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采用光学显微镜、扫描电镜和能谱分析研究了含W型10%Cr(质量分数)超超临界钢中δ--铁素体的组织形貌与微观结构. 结果表明, δ--铁素体的生成机理与奥氏体化加热温度密切相关. 当加热温度较低时, 极少量δ--铁素体在原奥氏体晶粒内部优先形核生长, 呈针状分布; 当加热温度较高时,δ--铁素体在原奥氏体晶界处形核, 快速生长, 呈多边形分布, 转变时发生溶质组元再分配. 两种形态的铁素体均明显降低了含W型10%Cr超超临界钢的冲击韧性, 但是, 针状δ--铁素体降低冲击韧性的幅度远大于多边形δ--铁素体;即使针状δ--铁素体的体积分数极少, 也会对钢的力学性能造成极大影响.

The tungsten–alloyed 10%Cr (mass fraction) steel, one of the advanced 9%—12%Cr steels, has been widely considered as a preferred candidate for making key components in ultra–supercritical (USC) steam turbines. Due to large amounts of ferrite former in the steel, the formation temperature of δ–ferrite is lowered down, and therefore δ–ferrite is apt to be produced during hot working. However, understanding of the formation mechanism of δ–ferrite and its influence on the mechanic properties of ultra–supercritical steels is still either ambiguous or conflicting. To clarify this problem, in this paper, the microstructure and morphology of δ–ferrite were investigated by optical microscope, scanning electron microscope and energy dispersive spectrum (EDS) analysis. Also, the mechanical properties including the tensile strength, ductility and impact toughness of the studied steel with various volume fraction of δ–ferrite were tested at room temperature. Experimental results indicate that the transformation mechanism of δ–ferrite is closely dependent on the austenitizing temperature. Extremely small amounts of acicular δ–ferrite preferentially nucleate and grow inside the prior austenite grains, if the austenitizing temperature is just a little higher than the equilibrium transformation point of δ–ferrite. While, as the austenitizing temperature increases further, some polygonal δ–ferrites subsequently form on prior grain boundaries and grow quickly. Meanwhile, the repartitioning of solute elements occurrs between δ–ferrite and prior austenite. Both acicular and polygonal δ–ferrites will damage the impact toughness of the studied steel. And in spite of its few amounts, the detrimental effect of acicular δ–ferrite on the mechanical properties, especially the impact toughness, is more severe than that of polygonal δ–ferrite. Additionally, the tensile strength and the area reduction of the studied steel decrease as the amount of δ–ferrite increases, while the elongation hardly changes with the amount of δ–ferrite increasing. As a conclusion, accurately controlling the austenitizing temperature to prevent from the formation of any δ–ferrite is not only necessary but also very important in obtaining perfect overall mechanical properties.