Datasheet

20 Kanthal Appliance Alloys Handbook.
Operating Life
The life of the resistance heating alloy is
dependent on a number of factors, among
the most important are:
– Temperature
– Temperature cycling
– Contamination
– Alloy composition
– Trace elements and impurities
– Wire diameter
– Surface condition
– Atmosphere
– Mechanical stress
– Method of regulation
Since these are unique for each applica-
tion it is difficult to give general guidelines
of life expectations. Recommendations on
some of the important design factors are
given below.
5. Design Factors
Oxidation properties
When heated, resistance-heating alloys form
an oxide layer on their surface, which slows
down further oxidation of the material. To
accomplish this function the oxide layer
must be dense and resist the diffusion of
gases as well as metal ions. It must also be
thin and adhere to the metal under tempera-
ture fluctuations.
The protective oxide layer on KANTHAL
alloys formed at temperatures above 1000 °C
1830
° F consists mainly of alumina (Al
2
O
3
).
The colour is light grey, while at lower tem-
peratures (under 1000 °C, 1830
° F) the
oxide colour becomes darker. The alumina
layer has excellent electrical insulating prop-
erties and good chemical resistance to most
compounds.
The oxide formed on NIKROTHAL
alloys consists mainly of chromium oxide
(Cr
2
O
3
). The colour is dark and the electrical
insulating properties inferior to those of
alumina.
The oxide layer on NIKROTHAL alloys
spalls and evaporates more easily than the
tighter oxide layer that is formed on the
KANTHAL alloys.
Results of several life tests according to
ASTM B 78 (modified) are given in Table 1
for KANTHAL and NIKROTHAL alloys.
In the table, the durability of KANTHAL
A-1 wire at 1200 °C 2190 °F is set at 100 %,
and the durability of the other alloys is re-
lated to that figure.
1100 °C 1200 °C 1300 °C
2010 ° F 2190 ° F 2370 ° F
KANTHAL A-1 340 100 30
KANTHAL AF 465 120 30
KANTHAL AE 550 120 30
KANTHAL D 250 75 25
NIKROTHAL 80 120 25 -
NIKROTHAL 60 95 25 -
NIKROTHAL 40 40 15 -
Table 1
Relative Durability Values in %,
KANTHAL and NIKROTHAL Alloys
(ASTM-test wire 0.7 mm 0.028 in)