Stainless steels 373Type 316L. It has been established that exhausts in Hyform 409 can be manufac-
tured on the same equipment as that used for mild steel, which is an important
factor in maintaining the cost of stainless steel exhausts at a relatively low level.
Whereas 80% of failures in mild steel exhausts were attributable to corrosion,
it was found that the remaining 20% were due to fatigue. Having improved the
corrosion performance very substantially, fatigue therefore becomes the predomi-
nant mode of failure in stainless steels. It was established that the limiting fatigue
stresses of Hyform 409 and mild steel were virtually identical. The thermal
conductivity and expansion coefficients were also very similar, indicating that
the 12% Cr material could replace mild steel without increasing the risk of
thermal fatigue. Therefore to ensure that a stainless steel exhaust system does
not undergo premature failure due to fatigue, careful consideration must be given
to design details and standards of manufacture rather than to the intrinsic mate-
rial properties. Extensive road trials have shown that attention to these details
can provide exhausts with excellent fatigue characteristics, thereby ensuring the
long-term integrity of stainless steel systems.
Whereas some car manufacturers fit complete 12% Cr exhaust systems as orig-
inal equipment, others have opted for hybrid systems involving a combination of
mild steel, aluminized mild steel and Hyform 409. In such systems, the 12% Cr
steel is restricted to the areas that constitute the greatest risk of failure by corro-
sion, i.e. the rear silencer boxes.
The exhaust manifold is generally made in cast iron and the material is perfectly
satisfactory in this application. However, cast iron manifolds are relatively heavy
components, and for weight reduction/improved fuel economy, consideration has
been given to their replacement with manifolds fabricated from stainless steel
strip. Type 304 has been used for this purpose and also some of the 17% Cr
steels that will be discussed in the following paragraphs on catalytic converters.
Catalytic converters have been fitted to cars in the United States since 1974 in
order to reduce the level of toxic products such as CO, NOx and unbumt hydro-
carbons in the exhaust gases. These devices are also being introduced gradually
in Europe and represent a major potential market for stainless steel. Typically,
a platinum catalyst mounted on a ceramic substrate is contained in a stainless
steel case, comprising a cylindrical shell flanked by inlet and outlet cones. In
early models, the operating temperatures were low and casing materials reached
a maximum temperature of 550-600"C. Given the experience with the 12% Cr
muffler grade in the United States, steels such as Type 409 were evaluated as
candidate materials for the early converters and proved to be perfectly satisfac-
tory. However, as the requirements of legislation have tightened, the operating
temperatures in the converter have been increased up to 900"C, which imposes
severe demands on casing materials. These include good elevated-temperature
strength to withstand the stresses produced by exhaust gas pressure and also
by the weight of the device itself. Additionally, the materials are required to
have good resistance to oxidation and scaling, not only for long life, but also
to avoid blockage and malfunction of the converter. Type 409 has a maximum
operating temperature of about 7000C and therefore more highly alloyed steels
are required for modem converter systems. Whereas austenitic grades such as
Type 304 (18% Cr, 9% Ni) have excellent creep rupture strength, coupled with