• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.39 no.1
• /
• pp.27-35
• /
• 2015

The material properties of crack growth models at an elevated temperature were derived from the results of numerous crack growth tests for Mod.9Cr-1Mo (ASME Grade 91) steel specimens under fatigue loading and creep loading at an elevated temperature. These crack growth models were needed for defect assessment under creep-fatigue loading. The mathematical crack growth rate models for fatigue crack growth (FCG) and creep crack growth (CCG) were determined based on the test results, and the models were compared with those of the French design code RCC-MRx to investigate the conservatism of the code. The French design code RCC-MRx provides an FCG model and a CCG model for Grade 91 steel in Section III Tome 6. It was shown that the FCG model of RCC-MRx is conservative, while the CCG model is non-conservative compared with the present test data. Thus, it was shown that further validation of the property was required. Mechanical strength tests and creep tests were also conducted, and the test results were compared with those of RCC-MRx.

• Tribology and Lubricants
• /
• v.27 no.6
• /
• pp.308-313
• /
• 2011

In this study, friction tests were performed in order to investigate the effect of sliding velocity and normal load on the friction characteristics of DLC (a-C:H) and WC/C (a-C:H:W) using a ball-on-disk type friction tester. DLC and WC/C were deposited on AISI 52100 steel balls. Friction tests against carburized SCM 415 Cr-Mo steel disks were carried out under various sliding velocity (0.1, 0.78, 1.56, 3.13, 6.25, 12.5, 25, 50 and 100 mm/s) and normal load (2.4, 4.8 and 9.6 N) conditions while the relative humidity was 20~40 % R.H. and air temperature was $16{\sim}24^{\circ}C$. As results, kinetic friction coefficients of DLC and WC/C were obtained under each test condition. The results show that the kinetic friction coefficients of DLC and WC/C generally increase with the increase in sliding velocity. And, under the same sliding velocity condition, the kinetic friction coefficients are almost constant regardless of normal load. In addition, the kinetic friction coefficients of DLC are lower than those of WC/C under the same test conditions.

• Korean Journal of Metals and Materials
• /
• v.50 no.4
• /
• pp.271-279
• /
• 2012

9-12% Cr steels have been used in thermal power plants which repeat start and stop operations. Major factors of fatigue life are temperature, frequency, stress ratio, holding time, microstructure, and environment. Normally, fatigue life decreases at high temperature, low frequency, high stress ratio, and long holding time conditions. A Mod.9Cr-1Mo steel, called G91, was developed at ORNL (Oak Ridge National Laboratory, USA) and was adopted as a high-temperature structural material in the ASME Code in 2004. However, its low-cycle fatigue and fatigue crack growth characteristics have been rarely studied. In this work, we have investigated the low-cycle fatigue crack growth behaviors of G91 steel under various test conditions in terms of temperature and stress ratio. As temperature and stress ratio increase, the crack growth rate becomes faster and striation distance also increases. On the other hand, the number of branch cracks decreases.

• Journal of the Korean Society for Heat Treatment
• /
• v.28 no.4
• /
• pp.171-180
• /
• 2015

The variation in microstructure and texture during continuous annealing was examined in a series of 1.6% Mn-0.1% Cr-0.3% Mo-0.005% B steels with carbon contents in the range of 0.010 to 0.030%. It was found that microstructure of hot band consisted of ferrite and pearlite as a consequence of high coiling temperature, and eutectoid carbon content was between 0.011% and 0.016%. Martensite ranged in volume fraction from 1.5% to 4.0% when annealed at $820{\circ}C$ according to the typical continuous annealing cycle. The critical martensite content for the continuous yielding was about 4% from stress-strain curves. The continuous yielding was obtained in the 0.030% carbon steel and 0.010% to 0.020% carbon steels revealed some yield point elongation ranging from 0.8% to 2.2% in as-annealed conditions. Higher tensile strength in the higher carbon steel is due to both increase in the martensite volume fraction and ferrite grain refinement. Decreasing the carbon content to 0.01% strengthened the intensities of ${\gamma}$-fiber textures, resulting in the increase in the $r_m$ value, which was caused by the lower volume fraction of martensite. The higher carbon steels showed the lower $r_m$ value of about 1.0.

• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.26 no.10
• /
• pp.1953-1960
• /
• 2002

The viscoplasticity theory based on overstress, one of the unified state variable theories, is generalized to model zero (no influence of loading rate) and negative (flow stress decreases with loading rate) as well as positive (flow stress increases with loading rate) rate sensitivity in a consistent way. On the basis of the long-time asymptotic solution the different types of rate sensitivity are classified with respect to an augmentation function that is introduced in the evolution law fur a state variable equilibrium stress. The theory predicts normal relaxation and creep behaviors even if unusual rate sensitivity is modeled. The constitutive model fir the behavior of a modified 9Cr-1 Mo steel at various temperatures is then compared with experimental data found in the literature.

• Proceedings of the KWS Conference
• /
• 2002.10a
• /
• pp.355-360
• /
• 2002

ElectroSlag Strip Overlaying (ESSO) process has been around since 1970. ESSO process had limited acceptance due to a few problems associated with the use of this process in its very early stage. Limited knowledge and, most significantly, poor quality of the equipment and welding flux gave the ESSO process a bad name. However, this process is well accepted today and used in North America, Europe and Japan. The ESSO process provides low dilution overlays at high deposition rates, excellent and consistent deposit chemistry with excellent surface quality, and virtually no defects. Capitan has taken this process one step further through extensive research and development of the process itself as well as the equipment. The improvement brought to the process warranted the issuance in May 2000 of an US patent. This study demonstrates the feasibility of this process with immediate positive production results. The main achievements of this work are as follows: $\textbullet$ Development of six various strip-flux combinations on three different base materials: carbon steel, $\frac{1}{4}$ Cr/.5 Mo and 2 $\frac{1}{4}$ Cr/l Mo, fully tested with: penetrant, ultrasound, bends, hardness, overlay chemistry, corrosion and hydrogen disbonding. $\textbullet$ 12" dia. 90 hot formed elbows from straight pipe electroslag overlayed with "1 layer" and "2 layer" Alloy 625 $\textbullet$ a very unique development of miniaturized overlaying equipment able to perform overlay in pipe with diameters as low as 10" (254 mm). This development has large applications in the field of offshore, petrochemical, refining, pulp and paper and power generation industries. The aftermath of this development was its immediate acceptance by major end users with the completion of four projects of overlayed pipe in the USA and Far East Asia.

• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.22 no.1
• /
• pp.44-51
• /
• 1998

In order to develop the evaluation methods of degradation for the Ni-Cr-Mo-V steel, which is in use for turbine rotor in nuclear power plant, the degraded materials were prepared by simulated degradation methods. The result of impact test and fatigue crack growth test shows that the FATT(Fracture Appearance Transition Temperature) and fatigue crack growth rate increased with the increase of degradation. And the result of new electrochemical polarization test method was suggested for the evaluating FATT, fatigue crack growth exponent and coefficient C values based on the results of relationship between corrosion current density(Icorr) & FATT, and the m & C and Icorr.

• Journal of the Korean Society for Heat Treatment
• /
• v.9 no.4
• /
• pp.229-233
• /
• 1996

This work has been carried out to reduce the softening of heat affected zone on laser surface alloying. Iron based rapid solidification material with $Cr_{5-10}$, $V_{1-3}$, $Mo_{3-7}$, $W_{2-5}$, $B_{7-8}$, $C_{2-3}$, $Si_{0.5-1}at%$ was alloyed on the surface of SM45C steel. The excellent softening resistance in alloyed and heat affected zone showed, which could be attributed to the formation of stable high temperature precipitates.

• KEPCO Journal on Electric Power and Energy
• /
• v.6 no.1
• /
• pp.41-47
• /
• 2020

The contribution of damage mechanisms to failure of steam turbine casing made of Cr-Mo-V steel was investigated. Creep-fatigue interaction on the HP side corner of turbine casing was revealed as the root cause of the catastrophic failure performed by metallurgical analysis. The steady-state pressure and transient thermal stress were analyzed based on the actual operating condition of the thermal plant. Damage of creep-fatigue interaction to crack initiation was evaluated with multiaxial effects. The contribution ratio of creep and fatigue to the crack initiation was estimated to 3:1. Temporary geometrical correct action with repair weld was executed. For long-term operation, design improvement of casing equipment for creep resistance should be needed.

• Journal of the Korean Society of Safety
• /
• v.28 no.3
• /
• pp.6-10
• /
• 2013

The Heat Recovery Steam Generator(HRSG) is a device recycling the exhaust gas of gas turbine in combined power and chemical plants. Since service temperatures was very high, the damage of HRSG tubes intensively occurred in superheater and reheater. The aim of this paper is to determine life and hardness relationship that addresses creep-rupture test and creep-interrupt test in modified 9Cr-1Mo steel. The measured life that consists of function of hardness was found to constant tendency.