Study of microstructure and mechanical properties of friction stir welded ferrite-martensite DP700 steel

Mahdi Mahmoudiniya^{1, 2*}, Leo A.I. Kestens^{2, 3}, Shahram Kheirandish¹, Amir Hossein Kokabi⁴

¹School of Metallurgy and Materials Engineering, Iran University of Science and Technology (IUST), Tehran, 16846-13114, Iran​​​

²Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, Delft, 2628 CD, the Netherlands​

³Metals Science and Technology Group, Ghent University, Ghent, Technologiepark 903 B-9052 Zwijnaarde, Belgium

⁴Department of Materials Science and Engineering, Sharif University of Technology, Tehran, 11365-11155, Iran

DOI: 10.5185/amlett.2019.2211

Publication Date (Web): Jan 10, 2019

Copyright © 2018 VBRI Press

E-mail: mmahmoudiniya@metaleng.iust.ac.ir

Abstract

In the present study, a 2 mm thick ferrite-martensite dual phase steel was subjected to friction stir welding. The welding was conducted by a tungsten carbide tool at a constant rotational speed of 800 rpm and various feed rates of 50, 100 and 150 mm/min. The microstructural features of friction stir welded joints were characterized by field emission - scanning electron microscopy as well as by transmission electron microscopy. The relationship between microstructure and tensile properties of the joints was investigated. Results showed that the stir zone of the welds consisted of bainite packets, exhibiting a different morphology compared to the ferrite phase and to the martensite phase. Microstructural examination of the heat affected zone showed that there is a softened region in the heat affected zone in all joints, irrespective of the welding speed. Decomposition of the martensite phase during tempering of the initial martensite of the base material was responsible for the observed hardness reduction. The decrease of the hardness in the softened zone was 28 ± 3, 21 ± 2.5 and 15 ± 3.2 HV for welding speeds of 50, 100 and 150 mm/min, respectively; whereby the base material exhibited a hardness of 275 ± 3 HV. The lower softening corresponded to the higher welding speed, i.e., under conditions whereby heat input to the weld was minimum. The tensile test results showed that the ultimate tensile strength of all welded joints is lower than the base metal and failure takes place in the softened region of the joints. The increment of welding speed increased the strength of the joint so that the weld conducted at the highest welding speed (150 mm/min) showed the highest tensile strength of 687 MPa, i.e. 95% of the strength of the base metal (723 MPa).

Keywords

Friction stir welding, ferrite-martensite dual phase steel, mechanical properties, microstructure.