Research And Application Of Relationship Between High- Temperature Stress Relaxation And Creep Of Heat Resistant Steel

The widely application and the development of higher temperature and higher pressure of high-temperature high-pressure apparatus in the modern industry promote to explore new acceptable materials in a relative short period, and attract more attentions of the service security of these materials. Creep strength, taking as a major indicator for the service security of a material, is predicted conventionally by the creep rupture test (CRT), which is considered to be with shortages of long period, large demand of material, high consumption of energy et al. These defects restrict the rapid cognition of creep rupture property of a material, and then reduce the speed of developing a new material. As a result, a rapid and accurate method for the creep property prediction is needed urgently. As a quick method for obtaining creep information, the stress relaxation test (SRT) can be used to predict the creep rupture property. Then, based on various heat resistant materials with typical FCC or BCC crystal structure, the current paper discussed the relationship between the creep rate during SRT (SRT's creep rate,?SRT) and the minimum creep rate of CRT (?min) and its influence factors, analyzed the creep mechanism, and proposed a method of how to predict creep rupture property with high precision by using SRT's data.It was found that there was a complex relationship in magnitude between the value of ?RT and?min, as it could be affected by various factors such as material, stress (?), temperature (T), et al. The effects of material was mainly due to the difference in microstructure between the steady state creep of CRT and the stable creep of SRT, but not due to the crystal structure. The temperature of T/Tm?0.60 and the stress of ?/?0.2?0.40 was the boundary condition to lead to the phenomenon of stable state?SRT equaling to ?min, and the relationship between stable state ?SRT and?min in a condition out of the range of T/Tm?0.60 and ?/?0.2 0.40 will be complicated. At the same time, the research also established a model to calculate ?in with stable state ?SRT.Through the analysis of stress exponent, apparent activation energy and apparent activation volume during the two kinds of creep, it was considered that the equivalence between stable state??SRT and ?min was not related to the power law break creep, but related to the power law creep in small stress exponent in a mechanism as dislocation climb, viscous slip, diffusion, et al. The increasing of apparent activation volume with the decreasing of applied stress during the stable state creep of SRT implied the stable state creep of SRT under low stress would rely much on the motion thermal activated. In addition, the parameter Tm is absolute melting point, and ?0.2 is the 0.2% proof stress.For the prediction of creep rupture property, the conventional method of based on the various kinds data of CRT (time to rupture and minimum creep rate) had similar predictions, but the results obtained by using CRT's data at high stress were not matched well with the real creep data. It implied that the conventional method to predict creep rupture property with high precision needed some long-time CRTs. Method of using the creep data of SRT, combining with some short time CRTs, could make a more accurate prediction than that obtained from conventional method with data of CRTs at high stress region only.