and Disadvantages of Braze Welding Braze
welding is faster than fusion welding, since the heat input required is much less.
The rod normally used for braze
welding has a melting point of about 8750
C (16000F). In the braze welding of steel,
the base metal must be heated
only to a temperature of about 9000C, rather
than to a temperature of more than 15000C.
The saving in time and
the saving in gas consumption may frequently be more than enough to counterbalance
the substantially higher cost
of the filler metal. The
reduction in heat input has other advantages, especially in the welding of cast
iron, which will be covered in the
next chapter. It minimizes the amount of preheating required. Since the bronze
filler metal is extremely ductile, it
can absorb stresses created during cooling which might, in the case of a cast
iron fusion weld, cause cracking of the
base metal or the weld. When used on steel, braze welding reduces distortion of
the base metal due to forces of
contraction and expansion. When
mild steel or cast iron are properly braze welded, the strength of the joint,
at normal temperatures, is likely
to be equal to, or even superior to,
the strength of the base metal. Braze
welding can sometimes be used to join dissimilar metals which cannot be successfully
fusion welded together.
Steel can be braze welded to cast iron. Copper can be joined to brass by a braze
weld. However, the joining
of dissimilar metals by any welding process is something to be approached cautiously.
The fact that you can turn
out what appears to be a good-looking weld is no proof that the overall result
is satisfactory. Essential properties
of one or both of the metals joined may have been adversely affected by the act
of welding. So
much for advantages. What are the disadvantages? One is quite obvious, although
often of no significance; you cant
match the color of the weld to the color of the base metal. Another, less obvious,
is that bronze loses strength at
relatively low temperatures. At 5000C, steel
and cast iron are nearly as strong as they are at room temperature
Any bronze has lost a great deal of its strength at 5000C.
Never use braze welding to repair parts that must operate
at temperatures above 2000C.
In Chapter 18 we shall talk more about
the technique and advantages of bronze-surfacing,
which is very closely related
to braze welding.