Introduction to Basic Manufacturing Processes and ... - always yours

Welding 337
be scattered. This scattering increases the spot size of the electron beam and consequently
there is lower penetration. As the vacuum increases, the scattering effect of the electron
beam decreases and hence, penetration increases. The other advantage of using vacuum is
that the weld metal is not contaminated.
Tungusten
filament
Cathode
Anode
High
voltage
Focus coil
Electron beam
Work piece
Fig. 17.30 Electron beam welding set up
The EBW process is mainly used for welding of reactive metals (nuclear reactor
components), titanium, zirconium, stainless steel, etc. for aero-space and automotive industries.
17.11 EFFECTS OF WELDING HEAT
During the process of welding, the metal is heated over a range of temperature up to fusion
and then allowed to cool. Heating and cooling causes metallurgical and mechanical effects on
the metal pieces being welded. The region of the base metal which is affected by metallurgical
change due to the welding heat is called the heat affected zone. In this zone, the metal has
been heated and cooled through a range of temperature great enough to cause changes in the
structure of the metal. The amount of change may be controlled by using the suitable welding
processes. Sometimes, the original properties may be restored by heat treatment after welding.
Although the mechanical strength of weld metals is at least equal to that of the base metal
but the metallurgical changes can reduce greatly the associated properties of the joint
particularly for shock and fatigue.
Mechanical effects of welding heat are the distortion and the residual stress resulting
from welding. The magnitude of these effects depends on the factors of design and welding
procedure. But the effects will always be present to a certain extent in any structure fabricated
or repaired by non-pressure welding. Because of expansion and contraction of the heated
portion the distortion and residual stresses set up. The base metal heated locally will expand
and up set owing to the restraint imposed by tile surrounding cold metal. This upset portion
of the base metal will contract on cooling beyond its original dimensions and thereby setup
internal stresses causing distortion. If the pieces being welded are not tree to move, high
residual stresses will be formed which may cause cracking during welding.