The investigation into the circumferential mechanical properties of tubular materials has been receiving increasing attention, since the tube hydroforming process has been used in the tubular materials forming field, because the circumferential mechanical properties have a significant effect on the hydroformability of the tubular materials. In the present study, a method for evaluation of the circumferential mechanical properties of the tubular materials with the flaring test was proposed. The expressions for the yield stress, strain hardening coefficient and exponent values of the tube were successfully derived based on the geometrical and mechanical relationships in the tube flaring test. To verify the reliability of this method, the calculated results of the yield stress, strain hardening coefficient and exponent values, obtained from the newly proposed method, were compared to the ones obtained with the conventional tensile tests. It was found that the method proposed in the current study is reliable, with high accuracy. The method is appropriate to evaluate the circumferential mechanical properties of the tubular materials.

In this study the hydraulic free bulge test with an internal pressure boosting velocities of 0.5 MPa/s and punch feeding speed of 0.2 mm/s was carried out to study the microstructure evolution of the transformation induced plasticity seamless steel tube in the hydroforming process. The microstructure of the selected parts of the tube after the hydraulic free bulge test were investigated with the optical microscope and transmission electron microscope. The transformation rate of the retained austenite to martensite of the selected parts were determined by the x‐ray diffraction. To study the deformation behavior and obtain the stress‐strain state of the tube, the finite element simulation of the hydraulic free bulge test was also carried out. The results show that the retained austenite in the microstructure of all the selected parts transformed to martensite after the hydraulic free bulge test. In the center part of the tube, the highest transformation rate of 82.84 % was obtained. The retained austenites distribute inside of the ferrite grains transform to martensite firstly under the compressive stress and strain state. The retained austenites distribute at the ferrite grain boundaries start to transforme into martensite when the tube is subjected to the tensile stress and strain.