Microstructural Effects on the Hydrogen Embrittlement of Dual-Phase Steels

Abstract

To decrease vehicle emissions, automotive manufacturers aim to decrease materials usage. Dual-phase (DP) steels are used in safety critical locations of a vehicle and with stronger DP steels, less material is needed. However, DP steels are known to be affected by hydrogen embrittlement (HE), which decreases the ductility and is thus of significant concern.

This work investigates the effect of heat treatment on the microstructure and HE of DP steel. Heat treatments are developed and conducted using dilatometry, with obtained microstructures characterized using scanning electron microscopy (SEM), optical microscopy (OM), electron backscatter diffraction (EBSD) and hardness measurements. Tensile specimens are heat treated using a Gleeble 3800-GTC thermo-mechanical simulator, before evaluating the HE effect by measuring the total absorbed hydrogen content using thermal desorption spectroscopy (TDS), and determining the static mechanical properties using slow strain-rate tensile (SSRT) testing. The fracture micromechanisms are investigated with fractography using SEM and EBSD with interrupted SSRT and fractured specimens.

The critical transformation temperatures are determined at 686±1 ⁰C and 571±10 ⁰C for Ar3 and Ar1 respectively. Intercritical annealing at 590 ⁰C, 630 ⁰C, and 670 ⁰C result in a martensite content of 24.97±6.43 %, 40.41±4.21 %, and 77.63±6.97 %, respectively. Hardness values of 240±31, 300±9, and 380±20 HV1 are obtained for the respective intercritical annealing conditions and are an accurate method for comparing the martensite content and microstructure of dilatometry and Gleeble specimen. The martensite distribution changes from a discontinuous network of lath martensite with small martensite islands at 590 ⁰C, to a continuous network of lath martensite without martensite islands at 670 ⁰C.

TDS yield an increase in the total absorbed hydrogen content of 1.16±0.11 wppm, 1.29±0.13 wppm, and 1.58±0.27 wppm at increasing intercritical annealing temperature, which is attributed to an increase in the ferrite/martensite interphase.

Through SSTT the optimal combination of ultimate tensile strength (UTS) and elongation is found at 630 ⁰C, with UTS of 1025±8 MPa and elongation of 12.6±0.6 %. For all specimens significant HE is observed, with an embrittlement index of 58.9±7.1 %, 89.2±1.1 %, and 85.8±2.5 % for increasing intercritical annealing temperature. The highest embrittlement at 630 ⁰C is attributed to dislocation pile-up around martensite islands, severely increasing embrittlement.

Fractographic investigation revealed the presence of quasi-cleavage fracture bands across the specimen cross-section. This is attributed to preferred crack propagation along martensite bands in the cross-section, with preferred crack nucleation at the martensite surface layer of the specimen short edge. EBSD of the fractured specimen revealed an increase in the kernel average misorientation (KAM) surrounding the ferrite/martensite interface and martensite islands, indicating an increased HE action due to the HEDE and HELP mechanisms.

It is concluded that there is considerable embrittlement of the heat-treated DP steel, which is significantly influenced by the martensite content and distribution. Since DP steels are used in safety critical components, controlling the distribution of martensite in the microstructure is vital, and avoiding surface layer martensite and martensite bands can reduce the HE effect.