Yu-Ting Tsai

Effect of interpass temperature on the microstructure and mechanical properties of multi-pass weld metal in a 550-MPa-grade offshore engineering steel

The influence of interpass temperature on the microstructure and mechanical properties of multi-pass weld joints (up to 36-mm thickness) by submerged arc welding (SAW) was studied from the perspective of offshore engineering. Optimal mechanical properties were obtained with the interpass temperature of ~130xa0°C. Decreasing interpass temperature from 130 to 80xa0°C increases the strength and hardness at the cost of impact toughness of the weld joint due to the formation of hard phases including bainite and martensite. Increasing the interpass temperature from 130 to 250xa0°C promotes a larger volume fraction of coarse M-A constituents and larger inter-spacing of high-angle boundaries, which, in turn, deteriorates the toughness. In addition, a large amount of M-A constituent necklacing prior austenite grains was observed in the reheated zone of all weld metals and was responsible for the low impact energy of the weld joint.

Investigation of idiomorphic ferrite and allotriomorphic ferrite using electron backscatter diffraction technique

A heat treatment for a vanadium-containing steel involving high-temperature austenitisation followed by two-stage isothermal holding was deliberately conducted to produce allotriomorphic ferrite (ALF), idiomorphic ferrite (IDF) and martensite. The resulting microstructures and crystallographic relationships with respect to the parent austenite grains were examined using the electron backscatter diffraction technique. Results indicated that IDF grains did not possess a specific orientation relationship (OR) with respect to the surrounding austenite matrix. The detailed substructures of IDF were also examined. In contrast, the ALF grains had an approximate Kurdjumov–Sachs OR with respect to one or both of the adjacent austenite grains; the latter case was termed a dual OR and was analysed in detail.

The application of convergent beam electron diffraction (CBED) analysis on transformation-induced plasticity (TRIP) steels

Convergent beam electron diffraction (CBED) in transmission electron microscopy (TEM) was applied to determine local carbon concentrations in low‐carbon transformation‐induced plasticity (TRIP) steels. High‐order Laue‐zone (HOLZ) lines were experimentally obtained for comparison with simulation results. A new procedure for calculating carbon content is thus proposed. Retained austenite (RA) is classified into three types by morphology; the relationship between the carbon content and the corresponding RA morphology is discussed based on CBED results. Furthermore, results of X‐Ray diffractometry measurements are also used for comparison.