The effect of Ti addition on the oxidation resistance of high-purity 19%Cr ferritic stainless steels has been investigated during isothermal heat treatment at temperatures between 1073 K and 1273 K in air. The microstructures of scale and the scale/metal interface were investigated in detail by means of SEM-EBSD and FE-TEM together with μ-EDS.

The Ti addition increases the oxidation mass gains but simultaneously improves the critical temperature of the oxidation resistance. The formed scale is mainly Cr₂O₃ regardless of the addition of Ti but Ti addition increases the thickness of Cr₂O₃. Considerable reduction in the grain size of Cr₂O₃ by Ti addition was recognized, which was inferred to increase the oxidation mass gain due to the easy diffusion through the grain boundary. Furthermore, Ti is oxidized beneath the scale/metal interface as internal complex oxides, such as Al₂TiO₅ and Al₂Ti₇O₁₅because of the small amount of aluminum in the steels used. In a somewhat deeper region from the interface, θ-Al₂O₃ forms where Ti cannot be oxidized. The oxygen getter effect by Ti atoms is postulated to be responsible for improving the oxidation resistance.

The purpose of this study is to investigate the influence of Nb content of Ti-xNb-7Al alloys (x=20-35 mass%, abbreviated as xNbA) on β (bcc)→α″ (orthorhombic) transformation with tempering through microstructure, isothermal age-hardening, and shape change of a U-shaped specimen with heating. Full α″-martensite structure was obtained on the quenched 20 NbA. With increasing of Nb content, residual β-phase increased, and single β-phase was obtained in the alloys of more than 25 NbA. From the result, the M_S temperature commonly decreases with increasing of Nb content. The hardness of 20 NbA and 23 NbA rapidly increased in a few seconds by isothermal aging at 450°C, and the hardness of the alloys of more than 25 NbA abruptly increased after an incubation period for 30-60 s. No compositional distribution between α″ and matrix in the aged 23 NbA was found by STEM-EDS analysis, but the obvious distribution of Nb was detected in 35 NbA. U-shaped specimen of 20 NbA exhibited no shape change by heating. On the other hand, the specimen of 23 NbA composed of β+α″ phases showed the shape recovery (SR) first due to work induced α″→β inverse transformation, then deformed toward the bending direction (SA: shape advance) due to β→α″ transformation. The specimens of more than 28 NbA composed of single β-phase exhibited only SA without SR. The starting temperature of SA (M_SA: β→α″) with heating increases with increasing of Nb content. Merging M_S (upper) and M_SA (lower), it is proposed that the whole M_S of xNbA alloy forms a bow shape curve. It is suggested that the low Nb alloys inside the M_S curve transform immediately without atom diffusion by tempering at 450°C, but the higher Nb alloys outside the curve need an incubation period to distribute Nb concentration with atom diffusion, and martensitic α″ transformation will abruptly occur in the local domains less than 25 NbA which is the inside of the curve.

In this study, thermal fatigue tests at maximum temperature 1073 K were performed using 13%Cr-Nb-Si and 18%Cr-Nb-Mo steels as representative heat-resistant ferritic stainless steels for automotive exhaust systems. The changes in the microstructure, the crystal orientation and the hysteresis loop during thermal fatigue in the temperature range from 473 K to 1073 K were investigated. As a result of comparing thermal fatigue life under these conditions, 18%Cr-Nb-Mo steel with high temperature strength was found to have a longer thermal fatigue life than 13%Cr-Nb-Si steel. During the thermal fatigue process, the material was softened by reducing of the amount of solute Nb, and the coarsening of Nb precipitation. By this softening, the form of the hysteresis loops changed with the increase in cycles. By considering the softening of the material, the change in the hysteresis loops could be predicted to some extent. Furthermore, by EBSD analysis, it was recognized that the dynamic recovery and recrystallization accompanied by the uniaxial and fine grain formation occurred during the thermal fatigue process. From the viewpoint of change of the microstructure, the thermal fatigue damage was quantified by the ratio of the low-angle grain boundary, and the change of this index with the progress of the cycle in 18%Cr-Nb-Mo steel had a smaller than 13%Cr-Nb-Si steel. It was thought that this point was caused by the retardation of recrystallization by solute Mo.