Austenitic stainless steels are of interest as structural materials for elevated temperature applications in power generation and chemical industries. For these applications, components of a structure are subjected to complex -stress-loading cycles at high temperature. Thus, understanding of the damaging process of high temperature creep-fatigue failure is of great importance to the optimum design of the components for improvement of reliability. It has been well established that grain boundary cavitation is the most serious damage mechanism of austenitic stainless steels under creep-fatigue interaction conditions. The carbides at the grain boundaries provide preferential sites for cavity nucleation owing to stress concentration during the fatigue cycle. Hence, these carbide characteristics, such as distribution, density and their morphologies etc., should be considered as important factors to determine creep-fatigue property. Moreover, earlier investigations have shown that carbide precipitation behavior and cavity formation are very sensitive to grain boundary characteristics.
In this study, systematic investigations of grain boundary carbide precipitation behavior and subsequent cavity formation with the consideration of grain boundary characteristics have been made, using AISI 304 and 316 stainless steels. Furthermore, on the basis of the relationship between the carbide character and cavity formation, the modification of carbide characteristics through the special heat treatment has been proposed and investigated to improve creep-fatigue resistance.
In 304 steel, a flat grain boundary is maintained throughout the aging treatment and all the carbides formed on the flat grain boundary are observed to have same triangular morphology. On the other hand, the grain boundary serration in 316 steel occurs at the early stage of aging treatment prior to the M23C6 carbide precipitation. The carbides formed at the serrated grain boundary are observed to have planar in shape and low density. The different morphology and distribution of carbides between 304 and 316 steels can be basically ascribed to the occurrence of grain boundary serration of 316 steel.
In 304 steel, it is observed that an increase in the misorientation between two adjacent grains results in a change in the carbide morphology from a plate-like to an acute triangular form. During the precipitation process, carbides preferentially hold the coherency with the one grain for which the one of {111} planes makes the smallest angle with the grain boundary plane. The carbides grow into the other grain at a later stage, having the lowest interfacial energy, which consequently resulted in the triangular carbide morphology. This investigation indicates that the precipitation behavior of carbide and its morphology are strictly determined by the minimization of energies for carbide nucleation and growth. After low cycle fatigue with a hold time at tensile peak strains, it is observed that cavity formation is more pronounced at random boundaries than at coincidence site lattice (CSL) boundaries. This result provides a good explanation that acute triangular carbides, which predominantly precipitate at random boundaries, are more likely to lead to cavity nucleation than the plate-like carbides precipitated at CSL boundaries because of their higher interfacial energy.
In 316 steel, microstructural observation reveals that the initially flat grain boundaries become serrated without the influence of carbide pinning points during aging treatment. Crystallographic features of the serrated grain boundaries were obtained through transmission electron microscope (TEM). Additionally, atomistic approach was applied to calculate the coincidence points on the serrated grain boundary across the two neighboring grains. Both investigations inform that by serration at the incipient stage of M23C6 precipitation, the interfacial free energy of the grain boundary can be lowered even though the grain boundary area is increased. It is also suggested that the occurrence of grain boundary serration is originated from the minimization of total free energy at a given temperature in the view of thermodynamics. The serrated grain boundaries can be considered as the stable configuration formed at a lower temperature while the straight grain boundaries at a higher temperature. High resolution TEM (HRTEM) investigations reveal the broad face of planar carbides formed at the serrated grain boundaries to be (11 1 ). These carbides probably possess low interfacial energy.
The occurrence of grain boundary serration is directly dependent on heat treatment condition, and involves the modification of carbide characteristics. The grain boundary serration leads to a change in the carbide characteristics as well as influencing grain boundary configuration, Le, morphology of carbide from an acute triangular to a planar loin! and the lowered density. Additionally, an array of carbide particles is changed from consistent to zigzag pattern in terms of their preference to one grain between two neighboring grains to share the coherency. Planar carbides on serrated grain boundaries have a lower interfacial energy than that of triangular carbides on straight grain boundaries. From these results, it is suggested that the enhancement of cavity resistance would be attained by the introduction of grain boundary serration and subsequent modification of carbide characteristics.
It is found that the grain boundaries are considerably serrated when a specimen is furnace-cooled, which has been proposed on the basis of investigation for the serration mechanism. The improvement of creep-fatigue resistance by the modification of carbides through the grain boundary serration is noticeably observed in both 304 and 316 steels. This result is understood by the fact that the modified carbides are favorable for the cavitation resistance, resulting in the retardation of cavity nucleation and growth. These design and control of optimum microstructure can be practically extended to apply to the real situations for better component reliability and performance of benefits. It is also expected that these results provide the basis of developing new alloys with good mechanical properties at high temperature.
오스테나이트계 스테인리스강에서 크리프-피로 변형시 주요 손상기구인 입계 cavity의 유용한 생성 site를 제공하는 입계탄화물의 체계적인 연구는 미비한 실정이다. 이는 스테인리스강이 오래 전부터 널리 사용되는 재료라고 생각해 볼 때 놀라운 일이 아닐 수 없다. 본 연구에서는 대표적이면서 고온 구조용 재료로서 가장 많이 적용되고 있는 304와 316 스테인리스강을 이용하여 서로 다른 모양과 분포를 갖는 각각의 입계탄화물에 대한 석출 및 제어에 대하여 고찰하였다. 결과들을 간략히 요약하자면, 304 스테인리스강의 경우 입계특성에 따라 탄화물의 모양과 분포가 달라짐을 확인하였으며, 결정학적인 고찰을 통해 탄화물들이 석출 및 성장시 시스템의 계면 자유에너지를 최소화하기 위해 삼각형 모양의 탄화물이 편평한 입계위에 생성된다는 것을 확인하였다. 아울러 크리프-피로 변형시 cavity 생성은 입계탄화물 및 입계특성과의 상관관계가 있음을 확인하고 이를 해석하였다. 한편, 316 스테인리스강에서는 시효처리 초기에 탄화물이 석출되기 전 입계 serration이 발생된다는 것을 세계최초로 발견하였으며, 입계 serration 발생에 대한 결정학적, 열역학적 고찰을 통해 낮은 온도에서 입계의 계면 자유 에너지(방위차)를 줄이기 위해 자발적으로 발생하여 serrated된 입계는 안정된 configuration이라는 것을 제시하였다. 이러한 입계 serration 발생은 등온 열처리 조건에 밀접하게 의존하는데, 높은 용체화 및 시효처리 온도 조건에서는 발생하지 않음을 확인하였다. 여러 가지 체계적인 미세구조 관찰을 통해 입계 serration은 입계모양 뿐만 아니라 탄화물의 모양과 밀도 및 정합배열에 상당한 영향을 준다는 것을 알 수 있었다. 게다가 serrated된 입계위에 형성된 탄화물들의 특성은 cavity 생성 저항성에 상당히 유리하도록 석출된다는 사실로부터 입계 serration 유도를 통한 미세구조 제어를 제안하였다. 입계 serration 기구에 대한 이론적 배경을 토대로 실제적으로 노냉을 시켰을 경우 입계가 심하게 serrated된다는 것을 확인하였다. 이러한 크리프-피로 특성 향상을 위한 미세조직 제어방안을 실제적으로 304 및 316 스테인리스강에 적용시켜 보았으며, 직접 실험을 통해 실제 크리프-피로 특성이 상당히 향상된다는 것을 검증하였다.
따라서, 본 연구결과의 의미를 따져 본다면, 입계특성을 고려한 입계탄화물 석출기구 규명 및 characterization, cavity 생성기구와 상관관계 제시, 입계 serration 기구 고찰 등의 학문적인 의미 이외에도 크리프-피로 저항성이 우수한 재료개발을 위한 미세구조 제어방법을 제안하는 등의 새로운 고온재료개발에도 본 연구결과를 직·간접적으로 응용할 수 있을 것으로 사료되어 매우 독창적이며 실용적인 연구로서 평가될 수 있다고 사료된다.
