含B、Co的9-12%Cr马氏体热强钢焊接中的切变型奥氏体化机理研究

9-12%Cr martensite creep-resistant steel has been widely used in thermal power plants. By adding B element, type IV crack in welding heat affected zone could be suppressed effectively. Further studies showed that the shear type austenitizing would occur during heating in welding with B and Co martensite heat resistant steel, which could be used to explain new phenomena of fine crystallization in the overheated zone and martensite lath recovered totally in the normalized zone. The mechanical properties test showed that the heat affected zone with shear austenitizing has both good toughness and excellent high temperature endurance strength. At present, it is accepted that B element could reduce internal energy within grain boundary, lath boundary and precipitates phase boundary, which could inhibit long-range diffusion of solute elements and is a major cause of shear austenitizing. The relevant mechanism is still controversial. In the presented application, mechanism of the shear austenitizing from the B and Co elements, including diffusion rate, phase transition speed and austenite stacking fault energy in high temperature, is going to be studied to ascertain necessary conditions for shear austenitizing. Besides, "broken austenite" would be used to assess the degree of shear austenitizing. The related research can enrich the theory of non-equilibrium shear austenitizing, can provide reference for composition design of high grade martensite heat resistant steel, can provide reference for forging, welding and heat treatment processes, can estimate metallurgical structure evolution in the process of service. This application has scientific and engineering significance.

9-12%Cr马氏体热强钢在火电构件中被广泛应用，通过添加B，可抑制服役中热影响区产生的Ⅳ型裂纹。进一步研究发现，含B、Co的马氏体热强钢焊接加热中发生切变型奥氏体化，出现过热区细晶化、正火区马氏体板条与焊接前原始板条几乎完全对应的新现象；力学性能测试表明发生切变型奥氏体化的热影响区同时具有优良的韧性与高温持久强度。目前认为元素B具有降低高温下晶界、板条界、相界等微观缺陷的内能而抑制溶质元素长程扩散是导致切变型奥氏体化的主要原因，但其机理尚存争议。本申请拟从B、Co元素对溶质扩散速率、相界推进速度及对高温奥氏体层错能的影响方面，探明产生切变型奥氏体化的冶金及热-力条件，并提出利用奥氏体“碎化”程度表征切变型奥氏体的组织热稳定性。相关研究可为高等级马氏体热强钢成份设计、控温控轧工艺研究、焊接及热处理工艺确定、服役中的组织演变规律预测提供借鉴与参考，具有科学与工程意义。

为满足清洁高效火电能源装备的需求，发展兼具优良成形制造和高使用性能的9–12% Cr马氏体热强钢已成为电站材料研发领域的重点内容。近年来出现的含硼马氏体钢因较高的蠕变强度及其焊接接头服役过程中不产生IV型裂纹极大提高了电站构件的服役安全性。但极低的硼含量且硼易与氮化合形成脆性相给钢材的制造带来了困难，同时焊后热影响的脆化问题也给接头服役安全性带来了威胁。本项目基于硼-钴协同作用的合金化思路开发了一批新型马氏体热强钢，并提出一套完整的铸造、锻造、热处理工艺路线。以控制非平衡加热条件下的奥氏体化过程为出发点，通过调整硼、钴复合比强化了焊接过程中热影响区的奥氏体记忆效应，并结合优化的焊后热处理工艺调控微观组织达到了增韧热影响区的目的。项目研究目标是丰富非平衡冶金学理论，并为国产高等级热强钢成分设计、制造工艺研究、焊接及焊后热处理工艺确定、接头制造可靠性保证提供借鉴与参考。