高强贝氏体钢HAZ的相变晶体学、M-A组元演变规律及其对韧性的影响机理

Because high strength bainitic steels have an excellent balance of high strength, good toughness and weldability, they are expected to become the main structural materials for the next generation in the field of engineering machinery, pipeline, offshore platform, and so on. However, their original microstructures and mechanical properties are disturbed by welding thermal cycle during the structural construction and the occurrence of deterioration of heat affected zone (HAZ) toughness becomes an important obstacle for their application of actual project. It is known that effective grain coarsening and the formation of M-A constituent that derive from phase transformation behavior during welding thermal cycle are two main factors to deteriorate HAZ toughness. Therefore, in this research the first focus is put on bainite transformation behavior. The effects of prior austenite grain size and transformation temperature on variant grouping mode of bainite are studied within the scale of prior austenite grains, the purpose of which is to reveal the coarsening nature of effective grain size in the crystallography. The morphology, micro- compositions and structure of M-A constituents are analyzed after subjected to different welding thermal cycles. In combination with the bainite crystallography, the relationship between the distribution of bainitic variants and the evolution of M-A constituents can be concluded. The nanoindentation technology is employed to characterize the micro-mechanical properties of M-A constituents. Combined with the strain distribution at micro-regions under the applied stress condition, the micro-mechanisms of M-A constituents stimulating the initiation of cleavage cracks can be discovered intuitively. Several comparable welding microstructures are expected to be designed to study their cleavage fracture behaviors. The effects of effective grain size and M-A constituents on the HAZ toughness are studied, respectively. Based on the classical Griffith theory, the main factor to control the HAZ low temperature toughness can be quantitatively resolved for different microstructures. Meanwhile, the potential mechanism about grain boundary characteristics hindering the propagation of second cleavage crack can be also revealed by serial sectioning method. These studies can offer some scientific theories for high strength bainitic steels to optimize HAZ toughness and to design acceptable welding system.

高强贝氏体钢焊接热影响区（HAZ）低温韧性的恶化是阻碍其工程应用的重要原因，而HAZ韧性的恶化又主要归因于有效晶粒粗化与M-A组元的形成。对此，本项目从热循环下的贝氏体相变行为入手，以原始奥氏体晶粒为尺度单元，研究原始晶粒尺寸和相变温度对贝氏体变体分组模式的影响，阐明有效晶粒尺寸粗化的晶体学本质。分析不同工艺下M-A组元的形态学、微区成分以及微结构特征，获得贝氏体变体分布与M-A组元演变规律的关系。采用纳米压痕技术表征M-A组元的微观力学性能，结合外应力作用下的微区应变分布规律，揭示M-A组元促进解理裂纹萌生的微观机理。巧妙设计多组对比性焊接组织，分别研究有效晶界与M-A组元恶化HAZ韧性的规律，基于经典的Griffith理论，定量解析不同焊接组织中控制解理断裂行为的主因，并系统阐述晶界特性阻碍二次解理裂纹扩展的作用机制。为优化高强钢HAZ韧性、设计合理焊接体系提供科学的理论依据。

焊接热影响区（HAZ）低温韧性的恶化是阻碍高强贝氏体钢工程应用的重要原因。通常，有效晶粒尺寸的粗化和硬脆相M-A组元的形成被认为是恶化HAZ韧性的主因。为了从根源上揭示这两种因素的形成原因及其相互关系，以便于协同调控HAZ韧性，本项目从焊接热循环下的相变行为出发，首先考虑焊接峰值温度的影响，获得CGHAZ和FGHAZ的焊接CCT相图，揭示了原始奥氏体晶粒尺寸对相变过程中碳扩散的影响，这决定了不同亚区M-A组元的形态、尺寸和形成位置；针对部分贝氏体相变组织进行晶体学分析，表明随相变温度的降低，贝氏体产物变体分组模式由Bain区分组模式到密排面分组模式转变，因此在贝氏体相变高温区生成的变体组分具有较为粗大的有效晶界尺寸；在不同尺寸的原始奥氏体晶粒下，FGHAZ晶粒由于母相空间的限制更容易出现优势变体选择现象，但相比于CGHAZ晶粒仍具有相对较小的有效晶粒尺寸，因此细化原始奥氏体晶粒仍是优化HAZ有效晶粒尺寸的首选途径。基于不完全贝氏体相变的组织特征，采用纳米压痕技术获得了M-A组元的纳米力学响应，基于Hertzian弹性理论解析载荷-位移曲线，M-A组元的弹性到弹塑性变形转变切应力是基体组织的2倍以上；当发生塑性变形后，M-A组元的塑性区大小是基体组织的2/5倍。因此，外应力作用下，M-A组元周围会出现应力集中现象而导致界面开裂行为。对比研究了不同焊接组织的冲击韧性，采用四种不同估算方法计算了其动态断裂韧性，ICCGHAZ的动态断裂韧性明显降低，结合断口形貌特征参数（如延性伸长区尺寸等），验证了以总体屈服载荷和峰值载荷的平均应力值为起裂应力值时所计算的结果较为合理；结合经典Griffith公式计算了M-A组元恶化ICCGHAZ韧性的临界尺寸（3.2μm）。基于对二次解理裂纹形貌和晶体学特征的分析，获得Σ3（如{111}59.5°）孪晶取向晶界具有有效抑制裂纹扩展的晶体学特征，对于粗大原始奥氏体晶粒来说，在同一Bain区变体的不同位置同时出现多条二次解理裂纹，微裂纹的扩展与合并造成试样的瞬断现象。此外，基于对焊接相变规律的理解，采用单道次平板焊接方法制备实际HAZ显微组织，结合断口形貌特征，分析了不同焊接工艺参数对显微组织和冲击韧性的影响，并获得了实验钢的合理焊接工艺参数范围；本研究结果为解决高强贝氏体钢焊接HAZ韧性的恶化问题提供了理论依据。