大型锻件中塑性夹杂物的变形机理及其影响

Large forgings were necessary basic components of major technical equipment and major projects for the country. But the economic development and national defense construction were seriously restricted result from the lower level of large forging process and the lack of high-end forgings manufacturing capacity. Large forgings serviced in harsh environments, so the mechanical properties of the large forging were high demanded and the defects size and density of the pores and inclusions were limited strictly. The overweight of inclusions defect detection was one of the main reason for the forgings unqualified. As the representative of plastic inclusions, manganese sulfide inclusion easily deformed with the plastic deformation of the matrix and leaded to a lot of forging cracks. However, there were many theories and techniques required to solve problems in the quality control of large forgings for plastic inclusions. The manganese sulfide inclusion was the research object in this project. The preparation methods of manganese sulfide cylindrical solid sample were studied, and the temperature control equipment for non-metallic material heated was designed. The high-temperature flow behaviors of manganese sulfide were accessed directly by experiment. The mechanical theory based on the damage model was established to analyze the evolution of the manganese sulfide inclusions and the fracturing deformation mechanism. The model was verified by in-situ experiments under different stress states. A new method was proposed to calculate the cross-scale analysis of inclusions in solving large forgings forming. It provides theoretical and technical support for large forgings quality control.

大型锻件是国家重大技术装备和重大工程所必需的基础部件，但我国大型锻件工艺水平较低，高端锻件制造能力不足，严重制约着我国的经济发展与国防建设。大型锻件服役环境恶劣，力学性能要求高，对其产品内部的孔洞、夹杂物等缺陷从尺寸、密度上限制严格，而夹杂物缺陷引起的探伤超标成为锻件不合格的主要原因之一。目前，以MnS为代表的塑性夹杂物高温流变应力无法测试，其锻造过程中的形变规律和对基体的影响机理不明晰，且无法进行有效的计算，导致生产中不可控。本项目拟针对典型塑性夹杂物MnS，研究MnS圆柱固体试样的制备，设计可用于非金属材料加热、控温的装置，并通过实验直接获得MnS高温流变行为；建立基于损伤力学的理论模型，分析MnS夹杂物的演变规律以及变形致裂机理，通过不同应力状态下的原位实验加以验证；提出一种新的方法，实现大型锻件成形过程中夹杂物的跨尺度计算与分析，致力于解决由塑性夹杂物超标带来的大型锻件制废问题。

夹杂物作为钢锭中不可避免的缺陷，不仅严重影响着大型锻件的使用性能，也提高了制造难度和工艺复杂性。作为最常见的塑性夹杂物，MnS的变形控制及其致裂机理研究进展缓慢，主要受到物性参数难以测定、缺少大型锻件缺陷跨尺度分析手段等因素制约。.本项目中设计并制作了一套可为烧结MnS试样进行热压缩实验的装置，研究了MnS的高温流变行为，分析了MnS夹杂物在金属基体中的演变行为，建立了基于GTN的MnS夹杂物高温损伤模型，开展了扫描电镜下原位拉伸试验，揭示了MnS夹杂物的致裂机理，并引入体胞概念，构建了钢锭与夹杂物之间的跨尺度计算系统。.MnS夹杂物对材料损伤行为影响与其应力状态有密切联系，较低的应力三轴度、较大的塑性应变下，MnS会加速孔洞的聚合。变形过程中MnS晶体状态对微孔洞产生机制有较大影响，单晶MnS多会产生脱粘，多晶MnS则易产生内部断裂。夹杂物的分布差异会影响材料的断裂形式，夹杂物含量高的区域会成为优先断裂区然后向其他夹杂物含量高的区域扩展并最终断裂。钢锭与缺陷尺寸相差较大时，体胞的边长不宜超过缺陷直径的15倍。基于体胞法的数值模拟与实验的结果对比可知，试件缺陷演化的结果与数值模拟结果吻合较好，亦表明体胞法能够较准确地模拟大型锻件缺陷的演化。.夹杂物的控制是大型锻件生产关注的重要问题之一，且MnS是易切削钢中重要的添加相，本项目对揭示MnS在加工过程中的演化以及对断裂行为影响有着促进作用，亦可为企业生产工艺制定提供理论依据。