Measurement of residual strain and microstructure of surface hardened S38C axles by neutron Bragg-edge transmission imaging

Hu, F. F.*; Qin, T. Y.*; Su, Y. H.  ; Ao, N.*; Zhou, L.*; Xu, P. G.   ; Parker, J. D.*; Shinohara, Takenao   ; Wu, S. C.*

High-frequency induction hardened S38C axles are widely applied in Japan Shinkansen and China bulletin trains. Due to well-designed surface treatment, these axles produce a large gradient change in the material structure from the axle surface to the core, resulting in a large-layer depth of residual strain and microhardness. It is well known that the residual strain of a real component is of great importance for its long-cycle service assessment, mainly at the reduced non-destructive detect interval and low maintenance cost. Fortunately, the neutron beam has a high penetrating capacity, which provides the advantage of using Bragg-edge transmission imaging to characterize the residual strain and microstructure inside large engineering components. In our study, ring shaped specimens of the axle were prepared. The neutron transmission imaging experiments were conducted at RADEN in J-PARC MLF under proposal Nos. 2022A0298 and 2023A0069. The obtained results reveal that the 0-5 mm area of the axle surface is compressive residual strain, while the core region is the combination of the tensile residual strain and the compressive residual strain. In the presentation, we will also analyze and compare the changes in residual strain and microstructure in defective S38C axles at various fatigue stages. We are expecting to employ pulsed neutron imaging data to assess the fatigue resistance of the axle.