The martensitic precipitation hardening stainless steel was developed in 1950's to overcome the limitation of fully austenitic and fully martensitic stainless steels on account of strength, ductility, toughness and high temperature performance capability. Precipitation-hardened stainless steels has ...
The martensitic precipitation hardening stainless steel was developed in 1950's to overcome the limitation of fully austenitic and fully martensitic stainless steels on account of strength, ductility, toughness and high temperature performance capability. Precipitation-hardened stainless steels has been gaining increasing importance as structural materials in a variety of applications in aircraft, chemical and nuclear industries. These steels possess good corrosion resistance and excellent mechanical properties that can be achieved by means of suitable quench-ageing heat treatments. Among these, the oldest and the most widely used steel is 0.07C-l7Cr-4Ni-5Cu composition, best known as 17-4PH steel. Although much engineering data on 17-4PH stainless steel are available and investigations on microstructures and mechanical properties of this alloy, heat treated to different conditions, have been reported in the literature, the emphasis in those investigations has been on the dctermination of suitable heat treatment conditions which result in the optimum combination of tensile strength, ductility and impact strength. Furthermore, the effect of the long-term high temperature exposure about service temperature on microstructures of this alloy has not been manifested in much detail, especially the change in δ-ferrite phase. In the present study, the systematic effect of heat treatment steps, such as homogenizing, solid solution treatment followed by aging treatment, on microstructures and mechanical properties of 17-4PH steel was investigated. Furthermore, the microstructural evolution and variation of mechanical properties depending on long-term aging at 400℃ was characterized in order to obtain a better understanding of the embrittlement phenomena on aging. In addition, after the long-term exposure, the homogenizing and aging treatment was employed to examine the possibility of the recovery treatment. As the homogenizing treatment time increased, the length of δ-ferrite decreased and elongated shape of δ-ferrite turned to sphere shape with the decrease of volume fraction. As the aging treatment temperature increased, the strength decreased while the toughness increased. It seemed that the this result was caused by the austenite phase reversion as well as coarsening of precipitates over 600℃. The long-term exposure test was carried out using the specimen which was homogenized at 1149℃ and solid solution treated at 1038℃ followed by aging at 480℃ The TEM microscopy revealed that the fine ε-Cu was precipltated in the martensite, after 70 hours exposure. After 400 hours exposure, the fcc-Cu precipitatcs in the size of about 10 nm were observed in δ-ferrite phase. It is considered that this precipitates in δ-ferrite caused the increase of strength and drastic decrease of elongation. As the exposed time elapsed over 1000 hours, the size of fcc-Cu precipitates was increased up to several tens of nano-motors and its numbers was increased also. Tt seemed that these increase of precipitates caused the increase of tensile strength up to 1562 MPa and drastic decrease of elongation up to 11 % after the exposure of 3060 hours. The long-term exposed specimen over 2000 hours was homogenized and aging treated again to manifest the possibility of recovery. The strength and elongation was restored after recovery treatment and the fcc-Cu precipitates were almost dissolved into the δ-ferrite matrix. It is well known that the martensite phase of 17-4PH steel is decomposed into the Fe-rich a and the Cr-enriched α' since the Cr concentration in 17-4 PH is within the spinodal line, and this α' phase induced the embrittlement. However, under the consideration of the results in this study, it seemed that the embrittlement phenomena on aging is caused by not only spinodal decomposition but also the nucleation and growth of Cu precipitates in δ-ferrite.
The martensitic precipitation hardening stainless steel was developed in 1950's to overcome the limitation of fully austenitic and fully martensitic stainless steels on account of strength, ductility, toughness and high temperature performance capability. Precipitation-hardened stainless steels has been gaining increasing importance as structural materials in a variety of applications in aircraft, chemical and nuclear industries. These steels possess good corrosion resistance and excellent mechanical properties that can be achieved by means of suitable quench-ageing heat treatments. Among these, the oldest and the most widely used steel is 0.07C-l7Cr-4Ni-5Cu composition, best known as 17-4PH steel. Although much engineering data on 17-4PH stainless steel are available and investigations on microstructures and mechanical properties of this alloy, heat treated to different conditions, have been reported in the literature, the emphasis in those investigations has been on the dctermination of suitable heat treatment conditions which result in the optimum combination of tensile strength, ductility and impact strength. Furthermore, the effect of the long-term high temperature exposure about service temperature on microstructures of this alloy has not been manifested in much detail, especially the change in δ-ferrite phase. In the present study, the systematic effect of heat treatment steps, such as homogenizing, solid solution treatment followed by aging treatment, on microstructures and mechanical properties of 17-4PH steel was investigated. Furthermore, the microstructural evolution and variation of mechanical properties depending on long-term aging at 400℃ was characterized in order to obtain a better understanding of the embrittlement phenomena on aging. In addition, after the long-term exposure, the homogenizing and aging treatment was employed to examine the possibility of the recovery treatment. As the homogenizing treatment time increased, the length of δ-ferrite decreased and elongated shape of δ-ferrite turned to sphere shape with the decrease of volume fraction. As the aging treatment temperature increased, the strength decreased while the toughness increased. It seemed that the this result was caused by the austenite phase reversion as well as coarsening of precipitates over 600℃. The long-term exposure test was carried out using the specimen which was homogenized at 1149℃ and solid solution treated at 1038℃ followed by aging at 480℃ The TEM microscopy revealed that the fine ε-Cu was precipltated in the martensite, after 70 hours exposure. After 400 hours exposure, the fcc-Cu precipitatcs in the size of about 10 nm were observed in δ-ferrite phase. It is considered that this precipitates in δ-ferrite caused the increase of strength and drastic decrease of elongation. As the exposed time elapsed over 1000 hours, the size of fcc-Cu precipitates was increased up to several tens of nano-motors and its numbers was increased also. Tt seemed that these increase of precipitates caused the increase of tensile strength up to 1562 MPa and drastic decrease of elongation up to 11 % after the exposure of 3060 hours. The long-term exposed specimen over 2000 hours was homogenized and aging treated again to manifest the possibility of recovery. The strength and elongation was restored after recovery treatment and the fcc-Cu precipitates were almost dissolved into the δ-ferrite matrix. It is well known that the martensite phase of 17-4PH steel is decomposed into the Fe-rich a and the Cr-enriched α' since the Cr concentration in 17-4 PH is within the spinodal line, and this α' phase induced the embrittlement. However, under the consideration of the results in this study, it seemed that the embrittlement phenomena on aging is caused by not only spinodal decomposition but also the nucleation and growth of Cu precipitates in δ-ferrite.
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#열처리 노출시간 스테인레스강 Heat Treatment Exposure Time
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