Stainless Steel for Automotive Exhaust

THERMAK® 17 Stainless Steel has excellent high temperature strength, thermal fatigue and oxidation resistance. It is stabilized with titanium and niobium and exhibits dramatic creep resistance. The dual stabilization prevents carbide sensitization exposure and makes the alloy thermally non hardenable. The improved thermal fatigue performance over other intermediate and high chrome ferritic stainless steels facilitates vehicle light weighting efforts for hot-end exhaust components. Since THERMAK 17 does not contain any molybdenum, it may represent an opportunity for savings for applications where Type 444 is currently supplied.

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Elevated Temperature Properties

Thermal Fatigue Performance

THERMAK 17 Stainless Steel has shown, in limited testing, to have 20 - 30% improved thermal fatigue life over intermediate chrome products currently used in automotive hot-end exhaust applications.


THERMAK 17 Stainless Steel is readily drawn and formed. The ULTRA FORM® Stainless Steel technology enhances formability and improves the consistency from one coil to another.

Corrosion Resistance

THERMAK 17 Stainless Steel shows improved oxidation resistance and oxide adherence when compared to other 17% chromium ferritics. The high silicon and manganese, copper added alloy reduces spalling when exposed to cyclic high temperature salt conditions creating a barrier of oxide that provides protection to the steel substrate. The dual stabilized alloy resists intergranular corrosion when welded and provides comparable corrosion resistance to equivalent alloys in acidic, high sulfate and lower temperature, cyclic salt environments commonly found in automotive exhaust applications.

Oxidation Resistance

THERMAK 17 Stainless Steel provides an improved oxidation resistance compared to other ferritic alloys such as Type 439 stainless steel. In addition to 17% chrome content, high silicon and high manganese contents improve high temperature oxidation resistance in air and exhaust gas atmospheres.

In continuous testing, 200 hours at 930 °C, THERMAK 17 Stainless Steel exhibited a uniform scale pattern and no spalling.


THERMAK 17 Stainless Steel is generally considered to be weldable by the common fusion and resistance welding processes. Grain growth in the weld heat affected zone, with reduced weld toughness, is a common occurrence in ferritic stainless steel weldments. Low weld heat input and elimination of weld discontinuities will provide maximum toughness in welded sections. Warming of welded parts prior to forming may also improve weld toughness. When a weld filler is required, 18 Cr-Cb Stainless Steel is recommended for applications where high temperature thermal cycling is expected. Filler wires 308L or 309L will provide improved weld ductility for lower temperature applications. Additional information concerning the welding of ferritic stainless steels may be obtained from the following publications:

1. ANSI/AWS A5.9, A5.22 and A5.4 (stainless welding electrode specifications).

2. “Welding of Stainless Steels and Other Joining Methods,” SSINA, (

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