Analysis of the friction stir weldability of high manganese TRIP steel

• Grant number: 12/03665-0
• Support Opportunities: Scholarships in Brazil - Scientific Initiation
• Effective date (Start): September 01, 2012
• Effective date (End): August 31, 2013
• Field of knowledge: Engineering - Materials and Metallurgical Engineering - Physical Metallurgy
• Principal Investigator: Haroldo Cavalcanti Pinto
• Grantee: Ricardo Henrique Buzolin
• Host Institution: Escola de Engenharia de São Carlos (EESC). Universidade de São Paulo (USP). São Carlos , SP, Brazil

Abstract

We propose a study of the friction stir weldability of novel austenitic high manganese steels with the effect of transformation-induced plasticity (TRIP).In spite of their extraordinary mechanical properties and high ductility, the high-Mn TRIP steels impose challenges to the fusion welding processes, when considered to the production of lightweight vehicle structures. This is due to their austenitic microstructure, which favors the distortion, as well as the formation of significant residual stresses in the weld bead and the heat-affected zones. Moreover, depending on the sulfur and phosphorus contents in those steels, the weld bead can become sensitive to the appearance of hot cracks during cooling, since these steels solidify with a fully austenitic structure. A TRIP alloy was produced in the foundry laboratory of the SMM-EESC-USP using a Fe-22%Mn-0.4%C chemical composition, which is suggested by the international scientific literature. The material was homogenized at 1150°C for 48h in order to minimize the microsegregation of Mn. Afterward, the ingots were hot-rolled at 1150°C in several steps with 10% reduction and finally were machined to a final thickness of 3.0 mm. The welded joints will be produced by evaluating the effect of the heat input during FSW through independent variations of the tool rotation speed and the welding speed. The welding procedures will be evaluated based on the mechanical properties of joints, such as micro-hardness profiles and tensile strength, on their microstructural gradients, the morphology of their fracture surfaces, as well with respect to the residual stress state. The technological challenge will be therefore to demonstrate the applicability of FSW for joining austenitic low cost and ultra-high-strength TRIP steels, aiming at innovating the metal structures used in the industry of mobility, such as the automotive, shipbuilding, and railway ones.(AU)