

Last issue's article on metal powder additions in submerged-arc welding reviewed equipment available and process characteristics. Part two considers powder feed techniques and the mechanical properties of the welds produced.

Forward feed technique

The metal powder, when laid in the joint ahead of the flux burden uses arc energy normally expended in achieving 'excessive' base metal fusion and therefore reduces weld penetration significantly. ^[9] In practice, the basic requirement for root penetration will restrict the amount of powder that can be added, depending on current level, welding speed and local joint geometry; excess powder addition can lead to lack of fusion defects. With modest metal powder addition rates, typically 5 kg/hr for single wire welding and 7.5 kg/hr for tandem wire welding, the reduction in penetration does not cause an undue risk of lack of fusion defects. ^[4]

Reduced penetration can also be exploited to achieve further increases in weld productivity in both single and multi-run welds. It has been demonstrated experimentally ^[9] that, for single run welding, the metal powder buffers the arc and allows the use of a high temperature ceramic backing strip for root run support. The ceramic strip is thermally insulating and restricts heat flow from the root region compared with a copper or steel backing bar ( Fig.7). Additional heat is therefore available to cause an increase in penetration and parent metal fusion, or alternatively this additional energy can be used to melt more metal powder and allow the welding speed to be increased. The practical benefits are illustrated by the following example.

Using single wire SAW with a metal powder addition, three single run welds were made in 12.5mm thickness steel plate. Using a copper backing the weld was completed using a travel speed of 350 mm/min and the weld metal cross sectional area was 160mm ² ( Fig.6). Using a ceramic backing and the same welding conditions the weld cross sectional area was increased to 190mm ² ( Fig.8), indicating a restricted heat flow from the root region and the availability of excess energy for melting greater amounts of metal powder addition. A further 30% increase in welding speed ( Fig.9) was achieved by this increase in powder addition rate.

The reduced penetration also provides a measure of tolerance to variations in root gap. ^[9] However, for single run welding it is necessary to modify the powder addition rate to compensate for increases in joint volume when the root gap widens to ensure joint filling. For multi-run welding the increased tolerance allows the root run to be deposited at a faster completion rate with SAW than other conventional procedures.

Current industrial practice for multi-run joints uses the SAW process for the fill and capping runs only. The root typically consists of one or two weld runs deposited using a manual or semi-automatic technique. The initial fill runs are then deposited using low arc energy SAW and then the weld is completed using a higher arc energy condition. ^[10] After welding, a reverse side 'punch-through' or sealing run is then made which penetrates some 8-l2mm, eliminating any potential defects in the root region. Recent work at The Welding Institute ^[8] has demonstrated that the initial 2-4 runs can be replaced by a single SAW run using the welding conditions typically used for fill runs. Burnthrough is avoided provided a metal powder is always present. However, the sealing run is still required to eliminate any possible root defects.

Magnetic attachment technique

The forward feed technique uses energy normally expended in producing 'excessive' parent metal penetration (both sidewall and root) for melting the additional filler metal. The amount of powder that can be melted therefore will depend upon the specific welding conditions. Too much powder ultimately leads to lack of penetration and unfused powder in the root, whilst the excess energy in the superheated tail end of the molten pool is not utilised.

In contrast, the magnetic attachment technique is considered to use heat available from both the 'excess-energy' sources. The metal powder is fed on to a slightly extended electrode extension, typically 55mm. Two delivery tubes are usually used, parallel to (for filling runs) or traverse to (for capping runs) the welding direction ( Fig.10). It is thought that when the metal powder is being pulled through the flux into the slag it is quickly heated to a temperature at which the heat of the slag and radiation from the arc would cause the powder to lose its magnetic properties. ^[11,12] Some of the powder will travel across the arc and some will travel around the rear of the arc cavity and be deposited in the tail end of the molten pool, where it is melted by the excess heat in this region. Ivochkin et al ^[11] found that, for a powder addition up to 65% of the wire deposition rate, there was no reduction in the amount of parent metal melted, indicating that the excess heat in the molten pool is used to melt the powder and not the heat used for 'excessive' parent metal fusion. Bead size (parent metal + deposited metal) is therefore increased when metal powder is added using the magnetic attachment technique whereas there is no significant increase in bead size when using the forward-feed method of powder addition.

The apparent increase in melting efficiency of the magnetic attachment technique over the forward feed technique means that higher powder addition rates are used industrially. ^[5] To date, this method has only been used with single wire SAW and a typical industrial addition rate is 9 kg/hr, 80% higher than that typically used for single wire welding with the forward-feed process. However, the additional productivity benefits to be made must be weighed against the increased cost of the more highly alloyed metal powder that may be required to produce high toughness weld metal.

The major advantages of this technique lie in its application to horizontal fillet welding, grout beading and small diameter circumferential seam welding.

Increases in productivity for the welding of flat position fillets can be achieved with both the forward-feed and magnetic attachment metal powder addition techniques, but the magnetic attachment technique allows the addition of more powder than the forward-feed technique and can therefore deposit larger fillet sizes. Also, only the magnetic attachment technique is applicable to horizontal-vertical (HV) as well as flat fillets, although the maximum leg length of an HV fillet is limited to 10mm.

For grout beading, the deposition of weld beads of specified height and width, the magnetic attachment technique is especially advantageous. Powder addition rates are higher than those used for welding, i.e. 14-18 kg/hr instead of 9 kg/hr, and grout beads some 9mm high and 25mm wide can be obtained in a single run. The forward-feed technique does not allow the use of such a high powder-to-wire ratio and would not therefore achieve the required bead size in a single run at an equivalent arc energy. ^[13]

For small circumferential seams, the forward-feed technique is not practicable because the powder is fed typically 50mm ahead of the arc and flux layer. Whilst the magnetic flux within the parent material is strong enough to hold the powder in a tight 'triangular' deposit, on small circumferential seams, a flux support may be necessary ( Fig.11). In this instance the magnetic attachment technique is the only practical method of powder addition.

Mechanical properties

Many welding procedures which include the use of a metal powder addition have been qualified for the fabrication of high integrity tubulars for offshore constructions operating in the North Sea. Mechanical property requirements are stringent and a minimum Charpy V notch impact toughness of 40J at -40°C is typically specified for the weld metal and HAZ in both the as-welded and postweld heat-treated conditions. Typical examples of mechanical property levels achieved are shown in Tables 3 and 4 where the metal powder has been added by the forward-feed ^[13] and the magnetic attachment ^[5] techniques respectively.

Table 3 Typical mechanical test data for tandem SA with iron powder ^[13]

Table 4 Typical as-welded weld metal fracture toughness properties achieved using magnetic attachment of metal powder addition in SAW ^[5]

The powder addition can be used to make significant alloy additions to the weld metal such that high arc energy welding will produce good weld metal properties ^[14] (27J at -40°C using an arc energy of 14 kJ/mm for single run welding of 30mm thick plate). But the HAZ toughness properties were low (27J at + 20°C in the same weld); therefore, to take advantage of this ability to develop tough weld metal at a high arc energy, it is necessary to select a parent steel capable of giving adequate HAZ toughness.

Summary

The use of a metal powder addition in SAW has now become an established technique for the welding of C, C-Mn and microalloyed steels. It is an inexpensive method of increasing the productivity of standard submerged-arc welding, not only by increasing metal deposition rates, but also through an increase in the melting efficiency of the filler metal, making the technique economically viable in comparison with other high metal deposition rate techniques, ^[10] e.g. tandem wire SAW. It seems surprising, therefore, that the process is not more widely used and that other fabrication industries, apart from the offshore and some vessel fabrication, have not utilised the technique and developed it for their own applications. The powder feeding equipment is simple to operate and can easily be attached to both fixed and portable equipment such as a tractor mounted SAW head. This opens up many areas of fabrication to the exploitation of metal powder additions where SAW is carried out on site, e.g. ship assembly and storage tank construction.

The development of powder additions to improve the productivity of SAW for the offshore industry arose because arc energy limitations were imposed to ensure specific mechanical property requirements. An arc energy limitation exists when welding a wide range of steels, including high tensile steels, high temperature creep resistant steels and stainless steels, to ensure satisfactory mechanical and metallurgical properties. This limitation can severely curtail the joint completion rate possible with SAW. The development of suitable metal powder compositions for the welding of these steels could allow significant increases in joint completion rate to be achieved. Furthermore, because of the reduced parent metal dilution, by the adoption of the forward-feed technique in particular, the risk of solidification cracking in susceptible materials, such as high carbon steel, could be reduced. ^[15]

The development of a wider range of consumables will require consideration of the effects on weld composition caused by interruption of the powder feed supply. Therefore wires, fluxes and welding procedure should be selected to provide the intended weld metal chemistry and properties without a metal powder addition. The composition of the metal powder can thus be used to enhance weld metal composition/mechanical properties, and the fabricator can still remain confident that should the metal powder supply be interrupted, the weld will still meet the required specification.

References