Good Morning All,
The extensive discussions over soldering, melting, alloying,
casting, heat-treating, etc. these past several years makes it clear
that a good many of our correspondents are artists. Our good and
gentle colleagues who don’t consider themselves engineers or even
technicians and really don’t want to become such. I feel driven to
make a point that I hope will aid us all in understanding and
communicating the issues of gasses, torches and fire. There are
untold variations and details that some of us could haggle over until
the goats come home to Topanga, and I’m going to make some rather
loose generalizations. However, I’m not trying to whack the hornet
nest so let’s not get involved in details, just let me make a point
that I think will help.
A torch is a device for the controlled mixing of a fuel gas with an
oxygen source. The mixture ratio is controlled by the valves and
openings within the torch and the pressures of the gasses applied to
them. The volume of mixture or rate of flow is largely determined by
the opening in the nozzle and the pressure of the aggregate mixture.
We light the mixture where it comes from the nozzle to make a flame
for our use. Let’s look at the flame.
The flame is a jet of mixed fuel and oxygen which is busy combining
chemically, breaking down the fuel molecules to form a residuum of
CO2 and water and minor “impurities”. We are interested in the heat
that this chemical union produces and we are interested in two
aspects–the temperature of the heat and the quantity of the heat. I
realize that this concept has been abused in the last few decades as
marketing forces have pushed various new technologies. For example
when the neodynium-YAG laser was introduced to eye surgery it was
termed a “cold laser”, but, good heavens, in the few nanoseconds of
its pulse discharge at its microscopic focal point the temperature
reaches about 9000 degrees Kelvin.! That’s more or less the
temperature of the surface of the sun, but the quantity of heat is
infinitesimal. This is my point, think separately of THE TEMPERATURE
OF THE HEAT and THE QUANTITY OF THE HEAT.
The TEMPERATURE is determined primarily by the type of fuel and
ratio of oxygen in the mixture. The QUANTITY of heat is determined
primarily by the volume or amount of mixture being burned. Jim
Binnion has given you the TEMPERATURE that can be achieved by each
fuel gas when mixed with the optimum ratio of pure oxygen and when
mixed with the optimum ratio of room air. Clearly all of them are way
above the temperature you want your work piece to reach for
soldering, So how do we understand what we want to acheive and how do
we get there?
In the case of a solder joint we want to raise the temperature of
the joint and the solder to it’s flow point without overheating the
work-piece and slowly enough that we have time to think and react.
This brings up some more technical factors. The rate of heat transfer
is mostly determined by the temperature differential, as from the hot
flame to the cool metal. The distribution of heat that is transfered
to the metal is a function of thermal conductivity. Silver and copper
have very high thermal conductivity while gold (and especially
platinum) have rather low rates of conduction. Now lets look at what
actually happens when soldering with our trusty torch.
At one extreme is platinum. We play our torch on the joint, the heat
stays near the joint but we need to get it really hot, so we use a
very hot but rather small flame. At the other extreme is fine silver.
The heat from our torch is rapidly conducted away to heat up the
entire work piece. The whole piece gets hot but our solder flow point
is much lower than the platinum example. For the silver piece we need
a fairly large QUANTITY of heat but not a really high TEMPERATURE;
with these requirements a tiny, super-hot flame can get you in
trouble much faster the a larger, cooler flame.
Think separately of TEMPERATURE and QUANTITY, it may help you (I