©
COPYRIGHT 2000 THE ESAB GROUP, INC. LESSON
IV weld metal
solidifies, it loses the ability to hold the hydrogen in solution and the hydrogen
is either expelled
into the atmosphere or moves throughout the weld zone. Steel and weld metal
are not as solid as they appear to
the naked eye, being filled with tiny submicroscopic pores.
The hydrogen atoms are smaller than
the crystalline structure of the steel or the weld metal,
and the hydrogen can move about somewhat
freely in the steel, just as air can move through a filter.
The hydrogen atoms move out of the weld metal into the heat affected zone.
The heat affected
zone (HAZ) is an area of critical importance in welding, especially in welding
high strength
steels. 4.2.0.2
The heat affected zone (See Figure
1) is that area of the weld joint that did not become
molten in the welding process, but underwent a microstructure change as a result
of the heat induced
by the arc. This zone
can become a weak link in the
normally very strong joint. First
of all, the grain struc- ture
of the HAZ is less refined
and therefore, weaker
than the sur- rounding
unaffected base
metal or the once molten
weld metal. And
secondly, if the HAZ
is permitted to cool too
rapidly in certain steels, a
hard brittle crystalline struc- ture,
known as Marsenite, is locked in
place. The relatively large pores of the
heat affected zone are a natural collect- ing
place for atomic hydrogen. When two hydrogen atoms meet, they immediately
unite to form
molecular hydrogen. The resulting molecules are larger than the crystalline
structure of the
metal and can no longer move about freely. As more and more hydrogen atoms
come into the
pores, form molecules, and are trapped, tremendous pressure can develop. Mild
steel and lower
strength steels are sufficiently plastic to move a little with the hydrogen pressure
and not cause
the steel to crack. Steels that have high hardness and high strength do
not have suffi- cient
plasticity to move with the pressure, and if enough hydrogen is present, cracking
of the steel occurs.
HEAT AFFECTED ZONE
FIGURE 1 SOLIDIFIED
WELD METAL HEAT
AFFECTED ZONE UNAFFECTED
BASE METAL
