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Extract from Course F18: Furnace brazing stainless steels for automotive
………………………There are two atmosphere categories that have to be considered.
atmospheres (including vacuum!).
1. Chemically active atmospheres
Atmospheres of this type react
with the oxides present on the surfaces of the parent
materials and brazing filler metals removing them in the process
In those situations where an in-line continuous conveyor furnace is brazing stainless
the ‘active’ ingredient of the atmosphere needs to be ultra-dry and ultra-pure hydrogen.
When mild steel
is being brazed under an exothermic atmosphere
‘active’ ingredients of the atmosphere are carbon monoxide and hydrogen.
In both of
these situations the mechanism of oxide removal is one of chemical reduction!
It is, however, very important for delegates to understand why it is very easy to braze
mild steel in a furnace that is normally processing stainless steel under hydrogen, but
why the brazing of stainless steel under an exothermic atmosphere is not possible.
Further, it is extremely important
to ensure that if consistently good results are to be
achieved one should never
process mild steel in a furnace that was installed with the
primary objective of using it to braze stainless steels! This is a matter to which we shall
return later in this monograph.
One of the fundamental requirements for any successful furnace brazing operation is to
ensure that the surfaces of the metals being brazed are free from oxide, or other films,
which may inhibit wetting when the filler material melts. Thus the ease with which
surface oxides can be removed from any given material is a function of the ease with
which the oxygen ions can be detached from the metallic ions present in the oxide.
Clearly, the degree of difficulty that is experienced depends upon the strength of the
chemical bond existing between the oxygen ions and the metal involved.
The strength of such a bond can be assessed in several different ways:
1. By the Heat of Formation
, Η, of the particular oxide in question. (However, this
will only provide an approximate guide).
2. More accurately, by the change in Free Energy
, in the system during the
3. From the maximum energy obtainable from the general chemical reaction:
where Me = metal and m = 1 mole of oxygen
As shown in Table 3
, metals like gold, silver and palladium possess a low heat of
value for their oxides and they can, as a result, be considered to be relatively
unstable and can thus be decomposed readily. The oxides of metals such as copper,
cobalt, nickel, iron and cadmium are higher on the stability scale and are therefore more
difficult to reduce. Of higher stability still are to be found the oxides of chromium,
manganese, titanium, aluminium and beryllium. In fact the various oxides of beryllium
have a far higher degree of stability than virtually any other element. Table
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