High-Temperature Austenitic Stainless Steel

Improvement in COv2 emission and efficiency of power plants calls for an increase in the operating temperature of such plants. The structural alloys employed are already at their potential limit, which in turn necessitates design of more advanced and efficient alloys. Such alloys must have satisfactory performance at high temperature with reasonable cost. Therefore, while Ni-based alloys can demonstrate the required response, their higher cost compared to their steel counterparts can limit their application. Such steels, however, require improvement in high-temperature mechanical strength, as well as oxidation resistance. To address the former, we hypothesize that a high density of low-energy, high-angle boundaries (e.g. coherent twin boundaries) will improve the high temperature strength, without significantly sacrificing stability or ductility. To this end, conventional 316 stainless steels were thermo-mechanically processed to create a high volume fraction of deformation twins in an austenite matrix with low dislocation density. The deformation twins were found to be thermally stable up to 800 °C, and in some case, they start to disappear at around 1000 °C. This microstructural feature was shown to be beneficial in improving the strength of steel at -100 °C to 500 °C temperature range, while maintaining an acceptable level of ductility. The enhancement of strength of this structure showed a peculiar resistance to increase in temperatures, i.e. the relative increase in yield strength increases as temperature is increased. This behavior was found to be a contribution of thermally-stable twin bands. This unique structure is also expected to perform well in creep condition, and be even further improved through precipitation Improvement in CO2 emission and efficiency of power plants calls for an increase in the operating temperature of such plants. The structural alloys employed are already at their potential limit, which in turn necessitates design of more advanced and efficient alloys. Such alloys must have satisfactory performance at high temperature with reasonable cost. Therefore, while Ni-based alloys can demonstrate the required response, their higher cost compared to their steel counterparts can limit their application. Such steels, however, require improvement in high-temperature mechanical strength, as well as oxidation resistance. To address the former, we hypothesize that a high density of low-energy, high-angle boundaries (e.g. coherent twin boundaries) will improve the high temperature strength, without significantly sacrificing stability or ductility. To this end, conventional 316 stainless steels were thermo-mechanically processed to create a high volume fraction of deformation twins in an austenite matrix with low dislocation density. The deformation twins were found to be thermally stable up to 800 °C, and in some case, they start to disappear at around 1000 °C. This microstructural feature was shown to be beneficial in improving the strength of steel at -100 °C to 500 °C temperature range, while maintaining an acceptable level of ductility. The enhancement of strength of this structure showed a peculiar resistance to increase in temperatures, i.e. the relative increase in yield strength increases as temperature is increased. This behavior was found to be a contribution of thermally-stable twin bands. This unique structure is also expected to perform well in creep condition, and be even further improved through precipitation strengthening. As for the oxidation resistance, alumina-forming austenitic stainless steels have been proposed as a more stable alternative than chromia-forming steels at high temperatures. There are a few successful alumina-forming system reports in literature, however, they require high levels of microalloying addition, and they were designed mostly in an ad-hoc manner. In this work, we developed an alumina-forming austenitic stainless steel that was designed to employ thermodynamic and kinetics of oxidation. This model also aimed to have the lowest extent of alloying, and to produce the “leanest” alumina-forming austenitic stainless steel composition. Short-term and long-term oxidation tests demonstrated the capability to form alumina scale. This alloy can used as the baseline for alumina scale formation studies, with further alloying additions

Role of thermally-stable deformation twins on the high-temperature mechanical response of an austenitic stainless steel

In the present study, a two-step thermo-mechanical processing consisting of cold work and heat treatment steps was performed to increase the operating temperature of 316 austenitic stainless steels. A hierarchical microstructure of thermally-stable, nano twin bands was achieved forming into bundles in elongated grains. The mechanical response of the samples with this microstructure was evaluated through uniaxial tension tests at temperatures ranging from 20 °C to 500 °C and compared with those from the fully annealed samples. The results demonstrate that such hierarchical microstructure leads to a significant increase in the elevated temperature yield strengths due to the presence of nano-twin boundaries and resulting decrease in dislocation mean free path and increase in dislocation storage capacity. In fact, the yield strength ratio of the twinned and annealed samples increases with increasing temperature up to 500 °C, indicating the effectiveness of pre-existing thermally-stable twin boundaries as the strengthening source at temperatures as high as 0.46 homologous temperature. The hierarchical microstructure also led to irregular serrations through dynamic strain aging in the stress-strain response at 500 °C, which is attributed to the bi-modal microstructural length-scales present in the structure affecting the diffusion distances during dynamic strain aging. This structure also increases the tensile strength, and without a total loss in ductility, even though the flow stress of the twinned samples surpasses the tensile strength of the annealed samples, especially at elevated temperatures. Total hardening rate is consistently higher in the twinned samples as compared to the annealed samples, indicating the positive role of nano-twin boundaries in the dislocation storage capacity at elevated temperatures. Overall, the present study clearly demonstrate the positive role of thermally stable nano-twins on the elevated temperature mechanical response of austenitic stainless steels

High-Temperature Austenitic Stainless Steel

Improvement in COv2 emission and efficiency of power plants calls for an increase in the operating temperature of such plants. The structural alloys employed are already at their potential limit, which in turn necessitates design of more advanced and efficient alloys. Such alloys must have satisfactory performance at high temperature with reasonable cost. Therefore, while Ni-based alloys can demonstrate the required response, their higher cost compared to their steel counterparts can limit their application. Such steels, however, require improvement in high-temperature mechanical strength, as well as oxidation resistance. To address the former, we hypothesize that a high density of low-energy, high-angle boundaries (e.g. coherent twin boundaries) will improve the high temperature strength, without significantly sacrificing stability or ductility. To this end, conventional 316 stainless steels were thermo-mechanically processed to create a high volume fraction of deformation twins in an austenite matrix with low dislocation density. The deformation twins were found to be thermally stable up to 800 °C, and in some case, they start to disappear at around 1000 °C. This microstructural feature was shown to be beneficial in improving the strength of steel at -100 °C to 500 °C temperature range, while maintaining an acceptable level of ductility. The enhancement of strength of this structure showed a peculiar resistance to increase in temperatures, i.e. the relative increase in yield strength increases as temperature is increased. This behavior was found to be a contribution of thermally-stable twin bands. This unique structure is also expected to perform well in creep condition, and be even further improved through precipitation Improvement in CO2 emission and efficiency of power plants calls for an increase in the operating temperature of such plants. The structural alloys employed are already at their potential limit, which in turn necessitates design of more advanced and efficient alloys. Such alloys must have satisfactory performance at high temperature with reasonable cost. Therefore, while Ni-based alloys can demonstrate the required response, their higher cost compared to their steel counterparts can limit their application. Such steels, however, require improvement in high-temperature mechanical strength, as well as oxidation resistance. To address the former, we hypothesize that a high density of low-energy, high-angle boundaries (e.g. coherent twin boundaries) will improve the high temperature strength, without significantly sacrificing stability or ductility. To this end, conventional 316 stainless steels were thermo-mechanically processed to create a high volume fraction of deformation twins in an austenite matrix with low dislocation density. The deformation twins were found to be thermally stable up to 800 °C, and in some case, they start to disappear at around 1000 °C. This microstructural feature was shown to be beneficial in improving the strength of steel at -100 °C to 500 °C temperature range, while maintaining an acceptable level of ductility. The enhancement of strength of this structure showed a peculiar resistance to increase in temperatures, i.e. the relative increase in yield strength increases as temperature is increased. This behavior was found to be a contribution of thermally-stable twin bands. This unique structure is also expected to perform well in creep condition, and be even further improved through precipitation strengthening. As for the oxidation resistance, alumina-forming austenitic stainless steels have been proposed as a more stable alternative than chromia-forming steels at high temperatures. There are a few successful alumina-forming system reports in literature, however, they require high levels of microalloying addition, and they were designed mostly in an ad-hoc manner. In this work, we developed an alumina-forming austenitic stainless steel that was designed to employ thermodynamic and kinetics of oxidation. This model also aimed to have the lowest extent of alloying, and to produce the “leanest” alumina-forming austenitic stainless steel composition. Short-term and long-term oxidation tests demonstrated the capability to form alumina scale. This alloy can used as the baseline for alumina scale formation studies, with further alloying additions