Information

F. Instructions for Treatment
page 17
The alloys delivered by
Isabellenhütte have good working
characteristics.
The following instructions should be
observed:
Winding
All wires should be treated as
carefully as possible, with a tension
below the yield point. The yield point
can be taken as being appr. 50%
below tensile strength in annealed
condition.
Aging
Even when winding a resistance
wire with little tension deformation
can take place. By simple bending,
mechanical stresses are built up in
the wire. They affect the electrical
resistance and should be minimized
by heat treatment.
Copper alloys can increase their
resistivity by deformation; for
ISOTAN
®
e.g. this increase can
achieve up to 7%. As regards
nickel-chromium alloys, deformation
reduces the resistivity, for ISAOHM
®
e.g. up to 10%.
Heat treatment – which is also
called artificial aging – is especially
required when precision resistors
are manufactured.
Aging is a stabilization process; it
can be accelerated considerably by
applying temperature well above
ambient temperature is determined
by the material and especially by the
insulation used.
In some cases artificial aging by
temperature cycling is an
advantage. Here temperature cycles
between 20°C and maximum aging
temperature are repeatedly run.
The aging temperature should not
exceed 140°C because of the
sensitivity to heat of the wire
insulation. Bare wires slightly begin
to oxidize above 100°C; this oxide
layer can be removed by pickling.
Pickling
The type of pickling bath is mainly
determined by the nature of the
alloy. For MANGANIN
®
e. g.
sulphuric chromic acid, for
ZERANIN
®
nitric acid, for ISOTAN
®
also nitric acid or persulphate pickle
is used; for nickel-chromium alloys
pickling in phosphoric acid has
proven its value. The immersion
period lasts for 1 to 10 minutes
depending on the alloy.
Soldering, Brazing and Welding
of Resistance Alloys
Soldering
The copper alloys manufactured by
Isabellenhuette can be soldered like
pure copper, using normal solders
and fluxes. Light abrasion of the
areas to be soldered is recom-
mended.
When soldering precision resistors a
leadfree solder (e.g. L-SnAg5
according to preferrably pure
colophony) should be used. Even
the smallest residual of aggressive
fluxes causes corrosion of the wire
thus alternation to the resistance.
Solders containing lead may be
subject to metallic alternations. The
melting point of the solder must be
such as to prevent softening over
time. In general, however, brazing
should be preferred.
ISAOHM
®
alloy can only be
soldered by using particularly
aggressive fluxes, as used for
soldering stainless steels (e.g.
Soldaflux Z by Degussa). In addition
the surfaces must be roughened
beforehand. But even then the bond
does not have the quality as with
copper alloys.
In all cases the flux must be
carefully removed.
Brazing
The soundest and most reliable
bonds are made by brazing. Since
the temperature coefficient and
resistivity of the alloys for precision
resistors are affected by heating, it
is recommended to use low melting
point brazing solders (L-Ag40Cd
DIN 1707, e.g. “Degussa 4003”) and
to keep the brazing time as short as
possible. Flux “h” of Degussa has
proved useful.
Welding
The alloys can be welded. It must
be kept in mind, however, that the
electrical and heat conductivities as
well as the melding points often
greatly differ from the respective
values of other materials.
General
Besides those mentioned above
normally no other peculiarities need
to be taken into consideration. It is
not possible to define “ideal”
conditions for soldering, brazing or
welding, since experience has
shown that the skill of the worker
plays a more important role than the
selection of components.
Cutting
While with copper-nickel alloys no
problems arise, nickel-Chromium
alloys and pure nickel are tough and
show high strength at high
temperatures; they thus tend to
“weld together” with the tools. This
must be taken into consideration
when drilling, threading or sawing.
Under certain circumstances it may
be more favourable to use hard, i.e.
un-annealed, alloys.