Information

B. Special Characteristics of Nickel-Chromium Alloys
The resistivity of nickel-chromium alloys shows a special
characteristic. At temperatures below 500°C it is affected by
the rate with which the alloy has been cooled from high
temperatures, e.g. after annealing, and it decreases with
increasing cooling rate. This behaviour is shown
schematically in the graph on the right.
The solid line represents the so-called state of equilibrium,
i.e. the resistivity of an annealed wire after slow cooling. The
dotted line indicates how the resistivity below 500°C
changes for lower values by rapid cooling. Rapid cooling
takes place e.g. with thin wires after strand annealing.
This effect is strongest for wires of nonferrous nickel-
chromium alloys, like NiCr8020. It is weaker for nickel-
chromium NiCr6015 and can be neglected for nickel-
chromium NiCr3020.
Effect of Pre-Treatment on the Temperature Dependence of the
Resistance on ISA-CHROM 80
®
.
In addition, since with normal strand annealing the cooling
rate increases with decreasing wire diameter, this effect will
become stronger, too, as the wire diameter becomes
smaller.
For NiCr8020 and NiCr6015, assuming normal annealing
conditions, the resistance decrease between 1 and 0.01mm
diam. amounts to appr. 1.3 resp. 0.5%. For ISAOHM
®
the
decrease amounts to appr. 5% because of a varying
composition.
For resistance wires of NiCr alloys no sliding resistivity has
been standardized; instead an average resistivity is quoted
(see also DIN 17471). It should be born in mind, however,
that this value for NiCr8020 and NiCr6015 is lower than the
value quoted in DIN 17470 for heating resistors.
page 4