Accelerating Effects of Cyclic Creep Due to the Alternative Load Compared with Constant Load for CD 304L

Abstract

In this paper, by using a series of uniaxial tests, accelerating effects of cyclic creep due to the alternative load are compared with constant load creep behavior of CD 304L. Austenitic stainless steel 304L is used mostly in power generation and petrochemical industries due to its high temperature creep-fatigue resistance. Test samples have been obtained from cold drawn bars and the material conforms to ASTM A276-05A specifications. Uniaxial tests have been carried out at temperatures of 687, 717, and 737°C under constant and alternating loads. The effects of alternating load and the hold time on the mechanical behavior of the material and fatigue or creep damage values have been studied. The results show the strong interaction between these two damage mechanisms. It has been observed that due to the high temperature level, creep damage is dominant even in tests with very short hold times. Also, it has been shown that by increasing the hold time, the creep strain rate increases and consequently, the creep lifetime reduces. Most importantly, the results highlight the major effect of the alternating loads in increasing creep strain rate and reducing the creep lifetime compared with the constant load tests with the same average stress. In other words, the creep lifetime under alternating load is much shorter than the creep lifetime at constant load test carried out with the average load value.Introduction. Many engineering parts, which find applications in power generation and petrochemical plants, operate at high temperature under mechanical loads. In practice, level of operating temperature or the mechanical load may vary with time. Therefore, different mechanisms of failure can occur, which depend on the nature and history of termomechanical loadings. The important failure mechanisms from this point of view are as follows:(i) fatigue, which is usually faced when the engineering part operates at temperatures below 0.5T melt (T melt is melting temperature in Kelvin) and at alternating load with low stresses. Engineering components are often subjected to fatigue loading under stress-controlled conditions. The existing models describe the fatigue of engineering components including the Goodman equation [1,2], the modified Goodman equations [1, 3], the Smith-WatsonTopper parameter [4], the Walker parameter [5], and Dowling equation [6];(ii) creep, which occurs at high temperatures (above 0.5T melt ) under mechanical loads. Creep implies a time-dependant plasticity: after a sufficient elapse of time, either the viscoplastic strain becomes so large that the original shape of the structure is altered, or the creep rupture occurs. Creep is critical in a number of applications, for example, components used in power-generating systems and chemical plants, where the service temperature is high. Different models have been proposed, which explain creep behavior of materials [7];