Orientation relationship between ferrite and austenite and its influence on ultrasonic attenuation in cast austenitic stainless steel
Authors: Luo Zhong-Bing  , Dong Hui-Jun  , Ma Zhi-Yuan  , Zou Long-Jiang  , Zhu Xiao-Lei  , Lin Li  ,   NDT & E Laboratory, Dalian University of Technology, Dalian 116085, China
Published: December 2020

Cast austenitic stainless steel (CASS) is widely used in important engineering components, which has a two-phase microstructure, i.e.austenite and ferrite. With slow cooling rate during solidification procedure, the austenite grain is coarse and the morphology of ferrite is complex. Due to the remarkable elasticity anisotropy of austenite, the resulting structural noise makes the recognition of macroscopic defects quite difficult in ultrasonic testing. To improve the signal-to-noise ratio, the ultrasonic testing frequency is generally small, about 0.5-2.0 MHz, and the ultrasonic scattering effect of ferrite is ignored. However, for submillimeter or even smaller defect and damage near the surface, the ultrasonic testing frequency should be increased to achieve a higher resolution. In these cases, how the ferrite influences the ultrasonic wave propagation behavior and the testing result is still not conclusive. Therefore, CASS Z3CN20-09M is studied as an example in this paper. Based on ultrasonic propagation modeling and “in situ” experimental design, the crystal orientation relationship between ferrite and austenite in CASS is studied and the factors influencing the ultrasonic scattering attenuation are clarified. The results would be helpful for clarifying the ultrasonic response mechanism of CASS and critical for the quantitative evaluation of small defects and early-stage damage.<br/>The orientation relationship between ferrite and austenite and its influence on ultrasonic scattering attenuation in CASS are studied. The crystal orientations and their relationships between two phases are characterized by the EBSD technique. A two-dimension anisotropic model is built based on the morphology of ferrite, and the ultrasonic propagation is calculated by the time domain finite difference method. The influences of orientation relationship and morphology on the longitudinal wave attenuation are analyzed and verified by “in-situ” experiments. Results show that ferrite grains with bar or island shape are distributed on the austenite grains. The orientation relationship between ferrite and austenite is mainly Kurdjumov-Sachs relationship, and only a minority of ferrite and austenite satisfy the Nishiyama-Wassermann relationship. Numerical simulation of the ultrasonic propagation under a testing frequency of 15 MHz indicates that the orientation relationships between two phases and ferrite morphologies present collaborative effects on the ultrasonic scattering attenuation, which could not be ignored. The factors influencing the ultrasonic attenuation in <101> austenite grain are quantitatively analyzed. It is found that in single austenite grains of CASS, the inhomogeneity of crystal orientation, the orientation relationship between austenite and ferrite and the ferrite morphology play an important role in determining the total ultrasonic attenuation.<br/>The results would provide supports for clarifying the ultrasonic response mechanism of CASS and developing the quantitative evaluation methods.