Research On Flakes’ Generation Mechanism And Pre-control In Heavy Forgings

As the core components of large complete sets of equipment, heavy forgings aregenerally dozens of tons to hundreds of tons and the succeed product rate is lower, whichbelonging to the State Science and Technology Development Plan proposed urgent need toaddress cutting-edge technology. When heavy forgings in high-temperature forming andcooling process, the hydrogen will be separated out and aggregated in forgings internalmicro defects and lead to cracks initiation because of high hydrogen pressure, whichcalled hydrogen embrittlement (also called flakes) that are the most knotty and dangerousdefects like ‘cancer’ in heavy forgings. Based on the hydrogen pressure theory andmacroscopic and microcosmic knowledge of flakes, this paper comprehensive studied theinfluence of hydrogen and residual stress on flakes’ initiation and propagation in Cr5backup roll heavy forgings, and the mechanical model of flakes’ initiation and propagationwas established. The analysis results and mechanical model of flakes provide a basis forflakes’ pre-control in heavy forgings.To reveal the mechanism of micropores’ hydrogen pressure and residual stress onflakes in heavy forgings, based on the hydrogen pressure theory, this papercomprehensively researched the micropores’ hydrogen pressure stress field and couplingeffect between them. Through simulating, the microstructure transformation and residualstress in Cr5heavy forgings varied with time in heat treatment were researched. Then, themicropores’ mechanical characteristics and hydrogen aggregation around microporesunder known heat treatment residual stress were studied. The analysis results gave thegeneral information of micropores’ hydrogen pressure and heat treatment residual stressimpact on the flakes’ initiation and propagation in heavy forgings.For hydrogen pressure magnitude has the important influence on micropores’ fractureand damage, based on the gas Sieverts Law and chemical potential balance betweendissolved hydrogen atoms in lattice and hydrogen molecules pressure in micropores withinsteel, all the original micropores’ hydrogen pressure magnitude and concentrationcalculation models were studied and compared. With considering the volume expansionwhen the hydrogen atoms react into molecules, a new comprehensive calculation model for micropores’ hydrogen pressure magnitude and concentration was deduced, which solvethe accurate calculation problem of micropores’ hydrogen pressure magnitude andconcentration with considering the influence of total hydrogen concentration, porosity andtemperature in heavy forgings.According to the micropores’ hydrogen pressure magnitude and concentrationcomprehensive calculation model, the forgings’ porosity which depends on the forgingcompaction effect was directly affect the micropores’ hydrogen pressure magnitude. Tostudy the variation of micropores’ hydrogen pressure magnitude in compaction process ofheavy forgings, the micropores’ hydrogen pressure and concentration comprehensivecalculation model was imported into the finite element software DEFORM through secondtimes development. Based on the plastic theory of porosity material, the micropores’hydrogen pressure and concentration under different forging reduction with KD methodwere researched. Based on the principle of powder metallurgy, the porous body sampleswere prepared, and their compaction results by upsetting were agreed with finite elementanalysis results very well, so the effectiveness of using porosity material to analysis theporous compaction effect was verified. From the view of reducing micropores’ hydrogenpressure magnitude, with simulation several flakes’ pre-control method in heavy forgingswere proposed.The reason of flakes’ initiation is not only micropores’ hydrogen pressure andresidual stress, but also the influence of hydrogen-induced embrittlement on themechanical properties of metals. With comprehensive considering the micropores’hydrogen pressure, total hydrogen concentration, hydrogen-induced embrittlement,hydrogen aggregation and residual stress, a calculation model for flakes’ initiation andpropagation in heavy forgings was established with cohesive model, and the flakes’initiation and propagation characteristic under different total hydrogen concentration andresidual stress were studied. According to the analysis results, a BP neural network forquick prediction flakes’ initiation in heavy forgings was established, which provides a neweffective method for pre-controlling flakes’ initiation in heavy forgings’ production.