• 연구논문집
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• s.31
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• pp.157-163
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• 2001

Evolution of microstructure due to service exposure to high temperature has a strong effect performance of heat resistant steels. In case of modified 9Cr-1Mo steels, precipitation of $Fe_2Mo$-type laves phases and coarcening of $M_23C_6$-type carbides is the primary cause of degradation of mechanical properties such as creep resistance, tensile strength and toughness. Creep tests have been carried out on pre-aging mod. 9Cr-1Mo steels to examine the effect of pre-aging and stress on the creep strength. Based on the results, a nondestructive procedure, where electrochemical technique that quantitatively detect laves phases and $M_23C_6$-type carbides in a material is used, has been proposed to evaluate a residual creep life of mod. 9Cr-1Mo steels.

• Journal of Magnetics
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• v.16 no.1
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• pp.42-45
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• 2011

The present work studies a nondestructive evaluation of the degradation of modified 9Cr-1Mo steel using a magnetic method based on the existence of the peaks of reversible permeability (RP) in the differential magnetization around the coercive force. The apparatus is based on detection of the voltage induced in a coil using a lock-in amplifier tuned to the frequency of the AC perturbing field. Results obtained for the reversible permeability and Vickers hardness on the aged samples showed the peak interval of reversible permeability (PIRP) and Vickers hardness decrease as aging time increased. The correlation between Vickes hardness and the PIRP could be used to evaluate degradation of modified 9Cr-1Mo steel.

• Proceedings of the KSME Conference
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• 2003.04a
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• pp.637-642
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• 2003

The Modified 9Cr-1Mo steel identified as T91, P91 and F91 in the ASME specification has been widely used for the construction of modern power plants. The available data on the influence of process parameters during manufacturing and fabrication on its properties are not sufficient. In this study, the influence of various thermal cycles on the hardness and the creep rupture strength was analyzed in the base metal and the weldments made in tube and pipe of a Mod.9Cr-1Mo steel. The low hardness, 155Hv, showed low creep rupture strength below the allowable stresses of T91 base metal in the ASME specification. This low value was attributed to the fully recovered dislocation structure and the weakening of precipitation hardening associated with the abnormal thermal cycles.

• Korean Journal of Materials Research
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• v.9 no.11
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• pp.1137-1143
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• 1999

Microstructure and mechanical properties of Mod.9Cr-1Mo and W added 9Cr-0.5Mo2W steels were investigated for liquid metal reactor (LMR) heat exchange tube. The tempering temperatures at which cell structure was formed were $700^{\circ}C$ for Mod.9Cr-1Mo steel and $750^{\circ}C$ for W added 9Cr0.5Mo-2W steel. indicating the recovery of dislocation was delayed by the addition of W. 9Cr-0.5Mo-2W steel had the same kinds of precipitates with Mod.9Cr-1Mo steel, but the W was included in the precipitates in 9Cr-0.5Mo-2W steel. Micro-hardness and ultimate tensile strength of 9Cr-0.5Mo-2W steel were higher than those of Mod.9Cr-1Mo steel. The impact property of Mod.9Cr-1Mo steel was superior to that of 9Cr-0.5Mo-2W steel.

• Korean Journal of Metals and Materials
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• v.47 no.5
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• pp.283-289
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• 2009

This paper dealt with a statistical analysis for evaluating the creep crack growth rate (CCGR) for Modified 9Cr-1Mo (ASTM Grade 91) steel. The CCGR data was obtained by the creep crack growth (CCG) tests conducted under various applied loads at $600^{\circ}C$. To obtain logically the B and q values used in the CCGR equation, three methods such as the least square fitting method (LSFM), the mean value method (MVM) and the probabilistic distribution method (PDM) were adopted and their CCGR lines were compared, respectively. In addition, a number of random variables were generated by using the Monte Carlo simulation (MCS), and the CCGR lines were predicted probabilistically. It was found that both the B and q coefficients followed a 2-parameter Weibull distribution well. In the case of the ranges of 10-90% for the probability variables, P(B, q), the CCGR lines were predicted. Fractographic study was conducted from the specimen after the CCG tests.

• Journal of the Korean Society for Heat Treatment
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• v.23 no.4
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• pp.198-204
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• 2010

Modified 9Cr-1Mo steels possess excellent high-temperature mechanical properties and are widely used in energy conversion industries. However, in-service materials degradation, such as softening, carbide-induced embrittlement, temper embrittlement, etc., can take place during long-term operation. Evolution of microstructure due to service exposure to high temperature has a strong effect on the performance of heat resistant steels. In case of modified 9Cr-1Mo steels, precipitation of $Fe_2Mo$-type laves phases and coarsening of $M_{23}C_6$-type carbides are the primary cause of degradation of mechanical properties such as toughness, hardness, tensile strength and creep resistance. This study was aimed at finding reliable parameter for assessing the integrity of modified 9Cr-1Mo steels. Characteristic parameters were attained between mechanical and ultrasonic properties.

• Proceedings of the KSME Conference
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• 2001.06a
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• pp.223-230
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• 2001

The Monkman-Grant (M-G) and its modified parameters were estimated for modified type 316LN and $9{\sim}12Cr-1Mo$ steels with chemical variations. Several sets of creep data were obtained by constant-load creep tests in $550-650^{\circ}C$ ranges. The relation parameters, m, $m^*$, C and $C^*$ were proposed and discussed for two alloy systems. In creep fracture mode, type 316LN steel showed domination of the intergranular fracture caused by growth and coalescence of cavities. On the other hand, the Cr-Mo steel showed transgranular fracture of the ductile type caused from softening at high temperature. In spite of the basic differences in creep fracture modes as well as creep properties, the M-G and its modified relations demonstrated linearity within the $2{\sigma}$ standard deviation. The value of the m parameter of the M-G relation was 0.90 in the 316LN steel and 0.84 in the Cr-Mo steel. The value of the $m^*$ parameter of the modified relation was 0.94 in the 316LN steel and 0.89 in Cr-Mo steel. The modified relation was superior to the M-G relation because the $m^*$ slopes almost overlapped regardless of creep testing conditions and chemical variations to the two alloy systems.

• Journal of the Korean Society for Heat Treatment
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• v.23 no.3
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• pp.156-162
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• 2010

The modified 9Cr-1Mo steel (P91 steel) is very popular as a boiler tube material of the USC (Ultra-Supercritical) power plants. The steels were tempered in the temperature range of 400 to $650^{\circ}C$ and the mechanical tests, such as impact and hardness tersts were performed at the room temperature for the tempered steels. A drop in the impact value (embrittlement) and the hardeness increase were simultanously observed in the range of temperature between $475^{\circ}C$ and $600^{\circ}C$, particularly at $550^{\circ}C$. TEM observation shows the hardening was caused by $M_2C$, resulting in the embrittlement. And the maximum volume fraction of $M_3C$ was also observed at $550^{\circ}C$, Therefore, the embrittlement seems to be caused by both the $M_2C$ and $M_3C$.

• Korean Journal of Materials Research
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• v.21 no.11
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• pp.592-595
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• 2011

It is well known that modified 9Cr-1Mo steel has a low thermal expansion and high thermal conductivity with excellent high temperature properties compared to austenitic stainless steel. For these advantages, the steel is very popular for the boiler tube of thermal power plants. Normalizing is commonly utilized to obtain martensite in this steel, which shows an unusual toughness for martensite. However, some accidents related to this steel have been reported recently, opening the necessity for further study. As a particular behavior of the steel, an abrupt drop of the impact value has been identified upon tempering at 750$^{\circ}C$ for about 1 hour. It is well known that $Fe_3C$ forms during autotempering and turns to $Cr_2C$ at an early stage and then transforms to $Cr_{23}C_6$. In this study, the cause of the abrupt drop of the impact value was investigated with an impact test, microstructural observation, nanodiffraction and phase analyses using instruments such as optical and transmission electron microscopes (TEM) with an extraction carbon replica of the carbides. The analyses revealed that the $M_2C$ that formed when retained for about 1 hour at 750$^{\circ}C$ causes a drastic decrease in the mechanical properties. The sharp drop in mechanical properties, however, disappeared as the $M_2C$ transformed into $M_{23}C_6$ with longer retention.

• Korean Journal of Materials Research
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• v.22 no.1
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• pp.46-53
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• 2012

In this study, mechanical tests and microstructural analyses including TEM analyses with EDX of precipitates in modified 9Cr-1Mo steel were carried out to determine the cause of embrittlement observed after heat-treatment, which limits the usage of the alloy for power plants. Mod. 9Cr-1Mo steel specimens at austenite temperature were quenched to the molten salt baths at $760^{\circ}C$ and $700^{\circ}C$, in which the specimens were kept for 10 min ~ 10 hr with subsequent air-cooling. Impact tests showed that the impact value dropped abruptly when the specimens were kept longer than 30 min at $\sim760^{\circ}C$ reaching to minima in about 1 hr, and then increasing at further retention. The tensile strength of the specimens reached the minimum value without much change afterward, whereas the values of elongation showed the same trend as that of the impact value. The isothermally heat-treated steel at $700^{\circ}C$ also showed a minimum impact value in about 1 hr. These results suggest that the isothermal heattreatment at 760 and $700^{\circ}C$ for about 1 hr induces temporal embrittlement in Mod. 9Cr-1Mo steel. The microstructural examination of all the specimens with extraction replica of the carbides revealed that the specimens with temporal embrittlement had $Cr_2C$, indicating that the cause of the embrittlement was the precipitation of the $Cr_2C$. In addition, TEM/EDX results showed that the Fe/Cr ratio was 0.033 to 0.055 for $Cr_2C$, whereas it was 0.48 to 0.75 for $Cr_{23}C_6$, making the distinction of the $Cr_2C$ and $Cr_{23}C_6$ possible even without direct electron diffraction analyses.