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Background: Attractive properties of TRIP-type bainitic ferrite (TBF) steel ascribe to its unique microstructure of lath structure bainitic ferrite matrix and interlath retained austenite films. This work is concerned with obtaining ultra high-strength hot forged TBF steel with high elongation and excellent strength-elongation balance. Methods: The effect of austempering temperature on the microstructure along with its retained austenite characteristics and tensile properties of a hot forged TBF steel was studied. A detailed investigation correlating the steel structure and its tensile properties was carried out. Results: Tensile strength ranging from 1058 to 1552 MPa was achieved when the hot forged steel was austempered at (325 -475°C). Conclusions: Ultra high tensile strength of 1058 MPa, large total elongation of 29% and excellent strengthelongation balance of 30 GPa % were attained when the steel was austempered at 425°C. The large total elongation of this steel is mainly due to the uniform fine lath structure matrix and the pronounced TRIP effect of a large amount of retained austenite films which prevents a rapid decrease of strain hardening rate at low strain and leads to a relatively high strain hardening at high strain level. Rapid transformation of blocky retained austenite at low strain in the hot forged TBF steel austempered at higher temperatures results in a rapid increase of initial strain hardening. In addition, the coarse microstructure that contains large blocks of retained austenite / martensite and the insufficient numbers of bainitic ferrite lathes and retained austenite films deteriorate the total elongation and the strength-elongation balance of the TBF austempered at 475°C.

This work compared the parameters of friction stir welding (FSW) and under water friction welding (UWFSW) on the weld joint, such as tool rotation speed, transverse speed, and wall thickness, and compare the ultimate tensile strength (UTS) weld joint using the experimental work for FSW and UWFSW was performed using a new modified fixture to eliminate the post-process problems. Experiments were performed at three levels of three parameters: Wall thickness, tool rotational speed and travel speed using milling machine center by UWFSW and traditional FSW. The full factorial approach was introduced for statistical research. UTS i s measured as a reaction, meaning that the ultimate tensile strength achieved by welding the UWFS weld was greater than the conventional FSW weld. As a predictive research tool, regression analysis and variance analysis were used. From the study, it was observed that with minimal tool transverse speed, high tool rotation speed for UWFSW, maximum tensile strength is given, and that of traditional FSW.