Search

Improved safety standards and reduced automotive body-in-white weight have led to a strong interest in martensitic press hardening steel (PHS). As the sensitivity to hydrogen embrittlement of martensite increases at higher strength level, the very small uptake of diffusible hydrogen by aluminized PHS during the austenitization stage of the hot press forming process is of concern. The hydrogen uptake was found to reduce the plasticity of the PHS considerably. The PHS with a higher strength was more susceptible to hydrogen-induced brittle fracture. The present work reports that vanadium additions, which serve to trap the hydrogen, considerably reduce the negative impact of the hydrogen uptake in aluminized 1800 MPa and 2000 MPa PHSs. The contribution also proposes a mechanism for the uptake of hydrogen during the processing of aluminized PHS.

The use of press hardening steel (PHS) in structural safety-related parts has experienced a rapid growth in the automotive industry, due to increased passenger safety standards, which require an improved vehicle intrusion resistance. Very small concentrations of diffusible hydrogen in ultra-high strength PHS deteriorate their mechanical properties. The focus of the present study was the analysis of the diffusible hydrogen uptake during the hot press forming process of the aluminized PHS and its impact on the mechanical properties of PHS. The effect of the paint baking on the properties of aluminized PHS was also evaluated by mechanical testing and hydrogen thermal desorption analysis. A mechanism for the hydrogen absorption of aluminized PHS was proposed.

The hydrogen-facilitated anodic dissolution of steel is an interesting experimental phenomenon, but the persistent gaps in this knowledge area are great. The changes in the Tafel slopes and the reaction rates of steel that has been cathodically charged with hydrogen are interpreted mainly in the context of hydrogen trapping and de-trapping behaviors of steel using a variety of electrochemical methods. This study reveals that the increase in the anodic current density and the decrease in the polarization resistance are attributed primarily to the hydrogen-induced lattice expansion. Based on the Tafel-slope change, the oxidation of hydrogen cation partly contributed to the increase in the total anodic current density together with the dominant anodic reaction of the steel dissolution. The electrochemical permeation measurements showed much slower effusion kinetics of the hydrogen that has been trapped at the e-carbide particles, and the trapping and de-trapping behavior at the fine particles are one of the controlling factors of the hydrogen-enhanced anodic dissolution of steel. From an engineering aspect, it is believed that the current study will provide an important insight into future perspectives on stress corrosion cracking failure occurring in various high-strength steels.

Various rapid heating methods have been developed to increase the productivity of press hardening steel. One of these methods is direct resistance Joule heating. This heating method results in the melting of the surface coating and the formation of a persistent liquid trail as a result of the high thermal conductivity and low melting temperature of the Al-10 pct Si alloy coating. This can be addressed by an alloying preheating treatment prior to the press hardening process.

The surface alloying of 55 wt.% Al–Zn coated press hardening steel during a press hardening heat treatment was evaluated. Austenitization resulted in the formation of a surface layer containing FeAl and Fe2Al5. The use of an increased heating rate during the austenitization cycle resulted in a high volume fraction of Zn at FeAl grain boundaries, and the presence of Zn islands in Fe2Al5 grains. These microstructural features suppressed the high temperature oxidation and evaporation of Zn. Zn was found to provide a pronounced cathodic protection, and the alloyed coating did not cause liquid metal induced embrittlement.