NOMENCLATURE
CHAPTER 4 Experimental investigation
4.2 Pre-stage of the experiments
In this section the details of pre-requirements of the actual welding are presented briefly.
4.2.1 Trial and error run
Trial and error experiments were performed to get the broad range of the welding parameters. One of the control parameters (viz. voltage, current and speed) was varied at a small increment while other parameters kept. During trial and error run it was found that for higher currents the weld arc was making a through penetration hole in the weld. On the other hand, lower ranges of current was not giving full penetration of the plate. Welding current directly affect the depth of penetration and
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extends of base metal fusion. The welding arc voltage has a direct influence on the weld bead geometry and external bead appearance. The welding speed has pronounced effect on the weld size and the penetration for a given combination of current and welding voltage. At a given current and voltage, the depth of penetration and hardness are effected by electrode diameter. From trial and error experiments a range of parameters was found and the range of each parameters were divided into sub levels as shown in the Table 4.1. Along with the variable parameters, the SAW setup has some fixed parameters which were kept constant.
Butt joints were welded. The pates were tack welded using shielded metal arc welding (SMAW) before final welding using SAW. A fixture was designed to keep and hold the samples using clamps. A flux filled reusable backing bar was used to provide support to the weld metal from the bottom side of the plate. The Figure 4.1 shows the SAW set up.
4.2.2 Arc initiation
Unlike in manual welding, arc starting can be difficult in SAW, because of the flux cover. Here a rolled ball of steel wool, approximately 10 mm in diameter was placed at the appropriate spot on the joint and the electrode wire is lowered on to it till it is tightly compressed. The flux is then applied and the welding is commenced. The steel conducts the current from wire to the work piece and at the same time melts away rapidly as the arc is formed. After completion of welding for terminating the arc the carriage travel is switched off. The brief pause between the second and third step prevents the wire from moving forward into the molten pool and from sticking to the molten puddle as it solidifies. Is also helps to fill the crater. A constant voltage rectifier type power source, with a 1000A capacity is used to perform the welding process. The polarity used here is the reverse polarity. The arc voltage is established by setting the output voltage on the source. The welding parameters & their levels used in the final experiments are shown in Table 4.1.
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Table 4.1 Welding parameters and their levels
Variable Parameters Fixed Parameters
Parameters Values in each Level Unit Parameters Values Unit L1 L2 L3 L4
Voltage 27 27.5 28 28.5 V Root Gap 2.5 mm
Current 470 475 480 485 A Wire Feed Automatic - Speed 22 24 26 28 m/h Welding Polarity - DCEP Stick-out 2.2 2.4 2.6 2.8 cm Plate Thickness 8 mm
Figure 4.1 Submerged arc welding set-up
4.2.3 Design of welding fixture
A fixture is a device used to hold the work in the workshop and manufacturing industry. It is used to securely locate (position in a required position or orientation) and give support to the work, ensuring that all parts produced using the fixture will remain fixed. A fixture improves the economy of production by allowing smooth operation and quick transition from part to part, and also reduces the necessity of skilled labor by simplifying the mounting problem of work pieces, and increasing conformity across a production run. Fixtures must always be designed with economics in mind, the purpose of these devices is to reduce costs, and so it must be designed in
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such a manner that the cost reduction outweighs the cost of implementing the fixture. The top view and front view of fixture with adjustable clamping which was used for the experimentation is shown in Figures 3.2 (a, b & c) respectively.
(a) (b)
(c)
Figure 4.2 (a) Designed fixture to hold the samples, (b) Top view of the fixture (c) Front view of the fixture
4.2.4 Backing bar
It is a metal/ceramic slab placed at the root of a weld joint for the purpose of supporting molten weld metal. Backing bar which is having a deep semicircular groove filled with the same flux as used for SAW is found to be very effective for providing support to the molten metal and to form
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the required bottom reinforcement. The finer the particle size of the flux used in the backing strip, the better is the formation of bottom reinforcement. The Figure3.3 shows the dimension of the backing bar used in the experiments.
Figure 4.3 Backing bar with dimensions
4.2.5 Materials and job specimen
Mild steel is selected for the work material. Mild steel plate of size 200 mm × 100 mm × 8-10 mm with 1.5-2.5 mm root gap was butt joined by SAW process. 150 mm × 150 mm × 8-10 mm plates with a webb height of 50 mm was used for fillet joints. The work piece dimensions are shown in Figure 4.4.
(a) (b)
Figure 4.4 Plate dimensions of (a) butt joint (b) fillet joint
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(a) (b)
Figure 4.5 Welded plated (a) butt (b) fillet
Composition of the base metal (IS: 2062, Gr. B mild steel) is shown in Table 4.2. The chemical composition of the work piece is tested in the metal testing laboratory.
Table 4.2 Chemical composition of the base metal Element Carbon
(C)
Manganese (Mn)
Silicon (Si)
Sulphur (S)
Phosphorus (P)
Percentage (%) 0.15 0.58 0.23 0.020 0.028
Continuous bare wires in the form of coils and dry fluxes in grain form were used for the SAW of mild steel, low alloy steels etc. Auto melt EH-13 copper-coated electrode wire with 3.15 mm diameter in coil form was used for welding. Chemical composition of the electrode wire and fluxes are given in the Table 4.3 & 3.4 respectively.
Table 4.3 Chemical composition of weld metal wire
C Si Mn P S Cr Ni Mo Cu Al
0.06 0.03 1.50 0.03 0.03 0.06 0.03 0.01 0.04 0.046
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Table 4.4 Composition of the flux
Composition SiO2+TiO2 CaO+MgO Al2O3+MnO Ca+F2
Percentage (%) 25 20 30 35