Discontinuous yielding behavior, strengthening effect and deformation mechanism of double wall copper-brazed steel tube under various continuous sectional cooling

Unlike traditional cooling, continuous sectional cooling (CSC) consists of various cooling modes, which is of decisive significance to the microstructure and performance of double wall copper-brazed steel tube (DWBT). In this work, three CSC (CSC-A/B/C) processes are chosen to reveal strengthening and deformation mechanisms of DWBT. The results show that there are significant differences in discontinuous yield behavior, natural aging phenomenon, and tensile properties of the DWBT among three CSC processes due to diverse grain size and precipitates, but a similar texture is obtained due to Kurdjumov-Sachs relationship. For discontinuous yielding, the propagation velocity of Lüders band is a key factor to determine the yield point elongation (YPE), which reduces with decreasing grain sizes due to the interaction between dislocation and grain boundary. Thus, the CSC-C (1120 °C→4s940 °C…360 °C→16s110 °C) with small grains has a longer YPE. Meanwhile, the precipitation strengthening dominates the highest strength and lowest elongation of the CSC-A (1120 °C…100 °C) due to abundant MC carbides. Compared with CSC-B (1120 °C…170 °C), the yield strength of CSC-C is higher while ultimate tensile strength is lower, which is attributed to different contributions of precipitation and grain boundary strengthening to both. The MC carbide causes the natural aging of DWBT. Moreover, the geometrically necessary dislocations (GNDs) and low angle grain boundaries (LAGBs) raise linearly with strain. The GNDs prefer to be distributed in small grains, and uniform grain can weaken the difference of GNDs distribution in different grain sizes. The LAGBs including dense dislocation walls or microbands are formed by the grain subdivision mechanism.