Ferritic stainless steels have their main use in the automotive industry in the manufacture of exhaust gas systems and are subject to embrittlement at high temperatures. To minimize the problem, alloys with higher chromium content and stabilized with titanium and niobium are used.
So, what are the phenomenas that influence embrittlement at 475 ° and 600 ° C in AISI 441 ferritic stainless steel for application in diesel engine exhaust systems?
Possible reason could be the sensitization (grain boundary corrosion).
Which occur between 500 C -800 C temperature range due to the formation of chromium carbides at the GBs, this decrease the strength levels at the GBs and also make steel vulnerable for oxidation at those regions.
Nb and Ti are added to the steel to take care of carbon present in it (form TiC and/or NbC) and not let the C react with Cr.
Short and sweet:
Possibilities:
1) u can have sigma phase formation on long exposure between 540-870 degrees C, Chromium should be more than 15 % for this to occur and Mo promotes it.
2) the spinodal decomposition of alpha iron rich phase and alpha prime chromium rich phase called as 475 degrees C embrittlement.
PS: Grain coarsening is a phenomenon occurring at 1000-1100 degrees C.exhaust gas temp max would be 800 C.
The sensitization explained above is associated with welding where ur Cr is gone as Cr carbides and u have what we call as IGC (corrosion issues) and in welding the temp reached is much higher around 1500-1600 degrees C.
BR
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