Good answer! Any other questions while Steve is on line? You in
the back row......;)
GS -
>
> Here's my best explanation of why headers fail with header wrap (I'm a
> metallurgist by day). With plain carbon steels, the material itself can
> actually suffer from thermal fatigue, temper embrittlement, and accellerated
> corrosion. Thermal fatigue is just the process of cyclic heating and
> cooling. This is common to all exhaust systems, but the problem is
> exaggerated by the thermal wrap, because the temperature which the header
> material is subjected to is higher than it would be without the header wrap.
> The heat which is normally dissipated throughout the engine compartment is
> held in the headers, causing them to be heated to temperatures where
> corrosion is accellerated, and thermal fatigue is greater due to larger
> gradients in the heating/cooling cycle. Temper embrittlement is caused by
> holding carbon steels in temperature ranges which can be sufficent to cause a
> phase change in the steel. This is often called re-crystallization, and
> causes normally ductile steels to become brittle, and fail in a brittle
> manner.
> The end result is a combination of these phenomena, which results in
> brittle, cracked headers with high levels of corrosion. In short, the header
> wrap will keep the heat out of the engine compartment, but at the cost of
> shortened header life.
>
> SteveM.
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