• Proceedings of the Materials Research Society of Korea Conference
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• 2006.05a
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• pp.37.2-37.2
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• 2006

• 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.

• Journal of the Korean Society for Nondestructive Testing
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• v.27 no.5
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• pp.417-425
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• 2007

The ferritic 9Cr-1Mo-V-Nb heat-resisting steel was experimentally studied in order to characterize its microstructural evolution during creep-fatigue by coercivity measurement. The creep-fatigue test was conducted at $550\;^{\circ}C$ with the tensile holding time of 60s and 600s, respectively. The coercivity decreased until the failure and the hardness monotonously decreased for the whole fatigue life. As the life fraction of creep-fatigue increased, the $M_{23}C_6$ carbide coarsened following the Ostwald ripening mechanism. However, the MX carbonitrides did not grow during creep-fatigue due to so stable at $550\;^{\circ}C$. The width of martensite lath increased because of the dislocation recovery at the lath boundaries. The magnetic coercivity has an influence on the microstructural properties such as dislocation, precipitates and martensite lath boundaries, which interpreted in relation to microstructural changes. Consequently, this study proposes a magnetic coercivity to quantify the level of damage and microstructural change during the creep-fatigue of ferritic 9Cr-1Mo-V-Nb steel.

• Journal of Power System Engineering
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• v.14 no.2
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• pp.84-89
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• 2010

The effect of forging condition on the microstructure and mechanical properties of 9Cr-1Mo heat-resisting steel was investigated. Microstructure of centrifugal casted 9Cr-1Mo heat resisting steel and forged heat resisting steel are consisted of martensite. With the increase of forging ratio, tensile strength and hardness increased, while elongation and impact value decreased. By increasing of forging starting temperature and finishing temperature, tensile strength and hardness increased, while elongation and impact value decreased. We obtained the optimum forging conditions as follow, forging ratio is 30%, forging starting temperature is $1200^{\circ}C$ and forging finishing temperature is $950^{\circ}C$.

• 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 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 Power System Engineering
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• v.14 no.6
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• pp.29-34
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• 2010

As the steam temperature of fossil power plant boiler is increasing, the use of 9Cr-1Mo high alloy material is prevalent and it is needed to investigate the characteristics of low cycle fatigue for high alloy and austenite stainless steel that has used up to recently. As a result of test, in 9Cr-1Mo high alloy steel, the relation of strain and fatigue life is non-linear and the crack mode of low cycle fatigue is brittle but in the austenite stainless steel, that of strain and fatigue life is linear and the crack mode of low cycle fatigue is ductile. Comparing the fatigue life between high alloy and austenite stainless steel, there is no consistent characteristics as to strains. But the fatigue life of 9Cr-1Mo steel is longer by 25% than that of STS304 stainless steel in the relatively low, 0.3% strain. In the other strain, the fatigue life of two materials is similar.

• Journal of Welding and Joining
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• v.22 no.5
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• pp.26-31
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• 2004

In order to propose the optimum welding condition for field application, the effects of welding heat input and cooling rate at PWHT on the mechanical properties were investigated. Submerged arc welding of 1.25Cr-0.5Mo steel for pressure vessel was conducted at welding heat inputs of 15.2kJ/cm, 30.9kJ/cm, and 44.8kJ/cm, and cooling rates of 184$^{\circ}C$/hr, 55$^{\circ}C$/hr, and 2$0^{\circ}C$/hr at PWHT. From the test results, as the welding heat input increase up to 30.9kJ/cm, the changes of microstructure and impact toughness were small. At the heat input of 44.8kJ/cm, however, toughness decreased obviously due to the coarsening of coarse-grained HAZ and formation of ferrite at bainite grainboundary of weld metal. On the other hand, cooling rates at PWHT did not effect on the changes in microstructure and mechanical properties. Even though tensile strength and impact toughness at all welding conditions of this study were above the minimum specification requirement, it was confirmed that heat input of 30.9kJ/cm was the optimum welding condition to improve welding performance by higher heat input.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.35 no.3
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• pp.251-257
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• 2011

This paper deals with the statistical properties of parameters B and q in the creep crack growth rate (CCGR) law, da/dt=B$(C^*)^q$, in Mod. 9Cr-1Mo (ASME Gr.91) steel which is considered a candidate materials for fabricating next generation nuclear reactors. The CCGR data were obtained by creep crack growth (CCG) tests performed on 1/2-inch compact tension (CT) specimens under an applied load of 5000N at a temperature of $600^{\circ}C$. The CCG behavior was analyzed statistically using the empirical equation between CCGR, da/dt and the creep fracture mechanics parameter, $C^*$. The B and q values were determined for each specimen by the least-squares fitting method. The probability distribution functions for B and q were investigated using normal, log-normal, and Weibull distributions. As far as this study is considered, it can be appeared that B and q followed the log-normal and Weibull distributions. Moreover, a strong positive linear correlation was found between B and q.

• Transactions of the KSME C: Technology and Education
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• v.4 no.2
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• pp.101-109
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• 2016

The material properties of heat resistant materials at power plants are affected by thermal aging as operating time is accumulated. In this study, the influence of thermal aging on yield strength, tensile strength and fracture behavior for Mod.9Cr-1Mo (ASME Grade 91) steel which is a material widely adopted for Generation IV nuclear energy system has been investigated and analyzed. Service exposed Gr.91 steel materials sampled from a piping system of an ultra-supercritical (USC) plant in Korea with accumulated operation time of 73,716 hours were used for material testing. The test results of the service exposed material specimens were compared with those of the virgin Gr.91 steel specimens. Those test data were compared with the material properties of ASME code and RCC-MRx code. Conservatisms of the material properties in the design codes have been quantified based on the comparisons of those from virgin and service exposed material specimens.