MANUAL ELECTRIC ARC WELDING

 

Purpose of the work: to become acquainted with electric arc welding processes, welding arc properties, design and operation of welding equipment, to become skilful at welding regimes determination and adjustment.

Theory. Welding is a process of permanent joints manufacture by setting up atomic bonds between working pieces while heating, deforming or heating and simultaneous deforming them. Local or general heating may be employed. Electric arc welding has found the most wide use due to its advantages over other welding processes: both high heat concentration and productivity, relative universality, various weld position in space, simple and inexpensive equipment, possibility to obtain stable and relatively high properties of weld metal.

The following kinds of electric arc welding are distinguished: alternating-current (a-c) and direct-current (d-c) welding; manual and automatic (machine) welding; unshielded arc, submerged arc and gas-shielded arc welding.

Electric arc with the temperature of 6000…8000 °C in its core is the heating source in arc welding. Electric arc is a powerful stable arc discharge in ionized atmosphere of gases and metallic vapors. Arc distance ionization occurs at the instant of arc initiation and is sustained while arcing as a result of collisions between moving electrons (towards a cathode) and either gas molecules or metal vapor atoms. Electric properties of the arc are described by the static voltage-current characteristic, representing the dependence between voltage and current intensity (strength) at stable arcing instant. The main power source characteristic is considered to be its external characteristic – the relationship between output terminal voltage and current intensity in welding circuit. The intersection point of these two characteristics is supposed to accommodate the stable arcing (welding) (Fig. 5.1).

In arc welding transformers of ТД, ТС, СТШ, TCK types may be employed as a power source; in d-с welding – welding generators of ПСО, АСБ, АСД, CAM types and also rectifiers of ВД, BKC, BCC, ВДУ etc types. Welding current is adjusted by means of supply chokes (a-c welding) or variable resistances (d-c welding).

Manual arc welding is performed by an arc arcing between an electrode and working pieces. It is possible to use either non-consumable (of tungsten or graphite) or consumable electrodes – metallic rods (1.6…12 mm in diameter) coated with luting. Luting serves for stabilization of arcing, protecting of molten metal, deoxidizing an alloying it. Chemical content of electrode material has to match the content of a metal, which undergoes welding.

Fig. 5.1. Static voltage-current characteristic of a power source (1) and an arc (2) with different arc length l

 

Electrodes of 3…6 mm in diameter are mainly used in manual arc welding. Voltage is sustained within the range from 16 to 30 V, welding current intensity – 120…350 A.

Welding regime is completely characterized by welding current intensity I_W and electrode diameter d_e. The thickness of welding pieces determines the electrode diameter, the last determines the welding current intensity (approximately I_W = (40…60) d_e). Quantity of surfacing metal in welding is independent upon arc voltage. Therefore, voltage is not considered as welding characteristic.

Useful power N_u, W, released while arcing is determined by the formula:

N_u=I_wU_a

where I_w is welding current intensity, A;

U_a, is arc voltage, V.

Efficiency of welding transformer h is usually equal to 0.84…0.87. Hence, consumed power should be determined as follows:

N_c = N_u/h