Cailiao gongcheng (Sep 2021)
Correlation of hardness with creep rupture strength, allowable stress and service/remaining life of Grade 91 heat-resistant steel
Abstract
The creep rupture strength data of Grade 91 steel specimens with various hardness values were used to study the correlation of hardness with creep rupture strengths and maximum allowable stresses of the steel. The results show that creep rupture tests at a certain temperature and stress applied to the specimens with a series of hardness values receive the results in both overestimation and underestimation of rupture properties, leading to some unrealistic effect on 105h creep rupture strengths determined in this way. An approach was thus proposed to determine the lower limits of hardness satisfying the maximum allowable stresses at given temperatures. It was found with this method that a hardness level of ≥ 201(205) HBW of any of the Type 1- and Type 2-Grade 91 components running at a temperature of ≤ 575(600)℃ can satisfy the requirement of the maximum allowable stresses at the corresponding temperature specified by ASME BPVC 2019, and a hardness value of ≥ 204HBW is effective for the grade 91 components with a wall-thickness of ≥ 75 mm running at a temperature of ≤ 575℃ to satisfy the requirement of the maximum allowable stresses at the corresponding temperature specified by ASME BPVC 2017. Therefore, the most recently modified specification brings, to some extent, about difficulty in continuously practicing the application of the lower limit of hardness values (190-250HBW) specified by ASME BPVC 2017-2019 because it is not satisfied with the requirement on the maximum allowable stress at some given temperatures. Thus, there is a need to raise the lower limit of hardness values to settle this issue in the future. In addition, the optimization of the function fitting the creep-rupture data currently used in the estimation calculation of service/remaining lives was studied, showing that the tendency of overestimation of rupture properties can be reduced by replacing the current power function with the logarithm one. Quite good fitness of the practical data with the logarithm function curves is contributed to separating the whole data group with a series of hardness values into the higher and lower hardness level groups in calculation. On this basis, the relationship of thickness, hardness and service life of the components with variable dimensions and hardness values can be obtained by integrating the technical parameters of the safety assessment, which is able to reflect the applicability, reliability and intuitiveness of this combination. The above results can be used as reference for both the revisions of the relevant technical standards and the practical applications of industry.
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