21/4Cr-1Mo鋼管材の高温クリープ特性について

Abstract

It is a well-known fact that ferritic steels are practically more useful than austenitic steels as tube materials, and it requires on the part of the users in general to raise as much as possible the creep resisting temperature of ferritic steels.However, 21/4 Cr-1Mo steel has been so far the highest class of materials of ferritic steel in practical use, obtainable in market and used most frequently. On the other hand, still higher class materials, such as 7Cr-1Mo or 9Cr-1Mo steel, have been under researches and have been developed but they have not yet attained the stage for practical use.In the super heater tubes for boiler with a steam temperature of 570°C, austenitic steels are used in general, because, when the metal temperatures of the tubes exceed 600°C, the strength of Cr-Mo steels should become insufficient. However, with the advancement of the development of ferritic steels, there is the possibility of either 7Cr-1Mo or 9Cr-1Mo steel coming into practical use up to the metal temperature of 570-620°C.In the case of the temperature exceeding 600°C, 18-8 Ti steel used for supperheater tubes in America but some cases of accidents have been reported on account of the expansion and destruction of tubes while in use.Causes of these accidents are due to such special properties of austenitic steels as difficulty to control grain growth and embrittlement of grain boundaries by precipitation of carbides. Those events could be prevented if there appear ferritic steels which can be used in practise in the temperature range. of 570-620°C.In that sense it is desirable that ferritic steels with higher heat resistance be developed, and as the first step of such development, it is necessary to ascertain the maximum heat resisting temperatures of 21/4Cr-1Mo steel tubes, which are always determined according to the ASME or JIS Boiler Construction Code. However, these code values are estimated from the creep data of comparatively short-time and we understand that so far there have been no researches for determining the adaptability of code values to the steels on sale from the results of long-time creep data.In the present studies, 105hr 1% plastic strain stress and 105hr creep rupture stress are estimated on the basis of 1-3×104hr long-time creep data for 21/4Cr-1Mo steel, and from these results maximum allowable stress is determined and compared with code values.The obtained results are given below:1) Descending of creep strength in high temperature is much more rapid than that of code values.2) At the temperature lower than about 570°C, creep strength is always over code values but is lower at the temperatures higher than that, and in the latter case it is necessary to select the maximum allowable stress lower than code values.3) It is considered to be reasonable practically that heat resisting temperature of 21/4Cr-1Mo steel is 570°C.It is also well-known that creep data are accumulated as Master curve by means of the method of Larson & Miller based upon the rate process theory, and the author advocates the possibility of the structural changes occurring during the holding of high temperature to be described on this Master curve by using the same parameter.As an instance, the case of 21/4Cr-1Mo steel was taken up exmple. From the isothermal diagram obtained by annealing the steel within the temperature range of 400-750°Cfor 1000hr, the author sought the structural change diagram by using P=T(23+logt). And by imposing in upon the Master curve based upon the creep data of 1-3×104hr, the tendency of strength changes can almost exactly be explained in relation to structural changes.The author named this creep strength-structural change diagram the“Double Diagram”.