The intense heat of the arc increases the temperature of the metal beneath the arc, which therefore reaches the melting point very quickly. As soon as a temperature difference exists, the heat energy starts to flow out of the weld area and heats the surrounding metal. In welding, the welder must use skill to control the input and distribution of heat.

Heat Input

The heat input is controlled primarily by the current setting. To some extent the heat input can also be controlled by the arc length. When welding, the heat input into the puddle is often controlled by the welding speed and by the weave or whip, as discussed further on in this book. However, more correctly, this is a matter of heat distribution.

Unfortunately, it is not possible to provide exact recommenda­tions for the current settings to be used in each case for the following reasons:

Since there are so many reasons why current settings vary, it is common practice for welders to evaluate current settings by deposit­ing a weld or a bead on a piece of scrap metal. Conducting this evaluation beforehand is a very important step in making a perfect weld.

For pipe welding, this evaluation should be conducted on plates in the vertical (3G) position. Weld the test plate uphill or downhill, depending upon the direction to be used to weld the pipe. The plate should be free from dirt and rust. Rust, when in contact with the molten pool of metal, will cause undercuts at the edges of the weld. In conducting the evaluation, the welder should try out different arc lengths. He must be cautioned to remember that after a few deposits are made on a small plate, the plate will become very hot and will, in many instances, give misleading results. Before this occurs, a differ­ent plate should be used to make the evaluation.

As a result of the evaluation, the welder should be able to determine the current setting that will give the best results for the conditions at hand.

The effect of the arc length, welding current, and the welding speed can best be understood by studying Fig. 3-1.

Heat Distribution

The distribution of the heat and the rate at which it is withdrawn from the weld zone are dependent on the following factors:

1. Conductivity of the work material

2. The mass of the metal surrounding the weld zone

3. The paths available for heat conduction

4. The use of the weave, or whipping, technique.

When compared to most other materials, all of the metals are good conductors of heat. However, all metals are not equal in their ability to conduct heat. For example, aluminum is a better conduc­tor of heat than stainless steel, as shown in Fig. 3-2. The metal in the area of the weld will cool more rapidly in the case of aluminum than in stainless steel. Moreover, the heat will disperse throughout an aluminum plate more rapidly.

A large mass of metal adjacent to the weld zone will tend to withdraw heat from the weld zone more rapidly than a small mass. For example, heavy plates and thick-wall pipes will tend to cool the weld more rapidly than thin plates or thin-wall pipes. As another example, the corner fillet in Fig. 3-3 can withdraw heat more rapidly

WELD QUALITY INSPECTION

EVEN PENETRATION

UNIFORM CROSS SECTION

A GOOD BEAD

SMOOTH, WELL FORMED BEAD

COMMON WELDING MISTAKES

WELDING CURRENT TOO 10*

•

TOO MUCH PILING UP OF WELD METAL

OVERLAPPING BEAD. HAS POOR PENETRATION

COLD AT EDGES

ШІІЮ CURRENT TOO HIGH

UNDERCUTTING ALONG EDGES WEAKENS JOINT

TOO MUCH SPATTER TO BE CLEANED OFF

WID€. FIAT BEAD

ARC TOO LONG (voltage too high)

BEAD VERY IRREGULAR

POOR PENETRATION

WELD METAL NOT PROPERLY SHIELDED

WELDING SPEED TOO FAST

BEAD TOO SMALL, SHAPE IRREGULAR

NOT ENOUGH WELD METAL IN CROSS SECTION

POOR PENETRATION

WELDING SPEED TOO SLOW

TOO MUCH PILING UP Of MID METAL

TOO MUCH PENETRATION

WASTES TIME AND ELECTRODES

Fig. 3-1. Weld quality inspection.

than the ordinary fillet. The bevel butt joint has the least ability to withdraw the heat.

Another factor to consider in estimating the rate at which he’at will be withdrawn is the number of paths available along which the heat can flow. In Fig. 3-4 the plates have the same thickness, but the heat will be withdrawn more rapidly from the lap joint than from the edge joint because the lap joint provides two paths or directions of heat flow as compared to one direction for the edge joint. The paths along which heat can flow for a few typical weld joints are shown in Fig. 3-5.

AA

Fig. 3-2. The temperature distribution foraluminum plate and a steel plate at two sections {AA and BB) of a plate while welding. At A A the melting point of stainless steel is shown by the higher temperature of the molten metal in the puddle. At. BB the temperature of the entire welded plate has increased in both cases but the temperature of the aluminum plate is more equal throughout.

BB

Fig. 3-3. The corner fillet will cause the heat to disperse from the weld more rapidly than the ordinary fillet. The ordinary fillet will disperse heat more rapidly than the butt joint.

ORDINARY FILLET BEVEL BUTT JOINT

Fig. 3-4. The edge weld has fewer paths than the lap weld, along which the heat can flow from the welded joint.

EDGE WELD

'ZZZZZZl

WS////W,

SOCKET OR SLEEVE JOINT

BORE a BACK WELDED FLANGE

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