Diffusion Of Hydrogen In Thick Clean Steel And Effect Of Hydrogen On Mechanical Properties

In this paper, the relationship between hydrogen content of pre-charged samplesand pre-charging time with cathodal charging method has been studied. The effect ofhydrogen traps in SM490B clean steel on the hydrogen diffusivity has been studied bythe electrochemical permeation methods. The effects of the hydrogen atom indifferent states on the mechanical properties have been investigated by tensile tests ofstatic hydrogen charging and dynamic hydrogen charging samples.Results of different discharging experiments show that when the pre-chargingcurrent density is constant, the hydrogen contents of the samples increase withpre-charging time. In a certain range, the relation of the hydrogen content with thepre-charging time for20g clean steel （i=10mA/cm²） isC_H=3.665t^1/2-20.1176（40mint≤1≤40min）by glycerin hydrogen test andC_H=1.542tt^1/2-0.7311（t≤120min）by cathodal charging method.Results of the electrochemical permeation methods indicate that the reversiblehydrogen trap decreases the apparent diffusion coefficient of hydrogen, while theirreversible hydrogen trap only delays the penetrating time of hydrogen. Results alsoshow that the apparent diffusion coefficient of hydrogen increases when the cathodecurrent density （i） is between1and8mA/cm2, but keeps constant after the i equal toor larger than8mA/cm2. In specific temperature conditions （30℃）, the diffusioncoefficient of hydrogen in the ideal lattice of SM490B clean steel is3.97×10^-5cm²/swith the absence of the hydrogen trap. The function of diffusion coefficient D ofSM490B clean steel with temperature T can be expressed asD=0.498exp﹙23.48kJ/mol/RT﹚Tensile tests of the hydrogen charging samples present that non-diffusiblehydrogen atom can reduce the yield strength of SM490B clean steel, but only a10%reduction. Diffusible hydrogen atom elevates yield strength of the SM490B steel. In addition, the non-diffusible and diffusible hydrogen atoms significantly reduce thetensile strength and ductility of the SM490B steel.The nucleation of flake resultsfrom hydrogen molecules formed by the hydrogen atoms in the vacancy clusters orlattice defects of the of SM490B sample, when the sample is pre-charged withhydrogen. The pressure produced by the hydrogen molecules causes a microcrack toform in the matrix. Diffusible hydrogen atoms induced by the stress will be enrichedin the matrix around the flakes, and which leads to crack propagation, so that theflakes can grow.