【作者】 范俊锴；

【导师】 杜凤山；

【作者基本信息】 燕山大学 ， 机械设计及理论， 2013， 博士

【摘要】 大型锻件作为大型成套设备的核心零部件，一般有几十吨到数百吨重，成材率较低，属于国家未来科技发展规划纲要所提出的迫切需要解决的前沿技术。大型锻件在高温成形及其降温过程中，由于氢析出并偏聚于锻件内部的微缺陷中，所产生的内高压和微裂纹极易导致零件突然断裂，称之为氢脆（又称白点），被视为大锻件的“癌症”，是大锻件质量控制中最为危险和棘手的问题。为揭示氢脆缺陷的产生机理，本文以大型Cr5支承辊锻件为对象，基于氢压原理和白点的宏微观特征，综合研究了氢和残余应力对白点萌生和扩展的影响，建立了白点萌生扩展的力学模型，为预控大型锻件白点的产生提供了依据。为揭示大型锻件微孔隙内氢压和残余应力对白点萌生的作用机制，在氢压原理的基础上，对锻件内微孔隙在氢压作用下的应力场以及多孔隙应力场的耦合作用进行了系统研究。通过对大型Cr5支承辊锻件的热处理组织转变过程的数值模拟，研究了锻件内残余应力分布及其对微孔隙周围的氢聚集的影响，结合残余应力下微孔隙的力学特征，从总体上把握了微孔隙氢压和残余应力对白点萌生扩展的重要影响。针对氢压强度对大锻件内微孔隙损伤致裂的重要影响，在气体Sieverts定律和微孔隙内氢压与钢中溶解氢化学势平衡的理论基础上，通过对比分析已有微孔隙氢压强度和氢浓度计算模型，考虑到微孔隙中氢原子转变为氢分子时体积的膨胀，自主建立了钢内微孔隙氢压强度和氢浓度的综合计算模型，解决了氢浓度、孔隙率和温度综合作用下微孔隙内氢压强度和氢浓度的精确计算问题。根据微孔隙氢压强度和氢浓度综合计算模型，钢中孔隙率的大小，即锻件的压实效果直接影响到微孔隙中氢压强度的高低。为研究大型锻件成形过程中微孔隙氢压强度的变化，通过二次开发将钢内微孔隙氢压强度和氢浓度综合计算模型导入到有限元软件DEFORM中，以多孔可压缩材料的塑性理论为基础，模拟研究了不同压下量下，采用KD锻造工艺时锻件内部微孔隙氢压强度和氢浓度的变化规律。基于粉末冶金的基本原理，制备了多孔隙试样，其镦粗压实效果与有限元分析结果基本一致，验证了采用多孔可压缩模型来分析孔隙压实效果的可行性。最后，以降低微孔隙中氢压强度为出发点，通过模拟研究，提出了大锻件生产中白点的预控方法。白点的产生原因不仅仅是微孔隙内高强氢压和残余应力的作用，还必须考虑氢致脆化对金属力学性能的影响。在综合考虑了微孔隙氢压强度、总氢浓度、氢致脆化、氢聚集和残余应力的基础上，基于内聚力模型建立了计算锻件内部白点萌生扩展的综合计算模型，模拟研究了不同残余应力和氢浓度条件下锻件内部白点的萌生扩展特性，并在此基础上建立了锻件内部白点萌生快速预报的BP神经网络模型，为实际生产中白点的快速预报提供了新的方法。更多还原

【Abstract】 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.更多还原