AMP_04_May_June_2021_Digital_Edition

FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A Y / J U N E 2 0 2 1 6 4 pressive residual stress after nitriding. The influences of silicon on the nitriding case microstructure required more advanced techniques to observe. Figure 2a shows a CTEM- BF micrograph of the nitride case microstructure in a high silicon condition of the experimental alloys. Only ferrite and cementite are observable in the CTEM-BF micrograph, which is based on diffraction contrast. Anomalous features in the microstructure were observed during focusing in CTEM-BF mode leading the researchers to seek out alter- native imaging modes. Accordingly, scanning transmission electron microscopy utilizing high angle annular dark field (STEM-HAADF) was employed to form micrographs based on atomic number contrast. The corresponding STEM- HAADF micrograph is shown in Fig. 2b. The dark features correspond to a lower average atomic number relative to iron. Additional energy dispersive spectroscopy (EDS) measurements revealed the dark areas had elevated lev- els of silicon and nitrogen. The lack of diffraction contrast from these silicon containing nitrides led to the conclusion that they are amorphous. Advanced STEM-HAADF techniques determined that increases in silicon content resulted in increases in the vol- ume fraction of the amorphous silicon containing nitrides. The increased volume fraction of these particles corre- sponded to significant increases in nitride case hardness and compressive residual stress. In a subsequent publica- tion it was shown that these improvements in nitride case properties conferred by elevated silicon contents signifi- cantly improved the fatigue performance after nitriding [5] . By employing advanced techniques combining imaging, diffraction, and spectroscopy, it has been shown that sil- icon alloying is a viable strategy to improve the perfor- mance of nitrided steels. SEM AND STEREOLOGY TO UNDERSTAND PROCESSING-MICROSTRUCTURE RELATIONSHIPS IN 52100 STEEL As discussed in the previous section, PAGS refine- ment is well established to improve fatigue performance. Fatigue cracks in these high strengthmicrostructures often nucleate intergranularly at prior austenite grain bound- aries. Thus, refinement of PAGS presumably results in a smaller nucleated fatigue crack. As this fatigue crack nu- cleationmechanismhas been observed to occur in a single fatigue cycle at stresses above the endurance limit, a sig- nificant fraction of the fatigue life is consumed by fatigue crack propagation through the case microstructure. Thus, factors that affect fatigue crack propagation resistance in the case are critical to consider. PAGS refinement also results in refinement of the martensitic microstructure. In carburized steels, the case microstructure is typically composed of plate martensite and retained austenite. Figure 3a shows a representative micrograph of this microstructure obtained from 52100 steel austenitized and quenched. To simulate the effects of PAGS on case microstructure, specimens from this 52100 steel were subjected to various austenitization heat treat- ments to vary the PAGS. Then, detailed scanning electron microscopy (SEM) and rigorous stereological methods were utilized to determine martensite and austenite vol- ume fraction and size parameters. Figure 3b shows ex- ample results of martensite plate thickness as a function of PAGS, where the plate thickness increases as PAGS in- creases. Similarly, the martensite plate length and size of retained austenite increase as PAGS increases. Quanti- tative stereological characterization of case microstruc- Fig. 3 — (a) Representative microstructure, consisting of plate martensite and retained austenite, in a quenched and tempered 52100 steel. (b) Martensite plate thickness as a function of prior austenite grain size in a 52100 steel. Taken fromM. Agnani et al. [6] . (a) (b) 11 12