宏微观视域下钢内白点愈合工艺及机理研究

Flakes are crack type defects in heavy forgings and large cross-section rolled steel products, and it have great danger. Lots of previous research focused on the initiation mechanism of flakes, and many pre-control methods were given. However, for heavy forgings, it is impossible to completely eliminate flakes, because of its huge structural dimension and widely distributed defects. Therefore, the study on healing mechanism and process of flakes within heavy forgings is very important. To study the mechanism of flakes’ healing in heavy forgings, the experimental analysis of decomposition and diffusion behavior for high hydrogen pressure under high temperature in flakes is needed, and the decomposition and diffusion theoretical model for high hydrogen pressure in flakes will be established. Under press-working, the heavy forgings can be divided into the macroscopic deformation zone and the microscopic deformation zone around flakes, and the influence of hydrogen’ decomposition and diffusion behavior in flakes on flakes’ closure will be analyzed. The coupling relationship of microscopic crystal plasticity theory around flakes and macroscopic deformation for whole forgings will be studied. Research on the diffusion welding characteristics of material under high temperature and high pressure conditions, and the flakes’ diffusion welding model under high hydrogen pressure will be built. Finally, the flakes’ theoretical healing model system will be established, and the best process system for flakes’ forging healing in heavy forgings can be formed with optimization of forging and heat treatment. This study has important significance to guide heavy forgings’ defect repairing and improve its yield, and the research in a leading position over the world.

白点是大型锻件或大截面轧件内部萌生的一种具有巨大危害的裂纹类缺陷。以往研究多专注于白点的萌生机理，从而提出预控白点的方法。然而对于大型锻件来说，由于其结构尺寸巨大，缺陷多且分布广泛，要完全的消除白点是不可能的，因此对锻件内已萌生白点缺陷的愈合机理及工艺的研究就显得十分重要。为研究大锻件白点的愈合机理，实验分析白点内高强氢压在高温下的分解扩散行为，建立白点内氢压分解扩散的理论模型。在压力加工条件下，将大锻件分为宏观变形区和白点周围细观变形区，分析氢在白点内的分解、扩散行为对白点闭合规律的影响。研究白点区细观晶体塑性理论与其外部宏观变形的耦联关系。研究高温及内压作用下材料的扩散焊合特性，建立内高压条件下的白点扩散焊合模型。优化锻造成形工艺及热处理条件，建立白点愈合的理论体系模型，形成大锻件白点锻合的最佳工艺系统。本项研究对指导大锻件缺陷修复和提高其成品率具有重要意义，研究成果处于国际领先地位。

白点是大型锻件或大截面轧件内部萌生的一种具有巨大危害的裂纹类缺陷。以往研究多专注于白点的萌生机理，从而提出预控白点的方法。然而对于大型锻件来说，由于其结构尺寸巨大，缺陷多且分布广泛，要完全的消除白点是不可能的，因此对锻件内已萌生白点缺陷的愈合机理及工艺的研究就显得十分重要。本项目首先研究了氢作用下内含孔隙缺陷的轧辊类锻件在使用过程中的疲劳行为，为高氢含量锻件的安全使用提供了理论基础；白点锻合的关键在于防止白点焊合后二次白点的萌生，基于氢的微观行为，研究了加热条件下氢的微观扩散特征；采用晶体有限元的方法，研究了晶粒形态对氢扩散的影响；建立了基于氢压理论、氢致损伤理论和氢聚集理论的白点萌生耦合分析模型，研究了氢浓度、缺陷形态等影响因素对白点萌生的影响；试验研究了氢作用下的微观缺陷萌生因素和氢致损伤问题。研究结果表明，氢的存在对大型锻件的疲劳寿命影响十分明显，其影响程度和缺陷形态、位置和氢浓度密切相关；温度是氢扩散的第一因素，要降低氢浓度就必须考虑氢陷阱中氢的影响；晶界是氢扩散的一条重要通道，研究氢的扩散行为必须以晶粒尺寸和晶界特征为基础；白点的萌生是钢内多种机制共同作用的结果，氢是其中的主要因素，单一采用氢压模型的白点萌生机理并不准确，还必须考虑到其它因素的影响。通过本项目的研究，建立了白点焊合多尺度层面及多机理耦合的理论模型和计算方法，对于白点焊合工艺制定具有重要的指导意义，对指导大锻件缺陷修复和提高其成品率具有重要意义。