PSI - Issue 79

Giuseppe Macoretta et al. / Procedia Structural Integrity 79 (2026) 508–516

509

1. Introduction

Advanced High-Strength Steels (AHSS) are increasingly adopted by the automotive industry to reduce the vehicle's weight and increase passengers’ safety [1–3]. Multiphase steels enable the exploitation of the optimal characteristics of each microstructural component, resulting in a beneficial blend of mechanical strength and ductility and allowing lightweight design and vehicle emissions reduction. Among the most recently developed multiphase steels, Quenching and Partitioning (QP) steels emerged as 3rd generation Advanced High-Strength Steels (AHSS) for automotive applications, [4–6]. QP steels are produced by applying rapid cooling from austenitization to a temperature comprised between the martensite start and martensite finish values, followed by a partitioning treatment at a higher temperature to promote the carbon diffusion [7]. The resulting microstructure is composed of martensite, ferrite, and Retained Austenite (RA), which provides an outstanding balance of strength and ductility, fundamental for the material's cold formability performances necessary for automotive body-in-white components [8,9]. However, this multiphase microstructure poses significant questions that have to be properly addressed for enabling its adoption in the automotive industry [5,10]. A primary constraint in the application of AHSSs is their inherent vulnerability to Hydrogen Embrittlement (HE) and delayed fracture, which is strongly impacted by the multiphase microstructure of QP steel [5,11,12]. RA can have a significant impact on the mechanism of HE due to its low hydrogen diffusivity, acting as an irreversible trap. RA could transform into martensite due to plastic deformation, the well-known TRansformation-Induced Plasticity (TRIP) phenomenon. This fresh martensite will be exposed to the elevated H content present in the former RA region and cause hydrogen-induced cracks, [11,13]. In this work, the effect of Hydrogen (H) intake on the static mechanical properties of the QP1180 was investigated by means of tensile SSRT on electrochemically charged specimens. Microstructural investigations, H permeation tests, and Temperature Programmed Desorption (TPD) allowed us to characterize the microstructure and H diffusivity and trapping parameters of the as-received material. Fractographic examinations revealed the influence of hydrogen on fracture behavior, showing a transition from ductile to predominantly brittle fracture modes at elevated hydrogen concentrations. The effective H concentration present in the specimen was measured after the tensile test via the hot extraction method, allowing us to correlate the specimen's mechanical properties and the H content.

Nomenclature

AHSS Advanced High-Strength Steels H Hydrogen HE Hydrogen Embrittlement RA Retained Austenite TPD

Temperature Programmed Desorption

SEM

Scanning Electron Microscopy

SSRT Slow Strain Rate Test

2. Material and Methods

2.1. Material

Specimens for mechanical and H diffusion tests were extracted from commercial QP1180 sheets having a thickness of 1.2 mm, which were provided in the quenched and partitioned condition and with hot-dip galvanized surfaces. Metallographic investigations were carried out on samples extracted in directions parallel and perpendicular to the sheet plane. Samples were ground up to 1200 SiC paper, polished with a 1 μ m diamond suspension, and etched with Nital (2%). A Leica DMI3000 optical microscope, capable of reaching a magnification up to 1000x, was employed to observe the material microstructure.