The influence of hydrogen on MS980, MS1180, MS1300 and MS1500 martensitic advanced high strength steels used for automotive applications

Abstract

Martensitic advanced high-strength steels (MS-AHSS) are used by automotive manufacturers to create lightweight, crashworthy cars. One issue when using high-strength steels such as MS-AHSS is hydrogen embrittlement. This issue needs to be addressed to determine the viability of these steels in automotive applications. In this study, different grades of commercially available MS-AHSS were studied with the aim to (i) investigate the influence of hydrogen on the mechanical and fracture characteristics of the steel in laboratory and service conditions; (ii) to characterise the microstructure of the steels, (iii) to understand hydrogen diffusivity and uptake in the steels; and (iv) to understand the hydrogen trapping behaviour of the steels. Microstructure analysis using the light microscope and scanning electron microscope revealed the presence of martensite and some amount of ferrite in MS-AHSS. The proportion of martensite was directly related to the mechanical strength of the steel. The influence of hydrogen on the mechanical and fracture properties of the four martensitic advanced high-strength steels was studied using the linearly increasing stress test and electrochemical hydrogen charging. The hydrogen influence increased with steel strength, decreasing charging potential, and decreasing applied stress rate. Increased hydrogen influence was manifest in (i) the decreased yield stress attributed to solid solution softening by hydrogen, (ii) the reduced macroscopic ductility, and by (iii) the change from ductile cup-and-cone fracture to macroscopically brittle shear fracture, attributed to a dynamic interaction of hydrogen with the dislocation substructure somewhat similar to the HELP mechanism. The influence of hydrogen on the mechanical and fracture properties of the MS-AHSS was studied in simulated service conditions; i.e. (i) immersed in 3.5 wt% NaCl solution, and (ii) at substantial applied stress rates,. There was little influence of hydrogen for the four MS-AHSS in 3.5 wt% NaCl. Similarly, there was little influence of hydrogen for hydrogen-precharged MS1300 and MS1500 subjected to tensile tests at substantial stress rates. The diffusivities of hydrogen in MS980, MS1300 and MS1500 were quantified. The use of a Pt counter electrode during cathodic hydrogen charging is not recommended. Further work was conducted to examine the influence of hydrogen on the properties of martensitic advanced high-strength steels under conditions relevant to automotive service such as: (i) in 3.5 wt% NaCl at different cathodic potentials, (ii) in acidified 3.5 wt% NaCl and (iii) at substantial stress rates. The hydrogen embrittlement susceptibility of the steels increased at (i) increasingly negative potentials and at lower pH in 3.5% NaCl, and (ii) at high charging potentials in 0.1 M NaOH at substantial stress rates. The hydrogen influence was manifested by a reduction in ductility, and the presence of brittle features on the fracture surface. A new thermal desorption spectroscopy apparatus was used to measure the hydrogen concentrations in the four martensitic advanced high-strength steels after hydrogen charging (i) electrochemically in 0.1M NaOH and 3.5 wt% NaCl, and (ii) in gaseous hydrogen. The hydrogen concentration increased with (i) increasingly negative charging potential, and (ii) increasing hydrogen gas pressure. A relationship was derived between equivalent hydrogen fugacity and the applied charging overpotential. Trap binding energies were 26.4 kJ mol-1 and 61.0 kJ mol-1 for the two desorption peaks, interpreted as dislocation and interface hydrogen traps. There was no strong correlation between the steel strength and hydrogen concentration. Hydrogen permeation experiments were used to investigate hydrogen trapping in the four commercial automotive martensitic advanced high-strength steels. Hydrogen trapping increased with increasing mechanical strength as indicated by (i) the decrease in the hydrogen diffusion coefficient and (ii) the increase in reversible hydrogen trap density. The measured trap densities were in the order of ~1017 to ~1018 cm-3. There was no correlation between hydrogen uptake and the mechanical strength of the steel. HE susceptibility in the MS-AHSS is associated with proficient reversible trapping of hydrogen by dislocations. This work has shown that there is little influence of hydrogen on steel properties in auto service despite the fact that there is an influence of hydrogen under more severe conditions of cathodic hydrogen charging.