Study On Flake’s Formation And Hydrogen Diffusion Behavior In Heavy Forgings Under Mesoscopic Views

Hydrogen is a harmful element in steel.Hydrogen diffusion and aggregation not only reduce the mechanical properties of materials,but also produce flakes and other defects.Flakes can block metal matrix continuity,causing fatigue and sudden breakage of the metal parts,which has great harm.How to effectively control flake’s formation is an urgent problem to be solved.This paper focuses on the factors of flake’s formation mechanism and the hydrogen diffusion problem of large forgings during the heat treatment process under the mesoscopic field of views.Which has great significance for controlling flake’s formation and improving forging quality.To find out the hydrogen damage behavior of the material,the hydrogen charging method was used to characterize the hydrogen content in the steel through different hydrogen charging currents and hydrogen charging time.The effect of hydrogen enrichment in the steel on Cr5 steel was simulated.The expansion characteristics of the flakes after hydrogen charging are investigated.The results show that a large amount of hydrogen accumulates on the surface of the sample after hydrogen charging to generate hydrogen bubbling,accompanied by longitudinal crack propagation along the bubble.The fractured dimples of the sample after hydrogen charging were significantly shallower,indicating that hydrogen charging significantly reduced the toughness of the sample.To reveal the flake’s information characteristics under multi-factor coupling conditions,based on cohesion model,hydrogen pressure theory and hydrogen enrichment theory under stress,a multi-crack analysis model of flake’s formation and propagation was established.The effects of hydrogen concentration,cohesion strength and multiple cracks on the flakes formation were studied.The critical hydrogen concentration of flakes generation and its changing trend were determined by successive cycles.In addition,orthogonal analysis was carried out to determine the main factors of flake’s generation.The results show that high concentrations of hydrogen was accumulated at the crack tip,which is great reduced the cohesive strength of steel.large cracks are the main factors leading to flake’s formation.Densemulti-crack decreases the critical hydrogen concentration of flakes information,and the coupling effect between cracks leads to the formation of large cracks through small cracks.The hydrogen concentration can be effectively reduced by heat treatment dehydrogenation process.Therefore,based on the hydrogen diffusion theory and hypothesis,the theoretical model of hydrogen diffusion was established,and the hydrogen diffusion behavior in the trap was simulated by means of finite element software.The effects of microstructure,heating rates and holding temperature on hydrogen diffusion were studied.And the effects of various factors on the change of hydrogen pressure in the trap and the possibility of generation of hydrogen-induced cracking are analyzed during the hydrogen diffusion process.The results show that high temperature can fully stimulate the activity of hydrogen molecules and promote the activation and diffusion of trapped hydrogen.The difference in hydrogen diffusion rates between different tissues leads to the enrichment of hydrogen at the phase boundary during the process of heating.Increasing the heating rate can reduce the hydrogen diffusion time,but a higher heating rate leads to the trap hydrogen pressure improving exponentially,which is likely to increase the possibility of generating cracks around cavity.