Submerged arc welding consumables - Part 1Job Knowledge

The submerged arc process is somewhat unusual in that the welding consumables, unlike the other fluxed processes of MMA or FCAW, comprise two components, the wire and the flux, that may be supplied separately.

Since both the wire and the flux will have an effect on the weld metal composition, and hence on the mechanical properties, the welding engineer is faced with choosing the appropriate wire/flux combination for the application. This article discusses some of the characteristics of wires and fluxes. The next article will review the specifications.

The welding wire is generally of a composition that matches that of the parent metal and wires are available for the welding of carbon and low and high alloy steels, stainless steels, nickel and copper/nickel alloys. In addition, submerged arc welding may be used for surfacing with corrosion or wear resistant coatings using both wires and flat strips. The wires may be solid or metal cored. Strips may be rolled or sintered.

Welding wires vary from 1.2mm ('thin' wire or twin wire submerged arc) to 6.4mm in diameter and are capable of carrying welding currents ranging from 150 to 1600amps. The wires for ferritic steels are generally copper coated to increase contact tip life, improve electrical conductivity and extend the shelf life. Stainless steel and nickel alloy wires are bright drawn and uncoated. The wire is supplied on reels weighing 10 to 50kg and can also be obtained in large pay-off packs weighing up to 500kg. The strip used for surfacing is supplied in 15 to 240mm widths but the thickness is a standard 0.5mm. As with the wire, strip is available in a range of coil weights.

Whilst the wire is relatively simple and is designed to match the parent metal composition and/or mechanical properties, the flux is far more complex. The functions of the flux are:

to assist arc striking and stability

to form a slag that will protect and shape the weld bead

to form a gas shield to protect the molten filler metal being projected across the arc gap

to react with the weld pool to provide clean high quality weld metal with the desired properties

to deoxidise the weld pool

provide deoxidants

in some circumstances, to provide additional alloying elements into the weld poolFluxes may be categorised in two ways: by the method of manufacture (fused or agglomerated) or by its activity (neutral, active or alloying). Within these broad groupings the fluxes may be classified further by their constituents, silica, manganese oxide, calcium fluoride etc.

Perhaps the most convenient method of classifying, however, is by reference to the 'basicity index' (BI) of the flux. The index is calculated by dividing the sum of the percentages of the basic constituents by the sum of the acid constituents. Calcium, magnesium, sodium, potassium and manganese oxides, calcium carbonate and calcium fluoride are the basic constituents of a flux; silica and alumina the acid constituents. Acid fluxes have a basicity index of 0.5 to 0.8; neutral fluxes 0.8 to 1.2; basic fluxes 1.2 to 2.5 and highly basic fluxes 2.5 to 4.0. The basicity of a flux has a major effect on the weld metal properties, most importantly the notch toughness. As a general rule the higher the basicity the higher the notch toughness.

Neutral fluxes are designed to have little or no effect on the chemical analysis of the weld metal and therefore on the mechanical properties. They contain low silica, calcium silicate and alumina and do not add significant amounts of silicon and manganese to the weld.

The acid fluxes contain substantial amounts of silica, silicates in the form of calcium and/or manganese silicate and manganese oxide. These fluxes react with the weld pool and will raise both silicon and manganese content of the weld together with a high oxygen content. The result of this is that the toughness of the weld is poor but the fluxes will tolerate rusty surfaces, will detach easily and give a good weld appearance. They are especially useful for single pass high speed welding such as fillet welding of web to flange girder joints.

The basic fluxes fill much the same role in submerged arc welding as basic coatings do in manual metal arc welding. They have a low silica content and are composed of varying amounts of calcium carbonate and/or fluoride, alumina, calcium, manganese and magnesium oxides and rutile.

This combination of compounds gives a clean, low sulphur, low oxygen weld metal with good to excellent notch toughness. As a general rule, the higher the basicity, the higher the toughness. The transfer of silicon and manganese into the weld metal is also limited. Such fluxes are preferred for the welding of high quality structural steels, pressure vessels, pipework and offshore structures where either good high or low temperature properties are required.

The fused fluxes are acid, neutral or slightly basic and are manufactured by mixing the constituents together, melting them in an electric furnace and crushing the solidified slag that is produced to give a flux with a glassy appearance.

These fluxes are homogeneous, resistant to moisture pick-up and mechanically strong so that they do not break down but maintain the required particle size. The high temperatures required by the melting operation mean that some constituents, particularly the de-oxidants present in the highly basic fluxes, decompose and are lost. This limits the range of applications of these fluxes to general structural work where sub-zero service temperatures will not been countered.

The agglomerated fluxes may be neutral, basic or highly basic. They are made from a wet mix that is corned, dried and baked to achieve a low moisture content. This low temperature process means that strong deoxidants and ferro-alloys can be incorporated without being lost. The binders used in the corning process, however, are hygroscopic so that moisture pick-up can be a problem on the shop floor. Baking of the flux prior to use may be necessary and the flux should be stored on the welding equipment in heated hoppers. The flux may also suffer mechanical damage during recirculation, breaking down to form a dust. Although a small particle size is capable of carrying a higher current, too many fines in the flux will give rise to gas being trapped between the slag and the weld pool. This will result in unsightly gas flats or pockmarking on the weld surface. To avoid this, the recirculating system should be equipped with filters to remove both large particles of detached slag and the fine dust.

Fluxes are supplied in bags, generally plastic, weighing from 25 to 40kg and in plastic drums of up to 250kg. Recently some suppliers have been packing the flux in hermetically sealed bags, aka vacuum packed electrodes. This method is useful in that the flux can be used straight from the bag with guaranteed low hydrogen levels and without the need to bake prior to use.

Submerged arc welding consumables. Part 2 - specificationsJob Knowledge Part 1 Part 3

Of all the arc welding processes, only submerged arc welding uses two completely separate components, both of which may have a major effect on the mechanical properties of the weld deposit. This makes the specifying of consumables somewhat complicated. It will not be possible therefore to cover all the alloy types in this brief article which will cover the carbon, carbon-manganese and low alloy structural steels only.

BS EN ISO 14171:2010 is the specification for Welding consumables: Solid wire electrodes, tubular cored electrodes and electrode/flux combinations for submerged arc welding of non alloy and fine grain steels. This standard replaces BS EN 756:2004.

The specification covers the classification of the wire chemical composition and the wire/flux combination. It also specifies the mechanical properties of all weld metal deposits in the as-welded condition.

This standard is a combined specification providing for classification utilizing a system based upon the yield strength and the average impact energy for weld metal of 47 J, or utilizing a system based upon the tensile strength and the average impact energy for weld metal of 27 J.

The classification is composed of:

A reference to the standard 'ISO 14171'

A symbol 'A' if the classification is based on yield strength and average impact energy is 47J or 'B' if the classification is based on tensile strength average impact energy is 27J.

And of five parts, plus a sixth supplementary part:

Part 1. A symbol indicating the process - in the case of submerged arc welding this is 'S'.Part 2. Two digits indicating either the tensile properties of a multi-run deposit or the tensile properties of the parent metal to be welded using a two run technique - see Tables 1 and 2.

Table 1A. Symbols for tensile properties - multi-run technique (classification based on yield strength and average impact energy 47J)

Multi-run Tensile Properties

Symbol

Min. Yield N/mm 2 Min. UTS N/mm 2 Min. Elongation %

35 355 440 - 570 22

38 380 470 - 600 20

42 420 500 - 640 20

46 460 530 - 680 20

50 500 560 - 720 18Table 1B. Symbols for tensile properties - multi-run technique (classification based on tensile strength and average impact energy 27J)

Multi-run Tensile Properties

Symbol

Min. Yield N/mm 2 Min. UTS N/mm 2 Min. Elongation %

43X 330 430 - 600 20

49X 390 490 - 670 18

55X 460 550 - 740 17

57X 490 570 - 770 17Note: 'X' is 'A' or 'P', where 'A' indicates testing in the as-welded condition and 'P' indicates testing in the post-weld heat-treated condition.

Table 2A. Symbols for tensile properties - two-run technique (classification based on yield strength and average impact energy 47J)

Two-Run Tensile Properties

Symbol

Min. Yield Parent Metal N/mm 2 Min. Tensile Strength of Welded Joint N/mm 2

2T 275 370

3T 355 470

4T 420 520

5T 500 600Table 2B. Symbols for tensile properties - two-run technique (classification based on tensile strength and average impact energy 27J)

Symbol

Min. Tensile Strength of Welded Joint N/mm 2

43S 430

49S 490

55S 550

57S 570Note that the two-run technique has two tensile results specified; one for the minimum yield strength of the parent metal, one for the tensile strength of the welded joint.

Part 3. Table 3 gives the temperature at which the average Charpy-V impact value of 47J or 27J may be achieved.

Table 3. Symbol for Charpy-V impact properties

Symbol

Temp. for Min Impact Energy 47J or 27J at °C

Z No requirements

A +20

0 0

2 -20

3 -30

4 -40

5 -50

6 -60

7 -70

8 -80

9 -90

10 -100Part 4. The symbol for welding flux type shall be in accordance with ISO 14174.

Flux type symbol

Flux Type Symbo

l

manganese-silicate MS

calcium-silicate CS

zirconium-silicate ZS

rutile-silicate RS

aluminate-rutile AR

aluminate-basic AB

aluminate-silicate AS

aluminate-fluoride basic

AF

fluoride-basic FB

any other type ZPart 5. Tables 4 and 5 in ISO 14171 contain a listing of the chemical composition of 22 wires and are too lengthy to include in full in this article. The wires all contain a maximum carbon content of 0.15% and range from plain carbon, through C-Mn, C-Mo, Mn-Mo to Ni and Ni-Mo. All are prefixed 'S' followed by a number from 1 to 4 denoting from 0.5% Mn (1) to 2% Mn (4). The addition of nickel and/or molybdenum is denoted by the chemical symbol of the alloy addition being included. Thus an S3 wire contains 1.5% Mn, an S2Ni1Mo 1%.

Part 6. (optional) The standards also provides symbols symbols given an optional symbol indicating the diffusible hydrogen content of the weld metal obtained in accordance with ISO 3690 (see Table 6 in the standard).

Examples of designations:The designation for an electrode/flux combination for submerged arc welding for multi-run technique depositing a weld metal with a minimum yield strength of 460 MPa (46) and a minimum average impact energy of 47 J at -30°C (3) produced with an aluminate-basic flux (AB) and a wire S2 would be:

ISO 14171-A-S 46 3 AB S2In addition to BS EN ISO 14171 which specifies the mechanical properties expected from a particular flux/wire combination, there is an additional specification, BS EN ISO 14174:2012, that specifies the fluxes in greater detail, including the application for which a flux may be used. The specification uses a total of seven symbols, four being compulsory and three optional. The first symbol,'S', identifies the flux as being intended for submerged arc welding and the second the method of manufacture. This may be 'F', a fused flux; 'A', an agglomerated flux and 'M', a mixture of fused and agglomerated. The third part gives an indication of the chemical constituents and uses the same notation as in Table 4 above. In addition BS EN 760 gives a range of percentages for each of the constituents in Table 1.

The fourth part gives a symbol for the application(s), Class 1 being intended for the welding of carbon and low alloy steels, including high strength structural and creep resistant steels. There is no alloying from this class of flux. Class 2 fluxes are for the welding of, and the surfacing with, stainless and heat resisting steels and nickel alloys. Class 3 is for use with hard surfacing weld metals, the flux providing such elements as carbon, chromium and molybdenum to the weld deposit.

The remaining three symbols are not compulsory and comprise, firstly, a number or chemical element symbol that defines what is termed in the specification as the 'metallurgical behaviour' of the three classes of flux mentioned above. Two digits then specify the pick-up or loss of silicon and manganese (in this order) to be expected when welding carbon or low alloy steels using flux Class 1, as shown in Table 5 below. Flux Classes 2 and 3 may be characterised by the use of a chemical symbol to identify the alloying element being added via the flux, eg Cr, if the flux is chromium compensating.

The current type is indicated by the addition of DC or AC to the symbols and finally an 'H', followed by a number, gives the weld metal hydrogen level expected from a correctly dried or baked flux eg H5.

A designation for a flux supplied in accordance with BS EN 760 may therefore be S A AF 1 55 DC H5 for an agglomerated alumina-calcium fluoride basic flux intended for the welding of carbon or low alloy steels, no pick-up or loss of silicon or manganese, used with DC welding current and with a hydrogen content of less than 5mls/100gms weld metal.

It must be remembered that the properties given by these designations are obtained from as welded, all weld metal specimens deposited using standard welding parameters of current, voltage and travel speed.

The properties achieved in a production weld may be entirely different due to the effects of dilution from the parent metal, higher or lower heat input, different wire diameters, preheat and interpass temperatures and post weld heat treatment. It is essential, therefore, that the suitability of a flux/wire combination is confirmed by procedure qualification testing.

Note also that flux/wire combinations supplied to the same specification designation by different manufacturers may not necessarily provide similar mechanical properties or weld cleanliness.

Submerged arc welding consumables. Part 3 - AWS specificationsJob KnowledgePart 1 Part 2

As with the BS EN specifications for submerged arc welding consumables, the American Welding Society (AWS) system also uses a dual flux type/wire composition designation to identify the flux/wire combination that will provide the required properties.

The AWS system is somewhat simpler than the BS EN method, particularly if the full flux descriptor is used as specified in BS EN 760 (see Connect article No. 88). There are, however, only two specifications that deal with both wire composition and the flux but an additional two specifications that cover bare wires for stainless steels and the nickel based alloys. These are ANSI/AWS A5.17 - Carbon Steel Electrodes and Fluxes and ANSI/AWS A5.23 Low Alloy Steel Electrodes and Fluxes. The bare wire specifications are ANSI/AWS A5.9 Bare Stainless Steel Welding Electrodes and Rods and ANSI/AWS A5.Nickel and Nickel Alloy Bare Welding Electrodes and Rods.

In AWS A.5.17 and AWS A5.23 the first part of the designation describes the flux type and may comprise up to six digits depending upon whether the flux is supplied with the tensile strength expressed in increments of 10 megapascals (two numbers where 43 represents 430MPa) or in pounds per square inch (1 digit ie 6 represents 60,000psi).

The first digit, the letter 'F', identifies the consumable as a submerged arc welding flux, the next letter 'S' is only included if the flux is made from or includes crushed slag. Omission of this letter 'S' indicates that the flux is unused and contains no crushed used flux introduced either by the flux manufacturer or the welding fabricator.

The next one or two digits specify the minimum tensile strength as explained above and this is followed by 'A' or 'P' for whether the test results were obtained in the as-welded, the A condition or post-weld heat treated, the P condition. The last digit identifies the minimum temperature at which a Charpy-V impact value of 27J can be achieved as in Table 1 below.

Table 1 Impact Test Requirements

Digit

Test Temperature Impact valueJoules°C °F

Zno impact requirements

27

0 -18 0 272 -29 -20 274 -40 -40 275 -46 -50 276 -51 -60 278 -62 -80 27

In AWS A5.17 there is a total of eleven wires, split into three groups of low, medium and high manganese. The first digit, 'E', identifies the consumable as a bare wire electrode. If supplemented by 'C' the wire is a composite (cored) electrode. The composition of the solid wire is obtained from an analysis of the wire. However, since the composition of a cored wire may be

different from that of its weld deposit the composition must be determined from a low dilution weld deposit made using a specific, named flux.

The next letter, 'L', 'M' or 'H' indicates a low (0.6% max), medium (1.4% max) or high (2.2% max) manganese content. This is followed by one or two digits that give the nominal carbon content. An optional letter 'K' indicates a silicon killed steel. There are a final two or three optional digits identifying the diffusible hydrogen in ml/100gms weld metal, H16, H8 or H4.

A full designation for a carbon steel flux/wire combination could therefore be F6P5-EM12K-H8. This identifies this as being a solid wire with a nominal 0.12% carbon, 1% manganese and 0.1 to 0.35% silicon capable of achieving an ultimate tensile strength of 60 k.p.i. (415MPa), a Charpy-V impact strength of 27J at -50°F (-46°C) in the post weld heat treated condition.

The classification in AWS A5.23 is, of necessity, rather more complicated as this specification covers a wide range of low alloy steels, a total of thirty one solid wires and twenty nine composite wire weld metal compositions. Within the confines of this brief article it will not be possible to cover in full the entire classification of the wires.

The flux designation is almost identical to that of AWS A5.17, except that a four, five or six digit identifier may be used. Why this additional sixth digit? Because some of the electrodes in the specification are capable of providing tensile strengths above 100,000 psi - in these cases the designation may be, for example, F11, identifying the flux as providing 110 ksi (760MPa) minimum tensile strength.

The classification of the wire comprises two parts - the first that of the wire, solid wires being prefixed 'E' and composite wires 'EC', the second part specifies the composition of the weld deposit. In Table 1 of the specification only the solid wires are listed. The wire classification commences with 'E' to identify a bare wire, the next letter places the wire in a 'family' of wires. 'L' or 'M' identifies the wires as being alloyed with copper, 0.35% max; 'A'as containing molybdenum, 0.65% max; 'B' as the creep resisting steels containing chromium and molybdenum; 'Ni' for those wires containing nickel. 'F comprises the Ni-Mo or Cr-Ni-Mo wires; 'M' triple de-oxidised Ni-Mo wires; 'W' aNi-Cu wire and 'G' not specified.

This use of wires to this latter 'G' designation may lead to problems as quite large changes can be made to the composition to achieve the required mechanical properties - a good example of this is where the NACE requirements for sour service of 248BHN or 1% nickel maximum are required. To

achieve the required tensile or impact strength the consumable manufacturer may increase the carbon or nickel contents above those used in the procedure qualification test and still supply to the same designation.

Table 2 in AWS A5.23 classifies both solid wire and composite wire/flux combinations by means of weld metal compositions but still using the identifying letters as for the solid wires described above. The prefix 'E' is, however, omitted thus a carbon/molybdenum deposit may be classified, for example, as A3, a Cr-Mo deposit as B4, Ni-Mo as F5 etc.

Thus a full designation for a flux/wire combination for an as welded 1% Ni/0.25% Mo weld deposit with an ultimate tensile strength of 80ksi and an impact strength of 27J at -60°F (-51°C) may therefore be F8A6-ENi1-Ni1 and for a similar deposit using a cored wire in the PWHT'd condition F8P6-ECNi1-Ni1.

As mentioned in earlier articles on the topic of consumable specifications, it must be remembered that the mechanical properties and compositions are determined from test pieces taken from absolutely minimal dilution welds made on specified parent plates with a standard set of welding parameters - heat input, preheat, interpass temperature, post weld heat treatment temperature and time. They may therefore NOT reflect the results obtained in a production weld and the designation cannot be relied upon to guarantee the properties required by the application.

Where these properties are important it is therefore essential that mechanical testing, chemical analysis etc are determined from test specimens made using parent materials and parameters representative of production welding.