High Temperature Corrosion of Cast Irons and Steels

Doktorsavhandling, 2004

During the last decades, tougher competition and stiffer government policies have forced the automotive industry into a race for safer, cleaner and more fuel-efficient vehicles at low cost. As a consequence, exhaust systems should resist to increased working temperatures. This trend towards higher peak firing temperatures is mainly intended to decrease fuel-consumption and exhaust gas emissions. Thus, the demands on mechanical and oxidation properties at high temperature of the alloys increase, and a better understanding is needed to improve these properties. This thesis concerns the high temperature corrosion of cast heat-resistant irons and steels, intended for exhaust manifolds and turbocharger housings, in dry air as well as in synthetic diesel and petrol exhaust gases. The examined alloys were two ductile irons, a SiMo (Fe3.9Si0.6Mo3C) and a Ni-Resist (Fe32Ni5.3Si1.8Cr2.1C), and two cast heat-resistant stainless steels, one ferritic (Fe18Cr1.4Nb2.1Mn0.32C) and one austenitic (Fe20Cr9Ni1.9Nb 2.7W0.5C). Primarily, coupons were oxidised for 50h at 650°C-1050°C. The effect of LCF on the oxidation process was also studied. Detailed analyses of the oxide scales, by means of XRD, SEM/EDX, and AES, were done to characterise the oxides growth. Models, based on thermodynamics and kinetics, were proposed for the oxidation of these alloys. Both cast irons developed a thick and porous outer Fe-oxide that nucleated preferentially at the former graphite nodules. The inner oxide on SiMo consisted of Fe-Si-oxide and (Fe,Si,Cr) oxides in an unoxidised (Ni,Fe) matrix for Ni-Resist. Both alloys failed in forming a continuous protective oxide layer at 650°C or 750°C within 50h due to slow diffusion processes. At 850ºC and above, interfacial SiO2 was formed. However, decarburisation, and for Ni-Resist enhanced evaporation of Cr species, led to accelerated oxidation rates and fundamental differences in the oxide structure. The oxidation behaviours of the two cast stainless steels were strongly related to the microstructures. Interdendritic non-Cr carbides initiated thick oxides. The ferritic cast steel exhibited an interesting combination of (Mn,Cr) spinel, an intermediate Cr2O3 layer and an interfacial SiO2 film. The scale remained adherent and protective at all temperatures in all gases. Water vapour-induced Cr-evaporation was reduced, plausibly by the outer (Mn,Cr) spinel. Segregation of Cr during casting accelerated the formation of the Fe oxide nodules in the centre of the dendrites in the austenitic alloy. The alloy was very sensitive to water vapour and susceptible to catastrophic oxidation at 1050°C in petrol gases. Low cycle fatigue promoted the nucleation of pits and nodules. Cracking and re-healing of the oxides accelerated the Si- or Cr-depletions leading to faster oxidation.