Influence of the Cooling Time on the Microstructural Evolution and Mechanical Performance of a Double Pulse Resistance Spot Welded Medium-Mn Steel

Abstract

In the present work, the influence of the cooling time on the mechanical performance, hardness, and microstructural features of a double pulse resistance spot welded medium-Mn steel are investigated. Curves of the electrical resistance throughout the welding revealed that the cooling time strongly influences the heat generation during the second pulse. A second pulse after a short cooling time re-melts the center, and heat treats the edge of the primary fusion zone. This desired in-process heat treatment leads to a modification of the cast-like martensitic structure by recrystallization illustrated by electron backscatter diffraction measurements and to a homogenization of manganese segregations, visualized by energy-dispersive X-ray spectroscopy, which results in an enhanced mechanical performance during the cross tension strength test. In contrast, during excessively long cooling times, the resistance drops to a level where the heat generation due to the second pulse is too low to sufficiently re-heat the edge of the primary FZ. As a consequence, the signs of recrystallization disappear, and the manganese segregations are still present at the edge of the fusion zone, which leads to a deterioration of the mechanical properties.

A review of the processing, microstructure and property relationships in medium Mn steels

“…A CE value of >0.45-0.5 has a 'poor' weldability [65] and will require special welding processes. Unfortunately, most medium Mn steels have a high CE and initial studies have found the resistance spot weldability of medium Mn steels to be poor with very low cross tension strengths [207][208][209]. A cross tension strength test is where two strips of steel are arranged in a '+' shape and spot welded together at the overlap.…”

“…The predominant failure mode was found to be interfacial failure which is undesirable. Mn segregation to dendrite boundaries in the FZ also creates a local region with high CE and brittleness [207,209].…”

Prediction of the Shear Tension Strength of Resistance Spot Welded Thin Steel Sheets from High- to Ultrahigh Strength Range

“…For example, Fig. 1 STS values of RSWed high-to ultra-high strength steels in similar [18, and dissimilar [12,23,25,36,38,43,50,78,79,82,87,96,98,100,101,115,119,125, joint configurations for the welding of a R m = 1500 MPa steel the same STS is required for the joint as for a R m = 1200 MPa steel grade (see also in Figs. 2 and 3).…”

“…Multiple welding current pulses also act as a post weld heat treatment (PWHT). Stadler et al [18] have found that the second welding current pulse remelted the center of the weld nugget of a 0.1 C, 6.4 Mn, 0.6 Si (wt%) medium Mn-steel, leading to a recrystallization and homogenization of the initial weld microstructure, thus improving the mechanical properties. For the optimization of welding process parameters design of experiments (DoE) methods have widely been used in RSW.…”

Microstructure and Local Mechanical Properties of the Heat-Affected Zone of a Resistance Spot Welded Medium-Mn Steel

“…Consequently, the modification of the microstructure of the FZ, for example, with a second pulse that acts as an in-process heat treatment, represents a well-documented approach to improve their mechanical performance [ 13 , 14 , 15 , 16 , 17 , 18 ]. This approach has already been successfully adopted to improve the properties of the medium-Mn steel investigated in the present work [ 19 ].…”