SAW, semi-automatic and fully-automatic, is used for making butt joints in the downhand position and for making fillet welds in T and lap joints in the downhand and horizontal-verti­cal positions as shown in Fig. 8.27.

Normally this process cannot be used in vertical and overhead position, because of the difficulty of preplacing the flux.

It is important to bear in mind that the SAW process demands accurate edge prepara­tion and fit-up. In MMAW, irregularities in this regard are taken care of by the manual welder, though they do result in increased welding time and a large consumption of electrodes. In SAW, on the other hand, the operation is automatic, welding currents are high and the arc is deeply penetrating. Moreover, since the joint is submerged under the flux, the operator is unable to adjust the procedure to accommodate joint irregularities. A poor fit-up in a butt joint
can cause the granular flux to spill through the root gap. It can also give rise to burn-through and slag inclusions.

Second pass Backing pass

Fig. 8.27 Joint and positions suitable for SAW

Second pass

Backing pass Fig. 8.28 Base metal backing for SAW

Shops using SAW are advised to make edge preparations with automatic thermal cut­ting equipment (oxy-acetylene or plasma-arc), or by machining. In the absence of such facili­ties, SAW becomes a slow and unproductive operation with frequent interruptions and in­creased proportion of weld rectification.

In SAW, the weld puddle is of large size and remains in a molten condition for a long time. The welding procedure must ensure that this molten puddle is supported and contained until it has solidified at the root of the weld. This precaution is a must when full joint penetra­tion has to be achieved in a butt joint. The technique used for this purpose is termed weld backing.

8.8.1 Weld Backing Techniques

The various commonly used techniques involve use of the following: (1 )Base metal backing;

(2) Structural backing; (3) Weld backing; (4) Backing strip; (5) Copper backing; (6) Flux back­ing; (7) Backing tapes.

1. Base metal backing. The root face is kept sufficiently thick as shown in Fig. 8.28, to support the weld pool without burn-through. This technique is used for square or partially bevelled butt joints, for fillet welds and for plug or slot welds. Care has to be taken that the root faces of grove welds are in close contact. The first pass, deposited sometimes with lower current, acts as a backing for the second pass deposited with higher current to get through penetration.

2. Structure backing. In certain cases where design permits, another structural member can serve as a backing for the weld, as shown in Fig. 8.29. It is very important that the contact surfaces of the joint are clean and the contact is intimate in order to avoid porosity and slag inclusions. The weld must also provide sufficient depth of fission in the backing member.

Fig. 8.30 Weld backing for SAW

3. Weld backing. The backing weld is deposited at lower current and with a moderately penetrating arc using the manual arc, CO2 shielded arc or flux-cored arc process (see Fig 8.30). It may be in one or more passes to obtain sufficient depth to support the submerged-arc weld. The backing weld may be retained in the joint if it is of suitable quality. If otherwise, it may be removed by oxygen on arc gouging, by chipping or by machining after the submerged-arc welds have been deposited. The resulting groove is filled up with a submerged-arc weld.

4. Backing strip. The backing strip is of metal that is compatible with the one being welded. The weld metal fuses into the backing strip, so that it becomes an integral part of the joint as shown in Fig. 8.31. In this case, it is termed a permanent backing. In case it is intended to be a temporary backing, it may be removed finally by machining. Suitable root opening must be kept to ensure full penetration. It varies between 1.6 and 4.8 mm, depending on joint thickness. It is important that the contact surfaces between the plates and the strip are clean and the contact is intimate; otherwise porosity and leakage of molten weld metal may occur.

5. Copper backing. Copper backing shown in Fig. 8.32 has several advantages. Its high thermal conductivity enables it to extract the heat rapidly from the molten weld pool. Also the molten steel weld metal does not fuse with the copper material. Hence it only serves as a temporary backing.

The copper backing bar is either as long as the joint; or it is of short length and designed to slide underneath the travelling arc. In still other applications, it may be in the form of a rotating wheel.

For high production applications, the copper bar is provided with internal water circu­lation to maintain it relatively cool. The bar is usually grooved as shown in the figure to obtain weld reinforcement on the underside of the joint. It is important to ensure that the copper bar has sufficient mass to prevent melting of the copper material, which can result in contamina­tion of the weld with copper. It must be borne in mind that mechanical properties of steel weld metal deteriorate when the Cu content exceeds a certain limit.

6. Flux backing. As shown in Fig. 8.33, dry granular SA flux is placed in a trough of flexible sheet material. This sheet material rests on a rubberised canvas hose, which can be inflated to hold the flux tightly against the back of the joint. This technique will be discussed in detail while describing the one-side SAW used in Japanese shipyards.

Fig. 8.32 Copper backing for SAW: (A) V-groove butt; (B) Square butt

7. Backing tapes. Ceramic back-up tapes consisting of a ceramic material on an aluminium foil backing are available in the U. S.A. The exposed aluminium foil edges are covered with pressures sensitive adhesive covered with a removable liner. Lengths of strips are 0.5 to

1.0 metre. These can be easily applied to joints or seams to provide shielding or back-up for oneside welding and root pass back-up for two-side welds to be deposited by TIG, MIG and other arc processes. By using these tapes, arc gouging and further backside joint operations such as griding are eliminated or minimised. They avoid the use of expensive and clumsy fixtures, back-up bars and gas purging of weld.

8.8.2 Butt Welds

To make a full penetration butt weld in sheet metal without burn-through, steel or copper backing bar must be used. The joint is then completed with a single weld pass deposited from one side.

With copper backing, a square butt joint without root gap is used. The procedure data are given in Table 8.1.

Table 8.1. Data for SA butt welds with copper backing

Plate thickness t, mm	Electrode dia. mm	Current amps. Electrode + ve	Voltage V	Speed mm/sec.
1.6	2.4	350	23	50
2.0	2.4	400	24	42
2.4	3.2	500	30	40
3.6	3.2	650	31	30

Joint fit-up with steel backing is shown in Fig. 8.34 which shows that a small root open­ing is helpful. The procedure data are given in Table 8.2.

Plates up to 12.7 mm thickness and with square edges can be butt welded with a single pass using a steel backing strip. It is advisable to keep a root opening, because when the edges are butted together tightly, the resultant weld has a high build-up. Alternatively, a grove can be provided. Procedure data are given in Table 8.2.

hose

Fig. 8.34 Joint fit-up for butt welds in sheet metal Table 8.2. Data for SA butt welds with steel backing

Plate thickness mm/sec. t, mm	Root opening g, mm	Electrode dia. mm	Current amps. Electrode + ve	Voltage V	Speed mm/sec
1.6	0-0.8	3.2	450	25	45
2.0	0-0.8	3.2	500	27	33
2.4	.6 1. - 0	3.2	550	27	25
3.6	0-1.6	3.2	650	28	20
4.8	1.6	5.0	850	32	15
6.4	3.2	5.0	900	33	11
9.5	3.2	5.6	950	33	10
12.7	4.8	5.6	1,000	34	8

Plates in the thickness range of 6.4-15.9 mm and with square edge butted together tightly, can be conveniently butt welded with two passes, one from each side as shown in Fig. 8.35. The first pass deposited at a lower current serves as a backing for the second pass. It is important that the two passes penetrate into each other sufficiently to prevent lack of fusion and slag inclusion in the central region. Procedure data are provided in Table 8.3.

Fig. 8.36 Parameters for two-pass 19 mm and 25.4 mm t butt welds Table 8.3. Data for two-pass square butt weld, one from each side

Plate thickness t, mm	Baking pass	Speed mm/sec.	Second pass	Speed mm/sec.
Electrode dia. mm	Current amps.	Voltage V	Electrode dia.	Current amps. mm	Voltage V
6.4	4.0	475	29	20	4.0	575	32	20
9.5	4.0	500	33	14	4.0	850	35	14
12.7	5.0	700	35	11	5.0	950	36	11
15.9	5.0	900	36	9	5.0	950	36	9

The above-described procedure can be extended to plates of 19 mm and 25.4 mm thick­ness by providing 60° V-groves on both the sides and sufficiently large root face as shown in Fig. 8.36. Procedure data are given in Table 8.4.

Table 8.4. Data for 19 mm and 25.4 mm t butt welds 18 mm t 25.4 mm t First pass Electrode dia., mm 5 5 Current (DC+), amp 700 850 Voltage, V 35 35 Speed, mm/sec 12 5.5 Second pass Electrode dia., mm 5 5 Current (DC+), amp 950 1,000 Voltage, V 36 36 Speed, mm/sec 6 7

When plate thickness increases further, it becomes necessary to increase the V-groove and deposit the passes, one from the first side and two from the second side as shown in Fig. 8.37. Typical procedure data for 32 mm and 38 mm plates are given in Table 8.5.

It must be pointed out that the above procedures are valid for fused silicate type fluxes, which are capable of taking high welding currents. These procedures are very economical and they result in minimum number of passes of large cross-sections and considerable dilution of the weld metal by the base metal. They are recommended for steels of good weldability having low carbon equivalent and in cases where special impact requirements for the weld metal are not specified.

Fig. 8.37 Parameters for three-pass 32 mm and 38 mm t butt welds

Table 8.5. Data for 32 mm and 38 mm t butt welds Plate First pass Second pass Electrode Current Voltage Speed Electrode Current Voltage Speed thickness dia. amps. V mm/sec. dia. amps. V mm/sec. t, mm mm mm 32 5 850 35 5.5 5 1,000 36 5 83 5 1,000 36 4 5 1,000 36 4 Third pass Electrode Current Voltage Speed dia. amps. V mm/sec. mm 5 850 35 4 5 950 34 3

For welding steels of difficult weldability, or where stringent weld metal impact requirements are specified, procedures involving basic type of flux, multiple passes of limited cross-sections deposited with low currents, and minimum dilution by the base metal are recommended. For plates of 16, 25.4 and 38 mm thickness, for example, the joint fit-up is made as shown in Fig. 8.38. First two passes are deposited manually with a 4 mm basic low-hydrogen type electrode. With these passes serving as a backing SA weld passes are deposited at a speed of 7 mm/sec using 4 mm diameter electrode, 550 amps, 28 V. The number of SA passes for 16,

25.4 and 38 mm thick joints are 5, 12 and 26 respectively. After the vee is filled up, the manual weld at the root is completely gouged out and the groove is filled up with a SA pass.

Fig. 8.38 Joint fit-up for multi-pass butt weld