AISI 321 stainless steel with a high content of Ti, as compared with AISI 304 stainless steel, has been widely used in the power-generation industry because of its strong corrosion resistance through the inhibition of grain boundary sensitization. The added Ti forms TiC preferentially to prevent the formation of chromium-rich carbide precipitates in the grain boundaries, which are deleterious to the creep life of a material.
Generally, carbide at a grain boundary provides a preferential site for cavity nucleation under creep-fatigue interaction conditions. It has been reported that creep-fatigue properties are affected by carbide morphology, interfacial free energy between a carbide and neighboring grain and carbide density on the grain boundary. When Ti is added in austenitic stainless steels, TiC precipitates are uniformly formed in grain and at the grain boundary. Especially, TiC at the grain boundary can affect cavitation during high temperature fatigue cycles.
Earlier research concentrated only on the improvement of corrosion resistance by controlling the content of Ti in AISI 321 stainless steel. However, since the control of grain boundary carbides is known to be a very important factor in determining the mechanical properties of materials it is necessary to understand carbide characteristics at the grain boundary. Although the aforementioned studies addressed carbide precipitation, they only illustrated carbide precipitation behaviors on the basis of phenomenological observations. Moreover, there have not provided a systematic explanation of the TiC carbide, which is the main precipitate in AISI 321 stainless steel. Therefore, the investigation of TiC morphology and the relationships of interfaces between TiC and neighboring grains at the grain boundary may be considered to be significantly important.
In this study, investigations of the interfacial relationships between TiC carbides and neighboring grains in AISI 321 stainless steel have been conducted. In addition, the interfacial relationships between $Cr_23C_6$ carbides and grains are addressed as a reference for comparison with the case of TiC. And creep-fatigue tests are conducted using specimens with TiC carbides, which are main carbides and with $Cr_23C_6$ carbides, which are intentionally precipitated as a reference to be compared with the effects of TiC carbides in AISI 321 stainless steels.
It is observed that the interfacial planes between TiC and neighboring grains have lower indices than those between $Cr_23C_6$ and neighboring grains. From these results, it is suggested that interfacial free energy between TiC and grains is lower than that between $Cr_23C_6$ and grains. Regardless of carbide types (TiC and $Cr_23C_6$), the morphology of carbides edge at grain boundary is formed polyhedral shape rather than rectangular shape to reduce interfacial free energy. Also the growth direction of grain boundary carbides is forward to high interfacial plane to have more stable interfaces at edge of grain boundary. These investigation indicates that morphology of carbides are determined by the minimization of interfacial free energy during carbide growth.
interfaces at edge of grain boundary. These investigation indicates that morphology of carbides are determined by the minimization of interfacial free energy during carbide growth.
Total strain range controlled creep-fatigue tests with a 30 minute tensile hold time at the maximum tensile strain were conducted at 600 ℃ in order to investigate effects of TiC and $Cr_23C_6$ carbides on the creep-fatigue properties in TiC and $Cr_23C_6$ aged 321 stainless steels with the same grain carbide density. After the tests, it is observed that creep-fatigue life of TiC aged alloy is longer than that of $Cr_23C_6$ aged alloy. The difference of creep-fatigue life between the two alloys is based on the stronger cavitation resistance of TiC aged alloy compared with that of $Cr_23C_6$ aged alloy. From microstructural observation, it is verified that formation and growth of cavities in TiC aged alloy are more retarded than those in $Cr_23C_6$ aged alloy. In creep-fatigue properties of TiC aged alloys with different TiC carbide density at grain boundary aged alloy, creep-fatigue life of TiC aged alloy is improved with decrease of carbide density at grain boundary as like creep-fatigue life of $Cr_23C_6$ aged alloy. However, the grain boundary carbides density of small TiC aged alloy is higher than that of $Cr_23C_6$ aged alloy, creep-fatigue life of small TiC aged alloy is longer than that of $Cr_23C_6$ aged alloy.
Cavity nucleation factors between TiC aged alloy and $Cr_23C_6$ aged alloy are compared to investigate cavitation resistances of the alloys with the carbide types. From the results of calculation, cavity nucleation factor of TiC aged alloy is lower than that of $Cr_23C_6$ aged alloy. It is considered that the types of carbides are a more influential factor than the density of carbides in determining the value of the cavity nucleation factor.
The control of TiC carbide precipitation is suggested to improve creep-fatigue properties and intergranular corrosion resistance. When AISI 321 stainless steel is furnace-cooled and aged at the higher aging temperature, growth of TiC carbide is accelerated by help of thermal diffusion. Creep-fatigue properties of TiC aged alloy is improved with decreasing of grain boundary carbide density. This design can be helpful to improve this alloy for better component reliability and performance of benefits. These results can also be the basis of developing new alloys with good mechanical properties at high temperature.
