钴合金化与铜合金化改善含W型马氏体耐热钢焊缝强韧性的研究

Tungsten-bearing martensitic heat resistant steels, most notably P92(9Cr-0.5Mo-1.8WVNbN), is recognized as the key material for the development and the application of USC coal-fired power plants. However, its weld metal is prone to the formation of δ-ferrite resulting weak Charpy toughness. Currently, Ni is added to suppress the formation of δ-ferrite, but, this approach is still not very satisfactory due to lowing creep rupture strength, lowing drastically Ac1 temperature, and leading to the brittleness of Laves precipitation during ageing.The project aims to explore the improvement in toughness and creep strength using Co and Cu as a substitute of Ni. The main works to be carried out include as follows: the effect of Co and Cu on δ-ferrite retention and Ac1 temperature;the effect of Co addition on the precipitation and coarsening of Laves phase and the depletion of W in matrix and its influence on the creep properties (evaluated by mesoscopic continuum damage mechanics, MCDM); test and verification of the Cu addition conception, the strengthening mechanism of nano-particle ε-Cu (as well as the determination of the critical transition size in the martensitic weld metals) and ε-Cu's effect on the precipitation of Laves phase, in order to enhance the toughness and creep strength simultaneously. This research work will utilize the 3DAP and HAADS-STEM. The new design of weld alloy is expected to achieve the excellent toughness and creep properties. This research will contributes to the alloying theory and practical knowledge to the advanced heat resistant steels, it will also provide the foundation for the development of series consumables and their technical standards.

以P92（9Cr-0.5Mo-1.8WVNbN）为代表的加W型马氏体耐热钢是制造超超临界火电机组的关键材料，其焊缝容易产生δ-铁素体，损害韧性。目前一般采取加入奥氏体元素镍抑制铁素体的形成，但存在损失热强性、Laves相时效析出脆化及降低Ac1点等弊端。本项目探讨以奥氏体元素钴或低成本的铜替代镍改善焊缝的强韧性，内容包括：钴和铜对焊缝Ac1和铁素体的影响；钴合金化对Laves析出粗化行为的影响，及引起的基体W贫化对热强性影响的细观损伤力学评价；提出铜合金化设想并加以验证，应用3DAP和HAADF-STEM技术分析纳米尺寸ε-Cu在马氏体焊缝中的析出强化作用，确定其临界尺寸，揭示和调控ε-Cu的诱导Laves析出效应，以改变其分布和特征参数，实现韧性和热强性的同步改善。此研究将为P92焊缝的增强增韧提供新途径，丰富和发展高等级耐热钢的焊缝合金化理论，为系列焊材的开发和技术标准的制定提供依据。

以P92钢为代表的含钨型马氏体钢是制造超超临界火电机组的关键材料，其焊缝容易产生有害的δ-Fe相。目前一般采取加镍抑制δ-Fe的形成，然而它存在损失热强性、加速M23C6析出和降低Ac1点等弊端。本项目以钴和低成本的铜替代镍，抑制δ-Fe以改善P92焊缝的强韧性，主要内容包括：钴和铜对AC1点的影响；钴对Laves相析出及强韧性的影响；铜强韧化的效果及作用机理；钴合金化与铜合金化焊缝的性能评价等。.采取理论计算和热膨胀法得到了钴和铜对焊缝AC1点和δ-Fe的影响规律，结果表明，钴对AC1点的影响很小，铜降低AC1点的效应小于镍，并且当铜含量超过0.7% 后，铜对AC1点的影响很小。钴和铜抑制δ-Fe的能力相当，在焊缝中加入0.8%以上的钴或铜，可避免δ-Fe的形成，保证焊缝在热处理后的冲击韧性高于56J。.通过高温时效实验揭示了钴对P92焊缝Laves相析出行为的影响，基于JMA模型建立了其析出动力学方程。证明了含钴焊缝的Laves相在晶界非均匀形核。钴促进Laves相的析出和长大，含钴焊缝在650℃/10000h时效后的Laves相面积分数较不含钴的焊缝增加约20%，且大于0.4μm的大尺寸颗粒比例明显增大，因此加剧了焊缝的时效脆化。钴降低焊缝的蠕变强度，原因在于它促进了Laves相的长大，进而加速蠕变空洞在晶界的形核和生长。.在焊后热处理和蠕变过程中，P92焊缝中的铜以ε-Cu在晶界和板条界析出，随着铜含量的增加，ε-Cu的面积分数和平均尺寸明显增大。在含0.86%Cu焊缝中，ε-Cu的稳定性较好，在650℃/100MPa蠕变4900h时的面积分数约1.0%，平均尺寸约0.25μm，改善了蠕变性能，其在650℃/100MPa的持久寿命较含钴焊缝延长17%。ε-Cu诱导Laves相的析出，促进Laves的形核和长大，因此加剧了高铜（1.69%）焊缝的时效脆化，并且促进孔洞形成而降低蠕变强度。.根据钴和铜对焊缝性能的综合影响，得到了P92焊缝具有最佳高温强韧性的钴和铜含量范围为0.8-1.0%。.研究成果深刻揭示了钴和铜对P92焊缝析出相行为及高温长期性能的影响，为焊缝的强韧化提供了新的思路和途径，丰富和发展了高等级耐热钢焊缝的合金化理论，为系列焊材的开发和技术标准的制定提供了依据。.