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

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).

Effect of Carbon Content and Intercritical Annealing on Microstructure and Mechanical Tensile Properties in FeCMnSiCr TRIP-Assisted Steels

“…The great advantage of the AFM compared with SEM is the possibility of imaging surfaces in different environments, without any vacuum or special sample treatment, with very high resolution [29]. The multiphase samples should present at least three phases: PF, RA, and BF (and possibly also M), but they are not easy to identify in a large AFM scan area.…”

“…The concept used for the identification of phases and/or microconstituents is like used with SEM, exploiting the particularity of Nital to preferentially dissolve different phases and microconstituents present in multiphase steels, being able to generate topographic images that allow their morphology to be analyzed in detail. The great advantage of the AFM compared with SEM is the possibility of imaging surfaces in different environments, without any vacuum or special sample treatment, with very high resolution [29]. The multiphase samples should present at least three phases: PF, RA, and BF (and possibly also M), but they are not easy to identify in a large AFM scan area.…”