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蠕变工况下的厚壁构件。本文对回火马氏体板条组织的高Cr钢进行了非破坏性的剩余寿命评估，结果如下：1.控制高Cr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）结构、晶界和亚晶界上的沉淀物、亚晶粒内的细颗粒以及W和Mo等溶质原子。在这些因素中，板条结构是决定钢的屈服应力的最重要障碍。它的恢复与蠕变变形的进展和随后的破坏密切相关。因此，板条组织的回复应作为寿命评估的指标。2.马氏体板条组织回复的结果是板条宽度随蠕变应变的增加而增加。板条宽度的相对增加Δλ/Δλ^* 与蠕变应变线性相关。Δλ是板条宽度相对于原始值的变化，Δλ^* 是断裂时的Δλ值。线性关系系数不随蠕变条件或合金成分的变化而变化。3.根据板条宽度与蠕变应变的线性关系，可以估算工程机械构件的蠕变应变。借助蠕变数据库，可以很容易地预测任意蠕变条件下的蠕变曲线。通过估算蠕变应变和预测蠕变曲线，可以评估转子钢的剩余寿命。4.基于板条宽度测量的转子钢剩余寿命评估方法适用于低温回火转子钢。而锅炉钢（如HCM 12 A）在高温回火时，晶界空洞的合并会导致脆性断裂。在这种情况下，应采用基于空穴增长的寿命评估。

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.