Development of Nondestructive Assessment Methodology for Residual Life of Martensitic Heat Resistant Steel

高铬铁素体耐热钢广泛应用于电厂厚截面构件的蠕变工况。本研究对回火马氏体板条结构的高铬钢进行了无损剩余寿命评估，结果总结如下：1。控制高铬铁素体耐热钢蠕变变形和断裂的因素有四个。它们是位错（板条）组织、晶界和亚晶界上的析出物、亚晶内的细颗粒以及W和mo等溶质原子。在这些因素中，板条组织是决定钢屈服应力的最重要障碍。其恢复与蠕变变形的进展和随后的破坏密切相关。因此，应将板条结构的这种恢复作为寿命评估的衡量标准。由于马氏体板条组织的恢复，板条宽度随蠕变应变的增大而增大。板条宽度的相对增加Δλ/Δλ^*与蠕变应变呈线性关系。Δλ为板条宽度与原始值的变化量，Δλ^*为破裂时的Δλ值。线性关系系数不随蠕变条件和合金成分的变化而变化。根据板条宽度与蠕变应变之间的线性关系，可以对某工程装置构件的蠕变应变进行评估。利用蠕变数据库可以很容易地预测任何蠕变条件下的蠕变曲线。剩余寿命可以通过估算的蠕变应变和预测的蠕变曲线来评估。基于板条宽度测量的剩余寿命评估方法适用于低温回火转子钢。然而，在高温回火的锅炉钢（如HCM12A）中，由于晶界空洞的聚并而发生脆性断裂。在这种情况下，应采用基于空腔生长的寿命评估。

High Cr ferritic heat resistant steel is widely applied to the thick section components of power plants to be used in creep regime. In this research, nondestructive residual life assessment of such high Cr steel with tempered martensitic lath structure was examined, and the results are summarized as follows :1.There are four factors that may control creep deformation and fracture of the high Cr ferritic heat resistant steels. They are dislocation(lath)structure, precipitates on grain- and sub-boundaries, fine particles within subgrains, and solute atoms such as W and Mo.Among these factors, the lath structure in the most important obstacle that determines yield stress of the steel. Its recovery is closely related to the progress of creep deformation and the subsequent failure. Therefore, this recovery of lath structure should be taken as the measure of life assessment.2.As a result of the recovery of martensitic lath structure, lath width increases with increasing creep strain. The relative increase in lath width Δλ/Δλ^* is linearly related to creep strain. Δλ is the change in lath width from the original value, and Δλ^* is the value of Δλ at rupture. The coefficient of linear relation does not change with creep condition or alloy composition.3.Based on the linear relation between lath width and creep strain, we can evaluate creep strain of a component of engineering plants. Creep curve under any creep condition can readily predict with the aid of creep database. Residual life can be assessed from the estimated creep strain and the creep curve predicated.4.The residual life assessment based on the measurement of lath width is applicable to rotor steel tempered at low temperature. However, in boiler steel(such as HCM12A)tempered at higher temperature, brittle fracture takes place by the coalescence of grain boundary cavities. Life assessment based on cavity growth should be employed in this case.