Thermodynamical Evaluation of Creep Rupture Data and the Extrapolation of Long-Time Rupture Strength for Heat-Resisting Steels

Abstract

It is not possible yet at present to relate completely the mechanical properties of materials with the knowledges of physical process. As the deformation process of creep begins to be explained by the dislocation theory and the electronmicroscopic observation, it is expected that the estimation of structural changes in creep and the extrapolation of long-time rupture strength will possibly be made by clarifying the relations between the mechanical factors, such as stress, rupture time, elongation in creep rupture test, and physical factors.
In this paper some information obtained regarding structural changes with regard to creep rupture strength are described, applying Siegfried's method to creep rupture data, which modified the method of O.D. Sherby and J.E. Dorn by introducing temperature dependence for the activation energy. That is, from the curves of stress/log Z(=H/2.3RT-log t_b) for ferritic steels containing 0.5% Mo, 1% Cr 0.5% Mo, 1% Cr 1% Mo 0.25% V, 1.25% Cr 0.5% Mo 0.25% V, 1.25% Cr 0.5% Mo 0.75% Si, 2.25% Cr 1% Mo, 5% Cr 0.5% Mo and 9% Cr, 1% Mo it was shown that a tempering which tends to spheroidize carbides was undesirable to the creep rupture strength, and that the addition of silicon will have an injurious effect on the creep rupture strength while vanadium is available at higher temperature. And 10⁵hr rupture strengths for the above-mentioned steels were estimated from the rupture data within 10⁴hr through the application of this method to the extrapolation of long-time rupture strength. The obtained values agreed approximately with the extrapolated values on ASTM report.
On applying this method to these ferritic steels, the values ΔH which are determined by W. Siegfried were used. In an attempt to evaluate the creep rupture data more specifically, the relationships between a value of ΔH and metallurgical factors-chemical composition, structure and grain size-for various materials must be explained precisely. For ferritic steels, it is considered that a mass effect will practically become an important problem in future.
In the case of applying this method to 18-8 Ti steel and 18-8 Cb steel, it was shown that the break points on the curve of log Z/stress corresponded to the rupture with certain changes of creep deformation process which is caused by the precipitation of carbides and σ phase. And there were some cases that a simple curve of log Z/stress was not fit to be drawn. It was difficult, therefore, to extrapolate the long-time rupture strength. The creep deformation process can be explained as a complex process which consists of various phenomena, i.e. dislocation movement, diffusion of vacancy, viscous flow at grain boundary and grain boundary migration. It is necessary to make some corrections in the values of ΔS and ΔH in the thermodynamical function based on the phenomenon which is found as most prominent. On the extrapolation of long-time rupture strength, it is necessary that further investigations must be carried out to promote the accuracy of the correction by the observation of structural and physical changes in the materials.