Poisson Effects on X-Ray Diffraction Patterns in Low-Temperature-Carburized Austenitic Stainless Steel
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LOW-TEMPERATURE carburization (as well as low-temperature nitridation) of austenitic stainless steel results in improved properties, such as greatly increased surface hardness, wear resistance, fatigue resistance, and corrosion resistance.[1,2] The lattice parameter of the near-surface region is expanded by the interstitial carbon and nitrogen. X-ray diffractometry (XRD) of this so-called ‘‘expanded austenite’’ (also called S phase) has shown consistent variations in the observed lattice parameter, ahkl, ahkl ¼
k ðh2 þ k2 þ l2 Þ0:5 2 sinðhhkl Þ
½1
inferred from different peaks (hkl), where k is the wavelength of the incident X-ray, and hhkl is the diffraction angle of peak (hkl). (Absent strain and assuming good sample alignment, a should be the same for any peak in an fcc material.) Typically, a200 is found to be larger than a111, a220, and a311. Unfortunately, many researchers have only reported a few XRD peaks, rendering the investigation into the origin of this phenomenon difficult. However, a recent study by Fewell and Priest[3] used synchrotron radiation to obtain diffraction information from low-temperature-nitrided 316 stainless steel up to the (640) peak. They thoroughly reviewed the possible mechanisms reported in the literature for the observed H. KAHN, Research Associate Professor, and G.M. MICHAL, F. ERNST, and A.H. HEUER, Professors, are with the Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, OH 44106-7204. Contact e-mail: [email protected] This article is based on a presentation given at the ‘‘International Conference on Surface Hardening of Stainless Steels,’’ which occurred October 22–23, 2007 during the ASM Heat Treating Society Meeting in Cleveland, OH under the auspices of the ASM Heat Treating Society and TMS. Article published online March 11, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
diffraction results, which are summarized next. A variation in the nitrogen content with grain orientation was discounted, because ahkl did not depend on the X-ray angle of incidence. Tetragonal, monoclinic, and triclinic distortions were not considered, because they all involve peak splitting, which was not observed. The remaining possibilities included rhombohedral distortion, stacking faults, or lattice strain due to compressive residual stresses generated by the interstitial nitrogen. (The lattice parameter expansion in the case is impeded by the restraint imposed by the nontreated ‘‘core.’’) Rhombohedral distortion does not involve peak splitting. The interplanar spacings, dhkl, of a rhombohedral lattice are described by 1 ðh2 þ k2 þ l2 Þsin2 ar þ 2ðhk þ kl þ hlÞðcos2 ar cosar Þ ¼ a2 ð1 3cos2 ar þ 2cos3 ar Þ d2hkl ½2 where a is the lattice parameter, and ar is the rhombohedral angle. To obtain a200 > a111, ar must be greater than 90 deg, implying that a220 > a311. The XRD peak shifts due to stacking faults were first proposed by Warren[4] and later discussed in detail by Velterop et al.[5] Both analyses predict an increase in a200 and a decrease in a400
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