Steels_ Metallurgy and Applications, Third Edition

374 Steels: Metallurgy and Applications

Table 4.20 Steels for catalytic converters

Grade Cr% Al% Nb% Ti% Zr%

409 11.5

430Nb 17 0.7

18CrNb 18.6 0.6

Armco 12SR 12 1 0.6

Uginox FK 17 12 x C

0.3

0.3

0.3

0.4

good scaling resistance at temperatures up to 900"C, they are expensive and
their high thermal expansion characteristics could also introduce large thermal
stresses and cyclic fatigue in constrained parts of the component. The lower
cost and thermal expansion characteristics of ferritic steels therefore offer greater
attraction as candidate materials for high-temperature converters.
The nominal compositions used in current catalytic converters are compared
with Type 409 in Table 4.20.
Thus the majority of the grades rely on higher chromium contents in order
to achieve improved scaling resistance compared with Type 409. However, both
silicon and aluminium are also beneficial in this respect and a high aluminium
content is included in Armco 12SR. Each of the above steels also contains
substantial amounts of strong carbide-forming elements which stabilize the mate-
dais against intergranular corrosion. However, it is interesting to note that these
steels employ dual stabilization, involving niobium plus either titanium or zirco-
nium. Whereas the latter are the more powerful stabilizing elements and also
ensure a martensitic-free weld, the inclusion of niobium ensures better impact
properties in the HAZ. The creep resistance of ferritic stainless steels is also
improved significantly by the precipitation of Fe2Nb (Laves phase). In addition
to its function as a stabilizing element, zirconium is also beneficial in improving
the oxidation resistance of these steels. Therefore significant benefits are obtained
from the inclusion of more than one stabilizing element in ferritic stainless steels
for operation at high temperatures.
Because of the very large number of cars involved and the substantial amount
of material in each converter, it is reported that General Motors is now the largest
consumer of stainless steel in the United States.

Architectural applications

With its aesthetic appeal, excellent resistance to atmospheric corrosion and low
maintenance requirements, stainless steel offers major attractions as an archi-
tectural material. However, whereas stainless steel has featured prominently in
the architectural sector in Japan and the United States, it is only recently that it
has begun to enjoy similar success in the UK and Europe. One of the earliest
architectural uses of stainless steel was in the cladding of the top section of the
320 m-high Chrysler building in New York in 1930. After a period of 60 years,
an ASTM examination has shown that the stainless steel on this building is still in