高铝铁素体耐热钢的强化机理与高温氧化行为

Ferrite heat-resistant steel becomes the preferred steel grades for supercritical and ultra-super critical turbine. Development of new steel grades is unified mainly around the adjustment of elements content and the addition of new alloy elements. High temperature oxidation resistance of traditional 9%~12% ferritic heat-resistant steel mainly depends on the Cr2O3 film. Excessive levels of Cr content will be detrimental to the creep properties. This study intends to add Al to ferritic heat-resistant steel. High temperature oxidation resistance can be improved due to the formation of composite oxide film. But excessive levels of Al content lead to formation of intermetallic compounds. This can lead to poor ductility of steel. We added Al and modifier to ferrite heat-resistant steel. Intermetallic compounds becomes micro scale and uniformly dispersed, and dispersed in the matrix as second phase particles. It is beneficial to the further improvement of high temperature creep resistance. This study focuses on number, shape, size and form of precipitated phases and intermetallic compounds during variable temperature process, clarifying its strengthening and toughening mechanism, and establishing its transformation kinetics model. The composition, morphology and the distribution of the layered structure of the composite oxide film were studied under actual operating conditions. Combined with high temperature steam oxidation kinetics, the high temperature oxidation mechanism of high aluminum ferritic heat resistant steel was studied. This study will provide theoretical support for the development of a new type of high aluminum ferritic heat-resistant steel.

铁素体耐热钢成为超（超）临界机组高温锅炉用钢的首选钢种，新钢种的开发主要围绕着新合金元素的加入和原有合金元素的调整来进行。传统铁素体耐热钢主要依靠Cr2O3膜提高抗高温氧化性能，但Cr含量过高会对蠕变性能不利。本课题拟向铁素体耐热钢中加入Al元素，Al2O3-CrO3复合氧化膜能提高合金的高温抗氧化能力。但Al含量较高时，会形成金属间化合物，导致钢的塑性变差。本课题中在钢中加入Al及改性添加剂，产生微观尺度并均匀散布的金属间化合物，使之作为增强第二相弥散在基体中，有利于高温蠕变性能的进一步提高。本课题对变温过程中的析出相、金属间化合物的数量、形状、大小及存在形态进行研究，阐明其强化机理，建立相应的相变动力学模型；并对其模拟工况下复合氧化膜的组成、形态等特征及层状结构分布进行研究，结合高温蒸汽氧化动力学，探明高铝铁素体耐热钢的高温氧化机制。为开发更优异性能的新型高铝铁素体耐热钢提供理论支持。

铁素体耐热钢越来越多地成为超（超）临界机组高温锅炉用钢，新型铁素体耐热钢种的开发主要围绕着合金元素的调整来进行。向铁素体耐热钢中加入Al、Ni元素，产生增强第二相弥散在基体中，进一步提高抗高温蠕变性能，所生成的复合氧化膜能提高合金高温抗氧化能力。对变温过程中的析出相的数量、形状、大小及存在形态进行研究，阐明其强化机理，建立相应的相变动力学模型；并对复合氧化膜的组成、形态等特征进行了研究，探明高铝铁素体耐热钢的高温氧化机制。研究结果表明：.1）利用Thermo-Calc热力学计算软件对试验钢的平衡析出进行模拟，高铝铁素体耐热钢中平衡析出相包括M23C6型碳化物、Laves相、Z相、σ相，Al、Ni含量变化对Laves相、M23C6型碳化物、Z相的相组成及组成元素含量影响相对较小，而σ相析出量则随Al含量增加有较大下降。.2）高铝铁素体耐热钢的强化效应主要是由于马氏体板条组织和沉淀物析出，沉淀析出物主要包括M23C6、Laves相、弥散分布的MX等。试验钢热轧板轧制的变形量越大，晶粒细化作用越强，强度呈现明显上升趋势，同时塑性下降。冷轧处理进一步增大变形量，析出大量第二相粒子引起局部位错强化。热处理析出碳化物在晶界使得位错扎钉作用，亚晶界阻碍位错运动，强化了增强效果。试验钢中Al含量的增加降低了热压缩下的流变应力，降低了钢的加工难度。获得了实验钢高温变形本构方程，峰值应力的实验数据与计算数据具有较好的吻合性，表明该本构方程具有较高的准确性。.3）高温氧化实验中试验钢氧化动力学曲线拟合后遵循抛物线规律。氧化层主要组成物包括Fe2O3、Fe3O4、Al2O3、(Fe,Cr)2O3。Al含量越高，Al-Ni比越大，试验钢表现出更强的抗氧化性。高温氧化过程中试验钢基体与氧化层中间部位有脱碳现象，成为氧化层向基体扩展的萌芽点。Al元素向基体表面方向扩散，优先发生选择性氧化，氧化孔洞随Al含量升高而降低，利于形成连续的氧化膜，提高氧化层的致密度。