铁素体系耐热钢焊接热影响区相变行为的CSLM原位观察

Phase Transformation Behaviors in the Heat-Affected Zones of Ferritic Heat-Resistant Steels Enabled by In Situ CSLM Observation

铁素体系耐热钢焊接时热影响区的相变行为会严重影响焊接件的室温组织,在后续严苛的条件下运行服役时会对电站安全稳定运行产生严重威胁。高温激光扫描共聚焦显微镜(CSLM)能够重现热影响区焊接热循环,实现对微观组织演变的原位观察以及相变动力学的量化分析,具有优化焊接工艺和促进耐热钢铁材料发展的潜力。本工作利用CSLM对P11、P22、P913种典型Cr-Mo铁素体耐热钢热影响区连续冷却过程中的相变行为进行了原位表征,研究板条组织的生长特征与相变动力学。结果表明,根据形核位置能量势垒的高低,贝氏体板条依次在原奥氏体晶界、夹杂物、晶粒内部畸变区域、先前的贝氏体板条、晶粒内部的自由表面上形核;同时,依附于原奥氏体晶界和先前贝氏体生长的贝氏体板条以及在原奥氏体晶粒内部呈放射状切变生长的马氏体板条均通过碰撞晶界或其他板条的方式停止生长,并形成交叉互锁的结构。随着温度的降低,粗晶热影响区中板条的生长速率明显高于细晶热影响区,这主要归因于过冷度的增加和原奥氏体晶粒尺寸的增大。

Fossil-fired thermal power generation has dominated China's electricity production for a long time,contributing to around 70% of the total capacity.Developing long-life ultra-supercritical thermal power units is essential for improving coal-fired power generation efficiency,reducing harmful gas emissions,and achieving national energy conservation and emission reduction targets.The assembly and manufacture of advanced heat-resistant steel grades are required to address the above demands,serving as crucial components driving the technological advancement of thermal power units.Heat-resistant steel grades P11,P22,and P91,which are Cr-Mo based ferritic,possess a range of highly attractive properties,such as excellent mechanical properties,excellent corrosion resistance,and relatively low construction costs.These steel grades are widely used in pressure vessels and pipelines focused on high-temperature applications.Fusion welding techniques are invariably necessary to weld such heatresistant-grade steels before they are positioned in high-temperature service.However,it is worth noting that drastic solid-state phase transformations in the heat-affected zones(HAZs)during thermal welding cycles can profoundly influence the heterogeneous microstructures of welded joints,determining their final mechanical properties to a large extent.Furthermore,it seriously threatens the safe and stable operation of thermal power plants.High-temperature confocal scanning laser microscopy(CSLM)revolutionized traditional metallographic experiments,enabling real-time morphology and quantitative analysis tracking.This innovation has facilitated investigations into the kinetic phase transformation process and microstructure evolution in steels at high temperatures.In this work,the kinetics of phase transformation and microstructural evolution in the HAZs of P11,P22,and P91 ferritic heat-resistant steels during continuous cooling processes were systematically investigated using CSLM.The results revealed that bainite laths preferentially nucleate in the order of increasing difficulty in the energy barrier on austenite grain boundaries,inclusions,internal grain distortion areas,previous bainite laths,and grain interiors.Meanwhile,the growth characteristics of bainite/martensite laths were documented as the phase transformation progressed.It is revealed that bainite laths attach to prior austenite grain boundaries and the previous bainite,while martensite laths grow radially inside the prior austenite grains.Both bainite and martensite laths cease growing when they encounter grain boundaries or other laths,eventually forming an interlocking microstructure.Additionally,the growth rates of bainite/martensite laths in the HAZs of P11,P22,and P91 ferritic heat-resistant steels exhibited considerable variations as the temperature decreased.The analysis revealed that as the temperature decreased,the growth rate of laths in the coarse-grained heat-affected zone was considerably higher than that in the fine-grained heat-affected zone,which can be attributed to the increase in the degree of supercooling and prior austenite grain size.