Phases quantification in DP600 steel welded by GTAW process using SEM and atomic force microscopy

G.Y Pérez Medina,H Lopez Ferreira,E Hurtado Delgado,A F Miranda Pérez

doi:10.1590/1980-5373-mr-2016-0172

Abstract

The automotive industry is constantly under several challenges in many aspects, such as development of new materials and improvement their manufacturability. In order to achieve light weight, reduced emissions and ensure conductor safety, advanced high strength steels (AHSS) are able to fulfill these requirements. Dual phase steels (DP) are well suited for light weighing car body constructions. The gas tungsten arc welding (GTAW) process is focused in literature as an alternative choice for joining AHSS steels; this study is held to disclose the exhibited microstructural constituents. In addition, quantitative determinations of the volume % of phases in the various weld regions were made. The relative amounts of lower bainite (LB), upper bainite (UB), and polygonal ferrite (PF) in the heat affected zone (HAZ) were determined by image analysis from optical microscopy (OM) and scanning electron microscopy (SEM). It was found that GTAW promoted the development of significant amounts of LB in the HAZ (50.89 %). In contrast atomic force microscopy (AFM) leads to quantify the different phases by the morphology, height and roughness, it was found that martensite (M) dropped down to 7.5% in the intercritical zone (IZ) although PF increases to 92.5% compared with the base metal (BM).

Highlights

In an attempt to reduce automobile vehicle weight and enhance passenger safety, multiphase advanced high strength steels (AHSS) steels such as Dual phase steels (DP) steel has been developed
In the heat affected zones and the fusion zone of the welded DP600 steel, the same phases were found, from these micrographs it is evident that the exhibited microstructure in the steel were upper bainite (UB) and lower bainite (LB) with some areas of polygonal ferrite (PF), The presence of bainites in the HAZ agrees with the scanning electron microscopy (SEM) observations of López et al.[23]
atomic force microscopy (AFM) images enabled the identification of M, tempered martensite (TM) and PF in the base metal and intercritical zone

Summary

Introduction

In an attempt to reduce automobile vehicle weight and enhance passenger safety, multiphase AHSS steels such as DP steel has been developed In recent years these steels have been subject of extensive interest because of their unique combination of mechanical properties, high strength associate with good formability[1,2,3]. In an ideal world “the best joining method is no joining at all” This quote captures a designers frustration with the need to include joints which create potential areas of weakness in the automotive body[7], the GTAW process is an economy welding method; very limited studies about the GTAW joint have been reported for the DP steels,[8]. The exhibited volume percentages of M and PF in the various welded regions of the BM and IZ were quantified in order to disclose possible phase transformation reactions

Microstructural and microhardness characterization

GTAW Process

Welded microstructure

Phase quantification

Microhardness

Conclusion