Effect of Retained Austenite and Non-Metallic Inclusions on the Mechanical Properties of Resistance Spot Welding Nuggets of Low-Alloy TRIP Steels

Víctor H Vargas Cortés,Cuauhtémoc Maldonado Zepeda,Gerardo Altamirano-Guerrero ,I Mejı́a ,Víctor Hugo Baltazar Hernández

doi:10.3390/met9101064

Abstract

The combination of high strength and formability of transformation induced plasticity (TRIP) steels is interesting for the automotive industry. However, the poor weldability limits its industrial application. This paper shows the results of six low-alloy TRIP steels with different chemical composition which were studied in order to correlate retained austenite (RA) and non-metallic inclusions (NMI) with their resistance spot welded zones to their joints’ final mechanical properties. RA volume fractions were quantified by X-ray microdiffraction (µSXRD) while the magnetic saturation technique was used to quantify NMI contents. Microstructural characterization and NMI of the base metals and spot welds were assessed using scanning electron microscopy (SEM). Weld nuggets macrostructures were identified using optical microscopy (OM). The lap-shear tensile test was used to determine the final mechanical properties of the welded joints. It was found that NMI content in the fusion zone (FZ) was higher than those in the base metal and heat affected zone (HAZ). Whereas, traces of RA were found in the HAZ of highly alloyed TRIP steels. Lap-shear tensile test results showed that mechanical properties of spot welds were affected by NMI contents, but in a major way by the decomposition of RA in the FZ and HAZ.

Highlights

Advanced high-strength steels (AHSS), transformation induced plasticity (TRIP)steels are increasingly becoming used in the transportation sector due to that their high mechanical properties allowing the use of thinner gauge sheets
Optical micrographs obtainedoffrom theMetals base metal of TRIP steels that were under study are
Multiphase microstructures composed of ferrite (F), bainite (B), martensite (M), Optical micrographs obtained from the base metal of TRIP steels that were under study are and retained austenite (RA) were found in all steels but in different percentages

Summary

Introduction

Advanced high-strength steels (AHSS), transformation induced plasticity (TRIP). Steels are increasingly becoming used in the transportation sector due to that their high mechanical properties allowing the use of thinner gauge sheets. TRIP steels mechanical properties are microstructure dependent and they offer an excellent combination of strength and ductility, to high energy absorption during deformation owing to the deformation-induced transformation of RA into martensite (TRIP effect) at elevated strain level [1]. Resistance spot welding (RSW) is the predominant process in sheet metal joining, in the Metals 2019, 9, 1064; doi:10.3390/met9101064 www.mdpi.com/journal/metals. Weldability of TRIP steels is affected by their rich alloying content; the designed multiphase microstructure is modified by the thermal cycle inherent to the RSW process resulting in decreased mechanical properties of the spot weld due the formation of hard phases and non-metallic inclusions. Mechanical performance in TRIP steel spot welds is affected by factors such as [3]: (i) weld nugget diameter,

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Conclusion