晶界和晶内协同强化抑制新型低合金耐热钢再热裂纹的合金化调控研究

The new grades T23 and T24 have been regarded as the ideal materials for supercritical boilers due to their excellent creep properties and low cost. However, the high susceptibility of reheat cracking in the coarse grained heat-affected zone (CGHAZ) has hindered their applications. Literature review has concluded that the reheat cracking is of intergranular and this project explore a novel alloy design idea to slow down or retard the occurrence of intergranular cracking. This project presents a new concept of suppressing reheat cracking by coordinating strengthening within grains and on grain boundaries simultaniously, in order to avert the current difficulty, e.g. the welding processing measures, such as lowering heat input, increasing preheat temperature and adding tempering weld, could not effectively prevent reheat cracking. With the optimization of carbon content and the addition of a small amount of boron, the well-distributed and stable second phase along grain boundaries will be obtained in the CGHAZ, which can strengthen the grain boundaries but not lose the dispersion strengthening within the grain. Therefore, the strength difference between the interior and boundaries of grain can be reduced, and the reheat cracking showed intergranular creep fracture is retarded. The main works to be carried out include as follows: the precipitation and coarsening of M23C6 carbides along grain boundaries in the CGHAZ, the effect of distribution of M23C6 carbides on the strength of grain boundaries, the evolution of depletion of W along prior austenite grain boundary, the relationship for the size of M23C6 carbides and the creep voids, effect of carbon content on the volume fraction of MX and M23C6 precipitates and its optimizing design, refinment of the size of M23C6 carbides and redcution of the denuded zone size by the addition of boron. This research will contribute to the welding metallurgical theory of the low alloy heat resistant steels, and achieve a new alloy design for the advanced heat resistant steel with both high creep strength and low susceptibility to reheat cracking.

性价比高的T23/24等新型低合金耐热钢是制造超超临界锅炉的理想材料，然而严重的再热裂纹倾向阻碍了其应用。本项目提出晶界和晶内协同强化抑制该类钢再热裂纹的设想，拟通过优化碳含量和添加活性元素硼使粗晶区得到均匀、细小和稳定的晶界第二相分布，在不损失晶内强化的前提下增强晶界，减少晶界和晶内强度的差异，遏制沿晶蠕变断裂，避免再热裂纹的发生，改变目前限制热输入、提高预热温度和回火焊等苛刻工艺难以防止裂纹的困顿局面。研究内容包括：粗晶区晶界M23C6相的析出和粗化特性；M23C6相的分布对晶界强化的影响；晶界附近合金元素贫化层的演化规律；M23C6相粗化对蠕变孔洞的影响；碳对晶界M23C6相和晶内MX相体积分数的影响及含量的优化设计；硼的细化M23C6相效应及减轻晶界附近合金元素贫化作用等。本项目研究将丰富和深化对低合金耐热钢的焊接冶金学认识，得到一种再热裂纹敏感性小的高等级低合金耐热钢合金设计。

以T23为代表的性价比高的新型低合金耐热钢是建造超超临界电站锅炉的理想材料，然而严重的再热裂纹倾向阻碍了其应用。本项目提出晶界和晶内协同强化抑制其再热裂纹的思想，通过优化碳含量和添加活性元素硼的合金化调控，使粗晶区得到分散和稳定的晶界第二相分布，减少晶界和晶内强度的差异，遏制再热裂纹的发生。主要内容包括：粗晶区M23C6相析出特性及对再热裂纹的影响；硼对M23C6相演化行为及对再热裂纹的影响；碳、硼含量的优化设计，改进型T23钢的性能评价等。.研究发现粗晶区在再热过程中，晶内在较短时间（24h）内析出M3C、M7C3和少量M23C6，晶界析出大量M23C6，而MX相在时效较长时间（24~48 h）后才在晶内大量析出。晶界析出M23C6是导致再热裂纹的主要原因，它一方面促进孔洞形核，另一方面导致晶界附近Cr、W和Mo等合金元素的贫化，进一步弱化晶界。在拉伸应力的作用下，应变集中于弱化的晶界，促进孔洞的产生，孔洞沿晶界长大聚集成孔洞链，形成微裂纹，最终造成晶间断裂。.发现硼元素在焊后以非平衡偏聚方式偏聚于粗晶区晶界，它既可起到直接强化晶界的作用，还能抑制再热过程中晶界M23C6的析出长大，使得晶界强度得以保持，缩小晶内与晶界的强度差，使整体塑性得到提升，从而降低再热裂纹敏感性。.发现碳元素和硼元素对T23钢再热裂纹敏感性的影响最为显著，得到了碳和硼含量的配比关系，并提出了T23钢的再热裂纹敏感性判据。提出了抗再热裂纹T23钢的合金设计，综合性能评价结果表明，改良型T23钢不仅有良好的再热裂纹抗性，还有优异的高温持久蠕变性能。.研究成果深刻揭示了新型低合金耐热钢再热裂纹形成机理及影响因素，为解决它们应用中的瓶颈问题提出了有效的解决措施。研究成果丰富和发展了低合金耐热钢焊接冶金及焊接性理论，对于高参数火电机组用高等级材料的开发具有重要的理论意义和实用价值。